Root-strength drill bit and method of making

An improved drill bit having a shank of larger diameter than the body of the drill has an intermediate section rearward of the point section of the drill of smaller diameter than the point, and a longitudinally elongated, enlarged diameter root section joining the rear end of the intermediate section to the shank. The reduced diameter of the intermediate section provides a clearance space, preventing rubbing of the wall of a hole drilled by the bit, while the enlarged diameter root section increases the resistance of the bit to breaking. An improved method of making the enlarged-root drill bit includes inclining a relieving wheel at a substantially smaller angle to the longitudinal axis of a blank than the angle at which the blank was fluted. In one embodiment having a front tapered tip made by the novel method, rearwardly directed teeth are formed at the rear plane of the drill point, by fluting and relieving steps alone, without requiring an additional machining step.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to twist drills and methods for their manufacture. 
More particularly, the invention relates to an improved type of drill bit, 
and to a novel method for making the improved drill. 
2. Description of Background Art 
Twist drills in a wide range of sizes have long been used in the 
fabrication of a large variety of manufactured products. One industry 
which now employs prodigious quantities of twist drill bits, or drills, is 
the electronics industry. The drills are used chiefly to make holes in 
printed circuit through which the leads of electronic components are 
inserted. Typical printed circuit boards used in relatively simple 
consumer electronic equipment may require the drilling of 100 to 5,000 
holes, in sizes ranging from 0.004 inch to 0.250 inch in diameter. More 
complex consumer electronic equipment, and industrial and military 
electronics apparatus, may employ many printed circuit boards, some of 
which may require drilling as many as 50,000 holes of various sizes 
through the board. 
The drilling of printed circuit boards imposes some rather special demands 
on drill bits used for that purpose. 
First, the small size of the holes which must be drilled in printed circuit 
boards requires that drills as small as 0.004 inch in diameter be used to 
make the holes. Thus, even when made of a high strength material such as 
steel or carbide, drills of such small diameter tend to break relatively 
easily. The costs of purchasing replacement bits and the additional costs 
resulting from production down-time for drill replacement, could be 
reduced if breakage of small drill bits could be made infrequent. A second 
demand placed on drills used in the manufacture of printed circuit boards 
arises from the nature of the materials used to fabricate the boards. A 
significant percentage of printed circuit boards is fabricated from a 
composite material. For example, some printed circuit boards are made of 
thermosetting resin such as expoxy, in which glass fibers are imbedded. 
The glass fibers are hard and abrasive. Thus, drills intended for use with 
such printed circuit boards must be made from a very hard material, such 
as high-carbon steel, or carbide. Otherwise, the drills would dull 
quickly, requiring excessively frequent replacement or re-sharpening. 
Manufacturing drill bits from a hard or refractory material such as 
high-carbon steel or tungsten carbide solves the problem of providing a 
drill which can form an acceptable number of holes in fiberglass-expoxy 
printed circuit boards before requiring re-sharpening. However, varying 
the composition of the drill to make it harder has the undesirable side 
effect of making the drill more brittle, at least with any presently known 
materials. Thus, making a drill harder to avoid premature dulling makes 
the drill more susceptible to breakage. 
A third demand placed on drills used for printed circuit board fabrication 
arises from another characteristic of i the materials from which printed 
circuit boards are made. The frictional heat generated by the drilling 
process is often sufficient to melt some of the resin matrix of a printed 
circuit board. The melted resin has a tendency to flow radially inwards 
into a freshly drilled hole, a problem referred to as "resin smear." Then, 
if a drill bit having a uniform body diameter, or one which tapers to a 
somewhat smaller body diameter near the front edge or cutting lip of the 
drill, is withdrawn from the hole, the body of the drill rubs the wall 
surrounding the hole. Rubbing the hole wall during drill withdrawal can 
cause the hole diameter to be non-uniform, and can also result in broken 
drill bits and additional wall heating and resin smear. 
To prevent side wall rubbing, most drill bits used in the printed circuit 
board industry are "back tapered." In a back tapered drill, the body of 
the drill tapers rearward to a small diameter "root", i.e., junction with 
larger diameter shank which is held in the chuck of the drilling machine. 
But the smaller diameter of the root makes the drill more susceptible to 
breakage at that point. 
