Abstract:
The invention provides a bit ( 10 ) for use with a rotatable tool, said bit including: a shaft ( 12 ) having a rearward end adapted to be held by a chuck of the rotatable tool, and a forward end ( 18 ) having cutting tips formed thereon; a reaming head ( 14 ) including a body having a bore ( 16 ) adapted to receive the shaft therethrough in a sliding fit, the body having a plurality of cutting blades ( 26 ) formed on at least one peripheral surface thereof; the bore and/or the shaft having formations thereon such that when the rearing head has the shaft operatively located in the bore the reaming head is rotationally locked into the shaft; the reaming head and the shaft being separable from each other to allow either component to be replaced independently of the other. The invention also provides a bit having a tip end and shank end for use with a rotatable tool, said bit having at least 4 blades ( 24 ) to engage ard drive a screw when said rotatable tool is rotated, each blade including at said tip end a bevel so as to form a cutter ( 22 ) at the extremity of said blade, each bevel and/or cutter being able to cut in the same rotational direction. The invention further provides a method of making a bit for use as a screwdriver and countersinker, said method including the steps of: 1) forming at least a four blade screw driving end onto one end of blank, 2) forming a bevel at the extremity of each blade, said bevel producing an inclined plane cutter.

Description:
FIELD OF THE INVENTION 
   This invention relates to a screw driving and countersinking bit of the type which can be used by rotatable power tools for not only starting and countersinking a hole in a surface which is to have a screw fastener inserted therein, but also which can be used to drive the screw into the surface. 
   BACKGROUND OF THE INVENTION 
   The applicant&#39;s prior filed international application PCT/AU98/00834 describes in detail the problems associated with firstly starting a screw hole through a hard surface with a drilling tool, and then driving a screw which is inserted into the start hole, with a screw driver. Generally two different tools are required for the two different operations and this significantly increases the time required to insert the screw into the surface. 
   Modern electric power tools can be rotated at speeds which vary between very low speeds, as may be used to drive a screw, and very high speeds which are used for drilling holes and surfaces. 
   SUMMARY OF THE INVENTION 
   The present invention provides a bit for use with a rotatable tool, said bit including:
         a shaft having a rearward end adapted to be held by a chuck of the rotatable tool, and a forward end having cutting tips formed thereon;   a reaming head including a body having a bore adapted to receive the shaft therethrough in a sliding fit, the body having a plurality of cutting blades formed on at least one peripheral surface thereof;   the bore and/or the shaft having formations thereon such that when the reaming head has the shaft operatively located in the bore the reaming head is rotationally locked to the shaft;   the reaming head and the shaft being separable from each other to allow either component to be replaced independently of the other.       

   Preferably the forward end of the shaft includes four blades orthogonally aligned or angularly equi-spaced, relative to each other such that said forward end is specifically adapted to be used as a Phillips head screw driver, or similar type screw drivers. 
   The reaming head preferably has a forward face of generally tapered or cone shaped configuration, tapering convergently towards the bore through the body, said forward face having cutting blades formed thereon. 
   The shaft can have a square, hexagonal, or other polygonal cross sectional shape with the bore being of corresponding shape, at least in part, to provide for the aforementioned rotational lock. The shaft can also have a taper alone its length with the bore having a corresponding taper to provide for the reaming head to be taper locked to the shaft. 
   The bit can further include a spacer located on the shaft rearward of the reaming head to space the reaming head away from the chuck in use. The shaft can have a shoulder thereon against which the reaming head can bear in use. 
   The cutting tip on the forward end of the shaft can be of a form described in application number PCT/AU98100834, or can be defined by the forward end of the orthogonally aligned, or angularly equi-spaced, blades which define the Phillips screw driver head. 
   The present invention also provides a bit having a tip end and shank end for use with a rotatable tool, said bit having at least 4 blades to engage and drive a screw when said rotatable tool is rotated, each blade including at said tip end a bevel so as to form a cutter at the extremity of said blade, each bevel and or cutter being able to cut in the same rotational direction. 
   The bevel can be formed so as to be confined to each blade. Alternatively the bevel can be formed so that while being applied to one blade, the bevel is also formed on an adjacent blade. 
