Note: Descriptions are shown in the official language in which they were submitted.
~L~99509
--1--
FLAT BOTTOM HOLE DRILL
... __ . . . .
ABSTRACT OF TH~ ;DISC~OSURE
.
BACKGROUND OF THE INVENTI:ON
Drills frequently are used to provide cylindrical
holes in metallic workpieces. As is well known, the
cutting or boring action of the drill is carried out
by an elongated substantially cylindrical member~ One
end of the drill member is securely mounted in the driving
apparatus of the drill assembly which rotates the cutting
member about its longitudinal axis. The opposed end of
the eIongated cutting member includes at least one cutting
edge. A flute extends away from the cutting edge and
provides a channel for removal of chips and stock that are
produced as the hole is drilled.
A twist drill is one type of elongated cutting
member commonly used on both wood and metallic stock.
Briefly, the twist drill provides a plurality of cutting
edges disposed symmetrically about the perimeter o the
drill and extending generally along the longitudinal axis
thereof. A flute extends from each cutting edge and
typically extends helically around the perimeter of the
cutting member, hence the term twist drill. Although
the cutting edges of the twist drill extend toward the
longitudinal axis, they terminate a distance away from
the axis.
The structural features of the twist drill have
both operational advantages and disadvantages. A
principle advantage is that the cutting forces are
evenlydistributed acros~ the dril]. Thus, in many
:, i
~199S09
materials, the twist drill will provide a precise hole,
with each cutting edge contributing an equal amount to
the cutting effort. More specifically, each cutting
edge will generate chips or helical strips of stock
material having a width equal to the radius of the drill,
and a thickness that is a function of the feed rate of
the drill into the stock divided by the number of cutting
edges. A principal disadvantage of the twist drill is
that the portion of the drill between the lon~itudinal
axis and the innermost point of the cutting edges performs
no cutting function. Consequently, the innermost portion
of the material to be drilled is urged into the area of
the cutting edges by an axial force exerted on the drill
itself. In effect, the drill must be pushed into the
material being drilled so that the material in the
centermost portion of that hole can be urged into the
path of the cutting edges.
The axial force required can be reduced by
minimizing the distance between the longitudinal axis
and theinnermostpoint of each cutting edge. Of course,
this distance cannot be reduced to zero because the
material immediately surrounding the longitudinal axis
provides the principal axial support for the twist drill.
The amount of axial force can be reduced somewhat further
by chamfering the central portion to encourage the
movement of the stock material into the area of the
cutting edges. Although these structural features
minimize the axial force required to advance the twist
drill, the axial forces are significant enough to make
the twist drill inefficient and ineffective for stocks
made of hard metallic substances, such as titanium. The
exertion of these axial forces not only leads to an
inefficient use of energy in a drilling operation, but
also contributes to excessive wear on the twist drill
itself. Thus) twist drills must be replaced frequently
thereby resulting in a substantial amount of down time
for the entire drilling assembly.
:
9~09
A conventional "gundrill" is structurally
distinct from the twist drill and, in some respects,
is more desirable for drilling holes in hard metals.
Briefly, the gundrill includes only one cutting edge
and one flute. The single cutting edge extends from
a point on the periphery of the gundrill through the
longitudinal axis and to a point intermediate the
opposite peripheral surface of the gundrill and the
central axis thereof. The cutting edge terminates at
an apex that is offset from the longitudinal axis of
the gundrill so that the cutting end of the gundrill
resembles an asymmetrical cone. The gundrill also
includes a flute extending generally in an axial
direction from the cutting edge to allow for removal
of chips of stock that are cut by the gundrill.
In operation, the portion of the gundrill extending from
the longitudinal axis to the periphery accomplishes all
of the cutting. Additionally, because the cut extends
the full length`of the radius, stock material does not
have to be forced toward the cutting edge, and the axial
force required is less than that required when using a
twist drill. Due to this center cutting feature, the
gundrill is more readily adapted to drilling holes in
hard metals and has been widely employed in certain
metal cutting applications such as drilling a hole
in a gun barrel.
