U.S. patent application number 13/486592 was filed with the patent office on 2012-11-22 for cutting tap and method of making same.
Invention is credited to Stephen M. George, Willard E. Henderer, Vladimir D. Volokh.
Application Number | 20120295519 13/486592 |
Document ID | / |
Family ID | 42098989 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120295519 |
Kind Code |
A1 |
Henderer; Willard E. ; et
al. |
November 22, 2012 |
CUTTING TAP AND METHOD OF MAKING SAME
Abstract
A method of making a cutting tap includes the steps of: grinding
a blank to form a threaded body portion at an axially forward end
of the cutting tap, grinding one or more flutes in the threaded
body portion to form cutting edges; grinding the threaded body
portion to form a first cutting thread and a second cutting thread,
the first cutting thread at a first distance from the axially
forward end of the cutting tap, and the second cutting thread at a
second distance from the axially forward end of the cutting tap;
and grinding a chamfer in the threaded body portion such that a
thickness of a section of material removed from the second cutting
thread is smaller than a thickness of a section of material removed
from the first cutting thread during a tapping operation.
Inventors: |
Henderer; Willard E.;
(EVANS, GA) ; George; Stephen M.; (EVANS, GA)
; Volokh; Vladimir D.; (MAALOT, IL) |
Family ID: |
42098989 |
Appl. No.: |
13/486592 |
Filed: |
June 1, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12250570 |
Oct 14, 2008 |
8210779 |
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13486592 |
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Current U.S.
Class: |
451/48 |
Current CPC
Class: |
B23P 15/52 20130101;
B23G 5/06 20130101; B23G 2200/30 20130101; B24B 3/18 20130101; Y10T
408/904 20150115; B24B 19/04 20130101; Y10T 408/9046 20150115; Y10T
408/9048 20150115 |
Class at
Publication: |
451/48 |
International
Class: |
B24B 1/00 20060101
B24B001/00 |
Claims
1-17. (canceled)
18. A method of making a cutting tap, comprising the steps of:
grinding a blank to form a threaded body portion at an axially
forward end of the cutting tap; grinding one or more flutes in the
threaded body portion to form cutting edges; grinding the threaded
body portion to form a first cutting thread and a second cutting
thread, the first cutting thread at a first distance from the
axially forward end of the cutting tap, and the second cutting
thread at a second distance from the axially forward end of the
cutting tap; and grinding a chamfer in the threaded body portion
such that a thickness of a section of material removed from the
second cutting thread is smaller than a thickness of a section of
material removed from the first cutting thread during a tapping
operation.
19. The method according to claim 18, wherein the chamfer is
non-linear such that a peripheral surface of the first and second
cutting threads is on a curved line.
20. The method according to claim 18, wherein the chamfer is formed
such that a chamfer angle of the second cutting thread is smaller
than a chamfer angle of the first cutting thread.
Description
CROSS-NOTING TO RELATED APPLICATIONS
[0001] This Application is related to application Ser. No.
11/582,805, entitled "Cutting Tap and Method of Making Cutting
Tap", filed Oct. 18, 2006, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The invention relates in general to a cutting tap, and in
particular to a cutting tap having a cutting edge geometry that
improves the resistance of the cutting edges to chipping and
fracture.
[0003] Mechanisms and machine components requiring screw threads
have a long history in technology. Specifically, the application of
screw threads as fastener components dominates over all other means
to join parts into assemblies. Although there are many ways to
generate screw threads both internal as well as external,
experience has shown that taps are the favored means to generate
the internal screw thread. There currently exist two tapping
methods to generate internal screw threads. The dominant tapping
method is by cutting and removing material from the walls of a hole
to produce a helical V-shaped screw thread. Alternatively, internal
screw threads can be created by displacing material to form an
internal screw thread. However, tapping by cutting material is
generally favored because this method requires lower torque and
produces a more perfect thread form.
[0004] The dimensional accuracy of the shape and size of the
internal screw thread controls the precision and fit of the screw
thread assembly. Additionally, the speed of tapping affects the
cost to produce an internal screw thread.
[0005] There are two materials used to manufacture cutting taps.
High-speed steel is widely used for taps because of its high
strength. However, cemented tungsten carbide is favored as a
material for manufacturing other cutting tools over high-speed
steel owing to properties such as higher hardness and high
temperature stability including the ability to retain hardness at
high temperatures. Typically, cutting tools manufactured from
cemented carbide can be used at cutting speeds that are at least
three times higher than tools manufactured from "high-speed" steel
and the life of the tool is longer.
[0006] Referring now to FIGS. 9-11, there is shown one flute of a
four-fluted prior art cutting tap that has a straight cutting face.
In general, the cutting tap generates an internal thread form by a
succession of cutting edges on the chamfered section of the tap
having a length L. Material is removed from the wall of the hole
until the final thread form is obtained with the first full thread
on the main body of the tap. This progressive formation of an
internal thread is illustrated in FIG. 9 by superimposing the
sections of material removed by each of the four flutes.