   The cutter preferably has a leading portion which is an edge. The cutter can also include in its lead portion a wedge shaped formation. The wedge shaped position being formed on one side by a side of said blade and on another side by the cut of the bevel formed on an adjacent blade. 
   A method of making a bit for use as a screw driver and countersinker, said method including the steps of:
         1. forming at least a four blade screw driving end onto one end of blank   2. forming a bevel at the extremity of each blade, said bevel producing an inclined plane cutter.       

   Step 2 can be performed simultaneously on two or more blades, or can be performed on one blade at a time. 
   The bevel can be formed so as to be present only on each blade extremity. Alternatively the bevel, whilst being formed on a first blade can cut into a second blade, thereby dressing the inclined plane cutter of said second blade before or after said bevel is formed in said second blade. 
   Preferably an under cut is formed by said bevel cutting into said second blade. 
   Alternatively the cut produces as surface which has its direction normal to the axis of rotation or an axis parallel to the axis of rotation of said bit. 
   Preferably said cut and a side of said blade on which said cut is located form on the lead portion of each blade a wedge shaped portion. 
   Preferably the rest of said blade away from said bevel includes another cutting edge extending from the tip end towards the shank end. 
   Preferably the bevel is formed by a grinding wheel. 
   The grinding wheel can have its axis of rotation parallel to the axis of said blade when said tip is viewed in plan view. 
   To form said cut in said second blade said axis of rotation of said grinding wheel is at an acute angle to the axis of said blade when viewed in plan view. 
   Preferably said bit is indexed 90° for said grinding wheel to form said bevel on each blade. 
   These and further features of the invention will be made apparent from the description of various embodiments of the invention given below by way of example. In the description, reference is made to the accompanying drawings, but this was a feature shown in the drawing should not be constituted as limiting on the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective, part cutaway, view of a drilling bit according to the invention; 
       FIG. 2  shows a similar view to that of  FIG. 1  of a second embodiment of drilling bit according to the invention; 
       FIG. 3  shows a cross sectional side view of a drilling bit according to the invention mounted in a drill chuck; 
       FIG. 4  shows a cross sectional side view of another embodiment of drilling bit according to the invention; 
       FIG. 5  shows a cross sectional side view of a further embodiment drill bit according to the invention; 
       FIG. 6  shows a cross sectional side view of a further embodiment of drilling bit according to the invention: 
       FIGS. 7 and 8  show side and plan view respectively of the forward part of a drill shaft similar to that shown in  FIG. 2 ; 
       FIGS. 9 and 10  show side and plan views respectively of the forward part of a drill shaft similar to that show in  FIG. 1 ; 
       FIGS. 11 and 12  show side and plan views respectively of a forward part of a further embodiment of drill shaft suitable for use with the invention; 
       FIG. 13  illustrates a front elevation of another Phillips head bit; 
       FIG. 14  illustrates a plan view of the bit of  FIG. 13 ; 
       FIG. 15  illustrates a perspective view of the bit of  FIG. 13 ; 
       FIG. 16  illustrates a perspective view of a Pozi-Drive bit similar to  FIG. 15 ; 
       FIG. 17  illustrates a front elevation of a manufacturing method of the bits of  FIGS. 13  to  16 ; 
       FIG. 18  illustrates a plan view of the manufacturing method of  FIG. 17 ; 
       FIGS. 19 and 20  illustrate a front elevation and plan view of another Phillips head bit; 
       FIG. 21  illustrates a perspective view of the bit of  FIGS. 19 and 20 ; and 
       FIG. 22  illustrates a perspective view of the bit of FIGS.  11  and  12 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Illustrated in  FIGS. 1 ,  9  and  10  a bit  10  has two main components, namely a shaft  12  and a reaming head  14 . The reaming head  14  is of larger diameter than the shaft  12  and the reaming head  14  has a bore  16  therethrough, through which the shaft  12  is inserted. The forward end  18  of the shaft  12  projects through and beyond the forward end  20  of the reaming head  14  so that the forward end  18  of the shaft  12  is the leading part of the bit  10 . 
   The forward-end of the shaft  12  has cutting tips  22  formed thereon which will cut into a surface against which the shaft  12  is pushed in use. There are four cutting tips  22 , each located at on the forward end of a blade  24 , the four blades being orthogonally aligned, or angularly equi-spaced, relative to each other so that the blades  24  for a screw driver of the Phillips head screw driver type, to drive in or remove Phillips head screws from a surface. Other similar screw driver configurations, such as a “Pozidrive”, could be used with the bit of the invention. 
