CA2967049A1

CA2967049A1 - Grinding tool for buttons on a rock drill bit

Info

Publication number: CA2967049A1
Application number: CA2967049A
Authority: CA
Inventors: Bo Thomas Sjolander; Bjorn Sjolander
Original assignee: CME Blasting and Mining Equipment Ltd
Current assignee: CME Blasting and Mining Equipment Ltd
Priority date: 2014-11-13
Filing date: 2015-11-12
Publication date: 2016-05-19
Also published as: AU2015345942A1; RU2017120139A3; WO2016074067A1; BR112017009828A2; SE1750742A1; RU2017120139A; US20170304983A1; CN107427984A; NO20170957A1; FI20175521A; CA2870784A1

Abstract

The present disclosure provides a grinding cup for detachable connection to the output drive shaft of a grinding machine for grinding buttons on drill bits or cutters, said grinding cup having top and bottom surfaces and consisting of a lower grinding section and an upper body section co-axial with said grinding section to form said grinding cup with a centrally disposed recess formed in the bottom surface of the grinding section having the desired profile for the button to be ground; the improvement characterized by said body section having a centrally disposed upright drive section formed on the top surface of the grinding cup, said drive section shaped and sized to both driveably engage within a corresponding recess at end of the output drive shaft of said grinding machine and provide axial and radial support and retaining means for detachably connecting the grinding cup to the output drive shaft of the grinding machine wherein said drive section has a non-circular cross section and is tapered from the top surface of the grinding cup to the free end of the drive section.

Description

TITLE: GRINDING TOOL FOR BUTTONS ON A ROCK DRILL BIT
BACKGROUND
The present disclosure relates to improvements in apparatus for, grinding the hard metal inserts or working tips of rock drill bits (percussive or rotary), tunnel boring machine cutters (TBM) and raised bore machine cutters (RBM) and more specifically, but not exclusively, for grinding the cutting teeth or buttons of a rock drill bit or cutter.
In drilling operations the cutting teeth (buttons) on the drill bits or cutters become flattened (worn) after continued use. Regular maintenance of the drill bit or cutter by regrinding (sharpening) the buttons to restore them to substantially their original profile enhances the bit/cutter life, speeds up drilling and reduces drilling costs. Regrinding should be undertaken when the wear of the buttons is optimally one third to a maximum of one-half the button diameter.
Manufacturers.have developed a range of different manual and semi-automatic grinding machines including hand held grinders, single arm and double arm grinding machines and grinders designed specifically for mounting on drill rigs, service vehicles or set up in the shop.
These types of machines utilize a grinding machine having a spindle or rotor rotated at high speed. A grinding cup or grinding pin, mounted on the end of the rotor or spindle, grinds the button and typically the face of the bit/cutter surrounding the base of the button to restore the button to substantially its original profile for

2 effective drilling. In addition to the rotation of the grinding cup, these types of grinding machines may include features where the grinding machine is mounted at an angle to the longitudinal axis of the button and the grinding machine is rotated to provide orbital motion with the center of rotation lying in the center of the grinding cup.. When grinding the buttons, the centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button.
= Longstanding problems with these types of grinding machines are vibration and noise due to high rotational speeds, wear, the requirement for large compressors for pneumatic systems and long grinding times per button, in the larger sizes.
The grinding cups conventionally consist of a cylindrical body having top and bottom surfaces. The bottom or working surface consists of a diamond/metal matrix having a centrally disposed recess having the desired profile for the button to be ground. The rim around the recess may be adapted, for example by bevelling, to remove steel from the face of the bit around the base of the button.
Water and/or air, optionally with some form of cutting oil, is provided to the grinding surface to flush and = cool the surface of the button during grinding.
The grinding cups are provided in different sizes and profiles to match the standard sizes and profiles of the buttons on the drill bits or cutters. Typically the button diameter varies from 6mm up to 26mm.

