Patent Publication Number: US-10774596-B2

Title: Rolling cutter stability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States national phase of International Patent Application Serial No. PCT/US2016/052727, filed Sep. 21, 2016 and titled “Improvements on Rolling Cutter Stability,” which claims the benefit of, and priority to, U.S. Patent Application Ser. No. 62/234,555, filed Sep. 29, 2015 and titled “Improvements on Rolling Cutter Stability,” which application is expressly incorporated herein by this reference in its entirety. 
    
    
     BACKGROUND 
     Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and cutter pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. 
     Drag bits, often referred to as “fixed cutter” drill bits, include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material that forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (which may be made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits. 
     In PDC bits, PDC cutters are received within cutter pockets, which are formed within blades extending from a bit body, and may be bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades. 
     A PDC cutter may be formed by placing a sintered carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treated under high pressure, high temperature conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn integrally bonded to the substrate. The substrate may be made of a metal-carbide composite material, such as tungsten carbide-cobalt. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.” 
     An example of PDC bit having a plurality of cutters with ultra hard working surfaces is shown in  FIGS. 1 and 2 . The drill bit  100  includes a bit body  110  having a threaded upper pin end  111  and a cutting end  115 . The cutting end  115  includes a plurality of ribs or blades  120  arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body  110 . Cutting elements, or cutters,  150  are embedded in the blades  120  at angular orientations and radial locations relative to a working surface and with a back rake angle and side rake angle against a formation to be drilled. 
     A plurality of orifices  116  are positioned on the bit body  110  in the areas between the blades  120 , which may be referred to as “gaps” or “fluid courses.” The orifices  116  are adapted to accept nozzles. The orifices  116  allow drilling fluid to be discharged through the bit  100  in selected directions and at selected rates of flow between the blades  120  for lubricating and cooling the drill bit  100 , the blades  120 , and the cutters  150 . The drilling fluid also cleans and removes the cuttings as the drill bit  100  rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters  150  may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel upward past the drill bit  100  toward the surface of a wellbore. 
     Referring to  FIG. 2 , a top view of a prior art PDC bit is shown. The cutting face  118  of the bit shown includes a plurality of blades  120 , and each blade has a leading side  122  facing the direction of bit rotation, a trailing side  124  (opposite from the leading side  122 ), and a top side  126  that faces the formation. Each blade  120  includes a plurality of cutting elements or cutters extending radially from the center of cutting face  118  and generally forming rows. Certain cutters, although at differing axial positions, may occupy radial positions that are in similar radial position to other cutters on other blades. 
     Cutters may be attached to a drill bit or other downhole tool by a brazing process. In the brazing process, a braze material is positioned between the cutter and the cutter pocket. The material is melted and, upon subsequent solidification, bonds (attaches) the cutter in the cutter pocket. Selection of braze materials depends on their respective melting temperatures, to avoid excessive thermal exposure (and thermal damage) to the diamond layer prior to the bit (and cutter) even being used in a drilling operation. Specifically, alloys suitable for brazing cutting elements with diamond layers thereon have been limited to a few alloys that offer relatively low brazing temperatures to avoid or reduce damage to the diamond layer and high enough braze strength to retain cutting elements on drill bits. 
     A factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Polycrystalline diamond may be stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond may be due to the substantial difference in the coefficient of thermal expansion of the binder material (e.g., cobalt), as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures. 
     SUMMARY 
     In some aspects, a cutting element includes a cutting end extending a depth from a cutting face to an interface surface opposite the cutting face and a spindle. The spindle is axially separated from the cutting end by a transition region, and the spindle has a spindle diameter at a spindle side surface that is less than a cutting end diameter and a guide length measured from a point of transition to the transition region to a retention feature. The guide length is longer than 75% of a total length of the spindle. 
     In some aspects, a cutting element assembly includes a cutting element having a cutting end, a spindle, and a retention feature disposed along a spindle side surface. The assembly also includes a sleeve having an inner diameter at an inner surface of the sleeve, an outer diameter at an outer surface of the sleeve, and a taper extending axially from a base of the sleeve a length along the sleeve. The taper is formed by a decreasing outer diameter, and the spindle is within the sleeve such that the taper axially overlaps the retention feature. 
     In some further aspects, a cutting element assembly includes a cutting element having a cutting end extending a depth from a cutting face to an interface surface opposite from the cutting face, a spindle. A spindle diameter at a spindle side surface is less than a cutting end diameter at a cutting end side surface. A transition region having a transition surface extends from a point of transition from the interface surface to a point of transition from the spindle side surface. A cross-sectional profile of the transition surface has at least one planar surface. A taper line measured from the point of transition from the interface surface to the point of transition from the spindle side surface forms a taper angle with a line tangent to the spindle side surface, and the taper angle ranges from 5° to 85°. The cutting element assembly may also include an outer support, where the spindle is within the outer support, and a retention feature between the spindle and the outer support. 
     In still additional aspects, a cutting element assembly includes a sleeve, a cutting element partially within the sleeve, the cutting element having a cutting end, a spindle, the spindle axially separated from the cutting end by a transition region, and a retention feature along a spindle side surface. The assembly also includes at least one seal between the sleeve and the cutting element, the at least one seal having a quadrilateral cross-sectional shape. 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Other aspects and features of the description and claimed subject matter will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a drag bit. 
         FIG. 2  is a top view of the drag bit of  FIG. 1 . 
         FIG. 3  is a partial cross-sectional view of a cutting assembly according to some embodiments of the present disclosure. 
         FIG. 4  is a partial cross-sectional view of a cutting element according to some embodiments of the present disclosure. 
         FIGS. 5 and 6  are cross-sectional views of cutting element assemblies according to some embodiments of the present disclosure. 
         FIGS. 7 to 9  are graphs of simulation results for cutting performance of cutting element assemblies. 
         FIG. 10  is a schematic illustration of a fatigue testing apparatus. 
         FIG. 11  is a force diagram for performing fatigue testing on a cutting element assembly. 
         FIG. 12  is a cross-sectional view of a cutting element assembly prepared for fatigue testing. 
         FIG. 13  is a graph of the results for fatigue testing on cutting element assemblies. 
         FIG. 14  is a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure. 
         FIG. 15  is a side view of a cutting element assembly having a bevel formed on the sleeve. 
         FIG. 16  is a perspective view of a cutting element assembly having a taper formed on the sleeve. 
         FIG. 17  is a partial view of a cutting tool according to embodiments of the present disclosure. 
         FIG. 18  is a side view of adjacent cutting element assemblies having a bevel formed on each sleeve. 
         FIG. 19  is a side view of adjacent cutting element assemblies according to embodiments of the present disclosure, each cutting element assembling having a taper formed on a corresponding sleeve. 
         FIG. 20  is a graph of the normal forces from a bit having cutting element assemblies with a taper formed on each sleeve and a bit having cutting element assemblies without a taper. 
         FIG. 21  is a graph of the workrate of circumferential forces from a bit having cutting element assemblies with a taper formed on each sleeve and a bit having cutting element assemblies without a taper. 
         FIG. 22  is a perspective view of a tool using cutting element assemblies of the present disclosure. 
         FIGS. 23 to 28  are cross-sectional views of cutting element assemblies according to embodiments of the present disclosure. 
         FIG. 29  is a perspective view of a seal according to embodiments of the present disclosure. 
         FIG. 30  is a cross-sectional view of a cutting element according to embodiments of the present disclosure. 
         FIGS. 31 to 33  are partial cross-sectional views of cutting elements according to embodiments of the present disclosure. 
         FIG. 34  is a cross-sectional view of a cutting element according to embodiments of the present disclosure. 
         FIG. 35  is a partial cross-sectional view of a cutting element according to embodiments of the present disclosure. 
         FIG. 36  is a graph of impact testing results for cutting elements having varying transition surface geometries. 
         FIG. 37  is a partial cross-sectional view of a cutting element having a radiused transition surface. 
         FIG. 38  is a partial cross-sectional view of a cutting element having a transition surface with at least one planar surface. 
         FIG. 39  shows a finite element analysis (FEA) simulation of a cutting element. 
         FIGS. 40-1 to 40-6  show FEA simulation results of stress concentrations for various cutting elements having a 13 mm cutting end diameter. 
         FIG. 41  is a graph of the maximum principle stresses of the FEA simulation results of  FIGS. 40-1 to 40-6 . 
         FIGS. 42-1 to 42-4  show FEA simulation results of stress concentrations for various cutting elements having a 16 mm cutting end diameter. 
         FIG. 43  is a graph of the maximum principal stresses of the FEA simulation results of  FIGS. 42-1 to 42-4 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure relate to cutting elements that are free to rotate about their longitudinal axes. In some aspects, embodiments of the present disclosure relate to cutting elements retained within a sleeve or cutter pocket such that the cutting elements are mechanically retained (and not rotatable) within the sleeve structure or cutter pocket. The cutting elements may be used in a drill bit or other cutting tool. 
     According to embodiments of the present disclosure, a cutting element may be partially within a sleeve or outer support member, where the assembled combination of the cutting element and sleeve may be referred to as a cutting element assembly. During operation of a cutting element assembly, drilling forces may displace or move the cutting element out of alignment within the sleeve, which may lead to failure of the cutting element assembly. By limiting the displacement of the cutting element within the sleeve of a cutting element assembly, the life of the cutting element assembly may be improved. In some embodiments, the length of the sleeve and a portion of the cutting element therein may be extended in order to limit displacement. In some embodiments, the tolerance or spacing between the interfacing sleeve and cutting element surfaces may be reduced in order to reduce the displacement of the cutting element within the sleeve. Further, in some embodiments, a cutting element assembly may include one or more seals between the interfacing sleeve and cutting element surfaces, which may provide damping towards impact forces and reduce lateral movement of the cutting element. One or more seals may also be used in a cutting element assembly to inhibit contaminant from entering the cutting element assembly and/or inhibit grease or lubricant, if used, from leaving the cutting element assembly. 