   The blades  24  are arranged in a cone shaped configuration, that is, they taper convergently towards the forward end of the shaft  12 . The reaming head  14  also has four blades  26 , each blade  26  having a cutting edge  28  on the leading edge thereof. The cutting edges  28  are formed by the lateral surfaces  27  and  29  being ground or cut so as to be at an angle of less than 90° and preferably approximately 85° to the side face  25  of the blade  26 . The cutting edges on the blades  24  are formed in a similar manner in that the lateral surfaces  23  on the blades  24  at an angle of less than 90§ and preferably approximately 85§ to the sides  21  of the blades  24 . When the bit  10  is rotated in the direction of a arrow “A” the cutting edges  28  provide a cutting edge for the blades  26 . The blades  24  on the shaft  12  include cutting edges  22  on the leading edges thereof. 
   The forward end  30  of the reaming head  14  is of tapered or cone shaped configuration with a forward end of each of the blades  26  being angled at a steeper angle relative to the shaft axis  32  than the corresponding taper angle of the blades  24 . The reaming head  14  can be used either to form a counter sunk hole for the head of a screw to be inserted into a surface, or, alternatively, the reaming head can be used to cut a relatively deep hole in a surface into which a screw can be inserted, and thereby creating a hole which will be able to receive a plug. 
   The reaming head  14  is slideable relative to the shaft  12  so that, when required, the reaming head can be removed from the shaft  12 . The reaming head  14  can be formed of a material which is different to from the material from which the shaft  12  is formed. The materials from which the respective components are formed will be selected so as to optimise the cost and wear characteristics of the components. Thus, for example, should the shaft  12  be formed of a relatively inexpensive material compared to that of the reaming head, then the shaft  12  can be replaced on a more frequent basis than that of the reaming head  14 . This will allow the reaming head  14  to be retained and reused when the shaft  12  requires replacement. Alternatively, for certain applications, the shaft  12  can be formed of a hard wearing and expensive material and therefore be more costly than the reaming head  14 . In such instances it might be that reaming head  14  requires replacement more frequently than the shaft  12 . 
   To allow for the independent replaceability of the shaft  12  and reaming head  14 , it is desirable that the reaming head  14  is a sliding fit on the shaft  12 . A locking arrangement is required between the shaft  12  and the bore  16  through the reaming head  14  to prevent relative rotation therebetween. In the preferred arrangement, the bore  16 , at least for a portion of its length, has a non-circular configuration, typically hexagonal or square. The shaft  12  will have a corresponding configuration so that when the reaming head  14  is located operatively on the shaft  12 , the two components will fit together and not be able to rotate relative to each other. Furthermore, the shaft  12  will preferably have a stepped configuration defining a shoulder  34 , as indicated in FIG.  1 . 
   As an alternative the forward pan of the shaft  12  can be tapered to provide a friction or taper lock between the shaft  12  and reaming head  14 . This is described in more detail below. As shown in  FIG. 1 , the rearward part  36  of the shaft is of hexagonal configuration and is located in a hexagonal portion of bore  16 , whereas the forward part  38  of the shaft is round and is located in a round portion of the bore  16 . 
   Turning now to  FIGS. 2 ,  7  and  8  of the drawings, a bit  10 ′ similar to that of the previous embodiment is shown except that the cutting tips  40  on the forward end of the shaft are more in the form of a typical blade. As shown, two of the blades  24  which are opposite each other on the tip together define a v-shape starting cutting tip  42  which is adapted to form starter hole in a surface to which a screw is to be inserted. The other two blades  24  have ends  44  which are set back from the starting cutting tip  42 . Plus, the starting cutting tip  42  will provide the initial cut when in use. The four blades  24  can be used in a manner of a Phillips screw driver to drive in a Phillips headed screw. 
   It will be noted that the cutting edges  28  in the embodiment shown in  FIG. 2  are located on the opposite sides of the blades  26  since the bit shown in  FIG. 2  is adapted to be rotated in the opposite direction to that of the previous embodiment to achieve a countersunk hole or depression. The cutting edges  28 , and edges on blades  24  are formed in a similar manner to the edges of the previous embodiment. 
