Composite sintered twist drill

A helically fluted twist drill apparatus in which offset, opposed veins of sintered abrasive particulate, such as diamond, are embedded within a drill blank made of a less abrasive material such as carbide. The non-aligned veins of abrasive material, in one embodiment, themselves intersect through juxtapositioning adjacent the point and web of the drill, and in another embodiment, rely upon the interpositioning of a third intermediate vein which, in turn, intersects at least part of both the first and second nonintersecting veins, to describe a more economical and effective highly abrasive material region at the drill point and web locations.

BACKGROUND 
The present invention relates to drilling tools and more particularly to 
helically fluted twist drills larger than approximately 0.125 inches in 
diameter. 
Helically fluted twist drills are the most commonly used drilling tools and 
are required to perform severe machining operations under extremely 
adverse conditions. The cutting end of a helically fluted twist drill 
includes a pair of cutting lips on opposite surfaces of an intermediate 
web, the width of which is typically 12 to 20% of the diameter, and a 
chisel edge extending obliquely across the center of the web. Such drills 
are typically long and slender and the helical flutes constitute a column 
eccentricity that further reduces rigidity under axial thrust load. 
The concept of oppositely directed cutting surfaces at one extremity of a 
slender shaft which is both axially and torsionally loaded creates 
conflicting material demands in the construction of the drilling tool. The 
material of the cutting lips should be as hard as possible to cut the 
workpiece and as heatresistant as possible to maintain a cutting edge at 
elevated temperatures. At the same time the material of the body and shaft 
must be both rigid and tough to resist deflection and to hold up under the 
loadings imposed. These varying requirements have resulted in compromises 
in material selection, since hard materials tend to be brittle, while 
tough materials tend to wear easily. 
To obtain an optimum combination of characteristics, i.e., hardness and 
wear-resistance at the cutting surfaces and toughness and rigidity of the 
body and shaft, it has been proposed to form the cutting surfaces of one 
material and the body and shaft of another. This has resulted in a variety 
of combinations, such as tungsten carbide or diamond inserts or tips on 
carbon steel or carbide shafts. These combinations, while individually 
useful, have a common disadvantage, i.e., the braze connection between the 
insert or tip and the shaft. Tungsten carbide can be soldered or brazed 
directly to the steel or carbide shaft. However, a diamond tip or insert 
must first be adhered to a carbide substrate which is in turn soldered or 
brazed to the shaft. Diamond particles are typically formed into a compact 
and bonded to a carbide substrate with a metallic catalyst in a high 
pressure/high temperature (HP/HT) press. However, at atmospheric pressure 
the metal which catalyzes the bonding of the diamond particles to each 
other and to the substrate in the press will also catalyze the 
back-conversion of diamond to graphite at temperatures above 700.degree. 
C., causing disintegration of the compact. Accordingly, a low temperature 
solder or braze connection is used to attach the substrate to the shaft. 
This braze connection limits the effective life of such drilling tools, 
since it is softer than either the substrate or the shaft. The braze thus 
becomes the weakest point of the tool construction and the limiting factor 
in the tool usage. 
In co-pending applications Ser. No. 515,777 and Ser. No. 793,202, a process 
is disclosed for depositing a vein of diamond particles in a groove in one 
extremity of a cemented carbide shaft. With this process the particles are 
bonded directly to each other and directly to the carbide material of the 
shaft, such that the connection between the particles and the carbide 
becomes the strongest part of the drilling tool. The process as disclosed 
has particular applicability to printed circuit board drills which have 
diameters of approximately 0.006 to 0.125 inches and in which the vein 
occupies the full width of the web which may be from 0.0012 to 0.030 
inches wide. However, the process has not been applicable to large drills 
since cracking of the particle mass of the vein is encountered at vein 
widths of approximately 0.030 inches and above. 
SUMMARY OF THE INVENTION 
The present invention avoids the problems and limitations of the prior art 
known processes and products by providing a composite sintered twist drill 
and process of manufacture which is particularly suited for helically 
fluted twist drills larger than approximately 0.125 inches in diameter. 
This construction facilitates the use of a maximum quantity of materials 
having the optimum strength characteristics for the shaft and structural 
parts of the drill along with a minimum quantity of materials having the 
optimum hardness characteristics for the cutting surfaces of the drill. 
The hardest material is located in narrow veins at the leading edges of 
the web and across the mid line of the web and is bonded directly to the 
material of the web.

