Patent Publication Number: US-9416593-B2

Title: Piston strike face and bit interface for percussion hammer drill

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of, and priority to, U.S. Patent Application Ser. No. 61/746,777, filed on Dec. 28, 2012 and entitled “PISTON STRIKE FACE AND BIT INTERFACE FOR PERCUSSION HAMMERS,” which application is hereby incorporated herein by this reference in its entirety. 
    
    
     BACKGROUND 
     In percussion or hammer drilling operations, a drill bit mounted to the lower end portion of a drill string rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter about equal to the diameter or “gage” of the drill bit. 
     A typical percussion drilling assembly is coupled to the lower end portion of a rotatable drill string and includes a downhole piston-cylinder assembly coupled to the hammer bit. The impact force is generated by the downhole piston-cylinder assembly and transferred to the hammer bit. During drilling operations, a pressurized or compressed fluid flows down the drill string to the percussion drilling assembly. A choke is provided to regulate the flow of the compressed fluid to the piston-cylinder assembly and the hammer bit. A fraction of the compressed fluid flows through a series of ports and passages to the piston-cylinder assembly, thereby actuating the reciprocal motion of the piston, and then is exhausted through a series of passages in the hammer bit body to the bit face. The remaining portion of the compressed fluid flows through the choke and into the series of passages in the hammer bit body to the bit face. The compressed fluid exiting the bit face serves to flush cuttings away from the bit face to the surface through the annulus between the drill string and the borehole sidewall. 
     In oil and gas drilling, the cost of drilling a borehole is very high, and is generally proportional to the length of time it takes to drill to the desired depth and location. The time to drill the well, in turn, is greatly affected by the number of times the drill bit or other component of the percussion drilling assembly is replaced before reaching the targeted formation. Each time a drilling assembly component is changed, the entire string of drill pipe—which may be miles long—is retrieved and removed section by section from the borehole or borehole. Once the drill string has been retrieved and the new component installed, the drilling assembly is lowered to the bottom of the borehole on the drill string, which again is constructed section by section. This process, known as a “trip” of the drill string, takes considerable time, effort and expense. 
     During drilling, the piston of the downhole hammer repeatedly impacts a drill bit strike face on the shank of a hammer bit. Given the magnitude of the repeated impact forces, large impact stresses occur in each member at the strike interface. In some cases, sustained impact forces may cause plastic deformation and even mechanical failure at either or both of the piston or drill bit strike faces. Additionally, fluid may become trapped between the strike faces, causing an uneven distribution of stress. So-called wash off may likewise cause mechanical failure. Replacing a damaged part due to such a failure may be costly to the operator, because the drill string may need to be tripped in order to replace the damaged part. 
     SUMMARY 
     In one or more embodiments disclosed herein, a percussion drilling assembly includes a hammer bit and an annular piston. The hammer bit and annular piston may each be at least partially located within a housing capable of being coupled to a drill string. An annular bit shank of the hammer bit may include a bit strike face and cutting structure at respective upper end and lower end portions. The hammer bit may be arranged and designed to move longitudinally within the housing. The annular piston may include a piston strike face positioned at a lower end portion thereof, which piston strike face may be arranged and designed to strike the bit strike face. At least one of the bit strike face or the piston strike face may have a toroidal curvature profile. 
     In one or more embodiments disclosed herein, a method of manufacturing a percussion drilling assembly includes forming a housing having upper and lower end portions. The upper end portion may be configured to couple to a drill string. A hammer bit may be formed with an annular bit shank with a bit strike face at an upper end portion, and a cutting structure at a lower end portion. A longitudinally movable annular piston may be formed with a piston strike face at the lower end portion. A toroidal curvature profile may be formed on the hammer bit strike face and/or the piston strike face, and the longitudinally movable annular piston may be positioned in the housing. The hammer bit may be positioned at a position that is axially lower relative to the longitudinally movable annular piston, such that the hammer bit strike face is opposite the piston strike face. 
