GB2106429A - Improvements in or relating to annular cutters - Google Patents

Abstract

An annular cutter (10) has a cylindrical side wall (14) provided with cutting teeth (16) spaced circumferentially around its lower end, the inner periphery of each tooth being radially relieved (42) to provide a relatively narrow margin (a) at the inner side of each tooth directly behind the cutting edge (22) thereof. <IMAGE>

Description

SPECIFICATION Improvements in or relating to annular cutters The present invention relates to annular cutters for forming holes in workpieces.

One of the common problems associated with annular cutters is wear and chipping of the radially innermost edges of the cutter teeth. It is believed that this problem arises from the fact that during a cutting operation metal chips frequently become lodged between the inner periphery of the cutter teeth and the outer periphery of a central slug in a workpiece that is produced by the cutter. These chips build up and become welded to the inner periphery of the cutter adjacent the leading edge thereof. The build up of metal at the inner periphery of the cutter teeth creates considerable friction and increases the torque required to rotate the cutter. It also results in the creation of additional heat and produces excess wear which hastens failure of the cutter by chipping and breakage.

According to the present invention there is provided an annular cutter having a body provided with a generally cylindrical side wall having a plurality of circumferentially spaced teeth around its lower end and a plurality of spiral flutes around its outer periphery extending upwardly from between successive teeth, each tooth having at least one radially extending cutting edge, there being gullets extending radially outwardly from the inner periphery of the side wall to said flutes between successive teeth to facilitate the discharge of chips cut by said cutting edges radially outwardly and upwardly into said flutes, each gullet having a radially extending trailing face extending upwardly from the radially inner portion of each cutting edge, the inner periphery of each tooth being relieved in a radially outward direction to provide a circumferentially extending clearance space between the inner periphery of the tooth and the central slug formed in the workpiece by the cutter, said clearance space being spaced circumferentially rearwardly from the trailing face of each gullet so as to form a narrow margin on the inner periphery of each tooth which extends rearwardly and upwardly from said radially inner portion of each cutting edge.

An annular cutter embodying the present invention can overcome the problem of wear, chipping and breakage of annuiar cutters resulting from build up of chips around the inner periphery of the cutter.

By embodying the present invention it is possible to form an annular cutter so that the inner periphery of the cutter body have a minimum surface contact with the centre slug being formed by the cutter.

The invention will be further described by way of example with reference to the accompanying drawings, in which:~ Figure 1 is a side elevational view of a cutter according to a preferred embodiment of the present invention, Figure 2 is a fragmentary plan view of the lower end of the cutter to a larger scale, and Figure 3 is a sectional view along the line 3-3 in Figure 1.

A cutter of the present invention is generally designated 10 and is formed with a shank 12 at its upper end from which an annular side wall 14 depends. The lower end of side wall 14 is formed with a plurality of circumferentially space cutting teeth 16. Between the successive teeth 16 the wall 14 is formed with upwardly extending helical flutes 18 which have a radial depth equal approximately to one-half the thickness of wall 14.

Thus, each flute 18 is radially juxtaposed to a web 20. In the preferred form of cutter according to the present invention each tooth 16 is formed with three radially and circumferentially staggered cutting edges; namely, an inner cutting edge 22, an intermediate cutting edge 24, and an outer cutting edge 26. Cutting edges 22, 24 are formed at the lower end of webs 20 and, to accommodate the chips formed thereby, the adjacent portion of the webs is formed with an inner gullet 28 and an outer gullet 30. Cutting edge 22 is defined by the lower end of the trailing face 27 of gullet 28 and cutting edge 24 is defined by the lower end of the trailing face 29 of gullet 30. Cutting edge 26 is defined by the lower end of the trailing face 31 of flute 18.

Cutting edges 22, 24 are interconnected by a circumferential shoulder 32 and cutting edges 24, 26 are connected by a radius 34. These three cutting edges are vertically staggered by reason of the fact that each tooth is formed with a pair of back-off faces 36,38 which extend circumferentially rearwardly from these cutting edges in an upwardly inclined direction, for example, at an angle of about 8 to 100. In addition, back-off face 36 inclined radially inwardly and axially upwardly and back-off face 38 inclines radially outwardly and axially upwardly. These two back-off faces intersect in a crest 40. If the cutter is to be subjected to a heavy chip load, back-off face 36 preferably inclines radially at an angle of +100 to 30 to the horizontal and back-off face 38 preferably inclines radially at an angle of about 20 to 250.

When each of the cutting edges 22, 24, 26 is vertically staggered a distance in excess of the depth of cut per each revolution of the cutter, each of these cutting edges will cut a separate chip. By reason of the radial inclination of cutting edges 22, 24, the chips cut by these edges will be directed upwardly through their respective gullets and into flute 18. Likewise, the chip cut by the other cutting edge 26 will be directed upwardly through the adjacent flute 1 8 so that all of the chips are discharged smoothly and readily upwardly through the inclined flutes 18.