Some printed circuit board drills are undercut to provide a space or relief 
between the body of the drill and the wall of a newly drilled hole. In an 
undercut drill, a substantial portion of the length of the body of the 
drill rearward of the tip is of smaller diameter than the tip of the 
drill. Again, the reduced diameter root of such undercut drills makes them 
more susceptible to breakage than drills which are front tapered, or are 
of uniform diameter. 
Jeremias, in U.S. Pat. No. 4,480,952, Nov. 6, 1984, Non-burring Drill For 
Composite Materials, discloses a drill having a pointed blade-like tip 
portion extending forward from a cylindrical base having the same diameter 
as the shank of the drill. An annular groove rearward of the base having 
transverse forward and rearward wall surfaces forms therewith a right 
angled, circular cutting edge. The blade-like tip portion is provided for 
initiating a hole in a composite workpiece. The circular, rear facing 
cutting edge is provided for removing fibers projecting from the walls of 
the hole upon retracting the drill through the hole. Neither the Jeremias 
patent, nor any other prior art which the present inventor is aware of, 
discloses a drill bit for printed circuit boards which provides a 
satisfactory solution to the problems described in the previous 
paragraphs. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide an improved drill bit, or 
drill, of increased root strength, thereby making the drill more resistant 
to breakage. 
Another object of the invention is to provide an improved drill in which 
the body of the drill tapers rearward to a larger diameter, longitudinally 
elongated root section at the junction of the body with the shank, and 
which also tapers forward to a larger diameter tip, thereby locating the 
smallest diameter portion of the drill intermediate the root section and 
the tip. 
Another object of the invention is to provide an improved drill having a 
reduced diameter body to minimize hole-rubbing or withdrawal, yet having 
an enlarged diameter root section which affords greater strength. 
Another object of the invention is to provide an improved drill having a 
front tapered rear root section, and a front-tapered, undercut tip. 
Another object of the invention is to provide a method for manufacturing a 
drill having a body which tapers to a smaller diameter progressing 
rearward from the tip, and thence to a larger diameter rearward to the 
root. 
Another object of the invention is to provide a method for manufacturing a 
drill having a front tapered tip, a reduced diameter intermediate body 
portion, a front tapered root section, and an undercut, reverse cutting 
tip, in which a separate undercutting step is not required. 
Another object of the invention is to provide a drill having a back tapered 
tip, a body rearward of the tip which tapers rearward to a minimum 
diameter and thence tapers rearward to a larger diameter, forward-tapered 
root section, and a method for making same. 
Various other objects and advantages of the present invention, and its most 
novel features, will become apparent to those skilled in the art by 
perusing the accompanying specifications, drawings and claims. 
It is to be understood that although the invention disclosed herein is 
fully capable of achieving the objects and providing the advantages 
described, the characteristics of the invention described herein are 
merely illustrative of the preferred embodiment. Accordingly, I do not 
intend that the scope of my exclusive rights and privileges in the 
invention be limited to details of the embodiments described. I do intend 
that equivalents, adaptations and modifications of the invention 
reasonably inferable from the description contained herein be included 
within the scope of the invention as defined by the appended claims. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention comprehends an improved drill bit, or 
drill, and a novel method of making the drill. While the drill is intended 
to overcome certain problems encountered in drilling printed circuit 
boards and other composite materials, the drill and method of making have 
a wider utility which will be apparent to those skilled in the art. 
A major structural feature of the novel drill according to the present 
invention is a body section which has a reduced diameter intermediate 
portion which tapers positively to a large diameter, longitudinally 
elongated portion, or root section, forward of the root. The larger 
diameter root section improves the breakage resistance of the drill, while 
the reduced diameter of the intermediate section permits the drill to be 
withdrawn from a drilled hole without scuffing or rubbing the wall 
surrounding the hole. In preferred embodiments of the drill according to 
the present invention, the intermediate section of the body is of uniform 
diameter, or tapers rearward negatively a slight amount, to a minimum 
diameter neck intermediate the tip and root section of the drill, and then 
tapers rearward positively to join the larger diameter root section. 
