Note: Descriptions are shown in the official language in which they were submitted.
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SELF-CHAMFERING DRILL BIT
TEC~IHICAL FIELD:
This invention relates to drill bits, and in particular,
to drill bits that automatically chamfer a hole being
drilled by the drill bit.
BACKGROUND ARTt
It is often desirable where holes are drilled or formed
in workpieces to countersink or chamfer the peripheral edge
of the hole. Usually the hole is drilled and then a
separate countersink: is used to form the chamfer. Drill
bits have been made in the past, however, where the
countersink has been made an integral part of the drill
bit. The countersink being spaced rearwardly or
longitudinally back from the tip of the drill bit.
In some applica,t~.ons, where holes are drilled right
through a workpiece,. it is desirable to countersink the
hole on both sides of: the workpiece. Where a separate drill
and countersink is uaed, three operations are necessary to
complete the task. l?first, the hole is drilled, and then
second and third ope=rations are necessary to countersink
the hole on either side of the workpiece. If a combined
drill and countersink is used, it is still necessary to
perform two operations. First, the hole is drilled and
countersunk on one s_Lde, and then the hole on the opposite
side of the workpiece has to be countersunk in a separate
operation.
In the past, dr=ill bits have been produced having
cantilevered, spring steel deburring attachments for
deburring a hole on either side of a workpiece through
which a hole is drilled. An example of such a drill bit is
shown in document US-A-2,437,822. However, the problem with
this type of drill bit is that the stiffness of the spring
deburring element is so critical to the cutting action it
has, that for practical purposes this type of element is
useless. For example, if one tries to make the deburring
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element stiff enough to actually chamfer a hole, it might
work for a specific type or strength of workpiece, but if
the workpiece is too hard, it will not work at all, and if
the workpiece is too soft, it will drill right through the
workpiece just enlarging the hole. One would have to have
a different spring deburring element for each type or
strength of material being drilled and chamfered.
Applicant has found that this problem can largely be
overcome by providing a cantilevered insert that actually
does the cutting, and a separate spring located below the
insert, the properties of which can be chosen to make the
drill bit perform properly over an acceptable range of
materials to be drilled.
DISCLOSU OF THE INVENTIOH~
The present invention is a self-chamfering drill bit
that drills a hole and chamfers it on both sides of the
workpiece with a single pass of the drill bit into and out
of the workpiece.
According to one aspect of the invention, there is
provided a self-chamfering drill bit including a shank
having a forward, distal end portion including a pair of
opposed, radially disposed cutting edges. The shank defines
a longitudinal groove extending rearwardly from the distal
end portion, and a cantilevered longitudinal insert located
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in the groove, characterized by the following. The groove
has a floor. The insert has a forward end portion located
adjacent to the shank distal end portion and spaced from
the groove floor in its normal position. The insert also
has a rearward end portion attached to the floor, so that
the insert forward end portion can deflect radially
inwardly from its normal position into the groove. A
cutting tip is mounted on the insert forward end portion to
project radially, outwardly from the shank and retract into
the shank groove upon inward deflection of the insert. The
tip has forward and rearward bevelled cutting edges. Also,
bias means is located below the insert for returning the
insert to its normal position after being deflected
radially inwardly.
BRIEF DESCRIPTION OF THE DRAWINGS~
Preferred embodiments of the invention will now be
described, by way of example, with reference to the
accompanying drawings in which:
Figure 1 is an elevational view of a preferred
embodiment of a self-chamfering drill bit according to the
present invention;
Figure 2 is an enlarged, exploded, perspective view of
the drill bit shown in Figure 1;
Figure 3 is an enlarged sectional view of a portion of
the drill bit of Figure 1 taken along lines 3-3 of Figure
1;
Figure 4 is an elevational view of the drill bit of
Figure 1 shown entering a workpiece;
Figure 5 is a view similar to Figure 4 showing the
drill chamfering the hole on the front side of the
workpiece;
Figure 6 is a view similar to Figures 4 and 5 showing
the drill chamfering insert retracted into the drill shank
as it passes through the workpiece;
Figure 7 is a view similar to Figures 4 to 6 showing
the drill chamfering insert emerging from the backside of
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the workpiece and starting to form the chamfer on the
backside of the hole;
Figure 8 is a view similar to Figures 4 to 7 showing
the drill passed all the way through the workpiece with the
chamfering insert in its normal position as shown in Figure
4; and
Figure 9 is a sectional view of a hole in a workpiece
chamfered on both sides after the drill bit has been
removed from the workpiece.
