Abstract:
A rotary cutting tool with an elongate body disposed about a longitudinal axis, the elongate body including a helical flute and a polycrystalline-diamond cutting tip. The cutting tip comprises an inner portion having an inner point angle and an outer portion having an outer point angle different from the inner point angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application under 35 USC §120 is a division of co-pending U.S. patent application Ser. No. 12/907,397, filed on Oct. 19, 2010, which is incorporated herein by reference in its entirety, and which itself claims priority to U.S. provisional application No. 61/329,707 filed Apr. 30, 2010, entitled “PCD Drill for Composite Materials”, which is also incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Field of the Invention 
         [0003]    The invention relates generally to rotary cutting tools and, more particularly, to rotary cutting tools, such as drills, having polycrystalline-diamond (PCD) cutting tips. The invention further relates to a method for forming a rotary cutting tool having a polycrystalline-diamond cutting tip. 
         [0004]    Background Information 
         [0005]    Polycrystalline-diamond (PCD) drills have historically been formed as straight fluted, facet point drills. More recently, PCD drills have been formed having helical flutes and more complex point geometries similar to solid carbide drills. One of the major uses of such highly engineered PCD drills is for drilling in composite materials, such as carbon fiber reinforced polymer (CFRP) titanium composites. Drills used for cutting such material require a high wear resistance to survive in CFRP while having a geometry that is effective to cut titanium. Aerospace customers, who commonly utilize such CFRP composite materials, further require that the burr height of the titanium portion of the drilled composite material be maintained around 100 microns. Known PCD drills produce a high quality hole in the first few holes, but rapidly begin to produce unacceptable burrs soon thereafter (typically about 5 holes or less). Accordingly, such drills must be replaced frequently at a high cost. 
         [0006]    There is, therefore, room for improvement in rotary cutting tools used for drilling CFRP-titanium, particularly in the quality of the holes cut and the durability of the cutting tool. 
       SUMMARY OF THE INVENTION 
       [0007]    Deficiencies in the prior art are addressed by embodiments of the invention which are directed to a rotary cutting tool, a polycrystalline-diamond cutting tip for use with a rotary cutting tool, and a method for forming a rotary cutting tool having a polycrystalline cutting tip. 
         [0008]    As one aspect of the invention, a rotary cutting tool is provided. The rotary cutting tool comprises: an elongate body disposed about a longitudinal axis. The body includes a helical flute and a polycrystalline diamond cutting tip. The cutting tip comprises: an inner portion having an inner point angle; and an outer portion having an outer point angle different from the inner point angle. 
         [0009]    The outer point angle may be greater than the inner point angle. The inner point angle may be in the range of about 110 degrees to about 140 degrees. The outer point angle may be in the range of about 145 degrees to about 180 degrees. The elongate body may be formed from a carbide material. The elongate body may comprise: a first end opposite the cutting tip; and at least two coolant passages passing therethrough, each coolant passage extending from the first end to the cutting tip. Each coolant passage may be generally helical in shape. 
         [0010]    As another aspect of the invention, a polycrystalline diamond cutting tip for use with a rotary cutting tool is provided. The cutting tip comprises: an inner portion having an inner point angle and an outer portion having an outer point angle different from the inner point angle. 
         [0011]    The outer point angle may be greater than the inner point angle. The inner point angle may be in the range of about 110 degrees to about 140 degrees. The outer point angle may be in the range of about 145 degrees to about 180 degrees. 
         [0012]    As a further aspect of the invention, a method for forming a rotary cutting tool having a polycrystalline-diamond cutting tip is provided. The method comprises: forming at least two coolant passages in a generally cylindrical tool body; forming at least two coolant passages in a tip portion, the tip portion being separate from the tool body; and coupling the tip portion to the tool body to form the rotary cutting tool. 
         [0013]    The tip portion may be coupled to the tool body via a brazing process. The at least two coolant passages may be formed in the generally cylindrical tool body by an extrusion process. The at least two passages may be formed in the tip portion via an EDM drilling process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  is a side view taken along a line generally perpendicular and within the same horizontal plane as the primary cutting edge and second cutting edge portions of the cutting end of a helical drill in accordance with a non-limiting embodiment of the present invention. 
