Note: Descriptions are shown in the official language in which they were submitted.
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STIFF DRILL
BACKGROUND OF THE INVENTION
1. Field ~f the Inventi~,Q,U
This invention relates to drills for use in
metal, and in particular to drills designed for high
feed rates that place a premium upon resistance to
bending.
2. Background and Descri8~?on of Related Art
When operating at the very high metal removal
rates desirable in high production drilling, the
heaviest component of the reaction to the cutting force
has been found to be axial in nature, amounting, in some
instances, to ninety percent (90%) of the cutting force.
In this kind of service, the maintenance of tolerances
15 on hole size is largely dependent upon the resistance of
the fluted drill body to both bending and buckling under
the cutting load, which, even with the rarely achievable
balanced distribution of the transverse cutting forces,
is largely a question of the stiffness of the fluted
20 drill body, as a beam and as a column, under the major
load components resulting from high feed rates.
The problem of deflection of metal drills has
largely been addressed in the prior art as one of
balancing the transverse forces upon the cutting tip for
25 the elimination, to the extent possible, of a transverse
resultant. While some prior art drills have approached
desirable stiffness by shortening the fluted drill body,
the significant criteria for drill stiffness appear to
have remained undiscovered by the prior art.
30 SUN~lARY OF THE INVENTION
In briefest terms, the invention accomplishes
maximum resistance to flexure of the fluted drill body
by rotatively aligning the fluted cross section from the
drill tip to its juncture with the shank, so that its
35 maximum resisting moment is available to oppose the
applied bending moment. Put another Way, the rotative
orientation at that juncture should place the neutral
axis of highest resistance to bending perpendicular to
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the line of action of any net transverse force and
moment at the drill tip, which, in the usual case of
diametrically aligned cutting edges, is accomplished by
aligning that neutral axis parallel to the cutting edges
5 of the drill.
DESCRIPTION OF THE DRAWINGS
In the drawings,
FIGURE 1 is a perspective view of a drill in
accordance with the invention;
10 FIGURE 2 is an elevational view of the drill
of Figure i, partially diagrammatic, showing the forces
acting upon it while cutting;
FIGURE 3 is a diagrammatic end view of the
drill as seen in Figure 2, showing thereon in more
75 detail the transverse forces acting upon the rake faces
of the cutting inserts in operation; and
FIGURE 4 is a sectional view of the drill
taken on the line 4-4 of Figure 2.
DETAINED DESCRIPTION OF THE INVENTION
20 While a properly ground, double-edged twist
drill can approach or even achieve zero net transverse
forces on the drill face before uneven wear of the
cutting edges upsets the balance of forces, the
transverse forces on the body of an insert drill are
25 inherently unbalanced when but two inserts are provided,
disposed respectively on opposite sides of the rota-
tional axis and each sweeping only a portion of the
radius of the cut. Insert drills therefore typically
exhibit a low length-to-diameter ratio for greater
30 resistance to bending in response to the net transverse
force at the cutting end. At high feed rates, however,
the axial forces on the drill body, compressive in
nature, can add a further couple in a plane at right
angles to the net transverse force acting at the cutting
35 tip. This tends to increase the radial deflection that
would be caused by the deflection of the drill due to
unbalanced transverse forces alone.
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The drill of the invention seeks to maintain
hole size and roundness within tight tolerance against
the applied moments by careful rotative orientation of
the fluted section of the drill body at the section of
5 maximum bending stress, which is experienced in the
fluted drill body adjacent to the flange of the gripping
shank near the location of the maximum moment of the
applied transverse load at the tool tip. Specifically,
the desired orientation is that which results from
t0 selection of the helix angle of the flutes so as to
place the neutral axis of maximum resisting moment
perpendicular to the line of action of the resultant
transverse force at the tool tip. In the symmetrical
"hour glass" configuration of Figure 4, the main axis of
15 symmetry is preferred as the neutral axis of the section
of maximum bending stress, which, in the case of
diametrically aligned cutting inserts, places the main
axis of symmetry in a common axial plane with the
cutting edges of the inserts.
20 In the drawings, the cylindrical tool body 10
is shown in Figure 1 in a three-quarter view toward the
cutting tip 12, i.e., illustrating a diametrical align-
ment of the cutting edges 14 of two oppositely facing
cutting inserts.l6 and 18. The ratio of the unsupported
25 length of the drill of the invention to its cutting
diameter is in excess of 3, preferably about 4,
measuring from the flange 20 of the gripping shank 22.
Cutting fluid is conveyed from the machine spindle to
the cutting tip 12 by an axial channel 26 which is
30 drilled from the gripping shank 22 of the drill body 10,
and which is joined by diagonal branch channels 28
drilled from the cutting tip.
In the illustrated case, chip flutes 24 recede
from the cutting tip 12 in the preferred left-hand helix
35 notwithstanding the facing of the cutting inserts 16 and
18 for right-hand rotation (clockwise when viewed from
the end opposite the cutting tip).