BEST MODE FOR CARRYING OUT THE INVENTION-
Referring firstly to Figures 1 and 2, a preferred
embodiment of a self-chamfering drill bit is generally
indicated by reference numeral 10. Drill bit 10 is a heavy
duty drill bit of a type that is used to drill holes in the
webs of steel railroad rails to allow these rails to be
joined together by having bolts pass through steel plates
spanning the joints between the rails on either side of the
rail webs. The holes drilled in these rails are typically
about 3 centimetres in diameter and 1.6 centimetres deep.
In the case of railway rails, it is necessary to
chamfer the holes on either side of the rail webs in order
to prevent stress cracks forming around the peripheral
edges of the holes. Drill bit 10 allows the holes to be
drilled and chamfered on both sides of the web in a single
operation, as will be described further below.
Drill bit 10 includes a shank 12 having a forward _
portion 14 and a rearward portion 16. Rearward portion 16
includes a flat 18 and is adapted to be gripped by a chuck
to turn drill bit 10. Forward portion 14 includes a
forward, distal end portion 20 including a pair of opposed,
radially disposed cutting edges 22 in the form of
replaceable carbide inserts. Shank 12 is provided with
internal passages 24 for the flow of cutting fluid or
coolant, either liquid or air, through drill bit 10.
Internal passages 24 communicate with outlets 26
strategically placed to distribute the coolant as needed.
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It will be apparent to those skilled in the art that
many variations can be made in the drill bit as described
thus far to suit the size of hole and type of material
being drilled.
Shank 12 includes a longitudinal groove 28 extending
rearwardly from distal end portion 20. Groove 28 has a
floor 30 with a raised rearward portion 32 on which is
mounted a spring 34 and a longitudinal chamfering insert 36
in a cantilever fashion by means of a cap screw 38 and
washer 40.
Insert 36 has a forward end portion 42 located adjacent
to the shank distal end portion 20. Forward end portion 42
is spaced from groove floor 30 in its normal position by
being mounted as a cantilever on floor rearward portion 32
as indicated in Figure 3.
Forward end portion 42 includes a carbide cutting tip
44 brazed or silver soldered thereto to project radially,
outwardly from shank 12 as seen best in Figures 3_and 4 to
8. Cutting tip 44 has a forward bevelled cutting edge 46,
a rearward bevelled cutting edge 48 and an intermediate,
outer bearing surface 50 located between the forward and
rearward bevelled cutting edges 46, 48. Cutting tip 44
bearing surface 50 has a transverse side edge portion 52
located between the forward and rearward bevelled cutting
edges 46, 48. This side edge portion 52 is rounded or
chamfered as seen best in Figure 3, the purpose of which
will be described further below.
As seen best~in Figure 2, the forward end portion 42
of insert 36 is bevelled and insert 36 has a transversely
enlarged rearward end portion 54 having a hole 56 to
accommodate cap screw 38. When the rearward end portion 54
is attached to floor rearward portion 32, the insert
forward end portion 42 and cutting tip 44 can deflect
radially inwardly from their normal positions into groove
28. In fact, cutting tip 44 retracts into groove 28 upon
inward deflection of insert 36 until bearing surface 50 is
flush with the outer surface of shank distal end portion
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20, as seen best in Figure 6. Insert 36 is narrower than
longitudinal groove 28 and off-set to one side of groove 28
as indicated in Figures 1 and 3 to allow cuttings to be
flushed out from underneath the cantilevered insert 36 and
5 spring 34 and for this purpose, one of the coolant outlets
26 communicates with groove 28. Insert 36 is formed of
A.I.S.I. or S.A.E. 4140 alloy steel heat treated to a
hardness of 46 to 47 Rockwell C.