     In one or more embodiments disclosed herein, a method of drilling with a percussion drilling assembly includes lowering a percussion drilling assembly into a borehole. The percussion drilling assembly includes a housing, a hammer bit, and an annular piston. The housing includes upper and lower end portions, and the upper end portion is capable of being coupled to a drill string. The hammer bit may be located in the lower end portion of the housing and configured to move longitudinally within the housing, and may further include an annular bit shank with a bit strike face and cutting structure at respective upper and lower end portions. The annular piston may be located within the housing and can have a piston strike face at a lower end portion thereof. The piston strike face may be arranged and designed to strike or impact the bit strike face, and the bit strike face and/or the piston strike face may have a toroidal curvature profile. The method may further include engaging the cutting structure of the percussion drilling assembly with a formation and causing the bit strike face to impact the piston strike face. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       One or more embodiments of a piston strike face and bit interface for percussion hammers are described with reference to the following figures. The figures are drawn to scale for certain embodiments; however, a person of ordinary skill in the art should appreciate in view of the disclosure herein that the illustrated embodiments are not to scale for each embodiment contemplated herein or within the scope of the appended claims. The drawings may therefore also represent schematic or exaggerated illustrations of other embodiments. 
         FIG. 1  is a cross-sectional view of a percussion drilling assembly coupled to a lower end portion of a drill string, the percussion drilling assembly having an annular piston in a lowermost position in accordance with one or more embodiments disclosed herein; 
         FIG. 2  is a cross-sectional view of the percussion drilling assembly of  FIG. 1  with the annular piston in an uppermost position in accordance with one or more embodiments disclosed herein; 
         FIG. 3  is an enlarged partial cross-sectional view of the percussion drilling assembly of  FIG. 1 ; 
         FIG. 4  is an enlarged partial cross-sectional view of a lower end portion of an annular piston and an upper end portion of an annular drill shank in accordance with one or more embodiments disclosed herein; 
         FIG. 5  is an enlarged partial cross-sectional view of a lower end portion of an annular piston and an upper end portion of an annular drill shank in accordance with one or more embodiments disclosed herein; 
         FIG. 6  is an enlarged cross-sectional view of a lower end portion of an annular piston and an upper end portion of a drill shank in accordance with one or more embodiments disclosed herein; 
         FIG. 7  is an enlarged cross-sectional view of a lower end portion of an annular piston and an upper end portion of a drill shank in accordance with one or more embodiments disclosed herein; 
         FIG. 8  is an enlarged cross-sectional view of a lower end portion of an annular piston and an upper end portion of a drill shank in accordance with one or more embodiments disclosed herein. 
         FIG. 9  is a reference figure to help explain terms used with the disclosure herein; and 
         FIGS. 10A-E  are examples of curve profiles which may define toroidal curvature profiles within the scope of one or more embodiments of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to various illustrative embodiments. One skilled in the art will understand in view of the disclosure herein that the following description has broad application, and the discussion of any embodiment is meant only to be illustrative of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. The embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. 
     Certain terms are used throughout the following description, and claims, to refer to particular features or components. As one skilled in the art will appreciate in view of the disclosure herein, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are to scale for some embodiments, but are not to scale for each embodiment contemplated herein or within the scope of the claims. Indeed, certain features and/or components may be shown exaggerated in scale or in somewhat schematic form relative to other embodiments. Some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including but not limited to . . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct or even integral connection, or through an indirect connection via other devices and connections. Further, the terms “axial” and “axially” mean generally along or parallel to a central or longitudinal axis, while the terms “radial” and “radially” mean generally perpendicular to a central longitudinal axis. 
     In one aspect, one or more embodiments disclosed herein relate to an interface between strike faces of an annular shank of a hammer bit and an annular piston. One or more of the strike faces may incorporate a toroidal curvature profile, and the curvature profile may increase wear resistance of the annular shank and the annular piston by removing stress concentrations and thereby resisting, if not preventing, plastic deformation. 