3 Several different methods have been used to connect and retain the grinding cups on to the grinding machine. The grinding cups were conventionally held in the grinding machine by inserting an upright hollow stem projecting from the top surface of the grinding cup into a chuck for detachable mounting. Special tools such as chuck wrenches, nuts and collets are necessary to insert, hold and to remove the grinding cup into and out of the chuck.
To eliminate the need for chuck wrenches etc. the use of a shoulder drive on the grinding cups was developed. A
diametrically extending recess at the free end of a hollow drive shaft of the grinding machine co-operates with a shoulder or cam means on the adjacent top surface of the grinding cup to provide the drive means. The stem = of the grinding cup, in order to provide axially and radial support, is inserted into the hollow drive shaft and maybe held in place by one or more 0-rings either located in a groove in the interior wall of the drive shaft or on the stem of the grinding cup. See for example Swedish Patent No. B 460,584 and U.S. Patent No.
5,527,206.
An alternative to the shoulder drive is shown, for example, in Canadian Patent 2,136,998. The free end of the stem of the grinding cup is machined to provide flat drive surfaces on the stem that are inserted into a corresponding drive part in the channel of the output drive shaft into which the stem is inserted. The grinding cup is retained in place by a spring biased sleeve which forces balls mounted in the wall of the output drive shaft into an annular groove on the stem of the grinding cup.

4 Other innovations are illustrated in U.S. Patent No.

5,639,273 and U.S. Patent No. 5,727,994. In these patents, the upright stem has been replaced with a centrally disposed hexagonal cavity provided in the top surface of the grinding cup. The cavity is shaped and sized to permit the output drive shaft of a grinding machine to be inserted into the cavity. The end of the output drive shaft has an end section having a corresponding hexagonal cross section to fit into the hexagonal cavity to provide the drive means. A second section of the output drive shaft having a circular cross section to fit into the cavity to provide axial and radial support.
Some manufacturers, in order to provide grinding cups that are compatible for use with other manufacturers' grinding machines provide adapters that connect their grinding cup to the output drive shaft of competitors' grinding machines.
Regardless of the method of connecting the grinding cup to the output drive shaft of the grinding machine, it is important to optimize the operational stability of the grinding cup. Lack of operational stability often results in vibration and resonance during grinding. Vibration and/or resonance also directly results in increased rates of wear to all moving parts such as bearings, joints, = etc. of the grinding apparatus and can potentially interfere with settings within the operating control circuits of the grinding apparatus. In addition, lack of operational stability results in increased wear to all key drive/contact surfaces of the output drive shaft (rotor) and grinding cup which provide consistent, proper alignment between grinding cup and or adapter and the rotor during operation. Operational instability and associated vibration and/or resonance is a major contributor to the deterioration of the preferred built-in profile of the cavity in the grinding section of the 5 grinding cup. This directly results in deterioration in the profile of the restored button. The net effect being a substantial loss in the intended overall drilling performance of the drill bit or cutter used.
These known drive systems provide means to axially and radially suppOrt the grinding cup in the grinding machine and separate drive means for transfer of torosional forces to rotate the grinding cup.
SUMMARY OF THE INVENTION
Accordingly the present disclosure provides a grinding cup for detachable connection to the output drive shaft of a grinding machine for grinding buttons on drill bits or cutters, said grinding cup having top and bottom surfaces and consisting of a lower grinding section and an upper body section co-axial with said grinding section to form said grinding cup with a centrally disposed recess formed in the bottom surface of the grinding section having the desired profile for the button to be ground; the improvement characterized by said body section having a centrally disposed upright drive section formed on the top surface of the grinding cup, said drive section shaped and sized to both driveably engage within a corresponding recess at end of the output drive shaft of said grinding machine and provide axial and radial support and retaining means for detachably connecting the grinding cup to the output drive shaft of the grinding machine wherein said drive section has a non-circular

6 cross section and is tapered from the top surface of the grinding cup to the free end of the drive section.
Another embodiment provides a series of grinding cups for 5 grinding working tips of rock drill bits, wherein said working tips have a diameter of about 6mm to 26mm and a desired profile, each of said grinding cups in said series having a lower grinding section and an upper body section connected to form a grinding cup having top and 10 bottom surfaces, a centrally disposed recess formed in the bottom surface of said grinding cup having the desired profile and diameter of the working tip, combination drive means and support means provided on the = upper body section of said grinding cup that cooperates 15 with the output drive shaft of a grinding machine, wherein the combination drive means and support means consists of an upright section centrally located on the top surface of the grinding cup in the form of a truncated elliptic cone sized to engage with a 20 corresponding elliptic conical recess at the free end of the output drive shaft of the grinding machine, retaining means provided in conjunction with the drive means for detachable connection of the grinding cup to the output drive shaft of the grinding machine during use.
Another embodiment of the present invention consists of a holder device for detachable connection of a grinding cup to a grinding machine, said grinding machine having a pneumatically; hydraulically or electrically driven motor 30 which drives an output shaft. Suitably connected to the output shaft by any conventional means is a holder device of the present invention. The holder device may be an integral extension of the output shaft or a separate attachment. The holder device consists of a rotatable W02016/074067 =