The novel method for making improved drills according to the present 
invention utilizes fluting and relieving wheels that are inclined at 
different angles to a drill bit blank. In prior art drill fabrication 
methods, a cylindrical blank is rotated about its own axis at a rate of 
about 1 rpm, while being ground by a circular fluting wheel whose 
rotational axis is inclined to the longitudinal axis of the blank. While 
the fluting wheel is being driven about its own axis at a relatively high 
speed of 3,500 rpm. to 5,000 rpm., it is simultaneously advanced along a 
line parallel to the longitudinal axis of the blank at a linear speed of 
approximately 1 to 2 inches per minute. Linear movement of the fluting 
wheel begins at the tip and advances rearward towards the shank of the 
drill. Linear motion of the fluting wheel parallel to the longitudinal 
axis of the drill blank, in combination with rotation of the blank about 
its own longitudinal axis, results in a helically disposed spiral groove 
or flute being formed in the body of a drill blank. The ratio between the 
angular rotation rate of the blank about its axis to the linear speed of 
the fluting wheel relative to the blank, is chosen to yield the desired 
helix angle. Typically, two diametrically opposed helical flutes are cut 
in the cylindrical wall surface of the drill blank in two separate cutting 
steps. 
A helical band defined at one side by an edge of a helical flute, and at 
another side by a parallel line lying in the uncut cylindrical body of the 
drill blank a short distance inwards from the intersection of the flute 
with the uncut body, is referred to as the margin of the drill. A 
two-fluted drill will of course have two such margins. 
In conventional drill manufacturing methods, portions of the drill body 
initially uncut during the fluting operation and lying between margins are 
ground to a smaller diameter by a relieving wheel. The difference between 
the smaller diameter relieved portions and the uncut margins is referred 
to as body clearance, and is provided to reduce wall rubbing. 
In conventional drill fabrication methods, the rotational axis of the 
relieving wheel is typically inclined at a very slightly smaller angle to 
the longitudinal axis of the drill blank than was the inclination of the 
fluting wheel. Thus, while a typical fluting wheel inclination might be 35 
degrees, the inclination of the relieving wheel might be 30 degrees to 35 
degrees. 
In the novel drill bit manufacturing method according to the present 
invention, the relieving wheel is inclined at a substantially smaller 
angle than the fluting wheel angle. Thus, if the fluting wheel inclination 
were 35 degrees, the relieving wheel inclination according to the present 
invention would be 20 to 25 degrees. This novel reduction in relieving 
wheel inclination relative to fluting wheel inclination results in the 
formation of a drill having an undercut intermediate portion rearward of 
the tip, and two sharp, rearwardly directed pyramidally shaped cutting 
teeth, one extending rearward from the intersection of each margin with 
the conically-shaped front point surface.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a conventional prior art drill bit is shown to 
facilitate a description of the difference between the present invention 
and the prior art. 
As shown in FIGS. 1A and 1B, a typical prior art drill bit 20 has a front 
tapered, conically shaped point 21, a body 22 of generally uniform 
diameter, and two spiral or helically disposed flutes 23 cut in the 
cylindrical wall surface of the body and extending longitudinally rearward 
from point 21 to a location slightly forward of a circular transverse 
plane referred to as a root 24. Body 22 tapers radially outward in a 
frusto-conically shaped transition section that extends rearward from root 
24 to join a shank 26 of larger diameter than body 22. 
As shown in FIG. 1B, two lips or cutting edges 27 are formed in point 21. 
Margins 28 are formed at junctions between tip 27 and the cylindrical wall 
of body 22, by grinding or relieving the body. 
FIG. 2 illustrates a problem encountered in drilling multi-layer printed 
circuit boards with conventional drills of the type shown in FIG. 1. 
As shown in FIG. 2, a multi-layer printed circuit board 30 consists of 
copper-foil conducting layers 31 sandwiched between insulating layers 32 
of fiberglass-filled expoxy resin. Subsequent to the drilling of a hole 
33, a layer of copper 34 is deposited on either or both upper and lower 
board surfaces 35 and 36, respectively. Copper layer 34 is also deposited 
on the inner cylindrical wall surface 37 of hole 33. Thus formed, copper 
layer 34 has the shape of an eyelet which forms a conducting path, or 
"VIA" 38 between various conducting layers 31 of the PC board 30. 