     Referring now to  FIGS. 1-3 , an example percussion drilling assembly  10  is shown in accordance with one or more embodiments disclosed herein. One having ordinary skill in the art will understand in view of the disclosure herein that any percussion drilling assembly may be substituted for the one here described and may fall within the scope of this disclosure. In some embodiments, percussion drilling assembly  10  may be coupled to the lower end portion of a drill string  11  and may include a top sub  20  and a driver sub  40 . A substantially cylindrical housing  30  may be axially disposed or positioned between top sub  20  and driver sub  40 . An annular piston  35  may be slidingly disposed in the cylindrical housing  30 . The annular piston  35  may include a piston strike face  91  at a lower end portion. 
     A hammer bit  60  may be slidingly received by driver sub  40 . The lower end portion of hammer bit  60  may include a cutting structure. The upper portion of hammer bit  60  may form an annular bit shank  160  which can include a bit strike face  93  at its upper end portion. A fluid conduit  50  may extend between top sub  20  and annular piston  35 . Top sub  20 , cylindrical housing  30 , annular piston  35 , driver sub  40 , fluid conduit  50  and hammer bit  60  may be generally coaxially aligned, each sharing a common central or longitudinal axis  15 . Fluid (e.g., compressed fluid) may flow through the inside of fluid conduit  50  and exit radially outward into ports in annular piston  35  to provide air to upper and lower piston-cylinder chambers that actuate annular piston  35 . Consequently, fluid conduit  50  may also be referred to as a “feed tube.” As is known to persons having ordinary skill in the art, percussion drilling assemblies may alternatively utilize an air distributor assembly, in which air is directed radially inward from an outer radial location into upper and lower piston-cylinder chambers. 
     Top sub  20  may be threadingly coupled between the lower end portion of drill string  11  and the upper end portion of cylindrical housing  30 . Top sub  20  may include a central through passage  25  in fluid communication with drill string  11 . As shown in  FIG. 3 , passage  25  may include a generally uniform diameter upper section  25   a , a lower enlarged diameter section  25   c , and a generally frustoconical transition section  25   b  extending therebetween. The upper end portion of fluid conduit  50  may be disposed in lower enlarged diameter section  25   c , and coupled to top sub  20  with a pin  22  extending through top sub  20  and fluid conduit  50 . The outer diameter of the fluid conduit  50  may be less than the diameter of lower enlarged diameter section  25   c , and thus, an annular region  25   d  may be formed between fluid conduit  50  and top sub  20 . 
     Referring again to  FIGS. 1 and 2 , the lower end portion of cylindrical housing  30  may be threadingly or otherwise coupled to the upper end portion of driver sub  40 . Annular piston  35  may be slidingly disposed in cylindrical housing  30  above hammer bit  60  and may cyclically impact hammer bit  60  at the interface of piston strike face  91  and bit strike face  93 . The central through passage  33  in annular piston  35  may slidingly receive the lower end portion of feed tube  50 . Annular piston  35  may also include a first set of one or more flow passages  36  extending from central passage  33  to a lower chamber  38 , and/or a second set of one or more flow passages  37  extending from an upper chamber  39  to central passage  33 . Lower chamber  38  may be defined by cylindrical housing  30 , the lower end portion of annular piston  35 , and guide sleeve  32 , while upper chamber  39  may be defined by cylindrical housing  30 , the upper end portion of annular piston  35 , and the lower end portion of top sub  20 . 
     During drilling operations, annular piston  35  may be reciprocally actuated within cylindrical housing  30  by alternating the flow of the fluid (e.g., compressed air, compressed nitrogen, etc.) between passage  36 ,  37  and chambers  38 ,  39 , respectively. More specifically, annular piston  35  may have a first axial position in which outlet port  51  is axially aligned with passage  36 , thereby placing first outlet port  51  in fluid communication with passage  36  and chamber  38 . The annular piston  35  may also have a second axial position in which second outlet port  52  is axially aligned with passage  37 , thereby placing second outlet port  52  in fluid communication with passage  37  and chamber  39 . The intersection of passages  33 ,  36  may be axially spaced from the intersection of passages  33 ,  37 . Thus, when first outlet port  51  is aligned with passage  36 , second outlet port  52  may be misaligned with passage  37 , and vice versa. It should be appreciated that annular piston  35  may assume a plurality of axial positions between the first position and the second position, each allowing varying degrees of fluid communication between ports  51 ,  52  and passage  36 ,  37 , respectively. 
