Method for shanking rock bits

A method for shanking a rock bit utilizing a shank locator and a cone locator, each with tapered surfaces to engage respectively the tail stock of a lathe and the cones of a rock bit, to accurately align the body and the cones of the bit concentric with the design centerline of the bit. The lathe and cone locators each have interior conical surfaces to assure accurate, coaxial alignment of the shank end and the cone end of the bit. A compensating chuck is used to grip the cone end of the bit body and firmly hold the normally uneven surfaces while the threads are being machined.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to earth boring bits and in particular to 
methods for threading bits after assembly and welding of the sections of 
which they are composed. 
2. Background Information 
Earth boring bits of the rolling cone type are often called "rock bits", 
even though some can drill geological formations softer than those 
commonly classified as rock. The rock bit has many of the features found 
in the original two cone bits of Howard R. Hughes, U.S. Pat. No. 930,759, 
Aug. 10, 1909. 
The body of a typical, contemporary bit is constructed of three sections, 
assembled to engage across 120 degree, machined faces and welded to form 
an integral body unit. A cone is mounted, prior to welding, on a 
cantilevered bearing shaft that depends from each section. 
In recent years attention has been focused on improvements to the welding 
and assembly procedures to improve the geometry, the dimensional accuracy 
and the integrity to design of the rock bit. 
A method to improve the accuracy of the assembly and welding of the 
sections of a rock bit is show in U.S. patent of George W. Baur, U.S. Pat. 
No. 4,209,124, Rock Bit Assembly Method, June 24, 1980. Here, a shank 
positioner and a fixed ring are positioned with a fixture to hold the 
sections and assembled cones to minimize slippage of the sections during 
assembly and welding. 
Another method to improve integrity to rock bit design parameters is 
disclosed in my previous U.S. Pat. No. 4,414,734, Triad For Rock Bit 
Assembly, Nov. 15, 1983. A specially shaped dowel called a triad is used 
to maintain the 120 degree faces of the sections in the intended 
relationship. This method further assures accuracy of alignment of the 
sections during assembly and welding. 
Still, there is need for additional accuracy, especially in machining the 
threads on the shank of bits for accurate alignment of the sections and 
cones with the design centerline or rotational axis of the bit. Even 
though the sections of the bit are accurately aligned and welded, 
performance of the bit suffers if the threads of the shank are misaligned 
from the design centerline. The resulting eccentric rotation of the bit 
can produce accelerated wear of the cones and of the supporting bearings. 
SUMMARY OF THE INVENTION 
The general object of the invention is to provide an improved method for 
machining the threads on the shank of a rock bit such that the threads are 
more nearly coaxial or homocentric with the design centerline of the cones 
and the bit body. 
Accordingly, the invention is an improved method of threading the shank of 
a rock bit that includes the formation of a positioner such as a dowel 
hole in each head section the bit, welding the head sections into an 
integral body after assembly with their cones, positioning the bit in a 
cone locator in a compensating chuck, placing a shank locator in the 
positioners in the head section, inserting the tail stock of a lathe in 
the shank locator, energizing the compensating chuck to clamp the bit, 
removing the tail stock, machining the threads and releasing the 
compensating chuck and the bit. The cone locator and the shank locator 
have centering configurations, such as a tapered or conical interior 
surface that assures accurate, coaxial alignment with the design 
centerline. 
The above as well as additional objects, features and advantages of the 
invention will become apparent in the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The numeral 11 in the drawing designates a lathe used in this instance to 
machine threads on the shank end 13 of a rock bit 15 having a plurality of 
cones 17 for rotation upon and drilling through geological formations. As 
shown in FIG. 1, the bit 15 is held in a horizontal position by tongs 19 
above the lathe by a hoist (not shown). 
While in the horizontal position, the bit 15 is positioned accurately on 
the centerline of the lathe 11, with the cones 17 and body of the bit 15 
located concentric therewith. Accurate location of the bit 15 on the 
rotational axis of the lathe is achieved by the use of a shank locator 21 
on the shank end 13 of the bit, and a cone locator 23 on the cone end of 
the bit. 