7 drive member having a free end adapted to extend axially away from the the grinding machine. The drive member is provided with a recess at its free end with a coaxial passageway extending from the recess the length of the drive member or part thereof and through which coolant fluid may be directed to a grinding cup supported thereon. The recess has a non-circular cross section and is tapered inwardly from the free end of the drive member.
Further features of the invention will be described or will become apparent in the course of the following detailed descTiption.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the disclosure may be more clearly understood, embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which:
FIGURE 1 is perspective view from the right side of one embodiment of a grinding apparatus having a grinding machine carried for vertical and horizontal adjustment by a support system, and means for holding the bit(s) to be = ground.
FIGURE 2 is a perspective view from the left side of another embodiment of a grinding apparatus suitable for mobile applications and having a griding machine carried for vertical and horizontal adjustment by a support system and means for holding the bit(s)to be ground.

8 FIGURE 3 is an enlarged view of a grinding machine of FIGURE 1 having a spindle assembly including an ouput drive shaft according to the present disclosure.
5 FIGURE 4 is an enlarged vertical cross section of the spindle assembly of the grinding machine of Figure 2.
FIGURE 5 is a front plan view of part of one embodiment of a grinding cup according to the present disclosure.
FIGURE 6 is a partial cross-section of the grinding cup of FIGURE 5 along line 5-5.
FIGURE 7 is a. top plan view of the grinding cup of FIGURE
= 15 5.
FIGURE 8 is a front plan view of one embodiment of a lower grinding section of the grinding cup of FIGURE 5.
20 FIGURE 9 is a perspective view of an embodiment of a holder device in the form of a spindle for a grinding machine of FIGURE 3 according to the present disclosure with the grinding cup of FIGURE 5 attached to the spindle.
FIGURE 10 is a perspective view of the spindle and grinding cup of FIGURE 9 with the grinding cup detached from the spindle.
30 FIGURE 11 is a front plan view of the spindle and grinding cup Of FIGURE 9.
FIGURE 12 is a cross section of the spindle and grinding cup of FIGURE 11 along line A-A.

9 FIGURE 13 is a side by side view in cross-section of four grinding cups in a series of grinding cups according to the present disclosure, each of said grinding cups sized to grind a different size of button.
DETAILED DESCRIPTION
With reference to the FIGURE 1 one embodiment of a grinding apparatus is generally indicated at 1. The grinding apparatus 1 includes a grinding machine 2, means for holding one or more bits to be ground generally indicated at 3 and a support system generally indicated at 4. The grinding machine 2, means for holding the bits 3 and support system 4 are arranged to permit relative movement between the grinding machine 2 and the bit to be ground to permit alignment of the grinding machine 2 with the longitudinal axis of the buttons on the bit. The grinding apparatus 1 has a control system, part of which is generally indicated at 5, having a programmable operator control panel 6 capable of directly or indirectly monitoring and adjusting one or more operational parameters. The operational parameters may include feed pressure, grinding cup RPM, grinding time and other parameters as noted herein.
In the grinding apparatus 1 shown in Figure 1, the grinding machine 2 is carried by support system 4 which includes an arm or lever system 7 journaled on a stand 8 attached to the rear 9 of an open box 10. The bit holder means 3 consists of a table 11 mounted within the box 10.
=
FIGURE 2 ilustrates a more compact grinding apparatus optionally suitable for mobile applications. The grinding apparatus is generally indicated at 20. The grinding apparatus 20 includes a grinding machine 22, means for holding one or more bits to be ground generally indicated at 23 and a support system generally indicated at 24. The 5 grinding machine 22, means for holding the bits 23 and support system 24 are arranged to permit relative movement been the grinding machine 22 and the bit to be ground to permit alignment of the grinding machine 22 with the longitudinal axis of the buttons on the bit. The