     In addition, hammer bit  60  may include a central longitudinal passage  65  in fluid communication with downwardly extending passages  62  having ports or nozzles  64  formed in the face of hammer bit  60 . Bit passage  65  may also be in fluid communication with piston passage  33 . As annular piston  35  moves axially upward relative to hammer bit  60 , guide sleeve  32  may maintain the fluid communication between bores  33 ,  65 . Compressed or other fluid exhausted from chambers  38 ,  39  into piston passage  33  of piston  45  may flow through bit passages  65 ,  62  and out ports or nozzles  64 . Together, passages  62  and nozzles  64  may serve to distribute fluid around the face of bit  60  to flush away formation cuttings during drilling and to remove heat from bit  60 . 
     During drilling operations, a fluid may be delivered down the drill string  11  from the surface in the direction of arrow  70 . In some cases, the fluid may be provided by one or more compressors at or above the surface of a borehole. The compressed fluid may flow down drill string  11  into upper section  25   a  ( FIG. 3 ) of passage  25 . As shown in  FIG. 3 , with a sufficient pressure differential across check valve  57 , closure member  58  may remain in an opened position allowing the compressed fluid to flow through annular region  25   d , inlet ports  56 , and down feed tube  50  to outlet ports  51 ,  52  and choke  55 . The flow of compressed fluid may be divided between ports  51 ,  52  and choke  55 . In particular, a first fraction of the compressed fluid may flow radially outward through ports  51  and/or ports  52 , as represented by arrow  70   a . A second fraction of the compressed fluid may flow through choke  55  into a central piston passage  33 , as represented by arrow  70   b . In general, the first fraction of the compressed fluid flowing through outlet ports  51 ,  52  may serve to cyclically actuate annular piston  35 , while the second fraction of the compressed fluid flowing through choke  55  may flow through passages  33 ,  65 ,  62  and exit hammer bit  60  via ports  64 , thereby flushing cutting from the face of bit  60 . Since the flow of compressed fluid through outlet ports  51 ,  52  may actuate annular piston  35 , outlet ports  51 ,  52  may also be referred to as “piston actuation” ports. 
     Returning now to  FIGS. 1 and 2 , at the same time, drill string  11  and drilling assembly  10  may be rotated. Mating splines  61 ,  41  on bit  60  and driver sub  40 , respectively, may allow bit  60  to move axially relative to driver sub  40  while simultaneously allowing driver sub  40  to rotate bit  60  with drill string  11 . The rotation of hammer bit  60  allows the cutting elements (not shown) of bit  60  to be “indexed” to fresh rock formations during each impact of bit  60 . Compressed or other fluid exiting hammer bit  60  through ports  64  may flow upward from the base of the borehole through the annulus between drilling assembly  10  and the borehole sidewall to the surface. 
     Although one example of a hammer bit having a bit strike face and annular piston having a piston strike face has been discussed, one having ordinary skill in the art will understand in view of the disclosure herein that any hammer bit and annular piston in a percussion drilling assembly may be used without departing from the scope of the disclosure herein. 