       FIG. 3  shows an example of a cutting element assembly  20  having a cutting element  24  partially within and retained to a sleeve  22 . Cutting element  24  may, in some embodiments, be formed of two components, a carbide substrate  26  and an ultrahard material layer  28 , such as a diamond table, on an upper surface of the carbide substrate  26 . A lower portion  27  of the carbide substrate  26  forms a spindle within the sleeve  22 . The substrate  26  may have an upper portion  29  extending axially above the spindle  27  from a radial bearing surface  30  to interface with the ultrahard material layer  28 . Further, a transition region  31  is formed between the radial bearing surface  30  and the spindle  27 . The cutting element  24  may be retained within the sleeve by a variety of retention mechanisms such as by retention balls, springs, pins, etc. Various examples of such types of retention mechanisms (as well as other variations on the cutting assemblies suitable for use in the present disclosure) include those disclosed in U.S. Patent Publication Nos. 2010/0314176 and 2012/0273281; and U.S. Pat. No. 7,703,559, the entire disclosures of which are incorporated herein by reference. 
     In some embodiments, the retention mechanism may limit the axial movement or displacement of the cutting element  24  with respect to sleeve  22 . In such embodiments, the cutting elements may be rotatable within the sleeve, i.e., about the longitudinal axis of the cutting element  20 . In other embodiments, the retention mechanism may limit the axial movement or displacement as well as rotational movement of the cutting element  24  with respect to sleeve  22 . 
     The sleeve  22  and cutting element  24  may have substantially the same outer diameter as each other, or in some embodiments, the sleeve  22  may have a greater outer diameter than the cutting element. As shown, the cutting element  24  may have an outer diameter  50 , and the radial bearing surface  30  may include a substantially planar surface extending to the outer diameter of the sleeve having a radial length  52 . The thickness  54  of the sleeve  22  may be selected based on the radial length  52  of the substantially planar surface of radial bearing surface  30  and the outer diameter  50  of the cutting element  24 . Further, as shown, the thickness  54  of the sleeve may vary along its length, for example, to form a taper  40 . The taper  40  is formed by a gradually increasing sleeve thickness  54  that extends from the sleeve base an axial length, where the axial length is greater than the sleeve thickness  54  measured at its greatest thickness. Tapers according to other embodiments are described more below. 
     The cutting element  24  has a cutting end  33  (including the upper portion  29  of the substrate and the ultrahard material layer  28  shown in  FIG. 3 ) that extends axially above the spindle  27  and sleeve  22  from the radial bearing surface  30  to a cutting face  34  of the cutting element  24 . The height of the axial extension of the carbide substrate  26  from the radial bearing surface  30  to the ultrahard material layer  28  may be referred to as axial extension  56 . Further, in the illustrated embodiment, ultrahard material layer  28  may have a thickness  58 , where the cutting end  33  has a depth equal to the sum of the thickness of the axial extension and the thickness of the ultrahard material layer. 
     The spindle  27  has a retention feature  32  formed along the spindle side surface. As shown in  FIG. 3 , the retention feature  32  may be a circumferential groove. In other embodiments, the retention feature may be, for example, one or more cavities, one or more protrusions, or one or more ridges. A diameter  55  of the spindle portion axially above the retention feature and a diameter  57  of the spindle portion axially below the retention feature may be equal or unequal. For example, in some embodiments, the diameter  57  of the spindle portion axially below the retention feature may be less than the diameter  55  of the spindle portion axially above the retention feature. The portion of the spindle  27  above the retention feature  32  and extending to the transition region  31  is referred to as the guide length of the cutting element. Further, the cutting element assembly  20  may have a total length  51 . According to embodiments of the present disclosure, a cutting element assembly may have a ratio of a total length of the cutting element assembly to a diameter of the cutting element assembly that is greater than 1:1, greater than 5:4, or greater than 3:2. In some embodiments, the ratio of a total length to a diameter of the cutting element assembly may be less than 5:1, less than 5:2, or less than 5:3. In some embodiments, the ratio may be greater than the ratios described above (e.g., greater than 1:1) and less than the other ratios described above (e.g., less than 5:1) (e.g., greater than 1:1 and less than 5:1). 
     According to embodiments of the present disclosure, a cutting element may include a cutting face, a radial bearing surface opposite from the cutting face, a cutting end extending a depth from the cutting face to the radial bearing surface, and a spindle, the spindle axially separated from the cutting end by a transition region, where the diameter of the spindle is less than the diameter of the cutting end. The spindle may include a guide length measured from a point of transition to the transition region to a retention feature. The guide length of a cutting element according to embodiments of the present disclosure may be longer than ½ (50%), ⅗ (60%), ⅔ (66.7%), ¾ (75%), or ⅘ (80%) of a total length of the spindle. The guide length of a cutting element may be shorter than 9/10 (90%), ⅞ (87.5%), or ⅚ (83.3%) of a total length of the spindle. In some embodiments, the ratio may be greater than the ratios described above (e.g., greater than ½ or 50%) and less than the other ratios described above (e.g., less than 9/10 or 90%). For instance, the ratio may be greater than ½ (50%) and less than 9/10 (90%). 
     According to embodiments of the present disclosure, a transition surface may be designed based on one or more dimensions of the cutting element. For example, referring still to  FIG. 3 , the transition surface  31  may be designed based on at least one of the diameter  55  of the spindle portion axially above the retention feature  32 , the total length  51  of the cutting element assembly, the total length of the cutting element, the radial length  52  of the radial bearing surface  30 , the outer diameter  50  of the cutting element  24 , or a combination of cutting element  24  dimensions, such as, for example, the radial length  52  of the radial bearing surface  30  and the total length of the cutting element  24 . In some embodiments, the transition surface  31  may also be designed based on the material properties of the cutting element  24 . Further, as described more below, the transition surface design may include, for example, selecting size, such as radial and axial lengths of extension, shape, such as planar and/or non-planar surfaces, angle of orientation from the spindle to the radial bearing surface, and, if a seal is included, seal placement. 
       FIG. 4  shows an example of a cutting element according to embodiments of the present disclosure, where the cutting element  400  has a spindle  402  axially separated from a cutting end  404  by a transition region  406 , a retention feature  401  disposed along the length of the spindle side surface, and a longitudinal axis  408  extending therethrough. The cutting end  404  extends a depth from a cutting face  405  to a radial bearing surface  403  and has a diameter  409 . In some embodiments, the cutting end may include a diamond table that forms the cutting face and a substrate that extends from the diamond table to the base of the spindle, thereby forming part of the cutting end, the transition region and the spindle. In other embodiments, a cutting element may be formed of more than two types of materials. For example, a cutting element may include an ultrahard material table forming the cutting face, a carbide or other cermet material forming a substrate, and one or more transition materials between the ultrahard material table and substrate, where a transition material may include a mixture of ultrahard and cermet materials or one or more cermets different from the substrate material. In yet other embodiments, the entire cutting element may be formed from a single material. 
     The spindle  402  has a total length  410  and a guide length  412 , where the total length is measured from the base  407  of the spindle to a point of transition  416  to the transition region  406 , and the guide length  412  is measured from the retention feature  401  to the point of transition  416  to the transition region. Thus, the lengths of the total length  410  of the spindle and the guide length  412  of the spindle are measured from the same axial point along the cutting element,  416 , and extend different axial distances along the spindle. As shown, the retention feature  401  is a circumferential groove formed around the spindle side surface. In such embodiments, the guide length  412  is measured from the wall of the circumferential groove axially closest to the cutting end  404 . In other embodiments, the guide length may be measured from the point of the retention feature axially closest the cutting end to the point of transition to the transition region. The point of transition  416  to the transition region from the spindle may be defined as the point at which the slope of the line tangent to the spindle side surface changes. In other words, a line tangent to the spindle side surface may have a substantially constant slope (excluding any surface alterations which may act as a retention feature), which extends to the point of transition  416  to the transition region surface. 
     According to embodiments of the present disclosure, the guide length may range from greater than 60% of the total length of the spindle, from 70% to 95% of the total length of the spindle, or from 75% to 90% of the total length of the spindle. For example, as shown in  FIG. 4 , the guide length  412  may be greater than 75% of the total length of the spindle. The guide length of a spindle may also be measured with respect to the total length of the cutting element, i.e., from the base  407  of the spindle to the cutting face  405 . According to some embodiments of the present disclosure, a guide length may range from greater than 50% of the total length of the cutting element, from 55% to 85% of the total length of the cutting element, or from 60% to 75% of the total length of the cutting element. For example, as shown in  FIG. 4 , the guide length  412  is greater than 60% of the total length of the cutting element  400 . Further, in some embodiments, the guide length  412  may be measured with respect to the cutting end diameter, where the cutting end diameter is the diameter of the cutting element at its cutting end, such as shown as  409  in  FIG. 4 . For example, according to embodiments of the present disclosure, a guide length  412  may range from greater than 60% of the cutting end diameter, from greater than 75% of the cutting end diameter, greater than 90% of the cutting end diameter, and in some embodiments, the guide length  412  may be equal to or larger than the cutting end diameter (e.g., 110% or 120% of the cutting end diameter). In some embodiments, the guide length  412  may be measured with respect to the diameter of the cutting element spindle, such as shown as  420  in  FIG. 4 , where the diameter may be an outer diameter measured along the guide length portion of the spindle or at the base of the spindle. For example, according to some embodiments of the present disclosure, a ratio of the guide length to the diameter of the cutting element spindle may include limits of 3:4, 1:1, 3:2, 2:1, or 3:1, where any limit may be used in combination with any other limit (e.g., a ratio between 3:4 and 2:1). 