As shown in Figure 2, the inner periphery of wall 14 is relieved radially outwardly to form a clearance space 42 so as to leave a narrow margin a extending circumferentially rearwardly and upwardly from the inner cutting edge 22 of each tooth. The clearance 42 is preferably formed by removing metal from the inner periphery of the cutter wall by means of a cylindrical cutter in which case the relieved portion is defined by a segment of a circumferential arc 44. The maximum radial depth of the relieved portion 42 is between 0.005 and 0.025 inch (0.127 and 0.635 mm), preferably in the range of about 0.010 and 0.01 5 inch (0.254 and 0.381 mm).In order to provide an escape path for any chip that might become lodged within the clearance space 42, the relieved portion should extend rearwardly into the inner gullet 28 of the next successive tooth so as to form an escape passage as indicated at 46 in Fig. 2. The margin a should have a circumferential width of between 0.010 and 0.060 inch (0.254 and 1.52 mm) preferably, about 0.015 inch (0.381 mm). When the relieved portion 42 is formed with a cylindrical cutter so that it becomes progressively narrower in a direction toward the cutting edge 22, the margin a preferably has a width on the order of 0.010 inch (0.254 mm).

However, when the relieved portion 42 is machined as a groove of substantially uniform radial depth, then, in order to avoid substantial weakening of the tooth, the margin a should have a width greater than 0.010 inch (0.254 mm) depending upon the radial depth of the clearance groove.

The trailing face 27 of each gullet 28 is inclined rearwardly and upwardly an an angle of about 600 to the horizontal. As pointed out previously, the lower end of this trailing face cooperates with the back-off face 36 to define the cutting edge 22. It is preferred to form the margin a so that it has a substantially uniform width in a direction parallel to the trailing face of gullet 28 so that, as the cutter teeth are sharpened by grinding the backoff faces 36,38, the margin a will remain of substantially constant width. The uniform width of the margin can be obtained by orienting the cutting tool which forms the relieved portion 42 so that its axis is inclined to the central axis of the cutter in a direction generally parallel to the trailing face of gullet 28 which in turn forms the front rake face of cutting edge 22.In Figure 2 the broken line 48 depicts in a horizontal plane the outline of a tilted cutter that produces the circular arc 44 of clearance space 42. The relieved portion 42 can extend upwardly the full height of wall 14.

However the desired results are obtained when the margin a has a height which corresponds generally to the vertical dimension of the teeth 1 6.

For example, as shown in Figure 3, the clearance space 42 provided by the relieved portion extends upwardly slightly beyond the upper end of the outer gullet 30.

As pointed out previously, one of the problems involved in using annular cutters of the general type described herein is the lodging of chips between the inner periphery of the cutter wall and the outer periphery of the central slug formed by the annular cutter. The provision of the narrow margin a substantially eliminates this problem. If a chip becomes wedged between the inner periphery of the cutter and the central slug, the high unit pressure exerted thereon by the narrow margin a results in fracture of the chip or at least wearing down of the chip sufficiently so that it will be discharged into the clearance space 42 and then radially outwardly through the escape passage 46 into the next successive gullet 28.

Thus, the narrow margin a prevents build up of chips between the inner periphery of the cutter and the outer periphery of the slug and the problems resulting from such build up are thus avoided.

Claims (11)

1. An annular cutter having a body provided with a generally cylindrical side wall, said side wall having a plurality of circumferentially spaced teeth around its lower end and a plurality of spiral flutes around its outer periphery extending upwardly from between successive teeth, each tooth having at least one radially extending cutting edge, there being gullets extending radially outwardly from the inner periphery of the side wall to said flutes between successive teeth to facilitate the discharge of chips cut by said cutting edges radially outwardly and upwardly into said flutes, each gullet having a radially extending trailing face extending upwardly from the radially inner portion of each cutting edge, the inner periphery of each tooth being relieved in a radially outward direction to provide a circumferentially extending clearance space between the inner periphery of the tooth and the central slug formed in the workpiece by the cutter, said clearance space being spaced circumferentially rearwardly from the trailing face of each gullet so as to form a narrow margin on the inner periphery of each tooth which extends rearwardly and upwardly from said radially inner portion of each cutting edge.

2. A cutter as claimed in claim 1, in which each tooth has at least two radially spaced cutting edges, the trailing face of said gullet extending upwardly from the radially inner cutting edge.

3. A cutter as claimed in claim 1 or 2, in which said clearance space extends circumferentially rearwardly on each tooth into the gullet of the next successive tooth so as to provide a chip escape passage from each clearance space into the next successive gullet.

4. A cutter as claimed in claim 1,2 or 3, in which said clearance space has a maximum radial extent of about 0.127 to 0.635 mm.

5. A cutter as claimed in claim 1,2,3 or 4, in which said margin has a circumferential width of about 0.127 to 1.52 mm.

6. A cutter as claimed in any preceding claim, in which said clearance space extends upwardly to at least the upper end of said gullet.

7. A cutter as claimed in any preceding claim, in which said clearance space is defined by an arcuate segment of a cylinder.

8. A cutter as claimed in claim 7, in which said margin has a circumferential width of about 0.254 to 0.381 mm.

9. A cutter as claimed in claim 7 or 8, in which said clearance space has a maximum radial extent of about 0.254 to 0.381 mm.

10. A cutter as claimed in claim 6, in which the trailing face of said gullet inclines upwardly in a circumferentially rearward direction and said margin is of substantially uniform width in a vertical direction.

11. An annular cutter constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.