   In  FIG. 3 , a slightly different arrangement of bit  10 A is shown. In this bit  10 A, the shaft  44  has a forward end  46  of rounded larger diameter than the rearward end  48  thereof. A rearward facing shoulder  50  provides a surface against which the reaming head  52  will bear when the bit  10 A is operatively installed in a drill chuck  54 . A cylindrical spacer  56  butts up against the forward end of the drill or chuck  54  and the rearward end of the reaming head  52  to ensure that the reaming head  52  does not slide rearwardly under pressure as the bit  10 A is used. Thus, the bit  10 A shown in  FIG. 3  is assembled by inserting the rearward end of the shaft  44  through the bore  58  in the reaming head  52 . The shaft  44  has, on its forward end, cutting tips  60  of a type similar to that depicted in  FIG. 1  of the drawings. It is envisaged that the forward part  46  of the shaft  44  will be round in cross section where as the rearward part  48  will be square or hexagonal and, likewise, the forward part of the bore  58  will be circular in cross section whereas the rearward part of the bore  58  will be square or hexagonal to provide rotational lock between the shaft and the reaming head. 
   In  FIG. 4  of the drawings, a bit  10 B is shown. In this bit  10 B the shaft  60  provides a shoulder  62  which faces forwardly whereas the bore has a rearwardly facing internal shoulder  64  against which in the assembled arrangement is spaced from the shoulder  62 . The forward part  66  of the shaft  60  is of circular configuration whereas the rearward part  68  of the shaft  60  is of non-circular configuration. The forward part  66  of the shaft  60  is preferably of tapered configuration, the angle of taper being indicated by angle  70 . The reaming head  72  has the same taper, which is such that when the tapered bore of the reaming head  72  is inserted onto the shaft  60 , the reaming head  72  will lock onto the tapered shaft of the forward part  66  thereby ensuring that the two components are locked together in use. A gap  65  is defined between the shoulders  62  and  64  which allows the reaming head  72  and shaft  60  to lock together in use by permitting the engagement of shoulders  62  and  64  prior to the tapers on the shaft  60  and bore in reaming head  72  taper locking. 
   To remove the reaming head  72  from the shaft  60  the reaming head  72  will need to be tapped off the shaft  60  to break the taper lock between the shaft  60  and the reaming head  72 . However, with the stepped shaft A arrangement shown in  FIG. 4 , it will be appreciated that pressure applied to the reaming head  72  during normal drilling operation will simply urge the reaming head  72  against the taper  70  and no inadvertent dislodgment of the reaming head  72  or the shaft  60  will thus take place. 
   In the bit  10 D shown in  FIG. 5 , the reaming head  80  has a cylindrical rearwardly extending extension  82  which is adapted to abut the forward face  84  of a drill chuck  86 . Thus, the cylindrical extension  82  will prevent the reaming head  80  becoming dislodged from the shaft  88  during use. 
   In the bit  10 E shown in  FIG. 6 , the reaming head  90  has a grub screw  92  for locking the reaming head to the shaft  94 . It will be noted that the shaft  94  has a recess  96  into which the grub screw  92  locates to positively lock the reaming head  90  to the shaft  94 . To remove the reaming head  90  from the shaft  94  the grub screw  92  can be disengaged from the recess  96 , thereafter allowing the reaming head to be slid forward off the forward end of the shaft  94  and thereby allowing either the reaming head or the shaft to be replaced independently of the other. It is envisaged that with the arrangement shown in  FIG. 6 , it is not necessary for the shaft  94  and the internal bore through the reaming head  90  to be of non-circular configuration. However, a non-circular arrangement is still the preferred arrangement. In  FIG. 6  is illustrated a gap  89 . This gap  89  can be replaced by the shoulders  62  and  64  being in abutment if desired. 
   The bit  10 F shown in  FIGS. 11 ,  12 , and  22  is similar to the previous embodiments and like parts have been like numbered. The difference between the bit  10 F and previous embodiments is that the cutting tip  98  is of bevelled configuration as shown leading to a point  99  on the axis  38  of the shaft. The forward part of the shaft is defined by four orthogonal blades  102 , each having a cutting edge  104  (formed in a similar manner to the previous embodiments) on the leading edge thereof. The shaft is tapered along its length as indicated at numeral  70  to provide the taper lock with the reaming head in the manner described above. The point  99  is present on a square pyramid. It will be noted that the planes of the bevelled portions  98 A which form the cutting tip  98  do not include the point  99 . 