Spring 34 is a rectangular plate spring attached to
floor rearward portion 32 by cap screw 38 passing through
a hole 58 in the rearward end- of spring 34. Spring 34
extends below insert 36 in a cantilever fashion parallel to
and in contact with insert 36. Spring 34 is formed of
tempered spring steel having a hardness of between 48 and
51 Rockwell C. Spring 34 and to some extent insert 36 form
bias means for returning insert 36 to its normal position
as indicated in Figures 1, 3 to 5 and 8 after being
deflected radially inwardly into groove 28 as indicated in
Figure 6.
In operation, referring next to Figures 4 through 9,
drill bit 10 is shown in Figure 4 starting to enter a
workpiece 60 which could be the web of a steel railway
rail. As drill bit 10 progresses into workpiece 60,
cutting tip 44 engages workpiece 60 and the forward
bevelled cutting edge 46 of cutting tip 44 makes a first
peripheral chamfer 62 on the hole 64 being drilled in
workpiece 60. As drill bit 10 progresses further into _
workpiece 60, chamfer 62 begins to act as a cam and cutting
tip 44 acts as a cam follower causing cutting tip 44 to
deflect inwardly down into groove 28 and discontinue the
cutting action of cutting tip 44.
As drill bit 10 progresses further into workpiece 60
as indicated in Figure 6, cutting tip 44 is fully forced
into groove 28. The bearing surface 50 and especially
chamfered side edge portion 52 of cutting tip 44 prevents
cutting tip 44 from enlarging or scoring the inside surface
of hole 64.
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When drill bit 10 advances completely through workpiece
60 and cutting tip 44 begins to emerge from hole 64 as
indicated in Figure 7, the rearward bevelled cutting edge
48 begins cutting a chamfer 66 on the backside of hole 64.
When drill bit 10 passes completely through workpiece 60 as
indicated in Figure 8, the drill bit is withdrawn, and as
cutting tip 44 again engages workpiece 60, chamfer 66 is
completed. However, by stopping drill bit 10 and holding it
in the position shown in Figure 7, chamfer 66 could be
completely formed, as desired.
The cutting speed and feed of drill bit 10 when
drilling through steel typically varies from about 450 to
1200 R.P.M. at a feed rate between 2.5 and 75 centimetres
per minute, depending upon the type of coolant used and the
size of cuttings or chips produced. It will be appreciated
that a skilled person could adjust the cutting speed and
feed rate to suit the particular application, as desired.
Having described preferred embodiments, it will be
appreciated that various modifications could be made to the
preferred embodiments described above. For example, the
shape of cutting tip 44 could be changed and the cutting
angles of forward and rearward bevelled cutting edges 46,
48 could be altered to change the angle and shape of
chamfers 62, 66. The type of carbide used for cutting tip
44 would be chosen to suit the material being drilled.
Also, the hardness and materials used for insert 36 and
spring 34, as well as the type of spring 34 used, could be
varied to suit the material being drilled, especially if
softer materials such as aluminum are being drilled. More
than one insert 36 could be used in any particular drill
bit if necessary or if it is desirable to speed up the
drilling operation. Also, the length of insert 36 and
spring 34 could be adjusted to suit the size of drill or
the material being drilled. Workpieces of all shapes and
configurations can be drilled with drill bit 10.
It will be apparent to those skilled in the art that
in light of the foregoing disclosure, many alterations and
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modifications are possible in the practice of this
invention without departing from the spirit or scope
thereof. Accordingly, the scope of the invention is to be
construed in accordance with the substance defined in the
following claims.
