This invention relates to the grinding or re-grinding (herein both referred to simply as "grinding") of twist drills of the type hereby defined as having a butterfly-shaped cross-section which twists along the length of the drill in helical form and having a generally conical tip.

Such twist drills can be ground satisfactorily freehand by a skilled artisan using a rotary grinding wheel. However, such freehand grinding does take considerable skill to produce the optimum tip form. In order to assist persons who do not possess such skill, there has been developed a jig for attachment to a grinding machine. However, this does not produce the optimum tip form, for the reasons described below with reference to Figures 1 and 3 in which: Figure 1A is an end view of an exactly conically ground twist drill;

Figures 1B and 1C are side views of the drill of Figure 1A viewed in the directions 1B and 1C, respectively, in Figure 1A;

Figure 2A is an end view of an optimally ground twist drill; Figures 2B and 2C are side views of the drill of Figure 2A viewed in the directions 2B and 2C, respectively, shown in Figure 2A;

Figure 3A is an end view of an improperly ground twist drill as produced by the known grinding jig; and

Figures 3B and 3G are side views of the drill of Figure 3A viewed in the directions 3B and 3C, respectively, shown in Figure 3A.

Referring to Figures 1 to 3, a right-handed drill 10 has a butterfly-shaped cross-section with wings 12a, b and helical flutes 14a, b therebetween. The leading edges 16a, b of the wings 12a, b are generally straight, whereas their trailing edges 18a, b have a more pronounced concave curve. At the centre of the butterfly-shaped cross- section, there is a narrow central region 20 connecting the wings 12a, b. It should be noted that many conventional twist drills have helical ribs formed on the periphery of the drill to reduce rubbing of the driii in the hoie it is cutting, but such ribs are not shown in the drawings for reasons of simplicity.

If the tip of the drill is ground so as to be exactly conical, with a conical half angle of 60°, as shown in Figure 1, then the angle

L (Figure 1C) of the leading edges 16a, b to the longitudinal axis 22 is 60°; the general angle T (Figure 1B) of the trailing edges 18a, b to the longitudinal axis 22 is also 60°; the peripheral edge 24 of the tip is perpendicular to the longitudinal axis 22; and the tip has a sharp point 26. As is well-known, such an exact conical shape of the tip produces rubbing of the tip with the workpiece and the drill will not cut.

In order to overcome this problem, in an optimally ground drill as shown in Figure 2, the tip is not exactly conical but instead: the angle L (Figure 2C) of the leading edges 16a, b to the longitudinal axis 22 is 60°; the general angle T (Figure 2B) of the trailing edges

18a, b to the longitudinal axis 22 is slightly less at 55°; and the peripheral edge 24 of the tip is slightly inclined to the perpendicular to axis 22. Thus, the leading edges 16a, b can cut into the workpiece. It will be appreciated that this form of grinding produces a ridge 28 at the extreme tip of the drill, rather than a point 26 as in Figure 1.

In addition to ensuring that the drill is symmetrical, one of the arts in grinding a "good" drill is to make the ridge 28 as short as possible, so that the drill is as nearly pointed as possible. Otherwise, the drill will tend to wander when it is "started" and will tend to cut an eccentric or wandering hole. The known drill grinding jig does not do this, but instead produces a tip form as shown in

Figure 3.

In Figure 3: the angle L (Figure 30) of the leading edges 16a, b to the longitudinal axis 22 is 60°; the general angle T (Figure 3B) of the trailing edges 18a, b to the longitudinal axis 22 is 60°; and the peripheral edge 24 of the tip is inclined slightly to the perpendicular to the axis 22. This tip form is produced by advancing the drill into the grinding wheel as the drill is rotated so that, the line of contact of the drill with the wheel moves from a leading edge 16a, b to the respective trailing edge 18a, b, but keeping the longitudinal axis 22 of the drill at a constant angle of 60° to the face of the grinding wheel. As is clear from a comparison of Figures 2 and 3, the ridge 28 in Figure 3 is not as short as is possible, and accordingly an inferior cut is produced by the drill of Figure 3.

The present invention aims to provide a jig which simplifies drill grinding and which assists in producing a drill tip form more

like that described with reference to Figure 2 than that described with reference to Figure 1 or 3.

In accordance with the present invention, there is provided a twist drill grinding jig for a grinding machine, comprising: a drill holder adapted to hold a drill; and a mount for the drill holder to present the tip of a drill held in the holder to the grinding machine; wherein the holder and mount act as a mechanism to permit the drill to be rotated to a limited extent about its longitudinal axis and to cause such rotation to be accompanied by a tilting of the longitudinal axis of the drill.