As shown in FIG. 2, blobs of melted resin 39 can sometimes form in wall 37 
of a hole 33 drilled with a conventional FT/FT drill of the type shown in 
FIG. 1. Then, when copper layer 34 is deposited in hole 33, resin blobs 39 
can block electrical conduction between one or more layers 31 and the 
eyelet 38. Usually, the existence of such resin barriers is not revealed 
until electrical continuity tests are performed subsequent to the plating 
process, necessitating costly re-work of a board 30. 
One solution to the problem described above is to use a drill having a 
front-tapered rear transition section, and a back-tapered tip, i.e., an 
FT/BT drill. An FT/BT drill 40 is shown in FIG. 3 being used to drill a 
stack of printed circuit boards 41, which may be single layer or 
multi-layer boards. The back-tapered body 22A of drill 40 decreases the 
length of body that contacts wall surface 37A of hole 33A. Thus, less 
rubbing of surface 37A occurs upon withdrawal of FT/BT drill 40 from hole 
33. This is desirable because less rubbing generates less heat and 
therefore less resin smear. 
Another prior art approach to reducing wall rubbing during drill withdrawal 
is shown in FIG. 4. In FIG. 4, an undercut drill 50 is shown drilling a 
stack of PC boards 41. Undercut drill 40 has a front cylindrical body 
section 51 of uniform diameter extending rearward a short distance from 
point 21B. Rearward of front section 51, body 22B of drill 50 is undercut 
to a smaller diameter, which is uniform over the longitudinal span of the 
body rearward to its intersection with transition section 25B at root 24B. 
Since only the relatively short front body section 51 of undercut drill 50 
contacts wall surface 37B of hole 33B upon withdrawal, wall rubbing is 
again reduced. 
As pointed out above, front tapered/back tapered (FT/BT) drill 40 of FIG. 
3, and undercut (FT/UC) drill 50 of FIG. 4 both provide reduced hole-wall 
rubbing, as compared to front tapered (FT/FT) drill 20 of FIG. 1. However, 
both of the latter two drill bits have roots 24A and 24B which are of 
smaller diameter than that of root 24 of an FT/FT drill for the same size 
hole. Thus, prior art drills shaped to reduce wall rubbing are inherently 
weaker, and therefore more susceptible to breakage, than front tapered 
drills. Also, prior art drills of the type shown in FIGS. 3 and 4 and 
described above do not fully solve the problem of melted resin blocking 
conducting paths in multi-layer boards. 
With the limitations of prior art drills of the type described above in 
mind, the novel drill according to the present invention and depicted in 
FIGS. 5 and 6 was conceived of. 
As shown in FIGS. 5 and 6, one embodiment 60 of a novel drill according to 
the present invention has a conically shaped point 61, and a body 62 which 
may be viewed as having been formed from a front tapered cylinder or 
frusto-conic body 62A depicted by dashed lines 62B. A short distance 
rearward of tip 63 of point 61, body 62 of drill 60 is undercut by a 
transversely disposed, annular wall 64. Drill 40 has an intermediate 
section 65 of appreciable length and generally cylindrical shape which 
extends rearward from transverse annular rear wall 64 of point 61. 
Helically disposed flutes 65A are formed in the cylindrical wall surface 
of intermediate section 65. 
The rear end of intermediate section 65 of drill 60 terminates in a 
junction section 66 which tapers radially outwardly in an arcuate curve to 
intersect a circumscribed frusto-conic surface 66A of a larger diameter 
than the intermediate section. The circular intersection line defined 
above defines the front circular boundary of a root section 67. The outer 
surface of root section 67 is coextensive with the frusto-conic surface 
66A. The rear boundary of root section 67 is defined by a circular 
intersection line 68 marking the intersection of the root section with a 
transition section 69. Transition section 69 is of frusto-conic shape and 
has an outer wall surface which tapers linearly rearward to a larger 
diameter shank 70. 
As described above and depicted in FIGS. 5 and 6, the novel design of drill 
60 permits the root section 67 of drill 60 to be of substantially larger 
diameter than the root of prior art drills of the proper size to drill the 
same size holes. Accordingly, drill 60 has substantially greater 
resistance to breakage than prior art bits. Also, the increased diameter 
of root section 67 of drill 60 relative to prior art drills, and the 
longitudinal elongation of the root section, increases the rigidity or 
deflection resistance of the novel drill bit. Thus, novel drill bit 60 is 
less likely to wobble, and is therefore less likely to make holes which 
are crooked, out-of-round, or improperly sized. 
