GB2260283A - Drill bit - Google Patents

Abstract

A drill bit 12 has a carbide insert 13 with side faces 15 and angled flanks 16 ending in a chisel edge 20. Grooves 21 are cut into each face at an angle of 20 to 35 DEG and preferably 27 DEG to faces 15 so as to cut away part of the original chisel edge. The groove depth is approximately one third of the thickness T of the insert. Each groove is joined to the adjacent insert face 15 ending in a flank cutting edge 17 by a radius 23 substantially equal to the thickness of the insert. The cutting edges 22 provided by the grooves have zero or near zero and negative rake angle. <IMAGE>

Description

DRILL BIT This invention relates to drill bits and particularly to sintered metal tipped masonry drill bits.

A large variety of sintered metal, e.g. metal carbide, tipped masonry drill bits are available for different purposes and an object of this invention is to provide such a bit having improved cutting efficiency when drilling hard material.

The invention is particularly concerned with the type of masonry drill bit in which the tip in the form of a sintered metal insert has been brazed into a slot in the normal web of the drill. The insert is then sharpened such that it has flank cutting edges and a chisel cutting edge similar to a conventional drill bit but has additional grooves extending respectively into opposite sides of the web between the flank cutting edges and the chisel edge to narrow the central portion of the web. The only drill bit of this type available has the grooves located substantially opposite one another and with each groove cut into an insert face symmetrically and with both groove sides making angles of approximately 45 degrees with the insert face; the sides of the groove join the face of the insert at a sharp angle.

These inserts and grooves are commonly ground with a negative rake of at least 30 degrees so that the supplementary edges at the top of the groove sides act as continuations of the chisel edge and have an extruding rather than a cutting action.

The drill bit described herein incorporates three features, each of which improves the cutting efficiency.

The features may be claimed independently although they are preferably included together.

According to one feature each groove is cut at an angle to the insert face in the range 20 to 350, preferably 22 to 300 and with advantage substantially 270.

This helps the supplementary edges of the grooves which join the chisel edge to the flank cutting edges to act as cutting edges rather than extrusion edges; the grooves serve to reduce the chisel edge to approximately one third of its original length, so that the area on which it acts first on contacting the material is reduced to about one ninth. For a specific applied drilling force, the effective drilling forces initially are therefore increased. This increases the effectiveness on hard materials.

According to another feature the supplemental cutting edges produced by the grooves have either zero or negative near zero ( no more than 5 degrees) rake angle. Again this helps these edges, unlike the leading chisel edge which has little or no cutting ability , to have the ability to cut material in a similar way to the cutting action at the periphery of the drill bit.

According to another feature each face of each groove is joined to the adjacent insert face ending in a flank cutting edge by a radius. This radius preferably substantially equals the insert thickness.

One embodiment of drill bit, in accordance with the invention, will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a side view of a drill bit, Figure 2 is an end axial view of the drill bit and, Figure 3 is a diagrammatic side view taken at right angles to the view of Figure 1.

A masonry drill bit has a flute 12 ending in a tungsten carbide tip 13 in the form of an insert braised into the flute 12. The insert has angled ends 14 and faces 15 with flanks 16 angled at 190 and having flank cutting edges 17. The point has a chisel edge 20 extending across the insert. Two grooves 21 are cut into opposing faces of the tip between the chisel edges 20 and the cutting edges 17 thereby reducing the chisel edge 20 to a length of about 1/3 of the insert width and reshaping the cutting edges 17 to produce supplementary cutting edges 22. Each of these extends from the chisel edge 20 at an angle of substantially 270 the associated face 15.As viewed along the bit axis towards the tip end, as seen in Figure 2, each groove face is blended into the adjacent insert face defining a flank cutting edge 17;as indicated by the arrow 23 > byLradius. The broken line 24 in Figure 2 would not really be seen but is indicated to show the edge of the groove# if this radiusing was not present. The insert has a width t and the radius is preferably substantially the same as the insert thickness.

Each groove extends into the insert by approximately one third of the insert thickness. The point angle of the tip is approximately 1320 and the depth of each' groove is approximately one third of the insert thickness.

As seen in Figure 3, the tip preferably has zero rake, where this rake is defined as the angle that the face 15 makes with the axis of the drill. In other words, the cutting face 15 will be at right angles to the plane of the work surface.

The grooves are made in such a way that the cutting faces 22 are also substantially zero rake or where drill body configuration inhibits zero rake, the angle is near zero and negative.

Comparative tests were carried out drilling various hard plate materials using firstly a known drill bit type as described above having 450 entry angle and different rakes and secondly a variety of drills in accordance with this invention having different entry angles, with and without a radius and with negative rake or zero rake. The following tables set out the results. In these tables the cutting performance is rated between 1 and 9 with 1 being extremely good and 9 being very poor. The cutting performance was judged on the speed with which the drilling was achieved, the type of swarf exiting from the cut and the ease of starting the cut without drifting. The pressure varied from 1 being a low pressure to 9 being a high pressure as judged by the physical exertion used. All tests were done at a drill rotation speed of 560 rpm.

MATERIAL HARD WELD DRILL TYPE AVERAGE PRESSURE AVERAGE CUTTING 450 angle entry 7 5 with negative rake (prior art) 450 angle entry 6 5 with zero rake 4 (modified prior art) 300 angle entry 1 - 2 1-2 with zero rake MATERIAL MANGANESE STEEL (11-14%) DRILL TYPE AVERAGE PRESSURE AVERAGE CUTTING 450 entry angle 4 - 6 4 - 6 different rakes (prior art) 300 2-4 5 with neg. rake 300 angle 3 3 zero rake 300 angle 3 2 zero rake + radius 270 angle 2 1 zero rake 240 angle 4 4 zero rake 200 angle 3 5 zero rake MATERIAL STELLITE WELD DRILL TYPE AVERAGE PRESSURE AVERAGE CUTTING Split Point 6 - 9 5 450 chisel with neg. rake 300 angle entry 4 - 6 2 - 3 zero rake 300 angle entry 3 3 zero rake 300 angle entry 3 2 zero rake + radius 260 angle entry 2 6 zero rake 200 angle entry 3 5 zero rake MATERIAL SPRING STEEL PLATE - APPROX. 64 ROCKWELL DRILL TYPE AVERAGE PRESSURE AVERAGE CUTTING Split point 7 - 8 5 with 450 angle entry neg. rake (prior art) 300 angle entry 2 2 zero rake

Claims (9)

1. CLAIMS 1. A drill bit comprising a flute having a relatively hard tip in the form of an insert with side faces and angled flanks ending in a chisel edge, in which a groove is cut into each face, at an angle of 20 to 350, so as to cut away part of the original flank cutting edge and chisel edge.
2. 2. A drill bit according to Claim 1 in which each groove is cut at an angle of 22 to 300.
3. 3. A drill bit according to Claim 2 in which each groove is cut at an angle of substantially 270.
4. 4. A bit according to any of Claims 1 to 3 in which each groove has a depth of approximately one third of the thickness of the insert to reduce the length of the chisel edge to approximately one third of its length before the grooves are cut.
5. 5. A bit according to any of Claims 1 to 4 in which each groove is joined to the adjacent insert face which ends in a flank cutting edge, by a radius.
6. 6. A bit according to Claim 5 in which the radius is substantially equal to the thickness of the insert.
7. 7. A bit according to any of Claims 1 to 6 in which the cutting edges produced by the grooves have zero, or near zero and negative, rake angle.
8. 8. A bit according to any of Claims 1 to 7 which resembles a masonry bit.
9. 9. A drill bit substantially as described herein with reference to or as illustrated in the accompanying drawings.