10 grinding apparatus 20 has a control system, part of which is generally indicated at 25, having a programmable operator control panel 26 capable of directly or indirectly monitoring and adjusting one or more operational parameters. The operational parameters of most interest are selected from the group consisting of feed pressure, grinding cup RPM and grinding time.
In the embodiment of the grinding apparatus 20 shown the grinding machine 22 is carried by support system 24 which includes an arm or lever system 27 attached to the frame 28.
In FIGURES 3 & 4, an embodiment of a grinding machine 2,22 suitable for use with the grinding apparatus 1,20 of either FIGURE 1 or 2. FIGURES 3 & 4 show the grinding machine 2,22 attached to plates below a control box 5,24.
The grinding machine 2,22 is locked in place by levers 36. A water-cooled electric motor, generally indicated at 35, has an exterior housing defining a chamber in which the rotor and stator are located. Within the wall of the housing are located a series of longitudinal channels for the cooling water. Ports permit water in and water out respectively. A drive coupling at the bottom of housing permits attachment of the spindle assembly 45.

11 The drive coupling is inserted into the mating drive coupling 50 on spindle assembly 45. The spindle assembly 45 has an output drive shaft 46 to which a grinding cup can be connected. The spindle assembly 45 is attached to the electric motor housing by bolts 47. Coolant water for delivery to the grinding cup surface is provided though connection 48. The electric motor 35 is preferably a three-phase motor and power is connected through connection 51 to connection box. A flexible splash cup 53 is placed around the output drive shaft 46 of spindle assembly 45. The motor can be hydraulic, electric or the like without departing from the substance of the present invention. The dimensions of the casing are such that the grinding machine may be handled manually if desired. For the latter purpose, the casing is provided with handles projecting diametrically oppositely outwardly from the casing. Suitably connected to the output drive shaft 46 by any conventional means is a holder device 54. In the illustrated embodiment in Figures 9-12, the holder device 54 is an integral extension of the shaft 46 which constitutes a rotatable elongate drive member 55. The shaft 46 and drive member 55 are provided with a coaxial passageway 56 extending along the length or part thereof and through which coolant/flushing fluid may be directed to a grinding cup 60 supported thereon. The configuration of the holder device is set out in FIGURES 9-12.
Figures 5-8 illustrate one embodiment of a grinding cup 60 for detachable connection to the output drive shaft of a grinding machine for grinding buttons on drill bits or cutters. The grinding cup 60 consists of a lower grinding section generally indicated at 61 and an upper body section 62 co-axial with said grinding section 61 to form said grinding. cup 60. A centrally disposed recess 63 is

12 formed in the bottom surface 64 of the grinding section 61 having the desired profile for the button to be ground.
The bottom (grinding) surface 64 of the grinding section 61 is formed from a material capable of grinding the tungsten carbide button bits. In the embodiment, the grinding surface is formed from a metal and diamond matrix. The peripheral edge 72 in the bottom surface 64 is beveled to facilitate the removal of steel from the face of the bit around the base of the button during grinding.
The body section 62 has a base 65 having a centrally disposed upright drive section 66 formed on the top surface 67 of the base 65. The drive section 66 is shaped and sized to driveably engage within a corresponding recess (see Figures 9 and 11) at end of the output drive shaft of said grinding machine. Retaining means, generally indicated at 68, for detachably connecting the grinding cup to the output drive shaft of the grinding machine are provided on the upright drive section. In the embodiment illustrated the retaining means 68 comrises an annular groove 69 in the outer surface 70 of the upright drive section 66 sized and shaped to retain an 0-ring (not shown).
The upright drive section 66 has a non-circular cross section and is tapered from the top surface 67 of the base 65 to the free end 71 of the drive section 66. In the embodiment illustrated in Figures 5-6, the upright drive section 66 has a truncated elliptical cone shape.
The peripheral edge of the bottom 79 of the drive section

