GB2278793A

GB2278793A - Hole cutting tool

Info

Publication number: GB2278793A
Application number: GB9409255A
Authority: GB
Inventors: Stephen Sheldon
Original assignee: BRADFORD TOOL AND GAUGE Ltd
Current assignee: BRADFORD TOOL AND GAUGE Ltd
Priority date: 1994-05-10
Filing date: 1994-05-10
Publication date: 1994-12-14
Anticipated expiration: 2014-05-10
Also published as: GB9409255D0; GB2278793B

Abstract

A hole cutting tool, in particular a trepanning tool suitable for producing holes in metal, comprises a hollow shaft having teeth 4 around the annular end of the shaft and fluting 5 originating between the teeth 4 and extending in a helical fashion along the length of the shaft. In order to facilitate the break-up of chips and to extend the life of the tool, the tool has one or more of the following features: a substantially axial rake angle, an end tooth angle ( gamma ) of around 29 DEG , and the front edges of the teeth located in front of centre-line of the shaft. <IMAGE>

Description

HOLE CUTTING TOOL The invention relates to a hole cutting tool, in particular, but not exclusively, a trepanning type drill for use in cutting metallic and other ductile materials. The prime application is for the construction industry, but other engineering applications are also envisaged.

A trepanning tool produces a hole by removing a ring of material corresponding to the hole. For this purpose it has a ring of teeth at the end of a hollow shaft, which are adapted to cut into a workpiece when either the tool or the workpiece are rotated about the axis of the tool. The exterior of the shaft of the tool is usually constructed in much the same way as that of a conventional twist drill. In other words the shaft is fluted, with one flute being present for each tooth at the end of the tool. The flutes are each cut as a helix extending away from the tool head along the length of the shaft, and advantageously act to carry away chips of the workpiece material, formed when producing a hole. The radially inner surfaces, or "floors", of the flutes form collectively a generally cylindrical surface, the "flute floor surface", interrupted by the helical ridges between the flutes.

In designing a trepanning or other drilling tool, several criteria are of importance, in particular the efficiency of cutting and the lifetime of the tool.

Often the two are inextricably interrelated. The tool geometry may be optimised by the selection of appropriate cutting angles on the teeth, but the performance of the tool will be hampered if this geometry fails to solve the problems of chip build-up.

Chips produced during metal-cutting fall into two main categories, namely continuous chips and discontinuous chips. The former are produced when ductile metals are machined, the workpiece material being sheared, and the chip flows up the fluting of the tool, forming a spiral. Discontinuous chips on the other hand are small chips which are naturally formed in the tooling of brittle material. They are preferred over continuous chips for a number of reasons. They do not become entangled between the tool and the workpiece and are easily swept up the fluting, they do not wear the cutting edge to the same extent and the swarf produced is less bulky. Accordingly, it is known practice where the material being worked is ductile to use a drill having a means of breaking continuous chips into discontinuous-type chips. This "chip-breaker" may take the form of a groove or a step in the drill.

It is an aim of the present invention to provide a trepanning-type drill which has a tool geometry optimised for cutting and also for the breaking up and removal of chips, thereby achieving an extended working life of the tool.

The present invention is concerned with a trepanning-type drill comprising a substantially cylindrical hollow shaft open at one end, a plurality of teeth formed by a corresponding plurality of gashes in the open end of the shaft and fluting originating between the teeth, i.e. in the vicinity of the gashes, and formed in the outer surface of the shaft. In conventional tools of this type the gash, which is a valley-shaped cut, is formed so as to follow the line of the fluting. This is thought to aid the passage of swarf from the teeth into the fluting and out of the drill bit. There are various aspects of the invention and according to the first of these the gash has an axial rake angle which is substantially nearer the axial direction than the fluting is; preferably it is substantially vertical, i.e. parallel to the axis of the tool.

The axial rake angle is the angle to the tool axis which is made by the intersection of the trailing wall of the gash with the flute floor surface. Normally this angle follows more or less the angle of the flutes themselves, because the gash is cut at about this angle and is usually quite steep.

In a second aspect of the invention the gash has an end tooth angle of around 29 , say plus or minus 5 and at any rate less than 40". The end tooth angle is essentially the angle to the radial plane of the bottom, or "valley floor", made by the gash, in other words the steepness of the gash. Again, this has hitherto been fairly high, about 40 , to aid passage of swarf.