DETAILED DESCRIPTION OF THE DRAWINGS 
The present invention adapts the formation of veins of abrasive particles 
bonded directly to the web to large diameter drills while retaining the 
advantage of greatly improved effective life realized in printed circuit 
board drills. In limiting the width of the veins and locating them at the 
leading edges of the web, extremely hard cutting edges are obtained. At 
the same time only the minimum quantities of abrasive particles are 
employed, with consequent savings in cost. 
Referring to FIG. 1 of the drawing, an embodiment of the invention is 
illustrated which is particularly applicable to drills with webs of up to 
approximately 0.030 inches wide. As shown, flutes 11 and 12 define the 
lands 13 and 14 as well as the web 15. Oppositely directed cutting 
surfaces or lips are located on the exposed linear edges of the web. 
Narrow elongated veins 16 and 17 of sintered abrasive particles are 
imbedded in the margins of the web at the leading edges thereof and form 
the actual cutting surfaces or lips. The veins 16 and 17 extend inwardly 
from the circumference to the respective lands along chords which are 
equally spaced from a diameter of the drill and overlap at the midpoint of 
the web. The widths of the veins are selected so that veins overlap 
laterally and axially at the midpoint to form a short vein section across 
the full width of the web at the midpoint. 
A generally cylindrical cemented carbide blank 21, FIG. 2, having a 
diameter approximately equal to that of the desired drill and a length of 
two or three times the diameter, is provided with a conical end 22 and a 
flat end 23. The material of the blank is preferably a tungsten carbide 
composition and is selected with emphasis on such characteristics as 
transverse rupture strength, toughness, ease of brazing or welding and 
ease of grinding, rather than wear resistance. As shown in FIG. 3, the 
conical end 22 is provided with a pair of grooves or slots 24 and 25 
approximately 0.010 to 0.020 inches wide and approximately 0.030 to 0.050 
inches deep. Diameter 50 of carbide drill blank 21 together with mid-axes 
24a and 25a, of vein slots 24 and 25 respectively, are additionally shown. 
Mid-axes 24a and 25a are substantially parallel to each other and, are in 
turn parallel to diameter 50 passing between vein regions 24 and 25, with 
each of the vein regions positioned on an opposite side of the drill blank 
diameter passing therebetween. The slots can be formed with a die when the 
blank is molded or by grinding of the finished blank. The slots extend 
from the periphery along parallel, spaced chords past the central axis of 
the blank so that the interior extremities overlap a short distance. A 
well blended mixture of abrasive particles, such as diamond or cubic boron 
nitride, is then packed firmly into the slots so as to completely fill 
them. The abrasive particles are preferably 1-3 micron, but can be as 
large as 6-12 micron. The blank is placed in a refractory metal can which 
is then capped with a refractory metal disc and vacuum outgassed. The can 
is then placed in a HP/HT press and subjected to pressures of 45 Kbar to 
75 Kbar and temperatures of 1200.degree. C. to 1600.degree. C. for 
approximately 1-10 minutes to sinter the abrasive particles. Apparatus and 
techniques for such sintering are disclosed in U.S. Pat. Nos. 2,941,248; 
3,141,746; 3,745,623; and 3,743,489 (incorporated herein by reference). 
When the abrasive mixture is well sintered with particle-to-particle 
bonding and is bonded directly to the cemented carbide, the can is removed 
from the press. The refractory metal is removed from the composite blank 
26 and the flat end 23 is ground to an angle of approximately 30.degree. 
to 45.degree. with the central axis to provide an enlarged attachment 
area. The composite blank 26 is then secured to an elongated cemented 
carbide or tool steel shaft 27, FIG. 8, such as by a high temperature 
braze connection. The composite blank and the adjacent portion of the 
shaft are then helically fluted and finish-ground to form the body 28, 
FIG. 9, of the desired twist drill. 
An alternative embodiment of the invention, which is applicable to larger 
diameter drills, is shown in FIG. 4. Similar to FIG. 1, the flutes 31 and 
32 defined the lands 33 and 34 as well as the web 35. Narrow elongated 
veins 36 and 37 of abrasive particles are imbedded in the margins of the 
web at the leading edges thereof and form the cutting surfaces or lips of 
the drill. The interior extremities of the veins are joined by a disc or 
plug 38 formed of sintered abrasive particles which is concentric with the 
central axis of the drill. A generally cylindrical blank 39, FIG. 5, of 
cemented carbide having two flat ends and a diameter approximately equal 
to that of the desired drill is provided with a central bore 41 in one 
end. A pair of oppositely directed slots or grooves 42 and 43, FIG. 6, are 
ground or otherwise formed along spaced parallel chords of the blank 
extending inwardly from the periphery and tangent to the bore 41. The 
material surrounding the bore is then ground at a uniform angle to form a 
frusto-conical end on the blank. The slots are approximately 0.010 to 
0.020 inches wide, and the slots and bore are approximately 0.030 to 0.050 
inches deep. As illustrated in FIG. 7, the slots 42 and 43 do not extend 
parallel to the longitudinal axis of the blank, but instead, converge 
toward the longitudinal axis at pre-determined angles A and B. The angles 
A and B are equal to each other and to the helix angle of the fluting. 
When the veins of particles are formed in the slots the veins extend into 
the material of the blank at the helix angle. As the blank and shaft are 
fluted the veins are exposed along their full depth. Fluting of the blank 
and the shaft is thus simplified since the hard material of the vein is 
not removed during the fluting process. While the bore, conical end and 
slots are described as drilled and ground, they may be formed in the blank 
during the molding process. As with the embodiment of FIG. 1, the slots 42 
and 43 and the bore 41 are firmly packed with a blended mixture of 
abrasive particles and sintered in a HP/HT press at 45 Kbar to 75 Kbar and 
1200.degree. C. to 1600.degree. C. for 1-10 minutes to bond the particles 
to each other and to the carbide of the blank. The composite blank is then 
secured to a shaft and the blank and shaft helically fluted and finished 
to form the twist drill.