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With the opposite handed helical configuration
illustrated, the stated alignment of cutting edges with
the neutral axis of greatest moment at the critical
section (Figure 4) is achieved with approximately three-
s eighths of a turn of the helix. For helical flutes of
the same hand as the cutting direction, approximately
five-eighths of a turn are needed to achieve the desired
orientation.
The reason for preferring the opposite handed
t0 chip groove helix, although optional from the standpoint
of the utility of this invention, is the theoretical
tendency of the tool body to contract its radial
dimension under high torsional load, rather than to
expand radially, as would be the case if the helix
15 turned in the direction of cutting rotation. These
considerations are thought to become significant only
under conditions of extraordinary cutting speed and feed
which seek nevertheless to hold close tolerances of hole
size and roundness.
20 Figure 3 of the drawings illustrates diagram-
matically the location of the transverse forces 30 and
32 acting respectively upon the faces of the cutting
inserts 16 and 18 and the torsional moment 34 which they
combine to produce. Figure 3 also illustrates diagram-
25 matically in broken line outline the cross section of
the tool body 10 at the point therealong of highest
bending stress experienced by the fluted drill body,
namely at the section indicated by the line 4-4 of
Figure 2, and shown in solid line in Figure 4, which
30 indicates the relative location of the cutting inserts
16 and 18 in broken lines. With the idealized
symmetrical form of cross section shown in Figures 3
and 4 for section 4-4 of Figure 2, the symmetry axis 36
at that section is placed in the same axial plane as the
35 cutting edge diameter 38 (Figure 3), with the facial
forces 30 and 32 on the cutting inserts 16 and 18, and
their resultant 40, perpendicular to that plane. This
arrangement places the maximum available moment of
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resistance to bending at section 4-4 in proper alignment
to resist the bending moment at that section caused by
the resultant transverse force 40 (Figure 2) exerted
upon the cutting tip of the tool. The arrow represent-
s ing the transverse resultant 40 points downwardly in
Figure 2 because of the greater face force upon the
inner insert 18.
Figure 2 also depicts an axial force 42 acting
upon the tool as though at its center, and the further
70 bending moment 44, resulting from the eccentric axial
loading of tool body 10 by the axial forces 46 and 48
acting upon the two cutting inserts 16 and 18
individually.
It will also be appreciated that any bending
15 of the tool body about the section 4-4 of Figure 2
caused by the moment of the transverse resultant 40
about that section, will shift the resultant axial force
42 in the direction of the bending force 40, causing the
offset resultant axial force 42 to produce yet another
20 couple 50 acting in a direction to increase the bending
moment of the transverse bending force 40, with
cumulative effect upon the radial deflection of the
cutting tip of the tool.
Both considerations, i.e., deflection due
25 to transverse load at the tool tip, and the coplanar
transverse-load reinforcing couple 50, are resisted
optimally in the tool of the invention by placing the
neutral axis 36 of maximum moment of inertia of the
section 4-4 of Figure 2 in a common axial plane with the
30 cutting edge diameter 38 so as to present that section s
maximum moment of resistance to the bending moment
caused by the unbalanced transverse resultant 40 and its
reinforcing couple 50.
This alignment coincidentally places the
35 secondary axis of symmetry 52 (Figure 4) perpendicular
to the plane of the moment 44 resulting from the
eccentric axial loading of the tool body 10 by the axial
forces 46 and 48 acting upon the outboard and inboard
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inserts 16 and 18, respectively. Fortuitously, this
results in the greatest resistance to bending in the
axial plane 36-38 that is consistent with orienting the
maximum available resisting moment at the section 4-4 of
5 Figure 2 to oppose the moment of the concentrated
transverse load on the tip of the tool.
The preceding detailed description of the
drill of the invention ignores the possibility of a
detractive effect, upon the preferred relative rotative
10 alignments discussed, of torsional deflection of the
fluted tool body from the moment 34 of the individual
face forces on the inserts. The absence of discussion
of that possibility simply reflects the tact that
analysis of torsional deflection, at the length to
15 diameter ratios earlier herein stated, showed it to be
negligible under heavy anticipated loads.
The preceding discussion of principle
proceeded upon the illustrative example of a drill of
two cutting inserts having their cutting edges
20 diametrically arrayed, which may be regarded as typical
for small drill sizes, but not necessarily exclusive.
The inserts of larger drills may be greater in number,
and conceivably be differently arrayed.
The principles herein disclosed are never-
25 theless applicable, the primary consideration being that
the critical section of the fluted tool body remote from
the tool tip be oriented rotatively to present its
maximum resistive moment to the bending~moment at that
section resulting from unbalanced transverse load at the
30 tool tip.
Similarly, while those principles are
disclosed here in connection with drills whose cutting
edges are provided by indexable/replaceable inserts,
they are equally applicable to drills having cutting
35 edges integral with the drill body.
The features of the invention believed new and
patentable are set forth in the following claims.
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