13 36 is the same as or smaller than the diameter of the top surface 67 of base 65.
=
A passageway 73 is provided in the drive section 66. The 5 passageway 73 is co-axial with the up-right drive section 66 and communicates with one or more openings (not shown) on the grinding surface 64 through which coolant/flushing fluid may be directed to the surface of the button being ground during grinding.
As shown in Figure 8, integral with and adjacent the top surface 74 of the grinding section 61 are means 75 to connect the grinding section 61 to the upper body section 62. The means 75 to connect the grinding section 61 to 15 the upper body section 62 can be formed integrally with the grinding section 61 and machined to the desired configuration or cast separately and attached to the top surface 74 of the grinding section 61. In the embodiment illustrated in Figure 7, the means 75 to connect the 20 grinding section 61 to the upper body section 62, consists of a generally cylindrical stub 76 centrally = located on the top surface 74 of the grinding section 61.
The stub 76 is intended to be inserted into a corresponding cavity on the upper body section 62 in a 25 manner (1) that will prevent the grinding section 61 from rotating or spinning free relative to the upper body section 62 and (2) that will support axial, radial, torsion and feed forces associated with the use of the grinding cup. In the preferred embodiment illustrated the = 30 stub 76 is press fit into the drive connection member and the grinding section 61 TIG welded to the upper body section 62. Alternatively a stub on the upper body section 62 could fit into a corresponding cavity on the lower grinding section. Some examples of other possible

14 connection methods are taper fits, threaded connections, adhesives, solder, friction welding and pins. The stub 76 = could have a non-circular cross-section. It may be desireable tb have stub 76 be in the form of an 5 elliptical tapered cone.
The bottom surface 77 of base 65 on the upper body section 62 has cavity 78 formed therein sized and shaped to accommodate the cylindrical stub 76 centrally located 10 on the top surface 74 of the grinding section 61.
In the embodiment illustrated grinding cup 60 is one in a series of grinding cups for grinding the working tips or buttons of rock drill bits wherein the working tips

15 or buttons have a diameter of about 6mm to 26mm and a desired profile. The base 65 of the upper body section 62 of each grinding cup 60 in the series may be a constant height across the series of grinding cups 60a, 60b, 60c and 60d as shown in FIGURE 13. Alternatively the base 65 20 may be a constant height thru a group in the series of grinding cups and vary in height for the next group.
Referring to Figs. 1-4, a grinding machine 2,22 is illustrated which includes a motor housing or casing 25 within which is suitably supported a rotary motor. The motor can be hydraulic, electric or the like without departing from the substance of the present invention.
The dimensions of the casing are such that the grinding machine may be handled manually if desired.
For the 30 latter purpose, the casing is provided with handles projecting diametrically oppositely outwardly from the casing.
The motor drives an output drive shaft.
Suitably connected to the output drive shaft by any conventional means is a holder device 54.
In the illustrated embodiment in Figures 9-12, the holder device 54 is an integral extension of the shaft 46 which constitutes a rotatable elongate drive member 55. The shaft 46 and drive member 55 are provided with a coaxial 5 passageway 56 extending along the length or part thereof and through which coolant/flushing fluid may be directed to a grinding cup 60 supported thereon, the grinding cup being shown in Figs. 5-8 and described in detail above.
An end portion 57 of the elongate drive member 55, 10 extending from its free end 59, is provided with a non-circular recess 106 adapted to accomodate a corresponding sized and shaped upright drive section 66 on grinding cup 60. As shown in Figures 9-16, the exterior wall 70 of the upright drive section 66 on grinding cup 60 is adapted to 15 driveably engage within recess 106 centrally disposed in the free end 59 of elongate drive member 55. In the embodiment illustrated the interior wall 107 of the recess 106 in drive member 55 is machined to an elliptical cross section and tapering inwardly from the end 59 of the drive member 55. This design permits the upright drive section 66 of grinding cup 60 to be inserted easily within recess 106 without the necessity of specific alignment of the drive member and grinding cup. The grinding cup can simply be pushed on the drive member 55. When positioned in recess 106, the drive member 55 will rotate the grinding cup without slipping when in use. During grinding, the feed pressure applied to the grinding machine results in the grinding cup 60 being pressed securely within recess 106 providing a rigid connection to reduce wobbling of the grinding cup 60 during grinding. Further the present disclosure maximizes the contact surfaces between the upright drive section 62 on grinding cup 60 and the interior wall 107 of recess 106 on drive member 55. This reduces the wear