     Referring now to  FIG. 5 , a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure is shown. The cutting element assembly  500  has a cutting element  510  according to embodiments of the present disclosure partially in a sleeve  520 . The cutting element may include a cutting end  512 , a transition region  514  and a spindle  516 . The cutting end  512  is defined as the portion of the cutting element between the cutting face  505  and the radial bearing surface  503 . The spindle  516  portion of the cutting element includes a retention feature  518  formed at a guide length  515  from the transition region  514  along a spindle side surface  517 . The retention feature  518  shown is a circumferential groove formed around the circumference of the spindle; however, other embodiments may have other retention features, such as a protrusion or ridge, and some embodiments may have more than one retention feature formed on the spindle side surface. 
     The sleeve  520  has an inner diameter  522  at the inner surface of the sleeve and an outer diameter  524  at the outer surface of the sleeve. As shown, the inner diameter  522  and outer diameter  524  of the sleeve may vary along its length, thereby forming a varying sleeve thickness. For example, the inner diameter  522  of the sleeve is relatively larger at the axial length corresponding with the retention feature  518  formed in the cutting element  510 , such that a space is formed between the retention feature and the increase in the inner diameter  522 . A retention mechanism may be within the space to retain the cutting element  510  in the sleeve  520 . According to some embodiments, the varying inner diameter of a sleeve may include a circumferential groove formed in the inner surface of the sleeve at an axial position corresponding with a retention feature formed in the spindle of a cutting element. For example, in some embodiments, a cutting element may have a circumferential groove formed around the spindle of the cutting element, and a sleeve around the cutting element may have a corresponding circumferential groove formed around its inner surface, such that at least a portion of the corresponding circumferential groove of the sleeve shares an axial position with the circumferential groove of the cutting element. A retention mechanism may be between the corresponding circumferential grooves to retain the cutting element within the sleeve. In other embodiments, differently shaped retention features formed in a cutting element may share an axial position with at least a portion of differently shaped retention features formed in a sleeve of a cutting element assembly. 
     Further, the sleeve has a length  526  measured between a top surface  525  and a bottom surface  527 , where the top surface  525  interfaces with the cutting element radial bearing surface  503 . The length  526  of the sleeve extends at least the sum of the axial length of the cutting element transition region  514  and the axial length of the cutting element guide length  515 . According to some embodiments, the length of the sleeve may be equal to the sum of the axial lengths of the transition region and spindle portions of a cutting element retained therein. In some embodiments, such as shown in  FIG. 5 , the length  526  of the sleeve may be greater than the sum of the axial lengths of the transition region  514  and spindle  516  portions. 
     The guide length of a cutting element spindle and a corresponding length of a sleeve in which the cutting element is positioned may be extended to increase stability of the cutting element assembly. For example, during drilling, a rotatable cutting element assembly may consistently be subjected to fluctuating drilling and vertical load. Due to tolerance differences between the rotating cutting element and the sleeve, the cutting element may move under the load and generate kinetic energy. Once the amount of kinetic energy generated passes a certain critical value, the cutting element may be considered unstable and its fatigue life may drop. Thus, stability of a cutting element assembly according to embodiments of the present disclosure may be quantified using an equation for kinetic energy of the cutting element assembly during performance, where the kinetic energy, Ek, is equal to the product of the net force, F, the cutting element assembly is subjected to during performance and the displacement, s, of the cutting element within the sleeve. In some embodiments, extending the guide length of the cutting element limits cutting element displacement, thereby reducing the kinetic energy and improving cutting element assembly stability. Referring now to  FIGS. 6-9 , finite element analysis was performed to test cutting element assembly performance with different guide lengths.  FIG. 6  shows the model of a cutting element assembly  600  having a cutting element  610  partially within a sleeve  620 . The cutting element  610  has a cutting end  612 , a transition region  614 , and a spindle  616 . The spindle  616  has a guide length  615  measured from the transition region  614  to a retention feature  618  formed along the spindle side surface. Parameters of the simulations included a cutting force  630  of 3,000 lbf (1360 kgf), a 20° back rake angle, and a 0.08 in. (2 mm) depth of cut. A displacement  613  was measured at the bottom tip, or cutting portion, of the cutting end  612  to compare movement of the cutting element within the sleeve  620 . 
       FIG. 7  shows the simulation results for the cutting element assembly having a guide length of 0.303 in. (7.70 mm), where the resulting displacement is 0.0073 in. (0.18 mm).  FIG. 8  shows the simulation results for the cutting element assembly having a guide length of 0.267 in. (6.78 mm), where the resulting displacement is 0.0099 in. (0.25 mm).  FIG. 9  shows the simulation results for the cutting element assembly having a guide length of 0.243 in. (6.17 mm), where the resulting displacement is 0.0113 in. (0.287 mm). Thus, as the guide length was increased, the simulated displacement decreased. Further, the simulated cutting element assemblies were manufactured and tested in the field, where the cutting element assembly having a displacement of 0.0073 in. (0.18 mm) survived and the cutting element assemblies having displacements of 0.0099 in. (0.25 mm) and 0.011 in. (0.29 mm) failed. 
     Referring now to  FIGS. 10-13 , fatigue and static testing was performed on cutting element assemblies to test cutting element stability. As shown in  FIG. 12 , the cutting element assemblies  120  were set up by brazing a sleeve  122  into a testing coupon  102 . A cutting element  126  was then installed into the sleeve  122 , where each cutting element  126  has a cutting end  127  and a guide length  128  measured along its spindle  129 , from the cutting element transition region to a retention feature formed in the spindle. Cutting elements having guide lengths of 0.303 in. (7.70 mm), 0.267 in. (6.78 mm), and 0.243 in. (6.17 mm) were tested. As shown in  FIG. 10 , the cutting element assemblies  120  were loaded into a testing apparatus  100  and a radial load was applied to the cutting ends of each cutting element.  FIG. 11  shows a force diagram of the cutting element assemblies  120  being tested. As shown, the cutting element assembly  120  was horizontally positioned in the testing apparatus  100  such that a back side  112  and a top side  114  of the sleeve  122  was fixed and a bottom side  116  of the cutting element assembly  120  was not supported. For static testing, a radial load  118  was applied to a top side of the cutting end  127  until the cutting element assembly failed. For fatigue testing, a radial load  118  ranging from 500-1500 lbf (225-680 kgf) was applied at a 20 HZ frequency for two million cycles.  FIG. 13  shows a graph of the results for the fatigue testing, where cutting element assemblies having a 0.303 in. (7.70 mm) guide length survived the 2 million cycles, cutting element assemblies having a 0.267 in. (6.78 mm) guide length failed after an average of about 270,000 cycles, and cutting element assemblies having a 0.243 in. (6.17 mm) guide length failed after an average of about 47,000 cycles. 
     According to embodiments of the present disclosure, a cutting element in a cutting element assembly may have a guide length measured from a point of transition to the transition region to the retention feature that is longer than 0.3 in. (7.6 mm). In some embodiments, a cutting element may have a guide length greater than 0.35 in. (8.9 mm). In some embodiments, a cutting element may have a guide length greater than 0.4 in. (10 mm). 
     Types of cutting element assembly failure that may result from lost stability of the cutting element may include broken sleeves and loss of the cutting element. Cutting element assembly failure experienced during field testing and lab testing included broken sleeves in some of the cutting element assemblies broke and lost cutting elements. 
     According to embodiments of the present disclosure, displacement of a cutting element within a sleeve may be reduced, thereby improving cutting element stability, by reducing the tolerance between the cutting element and the sleeve. Tolerance between the cutting element and the sleeve may be described according to the amount of space, or gap, formed between the cutting element and the sleeve. In other words, cutting element assemblies of the present disclosure may have a diameter of a cutting element spindle less than the inner diameter of a sleeve along a shared axial position such that a gap is formed between the cutting element spindle and the sleeve. According to some embodiments of the present disclosure, the ratio of a gap formed between a cutting element spindle and a sleeve along a shared axial position and the diameter of the cutting element assembly at the same axial position may range from about 0.0005:1 to 0.02:1. By decreasing the gap formed between the cutting element and the sleeve, tolerance in a cutting element assembly may be reduced. Such a gap ratio may reduce the gap by greater than 20%, greater than 30%, or greater than 40% compared to conventional gaps, thereby improving the stability of the cutting element in some embodiments. 
     Cutting elements of the present disclosure may be retained within a sleeve to form a cutting element assembly, or may be retained directly to a cutter pocket formed in a cutting tool. According to some embodiments of the present disclosure having a cutting element retained within a sleeve, the cutting element assembly may include the cutting element partially within the sleeve, where the cutting element is retained within the sleeve by one or more retention features. The cutting element may include a cutting end, a spindle, and a retention feature disposed along the spindle side surface. The sleeve may have an inner diameter at an inner surface of the sleeve, an outer diameter at an outer surface of the sleeve, and a taper extending axially from a base of the sleeve a length along the sleeve, where the taper is formed by a decreasing outer diameter. The spindle may be within the sleeve such that the taper at least partially axially overlaps the retention feature. 
