GB2302664A

GB2302664A - Rotary percussive drill bit

Info

Publication number: GB2302664A
Application number: GB9513137A
Authority: GB
Inventors: Anthony Roy Small
Original assignee: Armeg Ltd
Current assignee: Armeg Ltd
Priority date: 1995-06-28
Filing date: 1995-06-28
Publication date: 1997-01-29
Anticipated expiration: 2015-06-28
Also published as: GB9513137D0; GB2302664B

Abstract

A rotary percussive drill bit of cruciform shape has an aligned pair of arms 14 provided with facets 22A, 22B between a major tip insert 12 and gashes 19, the included angle * small Greek beta * between the facets 22A, 22B on each arm being substantially less than the cone angle * small Greek alpha * of the residual cone areas 21X on the other pair of arms 15 of the cruciform.

Description

ROTARY PERCUSSIVE DRILL BIT This invention relates to a rotary percussive drill bit of the type known as a crosshead bit in which the cutting end of a spiral drill rod has a cruciform cross-section without any twist, the leading end of which is generally conical with a cone angle similar to the included angle between leading cutting edges of a major tip insert secured in a cross-slot in one aligned pair of arms of the cruciform, the radially outer ends of the other pair of arms being provided with recesses in which are secured minor tip inserts having a similar included angle between their leading cutting edges, and the first-mentioned aligned pair of arms (which incorporate the cross-slot and the major tip insert) being appreciably thicker than the other pair of arms so as to provide adequate lateral support for the major tip insert (which plays the major role in any drilling operation) even after machining of the gashes between the adjacent arms of the cruciform that break into the spiral grooves of the drill rod.

During a drilling operation, and especially one in which the direction of drilling is downwardly, there tends to be an accumulation of drilling spoil between the residual cone areas of the leading end of the cruciform and the bottom of the hole being drilled, which accumulation "cushions" the drill bit and reduces its cutting efficiency.

The object of the present invention is to provide a rotary percussive drill bit of the type described above with which the abovementioned "cushioning" is alleviated.

According to the present invention, a rotary percussive drill bit of the type described is characterized in that the first-mentioned aligned pair of arms of the cruciform are provided with facets between the major tip and the gashes, the included angle between the facets on each arm being substantially less than the cone angle of the residual cone areas on the other pair of arms of the cruciform.

The resulting appreciable inclination of the facets away from the major tip improves removal of spoil into the gashes (and thence into the spiral grooves) thus diminishing the "cushioning" effect and so increasing the cutting efficiency substantially.

The leading facets (i.e. with respect to the direction of rotation of the drill) are advantageously substantially coplanar with the respective leading faces of the cutting edges of the major tip, to promote smooth flow of spoil from those cutting edges to the gashes.

During manufacture, the facets are preferably machined after machining the gashes and before machining the cross-slot or before inserting the major tip insert.

A prior art embodiment of rotary percussive drill bit and an embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an end view of the prior art embodiment; Figure 2 is a fragmentary side elevation seen in the direction of the arrow A in Figure 1; Figure 3 is a fragmentary side elevation seen in the direction of the arrow B in Figure 1, and Figures 4 to 6 correspond to Figure 1 and upper parts of Figures 2 and 3 but show an embodiment of the present invention, by way of example only.

In the prior art embodiment of rotary percussive drill bit shown in Figures 1 to 3, of the type known as a crosshead bit, the cutting end of a spiral drill rod 10 has a cruciform cross-section without any twist, the leading end of which is generally conical with a cone angle CC similar to the included angle (of 1200) between leading cutting edges 11 of a major tip insert 12 secured in a cross-slot 13 in one aligned pair of arms 14 of the cruciform, the radially outer ends of the other pair of arms 15 being provided with recesses 16 in which are secured minor tip inserts 17 having a similar included angle between their leading cutting edges 18, and the pair of arms 14 being appreciably thicker than the other pair of arms 15 so as to provide adequate lateral support for the major insert 12 even after machining of the gashes 19 between the adjacent arms of the cruciform that break into the spiral grooves 20 of the drill rod.

It will be appreciated that during a drilling operation, and especially one in which the direction of drilling is downwardly, there tends to be an accumulation of drilling spoil between the residual cone areas 21 and the bottom of the hole being drilled, which accumulation "cushions" the drill bit and reduces its cutting efficiency.

In contrast, in the rotary percussive drill bit in accordance with the present invention shown in Figures 4 to 6, and in which like reference numerals represent like parts to those in Figures 1 to 3, the first-mentioned aligned pair of arms 14 of the cruciform are provided with facets 22A, 22B between the major tip 12 and the gashes 19, the included angle between the facets 22A, 22B in each arm being substantially less than the cone angle 0( of the residual cone areas 21X on the other pair of arms 15 of the cruciform.

The resulting appreciable inclination of the facets 22A, 22B away from the major tip 12 improves removal of spoil into the gashes 19 (and thence into the spiral grooves 20) thus diminishing the "cushioning" effect and so increasing the cutting efficiency substantially.

The leading facets 22A (i.e. with respect to the direction of rotation of the drill) are substantially coplanar with the respective leading faces 23A of the cutting edges 11 of the major tip 12, to promote smooth flow of spoil from those cutting edges to the gashes.

The angle of the trailing facets 22B matches the angle of the respective trailing faces 23B of the cutting edges of the major tip, the included angle ss between the facets on each arm 14 (and between the corresponding faces of the respective cutting edge) being in this instance 900.

Claims

1. A rotary percussive drill bit of the type described characterized in that the first-mentioned aligned pair of arms of the cruciform are provided with facets between the major tip and the gashes, the included angle between the facets on each arm being substantially less than the cone angle of the residual cone areas on the other pair of arms of the cruciform.

2. A rotary percussive drill bit, as claimed in Claim 1, wherein the leading facets (i.e. with respect to the direction of rotation of the drill) are substantially coplanar with the respective leading faces of the cutting edges of the major tip.

3. A rotary percussive drill bit, as claimed in Claim 1 or Claim 2, wherein the facets are machined after machining the gashes and before machining the cross-slot or before inserting the major tip insert.

4. A rotary percussive drill bit substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.