As shown in FIGS. 5 and 6, the novel drill 60 according to the present 
invention may desirably be provided with rearwardly directed, 
pyramid-shaped reverse cutting teeth 71. Such cutting teeth are effective 
in removing resin blobs 39 of the type described above in conjunction with 
FIG. 2. A novel method of fabricating a novel drill bit 60, in which 
reverse cutting teeth 71 may be formed without requiring a separate 
undercutting operation, is described below. 
FIGS. 7A, 7B and 7C illustrate the steps required to fabricate a 
conventional prior art undercut drill by conventional prior art methods. 
As shown in FIG. 7A, a reduced diameter body portion 22B is first ground in 
a length of drill rod to form a blank. Then, as shown in FIG. 7B, the 
blank is ground to produce a cylindrical front section 51 and a reduced 
diameter intermediate section. FIG. 7C illustrates an undercut drill which 
has been fluted and relieved in a manner to be described below, and 
subsequently sharpened or pointed to form point 21B. 
FIGS. 8A-8D illustrate the steps in prior art methods of fluting and 
relieving a drill. As shown in FIG. 8A, a grinding wheel 90 rotating about 
its axis 91 is advanced parallel to the longitudinal axis 92 of drill 
blank 93, which is simultaneously rotated about its axis 92. The angle A 
between axis 91 of wheel 90 and axis 92 of drill blank 93 is typically 35 
degrees. 
After drill blank 93 has been fluted as shown in FIG. 8B, the fluted blank 
is ground by a relieving wheel 94 as shown in FIG. 8C. The angle B between 
axis 95 of relieving wheel 94 and axis 92 of drill blank 93 is typically 
chosen to be slightly smaller than angle A. Thus, a typical value for 
angle B would be about 30 to 35 degrees. FIG. 8D illustrates the 
appearance of drill blank 93 which has been fluted and relieved, but not 
yet pointed. 
FIGS. 9A-9D illustrate the steps in fluting and relieving a drill according 
to the novel method of the present invention. 
As shown in FIGS. 9A and 9B, the first step in fluting a drill according to 
the method of the present invention is substantially similar to the prior 
art method depicted in FIGS. 8A and 8B. 
Thus fluting wheel 90 is inclined at an angle C of about 35 degrees .+-.2 
degrees with respect to longitudinal axis 92 of drill blank 93. 
According to the method of the present invention, relief grinding of drill 
blank 93 is done substantially differently from prior art relieving-wheel 
grinding. Thus, as shown in FIG. 9C, relieving wheel 99 is inclined at a 
substantially smaller angle D with respect to longitudinal axis 92 of 
drill blank 93. A typical value of angle D suitable for the purposes of 
the present invention would be about 22 degrees .+-.2 degrees. While the 
ranges for angle C and angle D stated above are believed to be optimum, it 
is important to note that other mean values and ranges for angles C and D 
may be useful, and would be within the scope of the present invention, 
provided angle D were appreciably different from angle C. 
After a drill bit blank 93 has been fluted and relieved as depicted in 
FIGS. 9A-9D and described above, the front end of the drill blank is 
ground into a conically shaped point 61 in a conventional fashion. 
FIGS. 5 and 6 show a drill bit 60 fabricated by the novel method of the 
present invention. As shown in those figures, the method of the present 
invention can produce a drill bit 60 that is not only undercut, but which 
also has a pair of rearwardly directed, wedge or pyramid-shaped cutting 
teeth 71 extending rearward from the intersection of the two diametrically 
opposed margins 100 with front conical surface 101 of point 61. 
Importantly, rear cutting teeth 71 are formed solely by the fluting and 
relieving steps according to the present invention, and do not require an 
additional machining operation. Each of the two cutting teeth 71 has an 
upper triangular surface 102 coextensive with the front tapered front 
section 103 of drill 60. Each cutting point 71 also has a pair of 
trapezoidal-shaped side walls 104, and a rear triangular-shaped wall 105. 
FIG. 10 illustrated another embodiment 110 of an increased root-strength 
drill according to the present invention. Embodiment 110 is of the FT/BT 
variety, in which transition section 119 and root section 117 are both 
front tapered, while front section 133 is back tapered.