       FIG. 14  shows a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure, where a sleeve has a taper formed at its base. As shown, the cutting element assembly  700  has a cutting element  710  partially in a sleeve  720 . The cutting element may include a cutting end  712 , a spindle  714 , and a retention feature  716  disposed along a spindle side surface  718 . The sleeve  720  may have an inner diameter  722  at the inner surface of the sleeve and an outer diameter  724  at the outer surface of the sleeve. A taper  726  is formed in the sleeve  720  by an increasing outer diameter  724  that extends axially from a base  721  of the sleeve  720  towards a top surface  725  of the sleeve. The terms “base” and “top surface” may not always refer to the direction the terms describe, depending on the positioning of the cutting element assembly, but instead, the base of the sleeve refers to the surface of the sleeve axially farthest from the cutting end of an assembled cutting element, and the top surface of the sleeve refers to the surface of the sleeve interfacing with a radial bearing surface of the assembled cutting element. Further, as shown in  FIG. 14 , a sleeve outer diameter  724  may be substantially constant from the top surface  725  to the taper  726 , at which point the outer diameter  724  may gradually decrease to the base  721 . The inner diameter  722  of the sleeve may be substantially constant along its length. However, in some embodiments, a sleeve may have one or more retention features formed along its inner surface, where the inner diameter may vary at the one or more retention features. 
     The taper  726  extends a length  721  along the sleeve  720 , where the taper length is measured along the axial length of the sleeve having a changing outer diameter  724 , and a radial width  723 , where the radial width is measured across the thickness of the sleeve  720 . As shown in  FIG. 16 , the length of the taper  726  at least partially axially overlaps the retention feature  716  formed in the assembled cutting element  710 . In other words, at least part of the taper  726  and at least part of the retention feature  716  share a common axial position. In some embodiments, a taper formed in a sleeve of a cutting element assembly may extend a length such that it overlaps an entire retention feature formed in a cutting element assembled to the sleeve. In other embodiments, a taper formed at the base of a sleeve may not share an axial position with a retention feature formed in a cutting element assembled to the sleeve. For example, a cutting element assembly may have a sleeve with a taper formed along its outer surface and a cutting element partially within the sleeve, where the taper extends a length from the base of the sleeve and a retention feature is formed along the cutting element at an axial distance from the base of the sleeve that is greater than the axial length of the taper. 
     The length  721  of the taper  726  may range from about ¼ (25%) of the length of the sleeve to about ½ (50%) of the length of the sleeve  720 . In some embodiments, a taper length may be less than ¼ (25%) the length of the sleeve, and in some embodiments, a taper may be greater than ½ (50%) the length of the sleeve. The radial width  723  of the taper  726  may range from about ¾ (75%) to ¼ (25%) of the greatest thickness of the sleeve  720 . In some embodiments, the radial width of a taper may be less than ¼ (25%) the greatest thickness of the sleeve, and in some embodiments, a taper may be greater than ¾ (75%) the greatest thickness of the sleeve. 
     Further, an angle  727  of the taper  726  may be measured with respect to a line  728  tangent with the sleeve outer surface at its largest outer diameter  724 . The angle  727  of the taper  726  may depend on, for example, the thickness of the sleeve, the length of the sleeve, and the shape of the taper. For example, the shape of the taper shown in  FIG. 16  is formed by a planar surface having a constant slope (i.e., the constantly decreasing outer diameter); however, in other embodiments, a taper may be formed by a curved or stepped surface having a varying slope. According to embodiments of the present disclosure, the taper may have an angle ranging from 0° to 90°. In some embodiments, the taper may have an angle ranging from 0° to 20°. In some embodiments, the taper may have an angle ranging from 10° to 15°. 
     As used herein, a taper is different from what may be referred to as a bevel or chamfer. For example,  FIG. 15  shows a side view of a cutting element assembly  170  having a bevel  172  formed at the base  174  of its sleeve  176 , and  FIG. 16  shows a side view of a cutting element assembly  180  having a taper  182  formed at the base  184  of its sleeve  186  according to embodiments of the present disclosure. The taper  182  may have an axial length that is greater than its radial width, while the bevel  172  may have a radial width that is equal to or relatively close in value to its axial length. In other words, the bevel  172  may have an angle formed with respect to a line tangent to the sleeve outer surface of about 45°, or in some embodiments, ranging between 40° and 50°. Thus, the size of the taper  182  may be described based on its axial length along the sleeve  186  outer surface, while the size of the bevel  172  may be described based on either its axial length or radial width. As shown in  FIGS. 15 and 16 , a taper  182  formed in a sleeve extends a greater axial length along the sleeve outer surface than a bevel  172 . For example, while a bevel may have an axial length (and radial width) less than the thickness of the sleeve, a taper may have an axial length greater than the thickness of the sleeve. In some embodiments, a bevel may have an axial length within a range of less than 0.06 in. (1.5 mm), and in some embodiments, a taper may have an axial length greater than 0.2 in. (5 mm). According to embodiments of the present disclosure, a taper may have an axial length extending greater than 5% of the total length of the sleeve, greater than 10% of the total length of the sleeve, greater than 25% of the total length of the sleeve, greater than 50% of the total length of the sleeve, or greater than 75% of the total length of the sleeve. For example, the taper may have an axial length between 5 and 100% of the total length of the sleeve or in some embodiments, between 10 and 50% of the total length of the sleeve. 
     Providing tapers along the outer surface of a sleeve may allow for reduced spacing between cutting element assemblies, or an increased number of cutting element assemblies to be arranged on a cutting tool. For example, cutting element assemblies of the present disclosure having an increased length (due to the relatively large guide length of the cutting element) may be spaced apart on a cutting tool based on, for example, their position along the cutting tool, e.g., side rake angle and back rake angle, the material of the cutting tool, the size and type of the cutting tool, and, if any, the size of a taper formed along the outer surface of the sleeve, such that the cutting element assemblies do not contact each other and that there is sufficient material from the cutting tool surrounding them in order to hold them to the cutting tool. 
     According to embodiments of the present disclosure, a downhole cutting tool may include a tool body and at least two cutting element assemblies within cutter pockets formed on the tool body. The cutting element assemblies may be secured to the cutter pocket, for example, by brazing the sleeve to the cutter pocket, or by other means of attachment. Each cutting element assembly may include a sleeve having a taper extending an axial length from the sleeve base, where the taper is formed by a decreasing outer diameter of the sleeve. A cutting element may be partially within and retained to the sleeve by one or more retention features. The cutting element may have a longitudinal axis extending axially therethrough, a cutting end having a depth measured from a cutting face to a radial bearing surface, and a spindle axially separated from the cutting end by a transition region, where the spindle includes a spindle side surface and a retention feature disposed along the spindle side surface. The distance from the longitudinal axis at the cutting face of one cutting element assembly to the longitudinal axis at the cutting face of an adjacent cutting element assembly may be less than 3 times the radius of the cutting element assemblies. 
     Referring now to  FIG. 17 , a partial view of a cutting tool according to embodiments of the present disclosure is shown. A drill bit  1900  has a body  1910  and a plurality of blades extending from the body  1910 . Blade  1920  has at least two cutting element assemblies  1930  according to embodiments of the present disclosure in cutter pockets formed along a top face  1922  of the blade  1920  at the leading face  1924  of the blade  1920 . The cutting element assemblies  1930  may have a cutting element partially in a sleeve, where the sleeve has a taper formed along the sleeve outer surface. The cutting element may have a longitudinal axis  1932  extending axially therethrough, a cutting end having a depth measured from a cutting face to a radial bearing surface, and a spindle rotatably retained within the sleeve. The distance  1934  between two adjacent cutting element assemblies  1930  may be less than 3 times the radius of the cutting element assemblies, where the distance  1934  is measured from the longitudinal axis  1932  at the cutting face of one cutting element assembly to the longitudinal axis  1932  at the cutting face of an adjacent cutting element assembly. According to some embodiments, the distance  1934  between two adjacent cutting element assemblies may be less than 2.5 times the radius of the cutting element assemblies. For example, the distance may be between 2 and 3 times the radius of the cutting element assemblies. 
       FIGS. 18 and 19  show cutting element assemblies  2000 ,  2100  spaced apart from each other along a blade. Specifically,  FIG. 18  shows adjacent cutting element assemblies  2000  having a cutting element partially in a sleeve, where the sleeve has a bevel  2010  formed at the base of the sleeve, and  FIG. 19  shows cutting element assemblies  2100  having a cutting element partially in a sleeve, where the sleeve has a taper  2110  formed along the sleeve outer surface. The smallest distance  2020  between the adjacent cutting element assemblies  2000  in  FIG. 18  is measured between the closest points along the sleeve at the bevels  2010 , while the greatest distance  2030  between the adjacent sleeves is measured opposite the bevels, near the cutting end. The smallest distance  2120  between the adjacent cutting element assemblies  2100  in  FIG. 19  is measured between the closest points along the sleeve at the tapers  2110 , while the greatest distance  2130  between the adjacent sleeves is measured opposite the tapers, near the cutting ends of the cutting element assemblies. 
     Adjacent cutting element assemblies  2000  having tapers may be spaced closer together than adjacent cutting element assemblies  2100  without tapers, and in some cases, even when cutting element assemblies having tapers are longer than cutting element assemblies without tapers. For example, as shown in  FIGS. 18 and 19 , adjacent cutting element assemblies  2100  have a total axial length greater than the total axial length of adjacent cutting element assemblies  2000 , but may be spaced apart at equal or close to equal greatest distances. In the embodiments shown, adjacent cutting element assemblies  2000  may have a smallest distance  2020  of about 0.045 in. (1.14 mm) and a greatest distance  2030  of about 0.13 in. (3.30 mm), while adjacent cutting element assemblies  2100  may have a smallest distance  2020  of about 0.048 in. (1.22 mm) and a greatest distance  2030  of about 0.13 in. (3.30 mm). In other embodiments, adjacent cutting element assemblies having tapers may be spaced apart at lesser distances than adjacent cutting element assemblies without tapers, depending on the total axial length of the cutting element assemblies and their positioning on the blade. By forming tapers at the base of the cutting element assemblies, the cutting element assemblies may have a greater axial length (thereby improving cutting element stability) while also allowing for improved spacing between adjacent cutting element assemblies. 