As best seen in FIG. 2, the shank locator 21 is adapted to receive the 
tapered end 25 of the tail stock 27 of the lathe 11. Protruding from the 
shank locator 21 are three positioners, here dowels 29, one for each of 
the three sections which constitute the body of the most common, three 
cone type of rock bit shown in the drawings. Although not illustrated in 
the drawings, each of the sections of the bit 15 has a 120 degree machined 
face that mates with a similar face on the other sections. When 
subsequently welded after assembly of the faces, an integral bit body is 
formed, as illustrated in FIGS. 1 and 2. 
The construction of the shank locator 21 is better shown in FIGS. 4 and 5, 
consisting of an annular body 31 having a tapered or conical interior 
surface 33, three dowels 29 and a handle 33. In the cross sectional view 
of FIG. 5 the angle B is preferably thirty degrees and matches conical 
surface 25 of the tail stock 27. The use of these two matching surfaces, 
and accurately drilled dowel holes in each of the sections of the body of 
the bit 15, positions the shank end of the bit accurately on the 
rotational axis of the lathe 11. 
The cone locator 23 has a conical interior surface 35 at an angle A of 
preferably fifteen degrees, as may be best seen in the cross sectional 
view of FIG. 2 and also in the enlargement of FIG. 3. The outermost or 
heel teeth 37 engage the conical or tapered surface 35, which positions 
the cones 17 of the bit in identical relationships with the bearings upon 
which they are supported. As a result, the cone end of the bit, as well as 
the shank of the bit are accurately positioned on the rotational axis of 
the lathe. 
In view of the above description of apparatus the steps of the method may 
be more fully understood. Each forging or casting of which the head 
sections of the bit are composed is processed by machining a dowel hole in 
the upper end of the shank, which will subsequently mate with one of the 
dowels 29 in the shank locator 21 in a accurate and predetermined 
relationship to the machined 120 faces. 
Each of the head sections of the bit has a bearing formed by various 
procedures such as machining, the application of metallurgical treatments 
and grinding. The dowel holes described above are formed accurately with 
respect to these bearing surfaces, as well as the 120 degree faces of each 
head section. 
Cones 17 are assembled on the bearing surfaces of each head section with 
one of the known fastener means, such as the prior art ball retainers or 
resilient rings. 
Then, the 120 faces of the head sections are aligned and assembled by an 
accurate technique such as using a triad as disclosed in the previously 
mentioned U.S. Pat. No. 4,209,124. 
The head sections are then welded to form an integral body, using one of 
the known welding methods such as the electron beam process. 
Next, the welded bit is positioned such that the heel teeth 37 of the cones 
17 engage the conical or interior surface 35 of the cone locator 23. This 
urges the cones 17 to identical positions on their respective bearing 
shafts and, assuming accurate processing of the cones and the bearings on 
which they are supported, positions the design centerline of the cone end 
of the bit on the rotational axis of the lathe. 
The shank locator 21 is positioned such that the dowels 29 mate with the 
dowel holes formed in the upper end of the head sections of the bit. Then 
the tail stock 27 of the lathe and its tapered end 25 is urged against the 
conical surface 33 of the shank locator. This assures that the shank end 
of the bit and its design centerline is accurately positioned 
concentrically with the axis of the lathe. 
Once the bit is accurately positioned through use of the cone and shank 
locators, the jaws of a compensating chuck are energized to grip the body 
of the bit 15. A compensating chuck is one in which the chuck jaws grip 
the workpiece equidistant from the center of the chuck, through 
utilization of an actuator that permits the jaws to float and engage 
independently of the chuck center. One satisfactory compensating chuck is 
manufactured by N. A. Woodworth Company of Detroit, Mich., under the 
trademark "Universal Ball-Lok". 
After chucking the bit, the threads are turned by a cutting tool in the 
prior art manner, and then the jaws of the compensating chuck are released 
and the bit removed. 
It should be apparent from the foregoing description that I have provided 
an invention having significant advantages. The use of a method employing 
the above described shank locator 21 and cone locator 23, with a 
compensating chuck, enables more accurate machining of the threads of the 
shank of a rock bit with respect to the design centerline. This assures 
that the bit will rotate when drilling about an axis that coincides with 
that of the drill collars and drill pipe, avoiding damage to the bit that 
can occur when there is eccentric rotation. 
While I have shown and described my invention in only the preferred form, 
it should be apparent that it is not thus limited, but is susceptible to 
various changes and modifications without departing from the spirit 
thereof.