In a third aspect of the invention the cutting edge of each tooth is arranged so as to be in front of the centre-line of the shaft.

In a preferred embodiment the cutting edge of the tooth is positioned about 2.0 mm to around 2.5 mm in front of centre-line.

The invention can use these three main features independently but preferably combines them to achieve the desired chip-breaking properties in the drill.

It is also envisaged that the fluting should be helical and orientated with a clockwise rotation having an angle to the vertical (axial) of approximately 29".

The invention may be carried out in a number of ways, but one specific example will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a trepanning tool in accordance with the present invention, Figure 2 is an end view of the trepanning tool showing the teeth from above, Figure 3 is a magnified schematic view in the radial direction of a tooth of the trepanning tool, Figure 4 is a schematic tangential view of a tooth on the trepanning tool showing tooth top angle and the end tooth angle, Figure 5 shows the form of a grinding wheel suitable for providing the geometry of the gash between the teeth, and Figure 6 shows the angle of the tool during grinding of the gash.

As shown in Figure 1 the trepanning tool, shown generally at 1, essentially comprises an end piece 2 for fitting into a bush on a jig for example, and a hollow cylindrical shaft 3. At the upper end of the shaft 3 i.e. remote from the end-piece 2 are provided a number of teeth (in this case seven) shown generally at 4 which are cut into the rim of the cylindrical shaft 3. Each tooth 4 has a corresponding helical flute 5 milled out of the cylindrical shaft and extending in a clockwise direction down towards the end-piece 2. The angle of each flute to the vertical is approximately 29 to optimise the removal of waste material. The flutes are separated from one another by helical ridges formed when the flutes are machined. The upper end of each ridge 6 is continuous with and forms part of the structure of a tooth 4.

Each tooth 4 has two upper (i.e. end-axial) faces, namely a radially outer face 10 and an inner face 11, the front edges l0A, 11A of which form the cutting edge of the tooth 4. The outer face 10 is also the end face of one of the ridges 6. In this embodiment the two faces 10, 11 are arranged so that the plane of each face lies at 25 to the horizontal, i.e. to the radial plane (Figure 4), the outer face 10 inclining downwards to the outside of the shaft and the inner face 11 inclining downwards to the inside of the shaft. The edge or annular ridge 12 formed between the two upper faces 10 and 11 is arranged so as to be close to tangential in direction with respect to the cylinder inner surface. The front edge 11A of the inner face 11 is substantially straight while the front edge 10A of the outer face 10 is arcuate owing to the curved shape of the walls of the fluting, there being a sharpened point where the front edges 10A and 11A meet at the front of the tooth 4.

The front rake angle (normal) a, i.e. the angle between the radial plane and the ridge 12 (formed between the outer face 10 and the inner face 11), in this embodiment is positive i.e. sloping downwards from the cutting edge. In this case the angle is around 15 to optimise cutting efficiency and yet still support the cutting edge, though this may vary depending on the material of the workpiece.

The positioning and configuration of the teeth is illustrated most clearly in Figure 2. The back edge 11B of inner face 11, although slightly curved, is arranged substantially on a radial line drawn from the axis of the cylindrical body of the tool, in other words on centre-line. The front edge llA therefore is arranged significantly in front of centre-line, in this case approximately 2 - 2.5mm in front of centre-line.

In usual designs for this type of tool this option would not be considered since a cutting edge on or behind centre-line is seen to be desirable.

Between each pair of adjacent teeth a gash is cut into the flute 5 between the teeth. This gash 15 is dimensioned so as to be roughly the shape of a lopped half-cone, narrower at its end on the inner side of the cylinder and wider at the outer end where it opens into the flute 5. The end tooth angle Y, i.e. the angle of the line along the lowest part of the gash 15, is arranged in this embodiment at approximately 29 to the horizontal (the radial plane or the plane of the workpiece). This compares with a norm of approximately 40 for similar conventional tools. The 29 angle is illustrated in Figure 6 which shows the inclination of the tool during grinding. Furthermore the gash 15 is not cut so as to be symmetrical about a vertical line, but is angled so as to tend towards the curve of the helix of the flute 5.