     In some embodiments, an average reduction of about 21.5% in cutting element spacing, when comparing cutting element assemblies having the same axial length and same positioning (e.g., back rake and side rake) on the cutting tool, may be achieved by using tapers formed at the base end of the cutting element assembly sleeves. For example, in some embodiments, cutting element assemblies may have a spacing between an adjacent cutting element assembly, where the spacing is quantified by a spacing ratio of the distance between adjacent cutting element assemblies (as measured between the longitudinal axis at the cutting face of one cutting element assembly to the longitudinal axis at the cutting face of the adjacent cutting element assembly) to the axial length of the cutting element assemblies. In some embodiments having tapers formed at the base of the sleeve, adjacent cutting element assemblies may have a spacing ratio ranging between about 1:10 to 3:10 or in some embodiments, less than 2:10, while adjacent cutting element assemblies having the same axial length but without tapers may have a spacing ratio ranging, for example, between about 4:10 to 9:10. 
     Further, by spacing cutting element assemblies closer together, a reduction in normal and workrate cutting forces may be achieved. For example, as shown in  FIGS. 20 and 21 , cutting element assemblies having a taper formed at the base end of the sleeves encountered lower normal forces and lower workrate forces than cutting element assemblies without a taper positioned in the same area of the bit. When cutting element assemblies are spaced closer together, more cutting element assemblies may be assembled to a bit, and the cutting forces of the bit may be distributed to more cutting elements, thereby providing a reduced cutting force to each cutting element. 
     Cutting element assemblies having a sleeve with a taper formed at its base may or may not have additional features described herein used in combination with the tapered sleeve. For example, in some embodiments, a cutting element assembly may include a cutting element partially in a sleeve, where a tighter tolerance is formed between the cutting element and the sleeve and where a taper is formed along the outer surface of the sleeve. In some embodiments, a cutting element assembly may include a cutting element partially in a sleeve, where the cutting element has an increased guide length and where a taper is formed along the outer surface of the sleeve. In some embodiments, a cutting element assembly may include a cutting element partially in a sleeve, where a tighter tolerance is formed between the cutting element and the sleeve, where the cutting element has an increased guide length, and where a taper is formed along the outer surface of the sleeve. In some embodiments, a cutting element assembly may include a cutting element partially in a sleeve, where one or more seals are positioned between the cutting element and the sleeve, as described below, and where a taper is formed along the outer surface of the sleeve. 
     Cutting element assemblies having an increased guide length may be restricted in how close together they can be assembled to a cutting tool. As cutting element assemblies are spaced farther apart from each other, the decreased cutting element count may lead to an increased load distribution on each cutting element. By forming a taper along the sleeve of cutting element assemblies, the cutting element assemblies may be spaced closer together, thereby allowing for an increased cutting element count on a cutting tool. Reducing the gap between adjacent cutting element assemblies to provide an increased cutting element count may reduce the load on each cutting element, which may increase the life of the cutting tool. 
     Referring now to  FIG. 23 , a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure is shown. The cutting element assembly has a cutting element  2600  partially within a sleeve  2610 . The cutting element  2600  has a cutting end  2602 , a transition region  2604 , and a spindle  2606 , where the spindle  2606  is axially separated from the cutting end  2602  by the transition region  2604 . A retention feature  2620  is disposed along a spindle side surface  2608 , and at least one seal  2630  is between the sleeve  2610  and the cutting element  2600 . The seal  2630  has a quadrilateral cross-sectional shape and extends around the circumference of the cutting element  2600 . Seals having a quadrilateral cross-sectional shape may include, for example, a rectangle, trapezoid, or parallelogram cross-sectional shape. In one or more embodiments, the cross-section of the seal  2360  may have an aspect ratio of at least 3:1 or 4:1 in other embodiments. As shown, the seal  2630  is positioned within the transition region  2604  between the cutting element  2600  and the sleeve  2610 . According to embodiments of the present disclosure, a seal may be positioned within grooves formed in one or both of the sleeve inner surface and the cutting element side surface, where the seal fits partially within the groove, or a seal may be positioned along a flat surface of one or both of the sleeve inner surface and the cutting element side surface. For example, as shown in  FIG. 23 , the cross-sectional profile of the transition region  2604  includes a planar surface, where the seal  2630  is disposed along the planar surface of the transition region  2604 . The cross-sectional profile of the sleeve  2610  in the axial position corresponding with the cutting element transition region  2604  also includes a planar surface, where the seal  2630  is between the planar surfaces of the sleeve and cutting element within the transition region  2604 . 
       FIG. 24  shows a cross-sectional view of a cutting element assembly according to embodiments of the present disclosure. The cutting element assembly is similar to that disclosed in  FIG. 23  except that the seal  2730  has a circular cross-sectional shape and extends around the circumference of the cutting element  2700 . Further, the cross-sectional profile of the sleeve  2710  in the axial position corresponding with the cutting element transition region  2704  includes a surface having a planar cross-sectional profile, where the seal  2730  is between the sleeve surface with a planar cross-sectional profile and the transition region  2704  of the cutting element. 
       FIG. 25  shows a cross-sectional view of another cutting element assembly according to embodiments of the present disclosure. The cutting element assembly has a cutting element  2800  partially within a sleeve  2810 . The cutting element  2800  has a cutting end  2802  axially separated from a spindle  2806  by a transition region  2804 . A retention feature  2820  is disposed along a spindle side surface  2808 , and at least one seal  2830  is between the sleeve  2810  and the cutting element  2800 . In particular, the seal  2830  is within a groove formed around the side surface  2808  of the spindle  2806  portion of the cutting element  2800  and protrudes from the groove to contact an inner surface of the sleeve  2810  having a planar cross-sectional profile. However, in other embodiments, the seal may protrude from a groove in the cutting element side surface to fit partially within a corresponding groove formed in the inner surface of the sleeve, such as shown in  FIG. 27  and described below. The seal  2830  has a circular cross-sectional shape and extends around the circumference of the cutting element  2800 . 
       FIG. 26  shows a cross-sectional view of another cutting element assembly according to embodiments of the present disclosure. The cutting element assembly has a cutting element  2900  partially within a sleeve  2910 . The cutting element  2900  has a cutting end  2902  axially separated from a spindle  2906  by a transition region  2904 . A retention feature  2920  is disposed along a spindle side surface  2908 , and at least one seal  2930  is between the sleeve  2910  and the cutting element  2900 . In particular, the seal  2930  is within a groove formed around the inner surface  2918  of the sleeve  2910  and protrudes from the groove to contact the spindle side surface  2908  having a planar cross-sectional profile. The seal  2930  has a circular cross-sectional shape and extends around the circumference of the cutting element  2900 . 
     According to embodiments of the present disclosure, one or more seals may be between a cutting element and a sleeve along at least one surface of the cutting element and/or sleeve having a planar cross-sectional profile, such as shown in  FIGS. 23-26 . However, in some embodiments, one or more seals may be between sleeve and cutting element surfaces having a non-planar cross-sectional profile, e.g., between corresponding grooves formed in sleeve and cutting element. For example,  FIG. 27  shows a cross-sectional view of another cutting element assembly according to embodiments of the present disclosure. The cutting element assembly has a cutting element  3000  partially within a sleeve  3010 . The cutting element  3000  has a cutting end  3002  axially separated from a spindle  3006  by a transition region  3004 . A retention feature  3020  is disposed along a spindle side surface  3008 , and at least one seal  3030  is between the sleeve  3010  and the cutting element  3000 . In particular, the seal  3030  is between corresponding grooves formed around the inner surface  3018  of the sleeve  3010  and the spindle side surface  3008 . The seal  3030  has a circular cross-sectional shape and extends around the circumference of the cutting element  3000 . 
     One or more seals may be between a sleeve and a cutting element of a cutting element assembly, where the seal may have a circular cross-sectional shape, a quadrilateral cross-sectional shape, or other shape, such as a polygonal shape or an irregular shape including planar and/or non-planar sides. In some embodiments, a seal may have a cross-sectional shape that is different than the cross-sectional shape of the space formed between a sleeve and cutting element in which the seal is disposed. In some embodiments, a seal may have a cross-sectional shape corresponding with the space formed between a sleeve and cutting element in a cutting element assembly in which the seal is disposed, where the space may have a circular, polygonal, or irregular shaped cross-section. 
