GB2233920A - Fluted drill - Google Patents

Abstract

A hard metal drill is produced by machining axial flutes (2) in a generally cylindrical blank. The flutes have generally flat side faces (9,11) with a minimum radius of curvature at a radially inner transition region (13) between them. The faces (9) carrying the cutting edges are given a positive radial rake by machining a pair of diametrically opposite flutes offset to a diametrical plane parallel to the faces carrying the cutting edges. The radial rake of the cutting edges can be varied by machining different offsets. A relatively small stock of blanks is required, therefore to produce drills with a range of radial rakes. The flutes are widened at the cutting end by inclined faces 11 which decrease the chisel end width 12. <IMAGE>

Description

DRILLS This invention relates to fluted drills.

Drills conventionally have a plurality of cutting edges extending from a central radial portion of the drill at equal angular intervals, and flutes extending along the drill body from the cutting edges. Such fluted drills are conventionally made of a tool steel but in order to enjoy the superior cutting qualities of hard metals it is known to braze hard metal inserts into a fluted body to form the cutting edges, or to manufacture a complete drill from hard metal.

According to the present invention, there is provided a flat drill, ie. a drill with axial flutes, of a hard metal in which the flutes are formed by cutting from a generally cylindrical hard metal blank.

By so forming a complete drill body in hard metal it is possible to avoid technical production difficulties and to manufacture the drill at an economic cost, particularly when manufacturing a drill of a diameter less than 0.5 inch (12mm). Furthermore, it is possible to limit the stocks of finished drills that must be held, by machining the flutes in the cylindrical blank drills to meet customers requirements as they arise.

A further aspect of conventional drill design is that the cutting edges are at a negative radial rake.

There have been proposals from time to time to give a part of the cutting edges a positive rake (GB 172820 and US 4744705), but at the expense of having a greater negative rake towards the centre of the drill. In another example, a zero rake, ie. a radial cutting edge, is proposed (GB 788745) but this is described as having its advantages in cutting plastics such as Bakelite (Trade Mark) and has apparently not proved itself against materials more difficult to cut.

According to another aspect of the invention, there is provided a fluted drill having cutting edges at a positive radial rake and flutes which, along the drill shank rearwards of the tip of the drill, have a transverse cross-section with a minimum radius of curvature at a radially inner region of the flute, between a face of the flute carrying the cutting edge and a radially outwardly extending clearance face of the flute.

In a preferred form of the invention, such a positive rake geometry is provided on a flat-fluted drill made of hard metal, ie. a drill with zero axial rake.

In a further aspect, the invention provides a fluted drill having substantially straight mutually parallel cutting edges, wherein the cutting edges lie behind, in the cutting direction, the diametrical line parallel to the cutting edges.

An embodiment of the invention will now be described with reference to the accompanying schematic drawings in which: Fig.l is a side view of a drill according to the invention; Fig. 2 is a side view of the tip of the drill of Fig. 1, and Fig. 3 is a front end view of the drill of Figs.

1 and 2.

In Fig. 1 there is shown a hard metal drill having a pair of diametrically opposite straight flutes 2 extending along the body of the drill from a conical tip 4 which may have the conventional 1180 drill point. The drill body has a first shank portion 5 of one diameter from the tip 4 to a relief 6 at which the shank is undercut and at which point the body continues with a second shank portion 7 of increased diameter.

The drill illustrated is intended to produce holes of a depth equal to the length of the first shank portion 5. If operated at a high material removal rate, particularly when machining aluminium and its alloys, some material tends to be extruded outwards from the lip of the hole and the secondary cutting edge provided by the leading end of the second shank portion 7 is able to trim off this material in the same manufacturing operation.

In Figs. 2 and 3 it can be seen that the flutes of the drill each form a front face 8 having a substantially straight cutting edge 9 at the tip, and a clearance face 10.

