Abrasive tools

An abrasive tool such as a drill bit has a body with a working face (10). Undercut recesses (12) are formed in the working face (10). Cutting inserts (16) which present working ends (20) are located in the recesses (12). Locking means (24) in the recesses engage between the undercuts (14) and formations (22) on the inserts (16). This locks the inserts (16) against withdrawal from the recesses (12).

BACKGROUND TO THE INVENTION 
THIS invention relates to abrasive tools. 
Abrasive tools are known that comprise a body in which are mounted inserts 
which have a wear-resistant or abrasive layer. It is this layer that is 
used to perform a cutting or abrading action. The support for the 
wear-resistant layer may be in the form of a post of squat proportions. It 
is the mounting of the post in the working face of the tool body that can 
present problems to the manufacturer and the user. Until now, such posts 
have been secured to the body by providing a suitable complementally 
shaped recess or socket in the working face, locating the post therein and 
brazing the post to the sides of the socket or recess by means of a 
suitable braze alloy. Most commonly, the post and the recess are 
cylindrical. 
A disadvantage of this method is that residual stresses are induced at the 
circumferential region of the post at the area where maximum bending 
stresses also arise in use. These stresses are of the same order of 
magnitude as the strength of the braze alloy. 
Another disadvantage is that the high temperatures required in the brazing 
cycle tend to cause deterioration of the post, which is usually made of a 
carbide, and also of the abrasive layer. This is especially so when the 
abrasive layer is composed of a diamond composite that is not thermally 
stable. 
It may also happen that the impacts to which the tool are subjected in use 
may cause sufficient rise in temperature to lead to melting of the braze 
and consequent failure of the bond. 
Another method has been to shrink fit the post so that it is mechanically 
held in the recess or socket. The socket may be suitably tapered with its 
cross section decreasing with increasing depth into the body. 
SUMMARY OF THE INVENTION 
A first aspect of the invention provides an abrasive tool comprising a body 
having a working face, at least one recess which is formed in the working 
face and which has at least one side and a wide portion extending beyond 
the side, a cutting insert which presents a working end and which is 
located in the recess, and locking means in the recess that engages the 
wider portion of the recess and the insert, thereby to lock the insert 
against withdrawal from the recess. 
Typically each recess includes an undercut which forms the wider portion. 
In preferred versions of the invention, the locking means comprises a 
deformable locking insert which has been forced into the recess between 
the recess and the cutting insert so as to engage between the undercut and 
the formation on the cutting insert. 
In one preferred embodiment, the undercut is conically shaped, the cutting 
insert has a conically shaped foot portion received in the undercut and 
the locking means comprises a tubular sleeve forced into a position of 
engagement between the undercut and the foot. 
In another embodiment, the cutting insert includes a relatively enlarged 
waist, the recess includes an annular undercut and the locking means 
comprises a tubular sleeve forced into a position of engagement between 
the undercut and the waist. 
In yet another embodiment, the cutting insert includes a laterally 
projecting portion, the recess includes a lateral enlargement and the 
locking means comprises; a strip insert forced into a position of 
engagement between the laterally projecting portion and the lateral 
enlargement. In this case, the recess may include a groove in its side 
wall serving as a keyway for the laterally projecting portion of the 
cutting insert. 
Each of the embodiments summarised above may comprise anti-rotation means 
to prevent rotation of the cutting insert in the recess. Typically, the 
anti-rotation means comprises a key on the cutting insert and a key-way in 
the recess, or vice-versa. 
Another aspect of the invention provides a method of securing a cutting 
insert to the working face of an abrasive tool which has an undercut 
recess for receiving the cutting insert, the method comprising the steps 
of: 
a) inserting, into the undercut recess, a cutting insert which has a 
lateral formation thereon, and 
b) engaging locking means between the undercut and the lateral formation of 
the cutting insert, thereby to lock the cutting insert against withdrawal 
from the recess. 