     One or more embodiments disclosed herein are directed to an optimized strike interface between annular piston and drill bit shank for a percussion hammer. The optimized strike interface may enhance the life span of the components. According to one or more embodiments of the present disclosure, in reference to  FIG. 4 , annular piston  435  and/or annular bit shank  460  may have a toroidal curvature profile  490 ,  492  at the strike interface between them. When a component, such as annular piston  435  or annular bit shank  460 , undergoes an impact, the stress imparted to the component may be greater than the yield strength of the material (e.g., metal) making up the component. When this occurs, the component may be plastically deformed. In the case of metal, plastic deformation may, in addition to causing a permanent shape change, cause the metal to generally become harder but more brittle than it was before deformation. Additionally, internal stresses may be introduced within the material. This phenomenon is known as work hardening. As a result of the increased hardness and internal stresses, the component may be more prone to material failure by chipping or cracking. By optimizing the toroidal curvature profiles  490 ,  492 , the stresses imparted to both the annular piston  435  and the annular bit shank  460  may remain below the yield strengths of the components. The annular piston  435  and annular bit shank  460  may have increased resistance to plastic deformation and may therefore not plastically deform to remain within an elastic strain regime during drilling operations. 
     Additionally, because liquids are often present in the borehole, the annular piston  435  and the annular bit shank  460  may trap a portion of fluid between them upon impact. The presence of a non-compressible liquid at the interface between piston strike face  491  and bit strike face  493  (known as wash off) may increase localized stresses on a component. The toroidal curvature profile in one or more embodiments of the present disclosure may help reduce or prevent wash off by reducing the number of flat surfaces within the assembly. Also, a convex bit strike face  493  may provide for additional run-off of liquids moving by gravity alone. 
     For the sake of clarifying terminology used throughout the disclosure, reference is now made to  FIG. 9 . A toroid is a three-dimensional surface formed when a two-dimensional radial cross-section  901  is rotated about an axis  915 . For a circular toroid, a major radius R, measured between the axis  915  and the center-point  910  of cross-section  901 , is constant. As used herein, toroidal curvature profile means a three-dimensional surface swept out by a curve profile  920  when rotated about axis  915  at a constant major radius R. As defined herein, a toroidal curvature profile may refer either to a convex extension of a body—as in the bit strike face  493  depicted in  FIG. 4 —or a concave recess into a body—as in the piston strike face  491  depicted in  FIG. 4 . Furthermore, a curve profile height h as used throughout this disclosure is defined as a distance from center-point  910  to the highest point of the curve profile  920  when the curve  920  extends in a positive longitudinal direction. Curve profile height h is measured from the center-point  910  to the lowest point of the curve  920  when the curve profile  920  extends in a negative longitudinal direction. 
     Where curve profile  920  is an arc section from a circle, curve  920  may be defined in terms of a radius of curvature r, defined as the distance to the center of the circle of which the arc forms a partial circumference. One having ordinary skill in the art will understand that any curve  920  may be used to form a toroidal curvature profile, including, for instance, circular, elliptical, hyperbolic, and parabolic curves while remaining within the scope of this disclosure. Additionally, one having ordinary skill in the art will understand that curve  920  may be constructed from segments of more than one type of curve while remaining within the scope of this disclosure. For example,  FIGS. 10A-E  depict several illustrative curve profiles which may define toroidal curvature profiles within the scope of one or more embodiments of this disclosure. 
     Curve profile  1020   a  of  FIG. 10A  is illustrated as an elliptical curve  1020  having height h 1 . Curve profile  1020   b  of  FIG. 10B  is a compound curve profile having curve profile height h 2  made up of a first line segment  1021 , a circular arc  1022  having radius of curvature r 1 , and a second line segment  1023 . Curve profile  1020   c  of  FIG. 10C  is a compound curve profile having curve profile height h 3 —which is negative—made up of a first elliptical arc  1025 , and a second elliptical arc  1026 . Curve profile  1020   d  of  FIG. 10D  is a compound curve profile having curve profile height h 4  made up of a circular arc  1030  having a radius of curvature r 2 , a curvilinear portion  1031  and an elliptical arc  1032 . Curve profile  1020   e  of  FIG. 10E  is a compound curve profile having curve profile height h 5  made up of a first line segment  1035 , a first circular arc  1036  having a radius of curvature r 3 , a second line segment  1037 , a second circular arc  1038  having a radius of curvature r 4 , and a third line segment  1039 . 