   Illustrated in  FIGS. 13  to  15  is another bit  10 G having four blades  100  as in previous embodiments. The four blades  100  are in the Phillip&#39;s head configuration where opposed blades are in line and all blades are radially equi-spaced relative to each other. 
   The blades  100  have cutting edges  45  which are similar to those of previous embodiments such as those having cutting edges  104  as in  FIGS. 11 and 12 , which run along the leading side edges of the blades. The edges  45  extend along the blades  100  from the shank end to the tip  110 . The edges  45  are formed in the same manner as previous embodiments. That is the lateral faces  23  on the blades  100  at an angle of less than 90° and preferably approximately 85° to the side face  170 . 
   The difference between the bit  10 G and previous embodiments is that the tip  10  is constructed differently. Each blade  100  includes at its extremity a bevel  150  which is inclined at an acute angle  152  to the rotational axis  154  of the bit  10 G. The angle  152  is approximately 60° however an angle in the range of 45° to 85° could be used depending upon the hardness of the timber or other material with which the bit  10 G will be used to form a countersunk depression. 
   As can be seen from  FIG. 14  the bevel  150  is formed so that the orientation in plan view as in  FIG. 14  is somewhat skewed relative to the radial direction of the blade  100 . The skewing is produced by the method of manufacture. In addition to this the bevels  150  are formed so that the tip or apex  99 , which is effectively a point, is common to each of the planes which form the bevels  150 , that is the bevels  150 , or the planes which form them, all pass through the same point or apex  99 . 
   In the formation of the bevel  150 , as illustrated in  FIG. 17 , the angle  152  approximates the tangent to the grinding wheel  160  where the grinding wheel  160  engages the blade  100 . The shape of the bevel  150  is represented in the figures as being a planar surface but as can be seen from  FIG. 17  the bevel  150  will be curved with the same radius of curvature as the grinding wheel  160  unless the grinding wheel  160  is moved along this tangent. But to all intents and purposes this curvature is difficult to observe in a finished product due to the relative thinness of the blade  100  to which the bevel  150  is applied. 
   Due to the angle  162  between the axis of rotation  164  of the grinding wheel  160  and the radial direction axis  166  of the blade  100 , the grinding wheel  160  will remove material from or grind into the adjacent blade  100  which in this instance is the blade oriented 90° in a clockwise direction from the blade  100  receiving the bevel  150  in FIG.  18 . The angle  162  is the complement of the angle of the angle  152 , that is the angle  152  and  162  added together equal 90°. 
   When the bevel  150  is applied to each of the blades  100  there is formed a triangular surface  168  on an adjacent blade which intersects with the bevel  150  on the blade which receives the bevel  150 . When considering a single blade  100 , a cutting edge  172  is formed by the intersection of the surface  168  and the bevel  150 . Further, the surface  168  together with the side surface  170  (which equates to the side surface  21  of previous embodiment) of blade  100  with which it intersects forms a wedge like projection which can be better viewed in FIG.  14 . 
   It is preferred that when the axis of rotation  164  of the bit  10 G is oriented in a vertical direction the surface  168  is formed with its direction, as defined by a line perpendicular to the surface  168  being, perpendicular to a parallel vertical axis. The surface  168  is produced with this orientation by the grinding wheel  160  as illustrated in  FIG. 17  having its axis of rotation  164  in the horizontal plane when engaging a bit  10 G in a vertical orientation. 
   While this orientation is preferred, if desired the surface  168  can be an undercut surface (that is the surface  168  and bevel  150  on the same blade form an acute angle) or an overcut surface (that is the surface  168  and bevel  150  on the same blade form an obtuse angle) by angling the axis of rotation  164  of the grinding wheel  160  to the horizontal axis in one direction to produce an under cut or in the other direction to produce an overcut. It is believed that the undercut surface will be more useful than an overcut surface, but in some circumstances the overcut may be preferred. 
   The surface  168  together with a bevel  150  on a single blade  100  forms a cutting edge  170 . The cutting edge  172  is the first portion of the bit  10 G to engage the material which is to receive the countersunk hole. The wedge shaped projection formed between the surface  168  and  170  also assists in cutting the material and assists in reducing the amount of power required to countersink a hole. 