Although the single cutting edge and single
flute structure of the gundrill provides an efficient
center cut, it also results in an imbalance of forces
during a drilling operation. This imbalance is
particula~l~ critical during the early phases of a
drilling operation when the gundrill initially penetrates
the stock. More particularly, because the gundrill has
only one cutting edge, there is no symmetrical cutting
surface to balance the forces exerted by the stock
against the single cutting edge. The imbalance of forces
S~5~
with the gundrill frequently causes the gundrill to wobble
which, in turn, causes a ~Irunout~ phenomena Thus, the
walls of the hole bored by the gundrill are not parallel
to one another, but rather, bulge outwardly, particularly
near the entry point to the hole.
To offset the inherent runout effects of a
gundrill, gundrills are generally fabricated with wear
pads which are adapted to bear against the sidewall
surface of the drilled hole, and thereby function to guide
the gundrill after its initial entry. Also, to minimize
runout at the entry point of the hole, bushings frequently
are employed to properly guide the gundrill into the
desired location. The bushings are located on the surface
of the stock and surrounding the area to be drilled.
Thus, the wear pads bear against the bushing and improve
the alignment of the hole upon initial entry of the
gundrill into the stock. Nevertheless, despite the
use ofwear pads and bushings, the imbalanceof forces
inherent in the gundrill design frequently causes
misalignments that exceed the tolerances of many workpiece
specifications. This is particularly likely to happen
in workpieces made of very hard metallic substances. To
attain the proper tolerances in these workpieces, a
second reaming operation is often required. However,
this reaming operation, like the use of bushings, is
extremely time-consuming, in-efficient, and costly.
In certain drilling operations, it is
necessary to drill very precise holes for a short distance
into a very hard material. Additionally, it often is
desirable to provide a hole with a bottom that is
substantially planaror flat and which is generally
perpendicular to the side walls of the hole. More
specifically, some design tolerances require the actual
diameter of the hole to be in the range of -.0005 inches
to ~.0015 inches of the specified diameter. Also, in
many specifications, the bottom surface of the hole may
be non-planar provided the bottom surface includes a
~9~S09
shoulder or ledge adjacent to and perpendicular to the
side walls of the hole and extending a distance inwardly
therefrom approximately equal to one half the radius of
the hole. This non-planar bottom surface of the hole is
acceptable only if the distance between the central
portion and the peripheral shoulder portion of the hole
bottom is very small.
Holes with the above cited specifications are
required, for example, on the cutting head portion of an
oil well drilling appaxatus. In this type of apparatus,
a head constructed from a very hard material, such as hard
steel AISI 4340, has a plurality of precise shallow holes
drilled therein to accept bits that will cut through rock
during an oil drilling operation. Twist drills are
currently used by certain tool manufacturers to make these
holes. However, because of the extremely hard stock
material, the twist drills have a very short life, and
it often is necessary to use a reamer before achieving
the desired precision hole. This, of course, is extremely
expensive. It is also known to utilize gundrills for
these purposes. However, as mentioned above, the gundrill
is least precise during its initial penetration of the
workpiece. Consequently, it is extremely difficult to
obtain a precisely drilled shallow hole with a gundrill.
Therefore, to achieve these design specifications with a
gundrill, it is necessary to utilize a complex arrange-
ment oE bushings and also to subsequently ream the hole
after the drilling operation.
Accordingly, it is an object of the subject
invention to provide a drill than can precisely bore
holes in a variety of materials.
It is another object of the subject invention
to provide a drill that is particularly useful for
drilling precise shallow holes in extremely hard material.
It is a further object of the subject invention
toprovide a drill that can drill a hole have a
substantially flat bottom that is substantially perpendicular
~l9~SO~
--6--
to the side walls of the hole.
It is an additional object of the subject
invention to provide a drill that operates so as -to
balance the cutting forces during a drilling operation.
It is yet another object of the subject
invention to provide a drill that will reliably remove
chips of stock material from the drilled hole.
It is an additional object of the subject
invention to provide a drill that can precisely drill
holes without the use of bushings or the like.
It is still another object of the subject
invention to provide a drill that can precisely drill
holes without requiring a subsequent reaming operation.
It is a further object of the subject
invention to provide a drill capable of drilling through
hard metallic materials and which has a long life in
operation.
.
SUMMAR~ OF THE INVENTION
-
The subject invention is directed to a flat
bottom drill tha`t is structurally distinct from both the
twist drill and the gundrill. Although the subject drill
can be used to drill any size hole in any material, the
advantages of this invention over prior art drills are
particularly apparent in drilling shallow holes (e.g.,
less than one inch) in extremely hard material such as
titanium or very hard alloy steels (e.g., 35 to 40
Rockwell).