           [0016]      FIG. 2  is a top view of the cutting end of the drill shown in  FIG. 1 . 
           [0017]      FIG. 3  is a top view of the cutting end of a drill in accordance with another non-limiting embodiment of the present invention. 
           [0018]      FIG. 4  is an enlarged view of the top view illustrated in  FIG. 3 . 
           [0019]      FIG. 5  is a partial cross-sectional view taken along arrows “ 5 - 5 ” in  FIG. 3 . 
           [0020]      FIG. 6  is a side view of the drill illustrated in  FIG. 3  taken along arrows “ 6 - 6 ” in  FIG. 3 . 
           [0021]      FIG. 7  shows a semi-transparent view of a prior art drill showing the internal coolant passages. 
           [0022]      FIG. 8  shows a semi-transparent view of a drill in accordance with a non-limiting embodiment of the present invention showing the internal coolant passages. 
           [0023]      FIG. 9  is a top view of the cutting end of the drill shown in  FIG. 8 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Directional phrases used herein, such as, for example, left, right, front, back, top, bottom and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. Identical parts are provided with the same reference number in all drawings. 
         [0025]      FIGS. 1 and 2  show a portion of an example helical drill  20  in accordance with a non-limiting embodiment of the present invention. Drill  20  is configured to be rotationally driven about a center longitudinal axis A-A or to have an associated workpiece (not shown) rotate, or both the drill  20  and workpiece rotate relative to each other. Referring to  FIG. 1 , drill  20  is arranged such that a cutting end  22  is formed at the outer end of a shank  24 . Shank  24  comprises a first portion  24   a  preferably formed of carbide material and a second portion  24   b  preferably formed of PCD material disposed at or about the cutting end  22 . Carbon fibers contained in composite materials are highly abrasive and a PCD tool material helps to prolong the life and edge sharpness of the drill  20 . A sharp edge is critical to minimize unwanted damage to the machined composite material and further to minimize burr height when the drill  20  exits the metal of a CFRP-titanium composite. A blunt edge generally causes excessive delamination in CFRP and likewise is unfavorable when cutting titanium, leading to higher stresses and temperatures, eventually resulting in premature chipping of the drill, and damage to the workpiece. 
         [0026]    In the example embodiment shown in  FIGS. 1 and 2 , shank  24  is formed by first sintering the PCD material onto a small piece of carbide which is then brazed onto a larger piece of carbide, such as at braze line  24   c,  shown in dashed line in  FIG. 1 . However, it is to be appreciated that other methods or steps may be employed in forming shank  24  without varying from the scope of the present invention. 
         [0027]    Continuing to refer to  FIGS. 1 and 2 , shank portion  24  includes two chip discharge flutes  32 . The flutes  32  are formed from the tip of the cutting end  22  and extend rearward to adjacent a fastening shank portion (not shown) of the drill  20  that is adapted to be mounted in a machine tool, as commonly known in the art. The flutes  32  are generally symmetric and at equal intervals in the circumferential and axial direction and are disposed in a generally helical path oriented at a helix angle φ ( FIG. 1 ) with respect to longitudinal axis A-A. The flutes  32  ensure that composite fibers of the workpiece are cut well while minimizing delamination as the drill  20  enters the workpiece. The helix angle φ of the flutes also plays an important role in the hole cutting process. A low helix angle φ or a straight flute would not evacuate the metallic chips effectively, while a high helix angle φ would reduce the strength of the cutting edge. A preferred helix angle also enables appropriate curl of the cut chips. In at least one embodiment of the present invention such preferred helix angle φ was found to be about 22.5 degrees. Generally such helix angle φ was found to be in the range of about 18 degrees to about 30 degrees. It is to be appreciated that a differential helix could also be employed. In such embodiments, the local helix angle near the cutting edge is preferably within the given range, but the helix angle toward the shank may vary within or outside the range. 
