Method of forming closely sized openings

A method of forming precision openings in hardened steel workpieces is disclosed. The method generally includes drilling nominal size openings in the workpiece while it is in a soft condition. Thereafter, sizing plugs formed from a material having a strength greater than that of the steel body during a predetermined range of temperatures are press-fitted in the openings. With the plugs in position, the body is heat treated to harden it. Subsequently, the plugs are removed and the resulting openings are found to be of an extremely uniform size without the wide variations present when hardening is performed without the sizing plugs.

BACKGROUND OF THE INVENTION 
The subject invention is directed generally toward the metal working art 
and, more particularly, to a method of forming holes or recesses to close 
tolerances in high hardness steel. 
The invention is especially suited for use in manufacturing percussion 
drilling bits of the types used in rock drilling and mining operations and 
will be described with particular reference thereto; however, the 
invention is capable of broader application and could be used to produce 
roller cone bits, polycrystalline diamond compact (PDC) bits, or similar 
bits of the type wherein the cutting is performed by hard material inserts 
carried in a steel body or whenever it is desired to produce a high 
hardness steel body having closely toleranced holes or recesses formed 
therein. 
One type of bit often used in rock drilling operations comprises a steel 
drill body having a generally cylindrical shank. The axial end face of the 
body is provided with a multiplicity of cylindrical openings into which 
are press-fitted cylindrical inserts of sintered tungsten carbide or the 
like. The outer ends of the carbide inserts have a generally hemispherical 
shape and extend outwardly of the end face of the steel body. The inserts 
perform the actual cutting or drilling of rock through an abrading or 
localized crushing operation. 
As drilling takes place, the inserts are slowly worn away and bit life is 
generally dictated by the life of the inserts. In certain rock formations, 
however, the nature of the rock is such that the rock dust and particles 
generated during the drilling operation erode the face of the steel bit 
body. This erosion of the body can proceed significantly faster than the 
wear on the inserts and bit failure takes place not because of insert wear 
but due to loss of support for the inserts. This wear and loss of support 
is generally most severe with respect to the outer or "gage row" inserts. 
In an effort to overcome the noted problem, various attempts have been made 
at increasing the hardness of the steel body. However, such attempts have 
not been commercially successful. For example, heat treating the body to 
high hardness prior to forming the insert openings makes forming the 
openings an extremely time consuming operation since conventional 
drilling, or drilling and boring operations cannot be used. Rather, slower 
and more expensive processes such as electrical discharge machining and/or 
grinding must be employed. 
Similarly, attempts at forming the insert openings while the steel body is 
in a soft or fully annealed state and thereafter heat treating the body to 
high hardness have been unsuccessful. The reason for this has been that 
the holes do not retain their required tolerances following the heat 
treating. That is, after heat treating there are found to be relatively 
wide and varying differences in the diameters of holes which prior to heat 
treating were of uniform diameter. As a consequence, the carbide inserts 
cannot be properly press-fitted in the openings. 
As is apparent from the foregoing, a distinct need exists for a method of 
forming the required openings to close tolerances while also providing a 
drill body of high hardness. 
SUMMARY OF INVENTION 
According to the subject invention, a method is provided which overcomes 
the noted problems and allows the economical production of high hardness 
steel bodies with precision sized openings therein. 
In particular, in accordance with one aspect of the invention, the method 
generally comprises providing a steel body capable of being heat treated 
to a relatively high level of hardness. While the body is in a machineable 
hardness range of up to approximately 42 on the Rockwell C scale (HRC), 
the required openings are formed in the body. Thereafter, plugs having a 
configuration to fill the formed opening are placed in the openings. 
Preferably the plugs are formed of a material having a high strength which 
is retained throughout subsequent heat treating temperatures and a 
coefficient of thermal expansion which is different than that of the steel 
body. 
With the plugs in place, the steel body is heat treated to take its 
hardness to a significantly higher level. Following the heat treating, the 
plugs are removed. 
With use of the method described, the resulting openings are found to have 
a size which is uniform and within desired tolerance ranges. The effect of 
the plugs is to perform a "sizing" operation on the openings and to hold 
the openings to a size substantially equal to that of the plugs as the 
body goes through the normal heat treating steps. 
In accordance with a more limited aspect of the invention, the openings are 
cylindrical and the plugs are press-fitted into the openings while the 
body is at a first relatively low temperature. Following a heat treating 
process of heating and quenching, the body, with the plugs in place, is 
tempered to take the body to a hardness level below maximum hardness but 
still well above its fully annealed hardness. The plugs are removed 
following tempering but, preferably, while the body is still at a 
temperature well above the first relatively low temperature at which they 
were originally inserted. 