Structurally, the subject flat bottom drill
includes opposed primary and secondary cutting edges
extending from which are primary and secondary flutes.
The primary cutting edge extends across the longitudinal
axis of the drill from a point near the axis to a point
on the opposite side of the axis, approximately midway to
the periphery. The primary cutting edge is perpendicular
to the longitudinal axis of the drill, and defines one
axial end of the drill. The outermost portion of the
~19~5t~
radius on which the primary cutting edge is disposed is
offset in the longitudinal direction of the drill with
respect to the primary cutting edge so that it performs
no cutting function, as explained in greater detail below.
The primary flute defines a substantially pie-shaped wedge
removed from the body of the drill. This pie-shaped flute
decreases in radial depth as the flute extends axially
away ~rom the primary cutting edge in order to increase
the strength of the web supporting the cutting edges.
The secondary cutting edge of the subject flat
bottom drill is also perpendicular to the longitudinal axis
of the drill but is located on the side of the drill
opposite the primary cutting edge. Additionally, the
secondary cutting edge extends from the periphery of the
drill at least to a point midway between the periphery
and the longitudinal axis. The secondary cutting edge is
axially separated from the primary cutting edge so that
it follows the primary cutting edge into the hole. As
discussed below, the separation along the longitudinal
axis of the drill between the primary and secondary cutting
edges is a function of the feed rate of the drill.
In operation, the primary cutting edge cuts an
annular central section of the resulting hole extending
from the axis of the resulting hole to a point approximately
midway between the longitudinal axis and the periphery of
the hole. The secondary cutting edge follows the
primary cutting edge into the hole and cuts a section
extending from the periphery of the resulting hole to
a point approximately midway between the periphery and
the center. Thus, the primary cutting edge cuts the
center part of the hole and the secondary cutting edge
cuts the outer radial part of the hole. The relative
lengths of the primary and secondary cutting edges can be
varied according to the material being drilled, but are
selected to provide substantially equal and opposite forces
on the opposed cutting sides of the drill. Therefore, the
subject flat bottom drill of the subject invention provides
. .
~199S09
balanced forces that are not attainable by a conventional
gundrill. Furthermore, as mentioned abbve, the primary
cutting edge of the subject drill extends across the
longitudinal axis thereof. Consequently, unlike the
twist drill, the subject flat bottom drill provides a
center cutting section. This feature of balanced forces,
combined with a center cutting action, enables the
subject drill to drill an extremely precise hole into a
hard metallic stock.
The subject flat bottom drill differs
significantly from the twist and gundrills in that the
combination of primary and secondary, or inner and outer,
cutting edges ensures that the maximum chip width will be
equal only to approximately a quarter of the diameter of
the drilled hole. In both of the prior art drills
described above, the chip width generally equals the
radius of the drill. The wider chips provided by the prior
art drills have a tendency to jam in the flutes thereby
imbalancing the forces on the drill and contributing to the
runout phenomena described above. In the subject drill,
by reducing the size of the chips channel by each flute by
approximately 50~, the chips are more readily removed from
the flutes, thereby ensuring that the opposed forces on the
subject drill will remain balanced.
As explained above, both the primary and
secondary cutting edges of the subject drill are perpen-
dicular to the longitudinal axis of the drill. In most
instances, the axial separation between the primary and
secondary cutting edges will be very small. Therefore/
the hole drilled by the subject drill will have a bottom
surface that is substantially planar and perpendicular to
the side walls of the hole. More specifically~ the central
portion of the bottom surface of the drilled hole will
define a wall that is separated from the outer portion of
the bottom surface of the drilled hole by a distance equal
to the axial separation between the primary and secondary
cutting edges.
5~
BRIEF DESCRIPTION OF TI~E, DRAWINGS
Figure 1 is a perspective view of the preferred
embodiment of a flat bottom drill of the subject invention.
Figure 2 is an end view of the preferred
embodiment of the subject invention showing both the
primary and secondary cutting edges.
Figure 3 is a side elevational view of the
subject flat bottom drill taken along the direction
indicated by arrow 3 in Figure 2.
Figure ~ is a side elevational view of the
subject flat bottom drill taken along the direction
indicated by the arrow in Figure 2.