         [0028]    Cutting end  22  includes a pair of cutting edges  30  ( FIG. 2 ) formed along the intersecting ridge where forward flute wall surfaces  33  ( FIG. 1 ) intersect with a top flank  34 . Each top flank  34  includes forward surface sections  34   a  and rearward surface sections  34   b  on opposing sides of the drill  20 . Each cutting edge  30  has at least a first cutting edge portion  36  and a second cutting edge portion  35 , with the first cutting edge portion  36  extending radially from a central generally straight chisel edge  41  to the second cutting edge portion  35 , and the second cutting edge portion  35  extending radially outward to at least approximately near an outer margin  39  on the external radial circumference of the drill  20 . The chisel edge  41  is formed by intersecting peak surfaces  45 . The second cutting edge portion  35  extends radially outward to a third outer cutting edge portion  37 . The third outer cutting edge portion  37  extends radially outward from the second straight portion  35  to the drill margin  39  and axially rearward. The length of the chisel edge  41  in comparison to the diameter of the drill is designed to be approximately between 1%-10% of the drill&#39;s diameter. 
         [0029]    The above described symmetric design of the cutting edges  30  greatly facilitates stability in use of the drilling system. This characteristic is achieved by the neutral or balanced geometry of the cutting surfaces, which significantly decrease any tendency of the drilling system to wobble in use. However, it is to be appreciated that cutting edges  30  as well as other elements described herein as being symmetric in the example embodiments may also be asymmetric without varying from the scope of the present invention. 
         [0030]    The forward sections  34   a  of top flank  34  immediately adjacent all portions of the cutting edge  30  are oriented at a first relief angle generally between 5 degrees and 20 degrees, or about 10 degrees. Rearward sections  34   b  of top flank  34  are oriented at a greater second relief angle than the forward sections  34   a.  Rearward surface sections  34   b  are oriented at a second relief angle generally between 15 degrees to 50 degrees, 25 degrees to 40 degrees, or at about 20 degrees. In the embodiment illustrated in  FIGS. 1 and 2 , the first cutting edge portion  36  is convex and has a generally constant radius of curvature R when taken from a top view along the central axis, as seen in  FIG. 2 . The radius of curvature R is generally set to the range of from 8% of the external diameter of the drill, XD, to 20% of the external diameter XD when viewed from a top view taken along the central axis of the drill, as shown in  FIG. 2 . The radius of curvature R generally eliminates the sharp transition between cutting edges  30 , so that breakage of the cutting edges  30  can be prevented regardless of drilling conditions. It is contemplated that the first cutting edge portion  36  may also be other convex curvilinear geometries rather than a convex shape having a generally constant radius. It is also contemplated that the first cutting edge portion  36  could also be formed in other non-curvilinear shapes (e.g., without limitation, chamfers) without varying from the scope of the present invention. 
         [0031]    The drill  20  is preferably shaped by thinning at the cutting end of the drill  20 . The thinning is applied to a thick central core portion at the tip of the drill main body and a curvilinear first cutting edge portion  36  is formed by the thinning, the first cutting edge portion  36  extends from the central chisel edge  41  to the second cutting edge portion  35 . It is to be appreciated that in the embodiment shown in  FIGS. 1 and 2 , the first cutting edge portion  36  does not stretch to the center of the drill  20 . The first portion  36  of the cutting edge is formed at a position slightly spaced apart from the central axis of the drill to reduce weakening of the center of the drill caused by stress concentration. 
         [0032]    The thinning surfaces on the drill tip  22  of the present invention shown in  FIGS. 1 and 2  reach from the central core of the drill  20  to the sidewall  49  of the drill  20 . The first thinning surface  38  extends from the rear side of the chip discharge flute  32  to the rearward surface  34   b  of top flank  34 , when viewed from a top view taken along the central axis A-A of the drill  20  (as shown in  FIG. 2 ). In the embodiment of the invention illustrated in  FIGS. 1 and 2 , the thinning surface  38  is disposed to stretch from the external circumferential sidewall  49  to the central core of the drill  20  near the central axis A-A. 