Step b) may comprise locating a deformable locking insert between the 
cutting insert and the recess and causing the locking insert to deform 
into a position of engagement between the undercut and the lateral 
formation of the cutting insert. Conveniently, the locking insert is in 
the form of a tubular sleeve and the locking insert is caused to deform 
into the position of engagement by application of force thereto using a 
tubular punch.

DESCRIPTION OF EMBODIMENTS 
This invention has particular application to drill bits wherein the inserts 
are studcutters. The studcutter may be any one of various shapes in cross 
section, including but not,limited to square, oval or rectangular or even 
combinations thereof. 
The working end may be of various shapes from flat cylindrical to oval or 
bullet-shaped and may contain indentations or be discontinuous. 
The recesses shaped to accommodate the studcutter may be cast in the 
working surface of the drill crown or may be machined using a dovetail 
cutter in a milling machine, such as a CNC (computer numerically 
controlled) milling machine. Alternatively, round undercut holes may be 
made with a conventional adjustable boring head fitted to any suitable 
milling machine. 
A first embodiment of the invention will now be described with reference to 
FIGS. 1a and 1b of the drawings. 
With reference to FIG. 1a, the working face 10 of a drill crown contains a 
recess 12 which has been undercut to form a flared, in this case conically 
shaped portion 14 of expanded cross section. At the working face 10, the 
recess 12 has a diameter of dimension d. A studcutter 16 has a post with a 
cylindrical portion 18, carrying an abrasive working end 20, and a 
conically shaped foot 22 at the other end. The diameter of the foot is no 
larger than the diameter d and is preferably slightly less. 
In assembling the drill bit, the studcutter 16 is located in the recess 12, 
foot first, so that the foot 22 locates in the flared portion 14 of the 
recess. It will be noted that the relative dimensions of the recess and of 
the studcutter post are Such that a vacant space is left between the 
studcutter post 18 and the walls 26 of the recess. 
Thereafter, a deformable sleeve 24 is positioned around the studcutter and 
is forced axially into the recess 12. With the application of sufficient 
force, the lower portion of the sleeve 24 is caused to expand around the 
foot 22 of the studcutter. In practice, force is applied to the sleeve by 
means of a tubular punch which presses against the upper edge of the 
sleeve and drives the sleeve in a direction into the recess 12. 
To facilitate expansion of the lower portion of the sleeve 24, saw cuts 28 
are formed in the wall of the sleeve from its leading end 30. The sleeve 
material will typically be a relatively pliable material such as mild or 
stainless steel or a plastics material, so that it can be made to deform 
in a manner to conform to the shape of the space existing between the wall 
of the recess and the studcutter post. 
The studcutter assembly is shown in assembled form in the working face of 
the drill bit in FIG. 1b. The lower portion of the sleeve 24 has expanded 
to surround the foot 22 of the studcutter 16 and occupy the undercut, 
flared portion 14 of the recess 12. 
In this position, the sleeve 24 serves to lock the studcutter in the 
working face 10 with its working end 20 exposed and standing proud of the 
working face. Any force exerted applied to the studcutter in a direction 
to pull it from the drill crown will result merely in forces, indicated by 
the arrows in FIG. 1b, being applied to the wall of the recess. The wall 
26 exerts opposing forces on the studcutter and sleeve, thereby locking 
them in position in the recess. 
The studcutter secural method illustrated by FIG. 1a and 1b was tested in 
the laboratory for impact and torque resistance. The test for impact 
resistance was performed using a standard Izod impact tester made by 
Avery. Using the Izod tester, a pendulum-suspended impactor chisel was 
swung transversely so as to impact against the upstanding portion of the 
studcutter post. In each case, the studcutter post was machined to have a 
flat surface against which the corresponding flat surface of the impactor 
chisel could strike fully. The pendulum was given 300J of kinetic energy 
for each blow against the cutter post. 