Thus, the mechanism can cause a transition from a grinding angle of, for example, 60 degrees to the axis at the leading edges to a grinding angle of, for example, 55 degrees to the axis at the trailing edges to produce a good tip form.

Specific embodiments of the present invention will now be described by way of example with reference to the remaining drawings, in which:

Figure 4 is a plan view of a grinding wheel, grinding jig and twist drill;

Figures 5A and 5B are elevations, viewed in the direction V shown in Figure 4, with the jig in a position at the beginning and end, respectively of a grinding operation;

Figures 6A and 6B are a side elevation and an end elevation, respectively, of a mount;

Figures 7A and 7B are rear elevations corresponding to Figures 5A and 5B, respectively;

Figure 8 is a sectional side elevation of a drill holder;

Figure 9 is a side elevation of the drill holder and a setting tool;

Figure 10 is an end view of the setting tool alone, as seen in the direction 10 shown in Figure 9;

Figure 11 is a side view of an alternative drill holder;

Figure 12 is a sectional side view of Lhe Figure 11 hoider holding a chuck and drill;

Figure 13 is an end view of the Figure 11 holder; and

Figure 14 is a side view of an additional component.

Referring to Figures 4 and 5, a conventional grinding machine has a cylindrical grinding wheel 30 which is rotated at high speed about its horizontal axis 32. A mount 34 is secured either to a base 36 of the grinding machine or to a bench on which the grinding machine is mounted. A drill holder 38 can be engaged in the mount 34 and holds the twist drill by means of a collet arrangement described below so that the drill tip is presented to the grinding wheel 30 with the axis 22 of the drill at an angle of 60° to the axis 32 of the grinding wheel 30 as viewed in plan (Figure 1). Referring to Figure 6, the mount 34 has a stand 40 to which is secured a socket 42. The collar 42 is formed by an upper half- cylindrical shell 44 having an axis 46, a lower half-cylindrical shell 48 having an axis 50 and two generally triangular gussets 52, 54 all welded together to form the socket 42 having a passageway which is generally circular at the front end 56 of the socket and is generally oval at the rear end 58 of the socket. The angle between the axes 46, 50 is about 18 degrees, and the axis 46 of the upper shell is horizontally coplanar with the axis 32 of the grinding wheel (see Figure 5A). The gusset 52 nearer the grinding wheel 30 has, at the rear end 58 of the socket, a rearwardly projecting lug 60 which is generally in the horizontal plane containing the axis 46. The mount 34 may be of welded steel construction, or an integral plastics moulding. Referring to Figures 7 and 8, the holder 38 comprises a cylindrical body 62 into which the drill is fitted coaxially so that the drill tip projects by a predetermined amount P from the front end 64 of the body. An annular flange 66 is integrally formed at the rear of the body 62 and has a pair of diametrically-opposed cut-outs 68a, b formed therein. Preferably using a setting tool as described below, the drill is secured in the holder 38 so that the leading edges 16a, b of the drill tip are horizontal when the cut-outs 68a, b are horizontally aligned.

Referring back to Figures 4 to 7, the holder 38 is placed into the mount 34 such that the drill is horizontal (Figures 5A, 7A) and such that the lug 60 engages one of the ut-ouls 68a. As the holder 38 and drill 10 are advanced by hand in the direction A (Figure 5A) until the flange 66 abuts the rear end 58 of the mount, the wheel 30 begins to grind the appropriate leading edge 16a of the drill at 60 degrees to

the drill axis 22. Then, as the holder 38 and drill 10 are rotated by hand in the direction R (Figures 5A, 7A) the mechanism provided by the mount 34, lug 60, holder 38 and appropriate cut-out 68a cause the holder 38 and drill 10 to tilt progressively, so that the shank of the drill descends and the tip rises slightly, until the position shown in Figures 5B and 7B is reached. The rotation of the drill causes the grinding to progress from the leading edge 16a to the trailing edge 18a, and the tilting of the drill causes the angle of grind to decrease progressively from 60° to 55° with respect to the drill axis 22. Without moving the drill 10 relative to the holder 38, the holder is then retracted to disengage the lug 60 and cut-out 68a. The holder and drill are then rotated half a turn, and the lug 60 is engaged with the other cut-out 68b. The grinding process is then repeated on the other half of the drill (leading edge 16b to trailing edge 18b). According, an optimum tip form is achieved.