Figure S is a side elevational view of the
subject flat bottom drill taken along the direction
indicated by arrow 5 in Figure 2.
Figure 6 is a perspective view of the second
embodiment of the subject flat bottom drill.
Figure 7 is an end view of the flat bottom
drill shown in Figure 6.
Figure 8 is a side elevational view of the
flat bottom drill shown in Figure 6 taken along the
direction indicated by arrow 8 in Figure 7.
Figure 8 is a perspective view of a third
embodiment of the subject flat bottom drill in which
both the primary and secondary cutting edges are chamfered
to increase the thickness of the central web.
Figure 10 is an end view of the flat bottom
drill shown in Figure 9.
Figure 11 is a side elevational view of the flat
bottom drill shown in Figure 9 taken along the direction
indicated by arrow 11 in Figure 10.
Figure 12 is a perspective view of a fourth
embodiment of the subject flat bottom drill in which a
wall of the primary flute is concave to further facilitate
chip removal~
Figure 13 is a cross-sectional view of a
workpiece having a hole bored by the subject fla-t bottom drill.
~9S(~9
--10--
DETAILED DESCRIPTION OF THE PREFE~RRED EMBODIMENT
~ he subject flat bottom drill, which is designated
generally by the numeral 20 in Figure 1~ is a substantially
cylindrical structure designed to rotate about its
longitudinal axis. Preferably, the 1at bottom drill 20 is
abricated from a high strength medium alloy steel that
is case hardened to be very hard on the outside.
As shown in Figures 2 and 3, flat bottom drill 20
includes a primary cutting edge 22 and a secondary cutting
edge 24. Primary flute 26 is defined by surfaces 30 and
32, which form a generally wedge shaped section removed
from flat bot~om drill 20 and which extends away from
primary cutting edge 22. Surface 30 of primary flute
26 is aligned substantially in a radial direction. Surface
32 or primary flute 36 extends in a non-axial direction so
as to terminate at the peripheral surace 38 o 1at
bottom drill 20. Surfaces 30 and 32 of primary flute 26
intersect at an angle which preerably is in the range of
100 to 200. In the preferred embodiment as shown in
Figures 1-5, this angle equals approximately 110 . By
making the angle of intersection of surfaces 30 and 32
greater than a right angle1 chips of stock material
cut by primary cutting edge 22 are more reliably and
efficiently removed through primary flute 26. Addition-
ally, as shown most clearly in Figures 1 and 3, theportion of surface 32 in primary 1ute 26 nearest primary
cutting edge 22 can be chamered across the centerline
of flat bottom drill 20 to further enhance the channeliz-
ation of chips within primary flute 26. Primary flute 26
must be at least as long as the hole being drilled to
ensure efficient, reliable chip removal.
Turning to Figure 4, secondary flute 28 is
defined by sur~aces 34 and 26. Surface 34 of secondary
flute 28 is substantially parallel to surface 30 of
primary flute 26. Surface 36 of secondary flute 28
extends in a non-axial direction so as to terminate a
peripheral surface 38 of flat bottom drill 20. As
~99S09
--11--
with primary flute 26, surface 36 of secondary flute 28
intersects surEace 34 thereof at an angle that preferably
is in the range of 100 to 120. Secondary flute 28 is
at least as long as the hole being drilled to ensure
proper chip removal, but need not be precisely the same
length as primary flute 26.
As shown in Figures l, 2 and 3, primary cutting
edge 22 has an innermost end point 40 slightly to one side
of the longitudinal axis of flat bottom drill 20. The
lQ outer primary cutting terminus 42 of primary cutting edge
22 is on the opposite side of the longitudinal axis of
flat bottom drill 20 from innermost end point 40, and a
distance approximately halfway between the axis and the
periphery 38 of drill 20. As shown most clearly in Figure
3, primary cutting edge 22 is substantially perpendicular
to the longitudinal axis, and defines the most extreme
cutting end of flat bottom drill 20.
Adajcent to and radially aligned with primary
cutting edge 22 is a non-cutting edge 44 which is separ-
ated in a longitudinal direction from primary cutt1ngedge 22 by step 46. Non-cutting edge 44 extends from
the periphery 38 of flat bottom drill 20 inwardly to
step 46. The longitudinal separation between cutting
edge 22 and non-cutting edge 44 is indicated by distance
"dl" in Figure 3. As explained in greater detai.l below,
non-cutting edge 44 performs no cutting function.