Three conventional brazed assemblies were tested alongside four assemblies 
according to FIG. 1a and 1b and measurements were made of the energy 
required to break the studcutter post. The post of the three conventional 
assemblies broke on absorption of 18J, 22J and 23J respectively. Of the 
four assemblies of the present invention, two of the posts fractured at 
47J and 110J respectively. Even though impacted twice, the other two posts 
could not be broken at all, indicating that these posts could twice absorb 
the full 300J of available energy without fracture. 
Overall it was clear that studcutter posts secured in accordance with the 
technique proposed in FIG. 1a and 1b showed a far higher impact resistance 
than the conventionally brazed studcutter posts. 
Torque resistance tests were performed using a torque wrench with a 
suitable adaptor to engage the studcutter post, and measurements were made 
of the torque required to rotate the post in its mounting. 
In a test on a conventionally brazed studcutter post, no rotation was 
observed at a torque of 450 Nm. In a corresponding test on a studcutter 
post mounted in accordance with FIGS. 1a and 1b, rotational slippage was 
observed at 140 Nm. This indicated the desirability of anti-rotation 
devices in the technique of FIGS. 1a and 1b if high levels of torsional 
resistance are considered necessary. 
A second embodiment of the invention is shown in FIG. 2. In this case, the 
studcutter 16 has a post 18 with an end section 22 and a reduced diameter 
neck section 32. In practice, the illustrated shape is achieved by 
appropriate machining of the cylindrical post of a conventional 
bullet-shaped studcutter. Collets 34 are secured to the post 18 in the 
region of the neck section and create a flared foot for the post as 
illustrated. The collets may be secured in the illustrated positions by 
means of a suitable adhesive. 
The embodiment of FIG. 2 is then secured in a recess in the working face of 
a tool by means of a sleeve like the sleeve 24 used in the first 
embodiment. As before, the studcutter is therefore locked against 
withdrawal from the recess. Once again, it is anticipated that 
anti-rotation measures will be required if high levels of torsional 
resistance are to be provided. 
A third embodiment of the invention is illustrated in FIGS. 3a and 3b. 
Referring to these Figures, the post of a studcutter 40 has a thickened 
waist portion 42 between its end 44 and the abrasive working end 46. The 
recess 48 in the working face 50 of a drill bit has a similar shape. 
In use, the studcutter post is inserted into the recess 48 and an 
expandable sleeve 52, similar to the sleeve 24 of the first embodiment, is 
then forced into position around the post 40 and the thickened waist 
portion 42 to lock the studcutter post in the recess. 
As in previous embodiments, it is expected that anti-rotation measures will 
be necessary if high levels of torsional resistance are to be provided. 
A fourth embodiment of the invention is illustrated by FIGS. 4a, 4b, and 
4c. Referring to these Figures, a studcutter 60 has a post 62 having an 
abrasive working end 64. Close to the opposite end 66 is a projection 68. 
A recess 70 formed in the working face 72 of a drill bit includes an 
enlargement or "bubble" 74 formed at its lower end. The bubble 74 is 
shaped to accommodate the projection 68 on the post 62. A groove 76 
extends from the working face 72 to the bubble 74. 
In use, the studcutter post 62 is inserted into the recess 70. During the 
insertion process the groove 76 acts as a keyway to guide the projection 
68 into the bubble 74. Thereafter a strip insert 78 is forced down the 
groove 76 and around the projection 68, as shown in FIG. 4b, to lock the 
studcutter post in the recess. 
In this embodiment, the engagement between the projection 68 and the bubble 
74, via the insert 78, acts to prevent rotation of the studcutter post in 
the recess. Thus this embodiment can be expected to have a higher 
torsional resistance than the earlier embodiments. 
It will be appreciated that it would be a simple matter to include an 
anti-rotation device of the type illustrated in FIGS. 4a to 4c in the 
embodiments described previously, or in fact other anti-rotation measures, 
if high levels i of torsional resistance are necessary in a particular 
application. 
Note that the exposed top surface of the sleeve or strip in the relevant 
embodiments can be provided with an abrasive protective layer. Note also 
that the abrasive working end of each cutting insert is preferably 
provided by a diamond compact.