     In one or more embodiments of the present disclosure, at least one of the piston strike face or the bit strike face has a convex toroidal curvature profile. In one embodiment, as depicted in  FIG. 4 , the bit strike face  493  of annular bit shank  460  may have a convex toroidal curvature profile  490 . The axis of the toroidal curvature profile  490  may substantially correspond with axis  415 . The radial cross-section of the toroidal curvature profile may have a curve extending from an inner surface  495  to an outer surface  497  on the annular bit shank  460 . 
     Still referring to  FIG. 4 , the piston strike face  491  of annular piston  435  may have a concave toroidal curvature profile  492  in some embodiments. The axis of the concave toroidal curvature profile  492  may also substantially correspond with axis  415 . The radial cross-section of the concave toroidal curvature profile  492  may have a curve extending from an inner surface  496  to an outer surface  498  on the annular piston  435 . 
     Annular piston  435 , annular bit shank  460 , other components of a percussion drilling assembly, such as the cylindrical housing, or some combination of the foregoing, may be formed by any conventional process known in the art, and the toroidal cutting profile may be formed integrally or separately from the annular piston or hammer bit. For example, one or more embodiments of the present disclosure may include forming an annular piston or hammer bit by casting, molding, forging, rolling, grinding, milling, turning, cutting, routing, etc. 
     As shown in the embodiment depicted in  FIG. 5 , the bit strike face  590  of annular bit shank  560  may have a convex toroidal curvature profile  590 . The axis of the toroidal curvature profile  590  may substantially correspond with axis  515 . The radial cross-section of the toroidal curvature profile may have a curve extending substantially from an inner surface  595  to an outer surface  597  on the annular bit shank  560 . 
     In  FIG. 5 , the piston strike face  591  of annular piston  535  may also have a convex toroidal curvature profile  592 . The axis of the convex toroidal curvature profile  592  may also substantially correspond with axis  515 . The radial cross-section of the convex toroidal curvature profile  592  may have a curve extending substantially from an inner surface  596  to an outer surface  598  on the annular piston  535 . 
       FIGS. 6-8  are partial cross-sectional views of further embodiments of the present disclosure, and show illustrative interfaces between a lower end portion of an annular piston and an upper end portion of an annular bit shank. 
     In an embodiment according to the disclosure, as depicted in  FIG. 6 , the piston strike face  691  of annular piston  635  may have a convex toroidal curvature profile  692 . The axis of the toroidal curvature profile  692  may substantially correspond with axis  615 . The radial cross-section of the toroidal curvature profile may have a curve extending substantially from an inner surface  696  to an outer surface  698  on the annular piston  635 . 
     The bit strike face  693  of annular bit shank  660  may have a concave toroidal curvature profile  690 . The axis of the concave toroidal curvature profile  690  may also substantially correspond with axis  615 . The radial cross-section of the concave toroidal curvature profile  690  may have a curve extending substantially from an inner surface  695  to an outer surface  697  on the annular bit shank  660 . A thickness of the walls of the annular piston  635  may be about the same as, less than, or greater than (as shown) the thickness of the walls of the annular bit shank. 
     As shown in the embodiment depicted in  FIG. 7 , the bit strike face  793  of annular bit shank  760  may have a convex toroidal curvature profile  790 . The axis of the toroidal curvature profile  790  may also substantially correspond with axis  715 . The radial cross-section of the toroidal curvature profile may have a curve extending substantially from an inner surface  797  to an outer surface  795  on the annular bit shank  760 . As also illustrated, the piston strike face  791  of annular piston  735  may have a flat profile, meaning the piston strike face  791  may be generally planar and orthogonal to axis  715 . 
     As shown in the embodiment depicted in  FIG. 8 , the piston strike face  891  of annular piston  835  may have a convex toroidal curvature profile  892 . The axis of the convex toroidal curvature profile  892  may also substantially correspond with axis  815 . The radial cross-section of the convex toroidal curvature profile  892  may have a curve extending substantially from an inner surface  896  to an outer surface  898  on the annular piston  835 . As shown, the bit strike face  893  of the annular bit shank  860  may have a flat profile. As used herein, a flat profile refers to a profile of a surface (e.g., of the bit strike face  893 ), that is generally planar and orthogonal to axis  815 . 