The two front faces 8 are parallel to each other and lie behind, in the cutting direction, a diametrical plane D running parallel to them. The cutting edges 9 thus have a positive radial rake. At the tip of the drill, the flutes are widened by reliefs 11 formed as inclined runoffs at the ends of the clearance faces 10. The reliefs reduce the width of the so-called chisel edge 12 between the ends of the flutes, thereby reducing the energy requirement of the drill without significant loss of strength. The reliefs 11 also serve to break the swarf and so prevent the drill from seizing in a hole being machined.

The end faces 16 of the tip 4 are also relieved, the angle of relief being sufficient to ensure clearance for these faces as the drill advances into the hole.

The cross-section of each flute over the main part of its length has a sharp transition region 13 of very small radius at the radially inner junction between straight or substantially straight front and clearance f ces 8,10. The radius of curvature of the transition region at the tip is increased by the reliefs 11, as indicated at 14, but it remains very small compared with any curvature that may be given to the front and clearance faces. In general, by giving the transition region a substantially smaller radius of curvature than either the front or clearance faces, the positive radial rake of the drill can be achieved without any undesired reduction of the rigidity of the drill shank.

Drills of the form illustrated can be produced by first sintering hard metal cylindrical blanks, which can incorporate the stepped diameter shank shown in Fig. 1 should this be required. It is then possible to machine the flutes, e.g by grinding, very simply to form the cutting edges, and similarly to add reliefs to the clearance faces at the drill tip. The offset of the cutting faces from their diametrical plane parallel to these determines the degree of positive radial rake that the cutting edges have. It is therefore possible to hold a stock of undifferentiated blanks for any size of drill and form the radial rake, it being envisaged that this may vary between zero and 12 , to suit the material to be cut.

It may be required to provide clearance on the outer circumference of the drill shank, behind the front faces. Such clearance can also be conveniently formed by grinding the basic drill blank, as by the sectoral reliefs 15 shown in broken lines in Fig.3. It is also possible, however, to provide blanks that incorporate circumferential clearances, and also end face and clearance face reliefs.

Claims (14)

1. A fluted drill having cutting edges at a positive radial rake and flutes which, along the drill shank rearwards of the tip of the drill, have a transverse cross-section with a minimum radius of curvature at a radially inner region of the flute, between a face of the flute carrying the cutting edge and a radially outwardly extending clearance face of the flute.

2. A drill according to claim 1 wherein said flutes are directed axially.

3. A drill according to claim 1 or claim 2 made of Ward metal.

4. A drill according to claim 3 wherein the flutes are formed by cutting from a generally cylindrical blank.

5. A drill according to claim 4 wherein said blank is formed with reliefs in its outer periphery, said reliefs being disposed between the flutes in the completed drill.

6. A drill according to any one of the preceding claims wherein each said flute face carrying the cutting edge has in radial cross-section a substantially rectilinear form adjacent the cutting edge.

7. A drill according to claim 6 wherein each said flute face lies behind, in the cutting direction, a parallel face containing the drill axis.

8. A drill according to any one of the preceding claims having a substantially planar clearance face on each flute.

9. A drill according to any one of the preceding claims wherein, in the tip region of the drill, each flute is :dened by a relief face.

10. A drill according to claim 9 having a pair of radially opposed flutes in which said relief faces are inclined towards each other at a radially inner region of the drill to reduce the length of a central chisel edge at the drill tip.

11. A method of manufacturing a hard metal drill wherein a generally cylindrical blank is machined to form axial flutes in the blank, each said flute having a transverse cross-section with a minimum radius of curvature at a radially inner region of the flute.

12. A method according to claim 10 wherein a series of drills having different radial rakes on their cutting edges are produced from similar blanks by machining the flutes with their cutting edge faces substantially parallel to but at different offsets from a parallel plane containing the drill axis.

13. A drill constructed and arranged substantially as described herein with reference to the accompanying drawing.

14. A method of producing a drill substantially as described herein.