         [0033]    Each thinning on opposite sides of the central axis A-A is composed of two thinning surfaces, first thinning surface  38  and second thinning surface  44 . As seen in  FIG. 1 , the second thinning surface  44  runs basically parallel to the central axis A-A of the drill  20 . It is contemplated, in an alternative embodiment of the invention, that the second thinning surface  44  may be slightly angled forward or rearward with respect to the cutting direction of the drill  20  to provide a negative or positive rake. The first cutting edge portion  36  is formed along the intersecting ridge where the second thinning surface  44  intersects with the peak surface  45 . The first thinning surface  44  extends generally downward to a crease  46  formed with second thinning surface  38 . The first thinning surface  44  is preferably not a flat plane but instead a convex surface, as best represented by line  36  in  FIG. 2  (note, line  36  represents the cutting edge portion formed where the first thinning surface  44  intersects with the peak surfaces  45 ). 
         [0034]    The second thinning surface  38  generally is flat and planar and oriented at a constant rearward angle with respect to a plane intersecting the central axis A-A of the drill  20 . In one embodiment of the invention, the plane interesting the longitudinal axis A-A is also parallel to the second cutting edge portion  35 , although this central axis intersecting plane need not be parallel to the second cutting edge portions  35 . The rearward angle is generally between 30 and 50 degrees, alternatively, between 40 degrees to 45 degrees, or may be about 45 degrees. It should be appreciated that the second thinning surface  38  may be shaped other than flat and planar without varying from the scope of the present invention. 
         [0035]    A flank edge  43  represents an upper boundary of the thinning. The flank edge  43  is defined as the intersection between the second thinning surface  38  and the top flank rearward surface section  34   b.  The flank edge  43  is oriented at an angle θ with respect to the chisel edge  41  (see  FIG. 2 ). The angle θ is generally set between the range of from 75 degrees to 105 degrees, or within the range 85 degrees to 95 degrees or at about 90 degrees (as shown). 
         [0036]    An upwardly inclined peak surface  45  is associated with each of the top flank surfaces  34   a,    34   a  and cutting edges  30 ,  30 . As shown in  FIG. 1 , the first cutting edge portions  36  associated with peak surfaces  45  are generally oriented to form an inner point angle γ, which represents the angle of the peak surface  45  and associated first cutting edge portions  36 . In the example embodiment shown, cutting edge  30  on one side of the rotational axis A-A is symmetric with the cutting edge  30  on the opposite side of the rotational axis A-A. However it is to be appreciated that the cutting edges  30  could also be asymmetric without varying from the scope of the present invention. In the embodiment shown in  FIGS. 1 and 2 , the peak surfaces  45  are oriented generally at the same angle (not numbered) with respect to the rotational axis A-A. The inner point angle γ is preferably within the range of about 110 degrees to about 140 degrees. 
         [0037]    The inner point angle γ generally defines an inner point  50  near the central portion of the drill  20 . Such inner point  50  generally provides improved stability and enables good centering of the drill  20  as it enters a workpiece (not shown). By decreasing the inner point angle γ of the inner point  50 , thus making inner point  50  steeper, the start up, stability and reduction in wobbling of the drill may be improved as desired by configuring the angle γ as required for various applications. However, it is to be appreciated that while decreasing the angle γ generally improves the start up, stability and wobble reduction of the drill, such decreasing also generally weakens the peaked tip of the drill  20 . 
         [0038]    Continuing to refer to  FIG. 1 , the second cutting edge portions  35  are generally oriented to form an outer point angle Γ. Preferably, the angle Γ is within the range of about 145 degrees to about 180 degrees. The outer point angle Γ generally defines a peripheral, or outer point geometry (hereinafter referred to as outer point  52 ). The relatively flat geometry of outer point  52  ensures that cutting forces are directed generally axially along the drill  20  rather than laterally, and hence decreases the size of the burr that rolls off along the exit edge of a drilled hole. 
         [0039]    The third outer cutting edge portion  37  may be curvilinear and have a constant radius of rotation or may instead be chamfered. It is also contemplated that other embodiments of the drill might not have a third outer cutting edge portion  37 , but may consist of only a first cutting edge portion  36  and a second cutting edge portion  35  that extends radially outward from the first cutting edge portion  36  to the extreme margin of the drill forming a sharp corner thereat. 