     For example,  FIG. 28  shows a cross-sectional view of another cutting element assembly according to embodiments of the present disclosure. The cutting element assembly has a cutting element  3100  partially within a sleeve  3110 . The cutting element  3100  has a cutting end  3102  axially separated from a spindle  3106  by a transition region  3104 . A retention feature  3120  is disposed along a spindle side surface  3108  to axially retain the cutting element within the sleeve. A seal  3130  is in a space between the sleeve  3110  and the cutting element  3100 , where the seal  3130  and the space in which the seal is located have corresponding cross-sectional shapes. In particular, the seal  3130  and corresponding space between the sleeve  3110  and cutting element  3100  have an irregular cross-sectional shape, including a planar surface and a curved surface. As shown, the seal  3130  may be in a space that extends axially through the entire transition region  3104  of the cutting element assembly and partially into the spindle  3106  region. The seal  3130  fills the space and thus also extends axially through the transition region and partially into the spindle region, contacting the cutting element outer surface along the transition region  3104  and part of the spindle side surface. In such illustrated embodiment, the cutting element  3100  and the sleeve  3110  have differing geometries transitioning between the radial bearing surfaces and side surfaces, and the seal  3130  fills the volume of space created by such differing geometries. According to other embodiments, a seal may be in one region of the cutting element assembly, e.g., the transition region or the spindle region. In some embodiments, more than one seal may be in a cutting element assembly where at least one seal is in one or more regions of the cutting element assembly, e.g., one seal in the transition region and one seal in the spindle region or two seals in the spindle region, or other combinations of seal placements. 
       FIG. 29  shows a perspective view of a seal according to embodiments of the present disclosure. The seal  3200  has an inner surface  3202 , an outer surface  3204  opposite the inner surface, a top surface  3206  and a bottom surface  3208  opposite the top surface. Each of the inner surface  3202 , outer surface  3204 , top surface  3206  and bottom surface  3208  have a planar cross-sectional shape, where the cross-sectional shape of the seal is rectangular. However, as mentioned above, seals may have other cross-sectional shapes, including, for example, polygonal shapes having three, four, five or more sides, circular or elliptical shapes, or irregular shapes having multiple non-planar sides or a combination of planar and non-planar sides. Different shapes of seals may be used to fit within different shapes of spaces formed between a cutting element and a sleeve or outer support of a cutting element assembly. 
     Further, seals may be made of different materials including, for example, graphite, wear resistant fabric infused with low friction materials, e.g., graphite and polytetrafluoroethylene (PTFE), other polymers having similar properties to PTFE, rubber and rubber-like materials, e.g., synthetic materials having similar properties to rubber, low friction coefficient metal, castable or deformable materials, or combinations of such materials. For example, as shown in  FIG. 29 , the seal  3200  may be made of rubber, rubber-like material or polymer and have a metal core  3210 . In some embodiments, such as shown in  FIG. 28 , a seal  3130  may be made of a castable or deformable material, such as castable elastomers. 
     Cutting element assemblies may be subject to impact forces and damage due to lateral movement during drilling, which may lead to fracture or brakeage. Further, some cutting element assemblies may be subject to damage from formation cuttings getting between the cutting element and sleeve or outer support, which may accelerate wear between the cutting element and sleeve or outer support components. For example, debris may enter the cutting element assembly and wear the sleeve inner surface. Including one or more seals between a cutting element and sleeve or outer support may help dampen impact forces on the cutting element during drilling as well as reduce the cutting element lateral movement. Using one or more seals between a cutting element and sleeve or outer support may also help to prevent debris from entering the cutting element assembly. Further, in embodiments having grease or lubricant used between a cutting element and a sleeve or outer support, for example to help rotation of the cutting element within the sleeve or outer support, one or more seals may be used to seal the grease within the cutting element assembly. 
     Furthermore, the transition region of cutting element assemblies of the present disclosure may be designed to provide the cutting element with improved strength and impact resistance. For example, according to embodiments of the present disclosure, a cutting element assembly may include a cutting element partially within an outer support and axially retained within the outer support by a retention feature between the cutting element and outer support. The cutting element may have a cutting end extending a depth from a cutting face to an interface surface opposite from the cutting face, a spindle, where a spindle diameter of a spindle side surface is less than a cutting end diameter of a cutting end side surface, a transition region having a transition surface extending from a point of transition from the interface surface to a point of transition from the spindle side surface, where a cross-sectional profile of the transition surface has at least one planar surface, and a taper line measured from the point of transition from the interface surface to the point of transition from the spindle side surface, where the taper line forms a taper angle ranging from 5° to 85° with a line tangent to the spindle side surface. In some embodiments, cutting elements having a transition surface with a planar cross-sectional surface closest to the spindle at an angle between 5° and 85° from a line tangent to the spindle side surface may have improved strength and impact resistance when compared with cutting elements having a radiused transition surface. 
     Because the strength of a cutting element may depend on the strength of its transition region, transition surface design may be used to reduce cutting element failure. By providing cutting elements with an improved transition surface design, such as according to embodiments of transition surfaces disclosed herein, the overall strength of the cutting element may also be improved. 
     Referring now to  FIG. 30 , a cutting element  3700  according to embodiments of the present disclosure is shown. The cutting element  3700  has a cutting face  3702 , an interface surface  3704  (also referred to as a radial bearing surface upon interfacing with a sleeve) opposite from the cutting face  3702 , a cutting end  3706  extending a depth from the cutting face  3702  to the interface surface  3704 , a spindle  3708 , and a longitudinal axis  3701  extending through the length of the cutting element  3700 . 
     The interface surface  3704  may interface with a top side of a sleeve (shown as  21  in  FIG. 3 ) to form a radial bearing between the cutting element and the sleeve. A spindle diameter  3718  at the spindle side surface  3719  is less than a cutting end diameter  3716  defined by the cutting end side surface  3717 . The cutting element  3700  has a transition surface  3720  extending from a point  3722  of transition from the interface surface to a point  3724  of transition from the spindle side surface  3719 . The point  3722  of transition from the interface surface may be defined as the point at which the slope of the line tangent to the interface surface changes. In other words, a line tangent to the interface surface  3704  may have a substantially constant slope, where the interface surface extends from a cutting end outer surface to the point  3722  at which the slope changes. The point  3724  of transition from the spindle side surface  3719  may be defined as the point at which the slope of the line tangent to the spindle side surface  3719  changes. In other words, a line tangent to the spindle side surface  3719  may have a substantially constant slope, wherein the spindle side surface  3719  extends from a base to the point  3724  at which the slope changes. Further, the transition surface  3720  extends around the circumference of the cutting element  3700 , although because  FIG. 30  is a cross-sectional view of the cutting element  3700 , the cross-sectional shape of the transition surface  3720  is shown rather than its extension around the cutting element  3700 . 
     A taper line  3725  is measured from the point  3722  of transition from the interface surface  3704  to the point  3724  of transition from the spindle side surface  3719 . According to some embodiments, a taper line may substantially correspond with the transition surface, such as when the transition surface has a substantially planar cross-sectional profile. According to other embodiments, such as shown in  FIG. 30 , the taper line  3725  may have a different shape than the transition surface  3720 . The taper line  3725  is at a taper angle  3726  from a line  3728  tangent to the spindle side surface  3719 . Further, in embodiments having the line tangent to the spindle side surface parallel with the cutting element longitudinal axis, the taper line angle may be measured with respect to either the line tangent to the spindle side surface or the longitudinal axis. 
     The taper line angle  3726  may range from 5° to 85°. According to some embodiments of the present disclosure, the taper line angle may be within a range having upper, lower, or both upper and lower limits including any of 5°, 10°, 15°, 20°, 25°, 30°, 35°, 45°, 60°, 75°, or 85°. In particular example embodiments, the taper line angle  3726  may range from 25° to 35°. Further, in some embodiments, the taper angle  3726  may be designed based on the radial length of the interface surface  3704 , the total length of the cutting element  3700  and/or the axial length of the spindle  3708 . For example, in some embodiments, a transition surface may have a taper angle of greater than 30° when the ratio of the radial length of the interface surface to the total length of the cutting element is greater than 1:8. 
     A transition surface may include at least one planar surface and/or at least one non-planar surface in rotated profile view. For example, as shown in  FIG. 30 , the cross-sectional profile of the transition surface  3720  may include a curved surface transitioning from the interface surface  3704  to a planar surface. 
     In some embodiments, a transition surface may include a cross-sectional shape having more than one planar surface transitioning at angled connections. For example,  FIG. 31  shows a partial cross-sectional view of a cutting element having a transition surface  3820  formed of more than one planar surface  3820 . 1 ,  3820 . 2 ,  3820 . 3 . As shown, the planar surface  3820 . 1  may transition at an angle from a point  3822  of transition from the cutting element interface surface  3804 , the planar surface  3820 . 2  may transition at an angle from the planar surface  3820 . 1 , and the planar surface  3820 . 3  may transition at an angle from the planar surface  3820 . 2  to an angle at the point  3824  of transition from the spindle side surface  3819 . The transition surface  3820  angle of orientation may be defined by the taper angle  3826  formed between a taper line  3825  extending from the point  3824  of transition from the spindle side surface  3819  to the point  3822  of transition from the interface surface  3804  and a line  3828  tangent to the spindle side surface  3819 . Further, as shown, the planar surface  3820 . 3  closest to the spindle side surface  3819  may extend at an angle  3826 . 3  with respect to a line tangent to the spindle side surface. According to embodiments of the present disclosure, the angle  3826 . 3  may range from 25 to 35°. 
     According to some embodiments of the present disclosure, a planar surface closest to the spindle side surface may form a majority of a transition surface. In such embodiments, the angle of the planar surface with respect to a line tangent to the spindle side surface may be within 1%, 5%, 10%, or 15% range of difference from a taper angle formed between a taper line and the line tangent to the spindle side surface. 