   Illustrated in  FIG. 16  is a bit  10 H of a Posi-drive configuration (which includes a rib  101  between each of the blades  100 ) which is similar to the bit of  FIGS. 13  to  15 , and like features have been like numbered. The bit  10 H is formed in the same manner as the bit of  FIGS. 13  to  15  and will function in the same manner. 
   As described above it is preferred that the grinding wheel  160  will form a bevel  150  by approaching and or engaging the blade  100  of the bit at an angle other than 90°. This will form the surface  168  and in combination with the surface  170  forms the wedge shaped cutting formation. 
   If desired however, as illustrated in  FIGS. 19  to  21  the grinding wheel  160  can be made to approach and or engage the blade  100  at 90° which will be in alignment with or the same direction as an adjacent blade positioned at 90° therefrom. By approaching in this direction the grinding wheel or cutter will also grind or cut through the adjacent blade positioned at 90° therefrom as is illustrated in the bit  10 K of  FIGS. 19  to  21 . This will result in the presence of surface  168  of greater surface area than the previous embodiment with the surface  170  being the remaining side of the adjacent blade no longer intersecting the surface  168  to form a wedge shaped formation. Whilst this bit  10 K is expected to be effective in countersinking and screw driving functions it is expected that the embodiments illustrated in  FIGS. 13  to  16  will be more efficient, that is requiring lesser power to achieve a countersunk depression or hole to be plugged, due to the presence of the wedge shaped formation formed by the intersection of surfaces  168  and  170  in the embodiments of  FIGS. 13  to  16 . 
   The angle  152  between the plane of the bevel  150  and the rotational axis  154  of the bits  FIGS. 13  to  21  can be increased or decreased depending upon two man factors. Both factors are dependent upon the material which is to be countersunk and receive a screw. The first factor is the that the angle  152  can be decreased thereby lengthening the length of the bevel  150  in the radial direction. This can be beneficial in some circumstances to assist in the counter-sinking process, however such an arrangement may not provide sufficient bearing surface to drive a screw. A balance between the amount bearing surface and the angle required on the bevel  150  for countersinking can be identified, so as to provide an optimum arrangement for various timber and other type of material properties. 
   There may be many variations to described embodiments without departing from the scope of the invention. However, it is envisaged that an arrangement in which the reaming or counter-sinking head is replaceable on the drill shaft, with the drill shaft providing an arrangement for starting a drill hole can have significant advantages in many industries. In particular, the arrangement in which the drill shaft has a screw driver head formed thereon, preferably a Phillips or other similar shape head, adds to the versatility of the tool and allows for both the drilling operation and the screw inserting operation to be carried out using the same drilling tool and the same drill bit. This should significantly decrease the time required to do both drilling and screwing, particularly where the screw head is to be counter sunk. 
   Furthers the above description discusses the formation of surface  168  and whether it is undercut or overcut being produced by the angle formed between the rotation axis of the cutting/grinding wheel and the rotation axis of the bit. If desired, a grinding/cutting wheel can be used which has grinding and or cutting surfaces at acute angles to form undercuts or obtuse angles to form undercuts, in which case the rotation axis of the grinding/cutting wheel can be kept in the horizontal plane relative to the axis of rotation of the bit. 
   While the above bits of  FIGS. 7  to  21  can be used with a reamer head as described in relation to  FIGS. 1  to  6 , they can function as a countersinker and screw driver without such a reaming head. 
   While the bevels  150  described above in respect of  FIGS. 13  to  21  are substantially planar in nature, these could be replaced by bevels which are formed from twisted surfaces or part twisted surfaces, such a helical surfaces. With such a twisted surface the embodiment of  FIGS. 11 ,  12  and  22  for example, would not require a central square pyramid at the tip because it could be replaced by a surface which starts in approximately the same plane close to the tip  99  and then shortly thereafter begins to twist, thereby removing the step between the pyramid and the bevel. Such twisted surfaces however, are likely to render the manufacturing more costly in view of the complex machinery required to rotate the axis of the grinding/cutting wheel, or the bit blank, or both, during the cutting/grinding operation to achieve the desired twist. 
   It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.