         [0040]    With respect to  FIGS. 3-9 , wherein a second non-limiting embodiment of the present invention is depicted, it should be appreciated that like parts of the previously discussed drill will retain the same reference item numbers and these parts will not again be discussed at length. 
         [0041]    Of particular note,  FIG. 3  is a view similar to that of previously presented  FIG. 2 , but the chisel edge  141  is much shorter relative to the tool external diameter than the chisel edge  41  previously discussed. The enlarged view of  FIG. 3  found in  FIG. 4  highlights this feature. Additionally, as will be discussed, the first curvilinear cutting edge portion  136  has a positive axial rake angle. 
         [0042]    Referring to  FIGS. 3-6 , the drill  100  has a longitudinal axis A-A ( FIG. 6 ) which in the end view of  FIG. 4  is the center of the drill  100 . As shown in  FIG. 6 , drill  100 , like drill  20  previously described, includes a shank  124  having a first portion  124   a  preferably formed of carbide material and a second portion  124   b  preferably formed of PCD material disposed at or about a cutting end  122 . In a preferred embodiment, shank  124  is formed by first sintering the PCD material onto a small piece of carbide which is then brazed onto a larger piece of carbide, such as shown by dashed braze line  124   c.  However, it is to be appreciated that other methods or steps may be employed in forming shank  124  without varying from the scope of the present invention. 
         [0043]    A first peak surface  45   a  and a second peak surface  45   b,  intersect at, and are generally adjacent to, the central axis A-A and intersect to form the chisel edge  141 . An imaginary bisector line  102  extends radially through the central axis A-A perpendicular to the chisel edge  141  and defines a first tool half  103  on one side of the bisector line  102  and a second half  104  on the other side of the bisector line  102 . 
         [0044]    Each tool half  103 , 104  has a first curvilinear cutting edge portion  136  extending radially from the chisel edge  141  and a second cutting edge portion  135  extending radially outwardly from the first cutting edge portion  136 . When viewed from the cutting end  122  ( FIG. 6 ) the chisel edge  141  is curved to blend with the first curvilinear cutting edge  136  of the first tool half  103  and the first curvilinear cutting edge  136  of the second tool half  104 . It should be appreciated when viewing  FIG. 4  that the chisel edge  141  blends smoothly with the first curvilinear cutting edge  136  of the first tool half  103  and the first curvilinear cutting edge  136  of the second tool half  104  to provide a continuous “s” shaped connector between each of the first curvilinear cutting edges. 
         [0045]    Of particular interest in the subject invention is the fact that the first curvilinear cutting edge portions  136  adjacent to the chisel edge  141  of each tool half  103 , 104  each have adjacent surfaces which define a positive axial rake angle. In particular, the second thinning surface  144  ( FIG. 6 ) serves as the rake face for the first curvilinear cutting portion  136 . It should be appreciated that the positive axial rake angle X ( FIG. 5 ) between the second thinning surface  144  and the central axis A-A may be generally between 0 and 15 degrees and preferably is about 5 degrees. 
         [0046]    Additionally, the length L ( FIG. 4 ) of the chisel edge  141  is short relative to the external diameter XD ( FIG. 3 ) of the drill  100 . In particular, the length L of the chisel edge  141  is generally between about 1% and 4%, preferably about 2.5%, of the external diameter XD of the drill  100 . 
         [0047]      FIG. 3  illustrates a radius of curvature R of the first curvilinear cutting edge  136  and this radius of curvature R may generally be between about 8% to 20% of the external diameter XD of drill  100 . As previously mentioned and with respect to  FIG. 4 , the chisel edge  141  is curved to blend with the first curvilinear cutting edge portion  136  of both the first tool half  103  and the second tool half  104 . As a result, the chisel edge  141  and the adjacent first curvilinear cutting edge portions  136  assume an “s” shape. This “s” shape, along with the positive axial rake angle X of the first curvilinear cutting edge portion  136  provides an enhanced ability to center the cutting tool  100  and also provides additional stability to the cutting tool  100 . 