       FIG. 32  shows a partial cross-sectional view of a cutting element having a transition surface  3920  formed of a non-planar surface  3920 . 1  and a planar surface  3920 . 2  according to other embodiments of the present disclosure. As shown, the non-planar surface  3920 . 1  may transition at an angle from a point  3922  of transition from the cutting element interface surface  3904  and the planar surface  3920 . 2  may transition at an angle from the non-planar surface  3920 . 1  to an angle at the point  3924  of transition from the spindle side surface  3919 . The transition surface  3920  angle of orientation may be defined by the angle  3926  formed between a taper line  3925  extending from the point  3924  of transition from the spindle side surface  3919  to the point  3922  of transition from the interface surface  3904  and a line  3928  tangent to the spindle side surface  3919 . Embodiments of the present disclosure may have transition surfaces formed of various combinations of one or more planar surfaces and/or one or more non-planar surfaces, wherein the angle of orientation of the transition surface is defined by the angle formed between a taper line and a line tangent to the spindle side surface. 
     Further, the size of a transition surface, such as radial and axial lengths of extension, may be designed based on dimensions of the cutting element. For example, referring to  FIG. 33 , a partial cross-sectional view of a cutting element is shown, wherein the cutting element has an interface surface  4004  extending a distance radially inward from a cutting end side surface  4017 , a spindle side surface  4019 , and a transition surface  4020  extending from a point  4022  of transition from the interface surface  4004  to a point  4024  of transition from the spindle side surface  4019 . The transition surface  4020  has a cross-sectional shape with at least one planar surface  4020 . 1 , a radial length of extension  4021  and an axial length of extension  4023 . The radial length of extension  4021  is measured from the point  4022  of transition from the interface surface  4004  to the line  4028  tangent to the spindle side surface  4019 . In other words, the radial length of extension  4021  is equal to D−(T+J), wherein D is the outer diameter of the cutting element, T is the radial length of the substantially planar surface of interface surface, and J is the diameter of the lower spindle portion axially above the retention cavity. The axial length of extension  4023  is measured from the point  4024  of transition from the spindle side surface  4019  to the line  4029  tangent to the interface surface  4004 . According to embodiments of the present disclosure, the radial length of extension  4021  and/or the axial length of extension  4023  of the transition surface  4020  may be designed based on the radial distance of the interface surface  4004  and/or the axial length of the spindle. 
     The planar surface  4020 . 1  may extend a radial length  4021 . 1  and axial length  4023 . 1 , wherein the radial length  4021 . 1  of the planar surface  4020 . 1  is less than the radial length of extension  4021  of the transition surface  4020  and the axial length  4023 . 1  of the planar surface  4020 . 1  is less than the axial length of extension  4023  of the transition surface  4020 . According to embodiments of the present disclosure, a transition surface may include a cross-sectional shape with a planar surface, wherein the planar surface has a radial length ranging from 10% to 100% of the radial length of extension of the transition surface and an axial length ranging from 20% to 100% of the axial length of extension of the transition surface. In some embodiments, a transition surface may include cross-sectional shape with a planar surface, wherein the planar surface has an axial length ranging from at least 50% of the axial length of extension of the transition surface. 
     Referring now to  FIG. 34 , a cutting element  4100  according to embodiments of the present disclosure may have a cutting end  4106 , a spindle  4108 , wherein the spindle diameter  4118  is less than the cutting end diameter  4116 , and a transition surface  4120  connecting the cutting end  4106  to the spindle  4108 . The cutting end  4106  is defined by a plurality of outer surfaces, including a cutting face  4102 , a cutting end side surface  4117 , and an interface surface  4104  opposite from the cutting face  4102 . A taper line  4125  is measured from the intersection of the interface surface  4104  and transition surface  4120  to the intersection of the spindle outer surface  4119  and transition surface  4120  and extends an angle  4126  from a line  4128  tangential to the spindle outer surface  4119 . Further, the transition surface  4120  has a radial length of extension  4121  and an axial length of extension  4123 , wherein the radial length of extension  4121  is measured from the interface surface  4104  to a line  4128  tangent to the spindle side surface  4119  and the axial length of extension  4123  is measured from the spindle side surface  4119  to a line  4129  tangent to the interface surface  4104 . 
     According to embodiments of the present disclosure, the radial length of extension  4121  may range from 25 to 100% of the radial distance of the interface surface  4104 . In some embodiments, the radial length of extension  4121  may range from 1/20 (5%) to 1/10 (10%) of the spindle diameter  4118 . 
     The axial length of extension  4123  of the transition surface may range from 50% to 150% of the radial distance of the interface surface  4104 . In some embodiments, the axial length of extension  4123  may be less than 1/10 (10%) of the length of the spindle  4108 . 
     Referring now to  FIG. 35 , a partial cross-sectional view of a cutting element  4200  according to embodiments of the present disclosure is shown. The cutting element has a cutting end, a spindle, and a transition surface  4220  extending from an interface surface  4204  of the cutting end to a spindle side surface  4219 . A taper line  4225  is measured between the point  4205  of transition from the interface surface  4204  to the transition surface  4220  and the point  4215  of transition from the spindle side surface  4219  to the transition surface  4220 . The taper line  4225  is at a taper angle  4226  from a line  4228  tangent to the spindle side surface, wherein the taper angle  4226  ranges from 5° to 85°. As shown, the transition surface  4220  may have a cross-sectional shape including planar and non-planar surfaces  4220 . 1 ,  4220 . 2 ,  4220 . 3 ,  4220 . 4 . Particularly, the transition surface  4220  has a non-planar surface  4220 . 1  extending from the point  4215  of transition from the spindle side surface  4219  to a planar surface  4220 . 2 , a planar surface  4220 . 3  extending at an angle from the planar surface  4220 . 2 , and a curved surface  4220 . 4  connecting the planar surface  4220 . 3  to the point  4205  of transition from the interface surface  4204 . A line  4223  tangent to the planar surface  4220 . 2  closest to the spindle side surface extends at an angle  4226 . 2  from the line  4228  tangent to the spindle side surface  4219 . In the embodiment shown, a taper line  4225  may not align with the transition surface  4220 ; however, in other embodiments, the taper line may substantially align with the transition surface. For example, as shown in  FIG. 35 , the angle  4226 . 2  of the planar surface closest to the spindle side surface  4219  is less than the taper angle  4226 . According to embodiments of the present disclosure, a planar surface closest to the spindle may have a tangent line forming an angle with a line tangent to the spindle side surface ranging between 5° and 85°. In some embodiments, a planar surface closest to the spindle may have a tangent line forming an angle with a line tangent to the spindle side surface ranging between 25° and 35°. 
     A transition surface may have a cross-sectional shape with a planar surface that is located closest to the spindle that extends directly from the point of transition from the spindle side surface, or that transitions to the point of transition from the spindle side surface with a curved surface. For example, as shown in  FIG. 35 , a curved surface  4220 . 1  connects the planar surface  4220 . 2  closest to the spindle to the point  4215  of transition from the spindle side surface.  FIGS. 31 and 32  show embodiments with a planar surface  3820 . 3 ,  3920 . 2  closest to the spindle side surface  3819 ,  3919  that extends directly from the spindle side surface  3819 ,  3919 . 
     Referring now to  FIG. 36 , cutting elements having various transition surface geometries were tested. Cutting elements 1 through 4 included radiused transition surfaces, i.e., transition surfaces having non-planar or curved surfaces, and cutting elements 5 and 6 included a planar surface closest to the spindle that formed an angle of 30° with a line tangent to the spindle side surface.  FIGS. 37 and 38  show examples of the tested cutting elements having a radiused transition surface and a transition surface with a planar surface extending at a 30° angle from a line tangent to the spindle side surface.  FIG. 37  shows a partial cross-sectional view of a cutting element  4400  having a radiused transition surface  4420 , wherein the radius of curvature of the transition surface  4420  is about 0.04 in. (1 mm).  FIG. 38  shows a partial cross-sectional view of a cutting element  4500  with a transition surface  4520  having a planar surface closest to the spindle side surface  4519 , wherein the planar surface extends at a 30° angle from a line tangent to the spindle side surface  4519 . Both cutting element  4400  and cutting element  4500  have an interface surface  4404 ,  4504  extending an equal radial distance from a cutting end side surface  4417 ,  4517  to the transition surface  4420 ,  4520 . 
     As shown in  FIG. 36 , cutting elements (samples 5 and 6) having a planar surface closest to the spindle forming an angle of 30° with a line tangent to the spindle side surface outperformed cutting elements (samples 1-4) having a radiused transition surface in impact testing. The impact tests used for testing cutting elements 1-6 included holding the cutting elements 1-6 in a testing machine at a 20° back rake angle, while a steel bar anvil (having a hardness of 62 Rockwell Hardness C (HRC)) impacted the cutting end of the cutting element. Each cutting element was impacted five times with the steel bar anvil at a force interval. Results of the impact testing are shown in  FIG. 36 , where the cutting elements (samples 1-4) having a radiused transition surface failed during impact testing at 13,000 lbf (5,897 kgf), 17,500 lbf (7,938 kgf), 15,000 lbf (6,804 kgf) and 17,500 lbf (7,938 kgf) and energy of 40 J, 60 J, 50 J, and 60 J, respectively. The cutting elements (samples 5 and 6) having a planar surface closest to the spindle forming an angle of 30° with a line tangent to the spindle side surface did not fail before the impact machine reached its limit, at 20,000 lbf (9,072 kgf) and 70 J. 
     Referring now to  FIGS. 39 to 43 , FEA simulations were performed to test the bending strength of cutting elements having various transition surface geometries. Particularly,  FIG. 39  shows a simulated cutting element  4700  for FEA, where the cutting element  4700  includes a cutting end  4710 , a spindle  4720 , and a transition surface  4730  connecting the cutting end  4710  to the spindle  4720 . A vertical load  4740  of 10,000 psi (69,000 kPa) was applied to the interface surface, or back, of the cutting end  4710  to predict the bending strength of the transition area. 