Secondary cutting edge 24 is substantially
parallel to primary cutting edge 22, but is located on
the opposite side of flat bottom drill 20. ~ore
specifically, secondary cutting edge 24 is perpendicular
to the longitudinal axis of flat bottom drill 20, and
extends inwardly from the periphery 38 a distance at least
to the radial distance between the periphery 38 and outer
primary cutting terminus 42 of primary cutting edge 22.
As shown most clearly in Figures 3, 4 and 5,
secondary cutting edge 24 is offset from primary cutting
edge 22 in a longitudinal direction by distance "d2".
S09
-12-
The distance "d2" is a function of both the design
specifications of the desired hole to be drilled and
the feed rate of drill 20. In many applications, the
distance "d2" can be as small as .003 to .004 inches.
It is important to emphasize that the distance "dl",
which is the separation between non-cutting edge 44 and
primary cutting edge 22 measured along the longitudinal
axis is greater than the distance "d2" which is the
separation between secondary cutting edge 24 and primary
cutting edge 22, also measured along the longitudinal
axis of drill 20. Additionally, distance "d3", which
is the separation between secondary cutting edge 24 and
non-cutting edge 44 measured along the longitudinal axis
of drill 20, also is a function of feed rate of drill
20. More specifically, non-cutting edge 44 follows
primary and secondary cutting edges 22 and 24 into an
already drilled hole. sy this unique arrangement,
primary cutting edge 22 cuts the innermost portion of the
hole to be drilled, secondary cutting edge 24 cuts the
outermost portion of the hole to be drilled~ and edge 44
performs no cutting function.
The precise dimensions of primary and secondary
edges 22 and 24 and the separation "d2" measured along
the longitudinal axis will vary according to the
specifications of the material being drilled, but will be
selected to ensure that the forces on both the primary
and secondary cutting edges will be substantially equal.
Thus, the structure provides center cutting by primary
cutting edge 22, and also provides balanced cutting by
both primary cutting edge 22 and secondary cutting edge
24. In this manner, the subject flat bottom drill 20
provides the advantages of both the prior art gundrills
and twist drills with the disadvantages of neither.
In operation, primary cutting edge 22 is the
first portion of drill 20 to contact the workpiece. After
this initial contact, the rotation of drill 20 about its
longitudinal axis causes the portion of primary cutting
~g~505~
edge 22 between the longitudinal axis and its outer
primary cutting terminus 42 to begin cutting the
center portion of the hole. Because the entire portion
of primary cutting edge 22 from the centerline to outer
primary cutting terminus 42 contributes to the cutting
operationr the subject flat bottom drill 20 can be easily
fed into the s~ock material without excessive axial force.
Secondary cutting edge 24 advances into the
stock material after primary cutting edge 22 has drilled
a hole to depth "d2" (e.~g., the separation between the
primary and secondary cutting edges 22 and 24 measured
along the longitudinal axis). After the secondary
cuttin~ edge 24 contacts the stock material, rotation of
drill 20 causes secondary cutting edge 24 to drill the
outermost portion of the hole. As mentioned above,
distance "d2"t is a function of the feed rater and is
selected to ensure that the secondary cutting edge will
perform no cutting operation on the innermost part of the
hole.
Chips removed from the stock material by the
subject flat bottom drill 20 are such that each chip
is approximately half as wide as chips produced by
either gun or twist drills. More particularly, the
chips of stock material produced by the primary cutting
edge 22 will have a width substantially equal to the
distance from the center line to outer primary cutting
terminus 42 of primary cutting edge 22t which is
approximately one half the radius of flat bottom drill
20. ~imilarly, the chips of stock material produced by
secondary cutting edge 24 will have a width equal to
the difference between the radius of drill 20 and the
width of chips produced by primary cutting edge 22. Again,
these chips will be approximately equal to one half the
radius o~ flat bottom drill 20. This cutting pattern
differs signi~icantly from the cutting patterns of both
twist and gun drills, which produce chips having a width
equal to the radius of the drill. By segmenking the chips
~99509
-14-
into approximately equal inner and outer portions, the
subject flat bbttom drill 20 ensures that the chips can
be efficiently and reliabIy removed through primary and
secondary flutes 26 and 28. On the other hand, the chlps
produced by prior art twist or gundrills are wider than
chips produced by the subject flat bottom drill 20, and
therefore, are substantially more likely to jam in the
flutes of these drills. Chip material that is retained,
even temporarily, in the flutes of known drills can cause
an imbalance of forces that could contribute to runout,
thereby creating unacceptable tolerances for many design
specifications. With the subject flat bottom drill 20,
on the other hand, the chips are half as wide as the above
mentioned prior art drills, and therefore are easily
removed through flutes 26 and 28.