16 to all key drive/contact surfaces of the output drive shaft (rotor) and grinding cup which provide consistent, proper alignment between grinding cup and or adapter and the rotor during operation.
Retaining means may be provided in recess 106 of drive member 55 to detachably retain the grinding cup 60 so that grinding cup 60 will not fly off during use but can still be easily removed or changed after use. The retaining means may include one or more annular grooves in the interior wall of the recess 106 in drive member 55. 0-rings or expansion rings may be inserted into the grooves.
In the embodiment illustrated, both the grinding cup 60 and drive member 55 are fabricated from stainless steel to prevent corrosion and to facilitate clean up after grinding.
Having illustrated and described a preferred embodiment of the invention and certain possible modifications thereto, it should be apparent to those of ordinary skill in the art' that the invention permits of further modification in arrangement and detail and is not restricted to the specific semi-automatic grinding apparatus illustrated. For example the design of the upper body section of the grinding cup and the free end of the output drive shaft can be reversed. In this embodiment the free end of the output drive shaft has a non-circular cross-section and is tapered. The tapered end of the output drive shaft is sized and shaped to fit within a corresponding recess in the top surface of the upper body section of the grinding cup.

The present disclosure utilizes drive means to provide =
both axial and radially support and transfer torosional forces. A non-circular tapered connection provides both support and drive. The resistance of the connection to 5 slippage improves as the feed pressure increases, resulting in a relative increase in support and drive capacity as feed increases. However the grinding cup can still be easily removed from the output drive shaft after = grinding with no more than a light tap from the side.
It will be appreciated that the above description related to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations 15 are within the scope of the invention as described and claimed, whether or not expressly described.
=

Claims

18

1. A grinding cup for detachable connection to the output drive shaft of a grinding machine for grinding buttons on drill bits or cutters, said grinding cup having top and bottom surfaces and consisting of a lower grinding section and an upper body section co-axial with said grinding section to form said grinding cup with a centrally disposed recess formed in the bottom surface of the grinding section having the desired profile for the button to be ground; the improvement characterized by said body section having a centrally disposed upright drive section, said drive section shaped and sized to driveably engage within a corresponding recess at end of the output drive shaft of said grinding machine and retaining means for detachably connecting the grinding cup to the output drive shaft of the grinding machine wherein retaining means for detachably connecting the grinding cup to the output drive shaft of the grinding machine are provided on the upright drive section, and wherein said drive section has an elliptical cross section and is tapered from the top surface of the grinding section to the free end of the drive section and wherein the grinding cup is one in a series of grinding cups for grinding the working tips or buttons of rock drill bits wherein the working tips or buttons have a diameter of about 6mm to 26mm and a desired profile, and wherein the base of the upper body section of each grinding cup in the series is sized so the distance from the top surface of the base to the center point of the radius of the convex recess in the grinding section is constant across the series of grinding cups.

2. A grinding cup according to claim 1 wherein the upper body section has a base with the centrally disposed upright drive section formed on a top surface of the base.

3. A grinding cup according to claims 1 or 2 wherein means to connect the grinding section to the upper body section are provided on the top surface of the grinding section.

4. A grinding cup according to claim 3 wherein the means to connect the grinding section to the upper body section consists of a generally cylindrical stub centrally located on the top surface of the grinding section, the stub being sized and shaped to be inserted into a corresponding cavity on the upper body section in a manner that will prevent the grinding section from rotating or spinning free relative to the upper body section and that will support axial, radial, torsion and feed forces associated with the use of the grinding cup.

5. A grinding cup according to claim 4 wherein the bottom surface of the base on the upper body section has cavity formed therein sized and shaped to accommodate the cylindrical stub centrally located on the top surface of the grinding section.

6. A grinding cup according to claim 5 wherein the retaining means comrises an annular groove in the outer surface of the upright drive section sized and shaped to retain an O-ring.

7. A holder device for detachable connection of a grinding cup to a grinding machine, said grinding machine having a pneumatically, hydraulically or electrically driven motor which drives an output shaft, the holder device comprising a rotatable drive member having a tree end adapted to extend axially away from the the grinding machine, a non-circular recess in the free end of the drive member with a coaxial passageway extending from the recess the length of the drive member or part thereof and through which coolant fluid may be directed to a grinding cup supported thereon, the cross-section of the recess being tapered inwardly from the free end of the drive member and wherein said recess has an elliptical cross section.