As is apparent, the subject method allows the required openings to be 
formed using conventional drilling or drilling and boring operations while 
the steel body is in a soft condition. In addition the resulting product 
has both high hardness and closely toleranced openings ready for further 
use, such as the press-fitting of inserts, without requiring additional 
machining or grinding. 
Accordingly, a primary object of the invention is the provision of a method 
of forming products of the type described simply and economically. 
Another object is the provision of a method of economically producing 
percussion-type drill bits and similar drill bits in which the bit body 
has a high hardness and precision sized insert receiving openings. 
A still further object is the provision of a method of the type described 
which can be practiced using conventional manufacturing and heat treating 
facilities.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring more particularly to the drawings wherein the showings are for 
the purpose of illustrating a preferred embodiment of the invention only, 
and not for the purpose of limiting same, FIG. 1 shows, in somewhat 
diagrammatic pictorial form, a percussion-type rock drill bit 10. The 
illustrated bit 10 is merely typical of the general class of bits or tools 
to which the subject method is particularly suited. Broadly, the bit 10 
comprises a steel bit body 12 having a mounting shank 14 and an integral 
head portion 16. As is conventional, the shank 14 has a generally 
cylindrical configuration and may be provided with one or more slots or 
spline ways 18 which function to mount the bit in the associated chuck of 
a pneumatic drill motor, not shown. Such bits normally include 
longitudinally extending passages 20 to permit air or other pressurized 
fluid to be supplied to the cutting face 22 of the bit to flush rock dust 
and particles out of the hole being drilled. Similarly, one or more 
flushing channels 24 are normally provided about the periphery of the head 
16 to allow the flushing air and rock dust and particles to exit upwardly. 
The actual cutting or drilling is performed by a multiplicity of hard 
material cutting inserts 26 press-fitted into precision bored holes formed 
inwardly of the cutting face 22. Typically, the cutting inserts 26 are 
formed of sintered tungsten carbide and have a precision ground outer 
surface of a diameter to be tightly received and held in the openings 28 
formed in the cutting face 22. Additionally, as best shown in FIG. 2 the 
inserts 26 normally have a somewhat domed or semi-spherical outer end 
portion 27 which extends beyond the cutting face 22. 
As mentioned earlier, the cutting operation is performed by the outwardly 
extending hard material cutting inserts 26. The bit body primarily 
functions as a support for the cutting inserts and, normally, the life of 
the bit is determined by the life of the individual inserts 26. Under 
certain conditions, however, the bit body 12 is subjected to significant 
erosion and wearing by the rock dust and particles generated by the 
cutting operation. In certain rock formations the erosion is such that the 
bit life is severely limited by such erosion and the body 12 wears away at 
a rate significantly faster than the inserts resulting in loss of support 
for the inserts and failure of the bit even though the inserts themselves 
are still serviceable. To overcome this difficulty it has been desirable 
to make the drill body 12 as hard as possible to resist this erosive wear. 
The previous approaches and difficulties encountered in attempting to 
provide a hard steel bit body 12 have been discussed. 
In accordance with the subject invention this problem is overcome by 
forming the bit 10 in accordance with the sequence of operations generally 
shown in FIG. 3 and described hereinafter. In particular, according to the 
preferred embodiment of the subject method, a bit body 12 having the 
general configuration desired is provided and formed from a steel which is 
capable of being heat treated to a significantly high hardness level. In 
the preferred embodiment a steel such as AMS 6418 is used for forming the 
body 12. Other steels having the same general characteristics, such as 
AISI 4340, B.S. No. 970EN214, EN27 and EN30 or other alloyed medium carbon 
steels could also be used within the scope of the subject invention. The 
steel drill body in its machineable hardness range (e.g. up to 
approximately 42 HRC) is first drilled to provide the openings 28 in the 
cutting face 22. The holes 28 are drilled to a nominal size approximately 
equal to, or preferably fractionally less than, the desired final size 
used for the inserts 26 which are to be subsequently inserted. For 
example, holes for the percussion bit of FIG. 1 are drilled at a nominal 
size of 0.6255 inches. 
After the drilling operation is completed, plugs such as a plug 32 (FIG. 4) 
are press-fitted into the drilled openings 28. The plugs 32 are sized so 
as to completely fill the openings 28 and require at least some moderate 
force for the press-fitting step. In the subject embodiment, the plugs 
range in diameter from 0.6255 to 0.6278 inches. In addition, the plugs are 
formed from a material which is stronger than the steel of body 12. 