Turning to Figures 6, 7 and 8, an alternate
embodiment of the subject flat bottom drill 20 is shown
where the end most corner produced by surface 30 of
primary flute 26 and the periphery 38 of the flat bottom
drill 20 is chamfered to further ensure that the edge
extending radially outward from primary cutting edge 22
will perform no cutting function. This chamfering pro-
duces triangular surface 48 which is angularly related
to both thelongitudinal axis of flat bottom drill 20 and
the peripheral surface 38 thereof. The distance measured
along the longitudinal axis of drill 20 between triangular
surface 48 and primary cutting edge 22 is at all points
greater than the distance between primary and secondary
cutting edges 22 and 24, as measured along the longitudinal
axis of drill 20. Furthermore, the offset between primary
and secondary cutting edges 22 and 24 and triangular surface
48 is a function of the feed rate of flat bottom drill 20,
so that secondary cutting edge 22 only cuts the outermost
portion of the hole, and non-cutting edge 50 of triangular
surface 48 performs no cutting function.
In many applications, the stock material with
which the subject flat bottom drill 20 is used is extremely
~199S09
--15-
hard. To increase the strength and useful life of flat
bottom drill 20 for these particularly demanding
applications, surface 30 of primary flute 26 and surface
34 of secondary flute 28 may be offset away from each
other and away from the longitudinal axis of drill 20,
as shown in Figures 9 through 11. The primary and
secondary cutting edges 22 and 24, however, rema n in
the relative positions explained above by appropriate
chamfering. More specifically, as shown most clearly
10- in Figures 10 and ll, chamfered surface 52 extends
angularly from surface 30 of primary flute 24 to primary
cutting edge 22. Similarly, chamfered surface 54
extends angularly from surface 34 of secondary flute 28 to
secondary cutting edge 24. This embodiment provides a
stronger, wider web portion 56 as is apparent by
comparing Figure 2 to Figure lO or by comparing Figure
5 to Figure 11.
In the embodiment of the subject invention
shown in Figure 12, surface 32 of primary flute 26 and
surface 36 of secondary flute 28 are both p~ovided with
a concave configuration. Preferably, this particular
shape is provided by a pressing operation during the
fabrication of flat bottom drill 20. This concave
configuration of surfaces 32 and 36 is desirable for work
on certain metal stocks to provida a better movement of
chips through the primary and secondary flutes 26 and 28.
Figure 13 shows a typical hole drilled in a
piece of stock 58 by the subject flat bottom drill 20.
Stock material 58 is extremely hard material r such as
titanium or a very hard alloy steel. The cylindrical
side wall 60 of hole 62 is drilled to an extremely tight
tolerance by a single drilling operation with the subject
flat bottom drill 20. For example~ on a one half inch
diameter hole 62, the side walls are provided with a
35 tolerance of -.0005 inches to ~.0015 inches in a single
drilling operation. Hole 62 in stoc~ material 58 is
relatively shallow, such that side walls 60 may be no
~99S09
-16-
more than one-fourth of an inch to approximately two
inches in length.
The bottom of hole 62 is defined by an annular
shoulder 64 and circular central portion 66. The angle
formed by the intersection of cylindrical side wall 60
and annular shoulder 64 is substantially equal to 90.
Central circular portion 66 is substantially parallel
to annular shoulder 64, and therefore also is substant-
ially perpendicular to cylindrical side walls 60. These
perpendicular and parallel relationships result from the
orthagonal alignment of primary and secondary cutting
edges 22 and 24 to the centerline of flat bottom drill
20. The length of well wall 68 extending between
annular shoulder 64 and circular bottom portion 66 equals
distance "d2", which as explained above, is the
separation between the primary and secondary cutting
edges 22 and 24 measured along the longitudinal axis
of the subject drill 20.