         [0048]    As previously discussed, the drill  100  has a chisel edge  141  with first curvilinear cutting edge portions  136  that form a positive rake angle X with the longitudinal axis A-A of the drill  100 . It is also possible to produce such a cutting tool without the chisel edge having a positive rake surface but with the chisel edge  141  smoothly blended with the first curvilinear edge portion  136  to produce a smooth “s” shape. 
         [0049]    In drilling CFRP-Titanium, the drill  100  described herein produced holes having burrs of generally less than 50% of known drills while lasting approximately twice as long as known drills. 
         [0050]    Referring to  FIGS. 8 and 9 , another feature of the present invention is shown in contrast to an example of the prior art shown in  FIG. 7 . Referring to  FIG. 7 , an example prior art drill  200  having a carbide body  202  with a brazed tip portion  204  is shown. Coolant is provided to a pair of openings  206  in the brazed tip portion  204  by a pair of straight passageways  208  provided in brazed tip portion  204  that extend from a single opening  210  in the braze joint  203  to each of openings  206 . Opening  210  is disposed at the end of a single central coolant passage  212  that travels axially along the central axis A-A of the drill  200 . 
         [0051]    In contrast to the prior art design shown in  FIG. 7 ,  FIGS. 8 and 9  show a drill  300  having a coolant delivery system in accordance with a non-limiting embodiment of the present invention. Similar to the prior art layout, drill  300  includes a carbide body  302  having a brazed tip portion  304 . Coolant is provided to a pair of openings  306  in the brazed tip portion  304  by a pair of straight passageways  308  provided in brazed tip portion  304  that extend from a pair of openings  310  in the braze joint  303  such that each passageway  308  is disposed between a respective one of openings  306  and a respective one of openings  310 . Each of openings  310  is disposed at the end of a respective spiral shaped coolant passage  312  that travels in a generally spiral-like manner about the central axis A-A of the drill  300  along a helix angle δ relative to the central axis A-A. 
         [0052]    Passages  312  are formed when the carbide rods are initially extruded. The helix angle δ of the passages  312  is generally controlled by the required helix angle on the flute (i.e., the lead (or pitch) of the coolant hole is typically the same as the desired lead to get the necessary flute helix angle). In some cases there are allowed deviations, as long as the coolant does not intersect the path of the flute profile. Typically the coolant hole is placed generally between 30-80% of the drill radius in the radial direction, and circumferentially about 25 to 60 degrees from the edge of the cutting corner. 
         [0053]    Passageways  308  are typically formed in tip portion  304  prior to brazing onto carbide body  302 . Such passageways  308  may be formed via EDM hole drilling or other suitable processes. The passageways  308  are preferably aligned at an angle to meet the existing coolant holes in the rod tangentially, however, the passageways  308  could also meet at other angles (e.g., without limitation, could be parallel to the axis of the drill). 
         [0054]    In such new design, as shown in  FIGS. 8 and 9 , strength and rigidity is not compromised significantly, unlike the example of  FIG. 7  where a single central coolant passage  212  is utilized. Unlike the prior art there is no danger of a “weak” intersection at the load bearing areas near the core. The multi coolant passage design of the present invention allows for more coolant volume to be brought to the cutting edge. The multi coolant passage design also presents no limitations for smaller diameter drills. Furthermore, the multi coolant passage design generally does not increase manufacturing costs as two holes need to be made through the PCD material either way. 
         [0055]    Although described herein in conjunction with a PCD tipped drill, it is to be appreciated that the multi coolant passage design could also readily be applied to other applications that involve brazing a tip portion to an existing rod. Rod materials used in such applications may commonly include, for example, without limitation, carbide, ceramic, powdered metal, high speed steel, steel, and others. Tip materials used in such applications may include, for example, without limitation, carbide, cermet, ceramic, PCD, pCBN and others. 
         [0056]    Drills constructed in accordance with the present invention can be used in many applications throughout all industries but are particularly well suited for use in hole cutting operations involving composite materials (e.g., without limitation, CFRP-Titanium composites). 
         [0057]    Other applications, embodiments and variations to the disclosed embodiments described herein will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the appended claims. 
         [0058]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.