Preferably, for hot sizing, the material from which the plugs are made 
should have a coefficient of thermal expansion that is greater than the 
steel of body 12; however, other materials having high strength but 
coefficients of thermal expansion lower than the steel of the body can be 
used. Moreover, it is preferable that the plugs 32 retain their strength 
throughout the heat treating steps subsequently to be described. Yet 
another desirable characteristic is to form the plugs of a material that 
will not metallurgically react with the steel body. 
Many different materials could be used for forming the plugs 32. 
Specifically, however, plugs of sintered tungsten carbide have been used 
successfully and many commercially available ceramic materials as well as 
some hardenable high alloy steels could also be used. In still other 
arrangements, a slurry material, such as a ceramic, can substantially fill 
the holes and provide hot sizing thereof in a similar manner. As used in 
this application, the term plug will also encompass a non-rigid material 
such as a slurry that is placed into the holes for hot sizing. 
After the plugs have been inserted into the openings 28 and the entire bit 
body 12 with the plugs in the openings is heat treated to significantly 
increase the level of hardness of body 12. Specifically, the body and the 
plugs are raised in temperature to a point at or above the transformation 
temperature of the steel forming the body and held there for a period of 
time and subsequently quenched. Excellent results have been achieved with 
the AMS 6418 steel mentioned earlier by raising the body and plugs to 
1600.degree. F. and holding at that temperature for 2 hours followed by a 
conventional oil quench. 
Subsequent to the heat treating operation it is preferable to temper the 
entire assembly by raising it to a temperature in the range of 400.degree. 
F. for a period of 3 hours followed by air cooling. 
Following the noted treatment, the resulting steel body 12 has a hardness 
in the range of 47 to 50 on the Rockwell C scale. After the tempering step 
the plugs 32 are removed from the openings 28. The resulting openings 28 
in the hardened steel body 12 are found to have uniformly constant 
diameters without the tolerance variations encountered when an attempt was 
made to harden such bodies without the use of the noted plugs. In 
particular, with the starting hole and plug sizes mentioned above, the 
finished openings are extremely close to the diameter of the plugs. 
Removal of the plugs 32 is generally quite simple and often the plugs can 
merely be manually removed from the body. Under certain circumstances, 
however, it may be necessary to heat the body up into the 400.degree. 
tempering range in order to easily extract the plugs. At that temperature 
the difference in the coefficient of expansion of the plugs and the body 
facilitates removal of the plugs. 
Yet another approach would maintain the holes at the nominal size of 0.6255 
inches. Austenitic stainless steel plugs would then be machined to a 
nominal diameter of 0.6240 inches. Placement of these plugs is easily 
performed because of the reduced plug diameter. These stainless steel 
plugs exhibit a lower strength than the steel forming the body at room 
temperature. Additionally, the stainless steel plugs have a coefficient of 
thermal expansion greater than the steel body material and the plugs will 
not metallurgically react with the steel body material. 
As the AMS 6418 steel body and stainless steel plugs are raised to a 
hardening temperature of approximately 1600.degree. F., the strength of 
the steel body lowers at a faster rate than the strength of the stainless 
steel plugs. The higher coefficient of expansion of the plugs permits the 
plugs to expand at an accelerated rate in comparison to the expansion of 
the steel body. In fact, upon reaching the hardening temperature the 
stainless steel plugs will have a higher strength than the steel body and 
sizing will result since the diameter of the plug will be 0.0018 inches 
larger than the drilled hole diameter. Thereafter, cooling to room 
temperature reduces the stainless steel plug to its original diametrical 
dimension of 0.6240 inches. On the other hand, the drilled hole in the 
steel body has been hot sized to 0.6273 inches, thus facilitating ease of 
removal of the plugs. 
With the plugs removed and the drill body 12 in the high hardness 
condition, the drilling inserts 26 can be press-fitted into the openings 
in the usual manner. The resulting drill bit 10 thus has a full hard drill 
shank and head with the inserts 26 firmly mounted therein. The method thus 
allows this desired combination to be achieved without the necessity of 
machining or grinding the openings 28 in the body while it is in the hard 
condition. 
The invention has been described with reference to preferred and alternate 
embodiments. Obviously, modifications and alterations will occur to others 
upon the reading and understanding of this specification. It is intended 
to include all such modifications and alterations insofar as they come 
within the scope of the appended claims or the equivalents thereof.