The manner of fixing the drill 10 in the holder 38 is shown in Figure 8. The cylindrical body has a screw threaded counterbore 90 extending in from its rear end 62, and a correspondingly threaded locking knob 92. At the ends of the passageway provided through the body 62 and knob 92 there are 45° tapered inwardly directed flanges 94. A collet member 96 is fitted into the passageway and has a central annular portion 98, with four gripping fingers 100 projecting axially in each direction from the annular portion 98. The ends of the fingers 100 are 45° tapered and complement the tapers of the flanges 94. Thus, with a drill 10 passing through the collet member 96, as the locking knob 92 is tightened, the tapers cause the gripping fingers 100 to squeeze onto the drill and lock it in place.

The range of drill sizes which the collet member 96 will grip satisfactorily is limited, and preferably a set of collet members is provided with different internal diameters to fit different drill diameter ranges, for example 0 to 3 mm, 3 to 5 mm, 8 to 10 mm and 10 to 13 mm.

It is important, in grinding the correct tip form, that the rotary position υf the dr ill in the holder relative to the cut-out 68a,b and that the projection P of the drill from the front end of the holder is correctly set. This may be achieved by measurement and reference to a diagram, but is preferably achieved using the setting

tool 70 which will now be described with reference to Figures 9 and 10. The setting tool 70 is manually used and comprises a cylindrical sleeve 72 which is open at its rear end 74 and which has a bridging element 76 welded to its front end 78. The bridging element 76 has a pair of longitudinally extending portions 80a,b and extending therefrom a pair of portions 82a,b which are inclined at about 45° to the axis of the sleeve 72, and which meet to form a V-shaped trough 84. A lug 86 projects from the rear end 74 of the sleeve and, as seen in Figure 10 is offset by about 45° clockwise from the axis of the trough 84. In use of the setting tool 70, the drill 10 is placed loosely in the holder 38, and the holder 38 is inserted into the sleeve 72 of the setting tool 70 so that the lug 86 engages one of the cut-outs 68a,b in the flange 66 of the holder. The drill is then pushed forward and twisted as necessary so that the four points of intersection of the leading and trailing edges 16a,b;18a,b with the peripheral edge 24 of the drill contact the inclined portions 82a,b of the bridging element 76 of the setting tool. The locking knob 92 of the holder is then tightened to lock the position of the drill in the holder. This therefore places the tip of the drill in a predetermined position angularly and longitudinally with respect to the holder 38 and the cut¬ outs 68a,b therein.

Figures 11 to 13 show an alternative tool holder 100 comprising a pair of tube portions 102, 104 with a shoulder 106 therebetween. The tube portion 104 of larger diameter has a pair of slots 108 extending from its free end to adjacent the shoulder 106, so that the diameter of the larger tube portion 104 is flexible. The shoulder 106 has a pair of cut-outs 68a, 68b which perform the same function as described above. The smaller tube 102 has an external length and diameter similar to the length and diameter of the cylindrical body 62 of the tool holder 38 described above.

As shown in Figure 12, a standard size drill chuck 110 is a sliding and turning fit in the tool holder 100, with the larger diameter portion 112 of the chuck sliding in the larger diameter tube portion 104 and the smaliei diameter portion 114 of the chuck being slidable in the small diameter tube portion 102. With a drill 10 held in the chuck 110, the chuck can be positioned by twisting and axial sliding in the tool holder 100 so that the tip of the drill projects by

the correct amount from the holder and so that the angular orientation of the tip relative to the cut-outs 68a, 68b is correct, and then the chuck 110 can be clamped in the holder 100 by tightening an over-centre toggle clip 116 to squeeze the larger tube portion 104 against the chuck. The setting tool 70 described above with reference to Figures 9 and 10 may be used to assist setting of the position of the drill tip relative to the tool holder 100. Also, alignment marks 118a, 118b may be provided on the shoulder 106 to assist in setting or preliminary setting. Once the drill position is set, the tool holder 100 is used in a similar way to that described with the smaller tube portion 102 being inserted into the socket 42 of the mount 34, and one or other of the cut-outs 68a, 68b being engaged by the lug 60.

In order to steady the chuck 110 in the holder 100 when sharpening long drills, a cylindrical member 120 may be provided, as shown in Figure 14, whichføs an inner diameter which is a snug fit on the smaller portion 114 of the chuck and which has an outer diameter which is a sliding fit in the larger tube portion 104 of the holder 100. The member 120 therefore provides additional support for the smaller portion 114 of the chuck.