In accordance with the invention, the trailing part of the outer edge 20 of the gash 15 is orientated so as to be nearly vertical, i.e. close to a 0 axial rake. This results in the formation of a step area 22 between the gash 15 and the ridge 6 which contributes to the breaking up of chips of waste material, as illustrated in Figure 3, which shows a magnified view of one tooth. The gash 15 intersects the floor of the fluting at the edge 20, and the trailing part 21 of this edge is what forms the rake angle, which is near to 0 with respect to the axis though a deviation, say of up to 10 , is possible. The significant factor appears to be that the trailing edge 21 is not parallel to the flute, so that the wedge-shaped step area 22 is produced between the flute wall and the gash. This contrasts with prior art tools where the trailing wall of the gash is nearly contiguous with the trailing wall of the corresponding fluting.

In particular when in combination, the features consisting of an end tooth position 2.5mm in front of centre-line, a 29 end tooth angle, and a 0 axial rake on the gash 15 introduce an extended life expectancy of the tool with efficient cutting ability and increased breakdown of the swarf. Although specific values of angles and other measurements have been given in this embodiment, it is obvious that some variations from these specific values will be tolerated while still achieving the same advantageous effects. Indeed, it may be necessary to vary these to optimise the working of the tool in a particular application. The approximate range of values envisaged as achieving the aims of the invention are 2-3mm front of centre-line, 20-35 end tooth angle, and 0-10 axial rake.

The dimensions of the trepanning bit may be conventional, say in the range 29-55mm in terms of the axial extent of the fluted surface, and 14-65mm diameter. Likewise any conventional arrangement of teeth is possible. The example shown has teeth with two radial surfaces, inwardly and outwardly facing, but the invention could also be applied to single-plane teeth; the surfaces of such teeth are normally inclined alternately outwards and inwards to aid cutting.

Claims

Claims:

1. A hole cutting tool comprising a substantially cylindrical hollow shaft, open at one end, a plurality of teeth formed at the open end of the hollow shaft by a plurality of gashes formed in the end of the shaft and fluting originating between the teeth, wherein the axial rake angle, formed by the intersection of the trailing wall of each gash with the corresponding flute floor surface, is substantially nearer the axial direction of the shaft than the fluting.

2. A hole cutting tool according to claim 1 in which the axial rake angle is substantially parallel to the axis of the tool.

3. A hole cutting tool according to claim 1 or 2 having an end tooth angle defined as the angle of the floor of each gash to the radial plane of the shaft, of approximately 24C to 34 .

4. A hole cutting tool according to claim 1, 2 or 3 in which the cutting edge of the tooth is located in front of the centre-line of the shaft.

5. A hole cutting tool according to claim 4 in which the cutting edge of each tooth is located between 2.0 and 2.5mm in front of the centre-line.

6. A hole cutting tool according to any preceding claim in which the fluting is arranged helically around the shaft.

7. A hole cutting tool according to claim 6 in which the fluting is oriented with a clockwise rotation.

8. A hole cutting tool according to claim 6 or 7 in which the fluting has an angle of approximately 29 to the axis of the shaft

9. A hole cutting tool according to any preceding claim in which each tooth has two axial end faces and a generally circumferential edge formed where the two faces meet, the angle between the edge and the radial plane of the shaft being approximately 15".

10. A hole cutting tool according to any preceding claim in which the flutes are separated from one another by helical ridges, the axial ends of which are continuous with and form part of the teeth.

11. A hole cutting tool according to claim 10 in which an area of the flute floor is formed between each gash and the corresponding ridge.

12. A hole cutting tool comprising a substantially cylindrical hollow shaft, open at least at one end, a plurality of teeth formed at the open end of the shaft, and fluting originating between the teeth, wherein the end tooth angle, defined as the angle of the floor of each gash to the radial plane of the shaft, is between approximately 240 and 34 .

13. A hole cutting tool comprising a substantially cylindrical hollow shaft open at least at one end, a plurality of teeth formed at the upper end of the shaft, and fluting originating between the teeth, wherein the cutting edge of each tooth is located in front of the centre-line of the shaft.

14. A hole cutting tool substantially as described herein with reference to any of the accompanying drawings.