       FIGS. 40-1 to 40-6  show the results of the FEA simulations performed for cutting elements having a 13 mm cutting end diameter, wherein the darker regions indicate higher stress concentrations.  FIG. 40-1  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 10° angle from a line tangent to the spindle side surface.  FIG. 40-2  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 20° angle from a line tangent to the spindle side surface.  FIG. 40-3  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 30° angle from a line tangent to the spindle side surface.  FIG. 40-4  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 45° angle from a line tangent to the spindle side surface.  FIG. 40-5  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 60° angle from a line tangent to the spindle side surface.  FIG. 40-6  shows the simulation results for a cutting element having a radiused transition surface, i.e., a transition surface without a planar surface. 
       FIG. 41  shows a graph of the simulation results shown in  FIGS. 40-1 to 40-6 , wherein the maximum principle stress experienced by each cutting element is plotted. As shown, the cutting element tested in  FIG. 40-3  (having a transition surface with a planar surface closest to the spindle extending at a 30° angle from a line tangent to the spindle side surface) experienced the lowest maximum principle stress under the applied vertical load. 
       FIGS. 42-1 to 42-4  show results for FEA simulations performed for cutting elements having a 16 mm cutting end diameter, wherein the darker regions indicate higher stress concentrations.  FIG. 42-1  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 20° angle from a line tangent to the spindle side surface.  FIG. 42-2  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 30° angle from a line tangent to the spindle side surface.  FIG. 42-3  shows the simulation results for a cutting element having a transition surface with a planar surface closest to the spindle extending at a 45° angle from a line tangent to the spindle side surface.  FIG. 42-4  shows the simulation results for a cutting element having a radiused transition surface, i.e., a transition surface without a planar surface. 
       FIG. 43  shows a graph of the simulation results shown in  FIGS. 42-1 to 42-4 , wherein the maximum principle stress experienced by each cutting element is plotted. As shown, the cutting element tested in  FIG. 50.2  (having a transition surface with a planar surface closest to the spindle extending at a 30° angle from a line tangent to the spindle side surface) experienced the lowest maximum principle stress under the applied vertical load. 
     One or more embodiments described herein may have an ultrahard material on a substrate. Such ultrahard materials may include a conventional polycrystalline diamond table (a table of interconnected diamond particles having interstitial spaces therebetween in which a metal component (such as a metal catalyst) may reside, a thermally stable diamond layer (i.e., having a thermal stability greater than that of conventional polycrystalline diamond, 750° C.) formed, for example, by substantially removing metal from the interstitial spaces between interconnected diamond particles or from a diamond/silicon carbide composite, or other ultrahard material such as a cubic boron nitride. Further, in particular embodiments, the rolling cutter may be formed entirely of ultrahard material(s), but the element may include a plurality of diamond grades used, for example, to form a gradient structure (with a smooth or non-smooth transition between the grades). In a particular embodiment, a first diamond grade having smaller particle sizes and/or a higher diamond density may be used to form the upper portion of the inner rotatable cutting element (that forms the cutting edge when installed on a bit or other tool), while a second diamond grade having larger particle sizes and/or a higher metal content may be used to form the lower, non-cutting portion of the cutting element. Further, it is also within the scope of the present disclosure that more than two diamond grades may be used. 
     Thermally stable diamond may be formed in various manners. A typical polycrystalline diamond layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt, may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are generally found within the interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table. 
     To obviate this problem, strong acids may be used to “leach” the cobalt from a polycrystalline diamond lattice structure (either a thin volume or entire tablet) to at least reduce the damage experienced from heating diamond-cobalt composite at different rates upon heating. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344, which are incorporated herein by this reference in their entireties. Briefly, a strong acid, such as hydrofluoric acid or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. Suitable acids include nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid, phosphoric acid, or perchloric acid, or combinations of these acids. In addition, caustics, such as sodium hydroxide and potassium hydroxide, have been used to the carbide industry to digest metallic elements from carbide composites. In addition, other acidic and basic leaching agents may be used as desired. Those having ordinary skill in the art will appreciate that the molarity of the leaching agent may be adjusted depending on the time desired to leach, concerns about hazards, etc. 
     By leaching out the cobalt, thermally stable polycrystalline (TSP) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance. As used herein, the term TSP includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by filling the volume with a secondary material, such as described in U.S. Pat. No. 5,127,923, which is herein incorporated by reference in its entirety. 
     In one or more other embodiments, TSP may be formed by forming the diamond layer in a press using a binder other than cobalt, one such as silicon, which has a coefficient of thermal expansion more similar to that of diamond than cobalt has. During the manufacturing process, a large portion, 80 to 100 volume %, of the silicon reacts with the diamond lattice to form silicon carbide which also has a thermal expansion similar to diamond. Upon heating, any remaining silicon, silicon carbide, and the diamond lattice will expand at more similar rates as compared to rates of expansion for cobalt and diamond, resulting in a more thermally stable layer. PDC cutters having a TSP cutting layer have relatively low wear rates, even as cutter temperatures reach 1200° C. However, thermally stable diamond layer may be formed by other methods, including, for example, by altering processing conditions in the formation of the diamond layer. The substrate on which the cutting face is optionally located or formed may be formed of a variety of hard or ultrahard particles. In one embodiment, the substrate may be formed from a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the substrate, such as cobalt, nickel, iron, metal alloys, or mixtures thereof. In the substrate, the metal carbide grains are supported within the metallic binder, such as cobalt. Additionally, the substrate may be formed of a sintered tungsten carbide composite structure. Various metal carbide compositions and binders may be used, in addition to tungsten carbide and cobalt. Thus, references to the use of tungsten carbide and cobalt are for illustrative purposes, and no limitation on the type of substrate or binder used is intended. In another embodiment, the substrate may also be formed from a diamond ultrahard material such as polycrystalline diamond or thermally stable diamond. While the illustrated embodiments show the cutting face and substrate as two distinct pieces, one of skill in the art should appreciate that it is within the scope of the present disclosure the cutting face and substrate are integral, identical compositions. In such an embodiment, it may be desirable to have a single diamond composite forming the cutting face and substrate or distinct layers. Specifically, in embodiments where the cutting element is a rotatable cutting element, the entire cutting element may be formed from an ultrahard material, including thermally stable diamond (formed, for example, by removing metal from the interstitial regions or by forming a diamond/silicon carbide composite). 
     A sleeve may be formed from a variety of materials. In one embodiment, the sleeve may be formed of a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the outer support element, such as cobalt, nickel, iron, metal alloys, or mixtures thereof, such that the metal carbide grains are supported within the metallic binder. In a particular embodiment, the outer support element is a cemented tungsten carbide with a cobalt content ranging from 6 to 13%. It is also within the scope of the present disclosure that the sleeve and/or substrate may also include one more lubricious materials, such as diamond to reduce the coefficient of friction therebetween. The components may be formed of such materials in their entirely or have portions of the components including such lubricious materials deposited on the component, such as by chemical plating, chemical vapor deposition (“CVD”) including hollow cathode plasma enhanced CVD, physical vapor deposition, vacuum deposition, arc processes, or high velocity sprays). In a particular embodiment, a diamond-like coating may be deposited through CVD or hallow cathode plasma enhanced CVD, such as the type of coatings disclosed in U.S. Publication No. 2010/0108403. 
     In embodiments using a sleeve, such sleeve may be fixed to the bit body (or other cutting tool) by any means known in the art, including by casting in place during sintering the tool, or by brazing the element in place in the cutter pocket. Brazing may occur before or after the inner cutting element is retained within the sleeve; however, in some embodiments, the inner rotatable cutting element is retained in the sleeve before the sleeve is brazed into place. Other embodiments of a cutting element assembly may include a cutting element axially retained within an outer support, which may include, for example, a portion of the cutting tool on which the cutting element assembly is formed. 
     Cutting element assemblies of the present disclosure may be used on any downhole cutting tool, including, for example, a fixed cutter drill bit or hole opener.  FIG. 22  shows a general configuration of a hole opener  830  that includes one or more cutting element assemblies  840  of the present disclosure. The hole opener  830  includes a tool body  832  and a plurality of blades  838  at selected azimuthal locations about a circumference thereof. The hole opener  830  generally includes connections  834 ,  836  (e.g., threaded connections) so that the hole opener  830  may be coupled to adjacent drilling tools that comprise, for example, a drillstring and/or bottom hole assembly (“BHA”). The tool body  832  generally includes a bore therethrough so that drilling fluid may flow through the hole opener  830  as it is pumped from the surface (e.g., from surface mud pumps) to a bottom of the wellbore. The tool body  832  may be formed from steel or from other materials known in the art. For example, the tool body  832  may also be formed from a matrix material infiltrated with a binder alloy. The blades  838  shown in  FIG. 22  are spiral blades and are generally positioned at substantially equal angular intervals about the perimeter of the tool body. This arrangement is not a limitation on the scope of the disclosure, but rather is used merely to illustrative purposes. Those having ordinary skill in the art will recognize that any downhole cutting tool may be used. While  FIG. 22  does not detail the location of the cutting element assemblies, their placement on the tool may be according to the variations described above. 
     Although just a few embodiments have been described in detail above, those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from the apparatus, systems, and methods disclosed herein. Accordingly, such modifications are intended to be included within the scope of this disclosure. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. For example, any element described in relation to an embodiment herein may be combinable with any element of any other embodiment described herein. 
     In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not just structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke means-plus-function or functional claiming for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. Each addition, deletion, and modification to the embodiments that fall within the meaning and scope of the claims is to be embraced by the claims. Features and components of the various embodiments may be combined together in any combination, except where such features/components are mutually exclusive.