For very hard stock materials 58, such as hard
st:eel AISI 4340, the feed rate of flat bottom drill 20
into stock 58 would be relatively low. Hence, the
distance "d2" separating the primary and secondary
cutting edges 22 and 24 would be very small. For
example, in many applications on very h~rd stock materials
58 the axial separation between the primary and
secondary cutting edges 22 and 24 would be in the range
of .003 inches to .004 inches. Therefore, well wall 68
also would be very small and the hole 62 in stock
material 58 would have a nearly planar bottom surface.
During a drilling operation, the cylindrical
portion between dashed lines 70 extending from the surface
72 of stock 58 to circular central portion 66 would be
drilled entireIy by primary cutting edge 22. The
annular tubular section extending between dashed line 70
and cylindrical side wall 60 from surface 72 of stock 58
to annular shoulder 64 would be drilled entirely by
secondary cutting edge 24.
:
:~9~S09
The portions of stock material 58 that had
extended between dashed lines 70 and cylindrical side
wall 60 is cut entirely by secondary cutting edge 24
and is removed entirely through secondary flute 28.
mhese chips of stock material 58 have a maximum width
equal to the distance from dash lines 70 to cylindrical
side wall 60. As explained above, this distance also is
approximately one half the radius of flat bottom drill
20. The secondary flute 28, like the primary flute 26, has
a maximum width at least equal to the radius of 1at
bottom drill 20. Therefore, the narrow chips produced
by the secondary cutting edge 24 are efficiently and
reliably removed through secondary flute 28.
Again referring to Figure 13, the portion of
hole 62 between dashed line 70 would be drilled entirely
by primary cutting edge 22. Because primary cutting
edge 22 extends across the longitudinal axis of flat
bottom drill 20, the drilling of stock material 58
would commence immediately upon contact of primary
cutting edge 22 with stock material 58. Thus, unlike
twist drills, it would be unnecessary to exert a
substantial axial force to effect the drilling action.
All of the stock material 58 that had been between
dashed lines 70 would have been removed through
primary flute 26. The width of this material would be
equal to the distance from centerline of the drilled
hole to dash lines 70. As explained above, this
distance typically is equal to one half of the radius.
Because the primary flute 26 extends the full distance
from the centerline to the periphery 38 of flat bottom
drill 20, these narrow chips are efficiently and
reliabIy removed through primary flute 26. To further
R encourage th~e ~egress of chips, the entire drill can be
coated with ~.
In summary, a flat bottom drill is provided
that has opposed primary and secondary cutting edges~
The primary cutting edge extends across and is perpen-
:
119~509
~18-
dicular to the longitudinal axis of the flat bottom drill.
The secondary cutting edge faces the opposite direction
and is disposed on the opposite side of the subject flat
bottom drill. Additionally, the secondary cutting edge
is substantially perpendicular to the longitudinal axis
of the subject drill and extends inwardly from the
perimeter thereof a distance at least equal to the
difference between the radius of the drill and the
length of the primary cutting edge. The secondary
cutting edge is offset from the primary cutting edge
axially a distancewhich is a function of the feed rate
of the drill. The relative sizes of the primary and
secondary cutting edges is selected to assure substantial
balancing of forces about the longitudinal axis o the
subject flat bottom drill. During a drilling operation,
the primary cutting edge contacts the stock material
initially and cuts the centermost portion of the hole in
the stock material. Typically, the radius of this
centermost portion will equal approximately half the
entire radius of the hole. The secondary cutting edge
follows the primary cutting edge and drills the outer-
most portion of the hole. Each cutting edge produces
chips of stock material having a width approximately
equal to one half the radius of the subject drill. The
chips produced by the primary cutting edge are
efficiently and reliably removed through the primary flute.
The chips produced by the secondary edge similarly are
efficiently and reliably removed through the secondary
flute. By producing narrow chips that are segmented to
opposite f utes, the chips move freely thereby precluding
any cha`nce that the chips could contribute to an imbalance
of forces about the centerline of the subject flat bottom
drill. This structure assures that very precise holes
can be drilled in a very hard material by a single drilling
operation withoutthe use of excess equipment such as guide
bushings.
While the preferred embodiment of the subject
9~iiO9
--19--
invention has been described and illustra-ted, it is
obvious that various changes and modifications can be
made therein withbut departing from the spirit of the
present invention which should be limited only by the
scope of the claims.