     In  FIGS. 4-8 , the cross-section of the curved profile is depicted as a circular arc, but as previously discussed, one having ordinary skill in the art will also understand in view of the disclosure herein that an elliptical, hyperbolic, parabolic, other curve profile, or some combination of the foregoing may be used and may be suitable for reducing stress concentrations and may still conform with the present disclosure. 
     Additionally,  FIGS. 4-8  illustrate the piston strike faces and the bit strike faces as having equal radii of curvature. As discussed herein, one having ordinary skill in the art will understand in view of the disclosure herein that the radius of curvature—and indeed the profile shapes—of a piston strike face and a bit strike face may not be the same, match, or mate. For instance, a radius of curvature of a concave strike face may be larger than a radius of curvature of a corresponding bit strike face. Alternatively, when two convex strike faces are used, they may be of different radius of curvature. Moreover, some strike faces may not have a radius of curvature, such as where the strike face has a flat profile. According to one or more embodiments of the present disclosure, a toroidal curvature profile disposed on a strike face may have a radius of curvature of between 0.5 inch and 150 inches (13 mm to 3.81 m). In one or more other embodiments, a toroidal curvature profile disposed on a strike face may have a radius of curvature between 1 inch and 4 inches (25 mm to 102 mm). 
     According to one or more other embodiments of the present disclosure, a toroidal curvature profile disposed on a strike face may have a curve profile height h of between 0.0005 inch and 0.040 inch (13 μm to 1 mm). In one or more other embodiments, the curve profile height h may be between 0.0005 inch and 0.4 inches (13 μm to 10 mm). 
     A method of making a percussion drilling assembly is now described. Such method may include positioning or otherwise disposing a longitudinally movable annular piston at least partially in a housing, with the longitudinally movable annular piston including a piston strike face at a lower end portion. The housing may include a lower end portion and an upper end portion, the upper end portion capable of being coupled to a drill string. A hammer bit may be positioned or otherwise disposed at least partially within the housing, and potentially axially lower than the longitudinally movable annular piston so that a bit strike face is opposite the piston strike face. The bit strike face may be at an upper end of the hammer bit. One or both of the bit strike face or the piston strike face may also have a toroidal curvature profile thereon. 
     In one or more embodiments, a method may also include forming the housing with upper and lower end portions, the upper end portion being capable of being coupled to a drill string. A method may include forming a longitudinally movable annular piston with a strike face at a lower end portion thereof. A method may include forming a hammer bit with a bit strike face at an upper end portion and a cutting structure at a lower end portion. A method may include forming a toroidal curvature profile on one or more of a bit strike face or a piston strike face. 
     In one or more embodiments, a method may also include selecting a toroidal curvature profile to distribute impact stress and reduce, if not prevent, plastic deformation in the annular piston and/or the annular bit shank. A method may also include selecting the toroidal curvature profile to reduce or minimize wash off. 
     A method of drilling with a percussion drilling assembly is also described. Such method may include lowering or otherwise placing a percussion drilling assembly in a borehole. The percussion drilling assembly may include a percussion drilling assembly such as those described herein. An example percussion drilling assembly may include a housing having a lower end portion and an upper end portion, the upper end portion capable of being coupled to a drill string. The percussion drilling assembly may include a hammer bit in the lower end portion of the housing, which hammer bit may be configured to move longitudinally within the housing. The hammer bit may include an annular bit shank with a bit strike face at its upper end portion and a cutting structure at its lower end portion. The percussion drilling assembly may further include an annular piston disposed in the housing, the annular piston having a piston strike face at its lower end portion. At least one of the bit strike face or the piston strike face may have a toroidal curvature profile. The method may also include engaging or contacting the cutting structure of the percussion drilling assembly with a formation and causing the bit strike face of the annular bit shank to impact or strike the piston strike face of the annular piston. 
     Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from embodiments disclosed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only 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 35 U.S.C. §112, paragraph 6 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.