[0007] As shown in FIG. 10, the prior art, cutting tap has a
straight cutting face that is inclined relative to a radial
reference line that travels from the cutting edge at the major
diameter to the center of the cutting tap at a cutting angle (or
rake angle) A1. In FIG. 10, the cutting angle A1 is defined as the
included angle between a line passing along the surface of the
cutting face and the radial reference line. The cutting angle A1 is
positive when the inclination from the radial reference line is in
the counterclockwise direction as viewed in FIG. 10. The cutting
angle A1 is negative when the inclination from the radial reference
line is in the clockwise direction as viewed in FIG. 10.
[0008] The magnitude of the cutting angle A1 has an influence on
edge strength of the prior art cutting tap. In this regard, one can
increase the strength of the cutting edge by reducing the cutting
angle A1 (i.e., making the cutting angle A1 more negative).
However, while a reduction in the cutting angle A1 will increase
the strength of the cutting edge, the amount of cutting force
necessary to tap (or cut) the threads increases with the reduction
in the cutting angle A1. When taps of the prior art are
manufactured from cemented carbide, the cutting edges are very
prone to chipping because carbide has low strength as compared to
high-speed steel. Specifically, the cutting edges that are most
prone to chipping are the narrow edges on the chamfer that approach
and include the first full thread after the chamfer. The narrow
full threads after the chamfer are also prone to chipping because
they have a small included angle. The wider edges on the entry part
of the chamfer are far less prone to chipping because they are not
as narrow as the cutting edges of the full threads.
[0009] It should be appreciated that the above description of the
obstacles connected with the cutting angle A1 of a cutting tap that
has a straight cutting face also exist for a cutting tap that has
an arcuate cutting face. In this regards, for a cutting tap that
has an arcuate cutting face, a chordal hook angle corresponds to
the rake angle A1 for the cutting tap with the straight cutting
face. The chordal hook angle is defined as the angle between a
radial reference line between the major diameter to the center of
the cutting tap and a chord between the distal cutting edge and the
minor diameter of the cutting tap.
[0010] As shown in FIG. 11, the cutting edges of the conventional
cutting tap are prone to chipping, especially the narrow cutting
edges on the chamfer that approach and include the first full
thread after the chamfer (illustrated by the third chamfered thread
in FIG. 11). The wider cutting edges on the entry part of the
chamfer are less prone to chipping (illustrated by the first and
second chamfered thread). Prior art taps have a chamfer defined by
a single straight line at a chamfer angle A2 with respect to the
axis of the tap. Because the chamfer is straight, the thickness T1
of the sections of material removed by each chamfered cutting edge
remains constant.
[0011] Because taps are geometrically weak, especially the cutting
edges, they are prone to chipping. Because cemented carbide has
lower strength than high-speed steel, taps made from cemented
carbide are more prone to chipping than taps made from high-speed
steel. Therefore, it is not possible to currently use taps made
from cemented carbide in some applications where high-speed steel
taps can be used.
BRIEF SUMMARY OF THE INVENTION
[0012] Briefly, according to an aspect of the invention, there is
provided a cutting tap comprising a body having an axial forward
end and an axial rearward end and a central longitudinal axis, the
body having a fluted section at the axial forward end, the fluted
section including a chamfered fluted section extending from the
axial forward end of the body and terminating at a first full
cutting thread, the chamfered fluted section comprising a first
cutting thread located a first distance from the axial forward end
of the body and a second cutting thread located a second distance
from the axial forward end of the body, the second distance being
greater than the first distance, wherein the chamfered fluted
section is shaped such that a thickness of sections of material
removed by the second cutting thread is smaller than a thickness of
sections of material removed by the first cutting thread.
[0013] According to another aspect of the invention, there is
provided a cutting tap comprising a body having an axial forward
end and an axial rearward end and a central longitudinal axis, the
body having a fluted section at the axial forward end, the fluted
section including a chamfered fluted section extending from the
axial forward end of the body and terminating at a first full
cutting thread, the chamfered fluted section comprising a first
cutting thread located a first distance from the axial forward end
of the body and a second cutting thread located a second distance
from the axial forward end of the body, the second distance being
greater than the first distance, wherein a peripheral surface of
the chamfered Doted section is non-linear such that the thickness
of sections of material removed by the second cutting thread is
smaller than the thickness of sections of material removed by the
first cutting thread.
[0014] According to yet another aspect of the invention, there is
provided a cutting tap comprising a body having an axial forward
end and an axial rearward end and a central longitudinal axis, the
body having a fluted section at the axial forward end, the fluted
section including a chamfered fluted section extending from the
axial forward end of the body and terminating at a first full
cutting thread, the chamfered fluted section comprising a first
cutting thread located a first distance from the axial forward end
of the body and a second cutting thread located a second distance
from the axial forward end of the body, the second distance being
greater than the first distance, wherein the first cutting thread
forms a first chamfer angle with respect to the central
longitudinal axis, and wherein the second cutting thread forms a
second chamfer angle with respect to the central longitudinal axis,
the second chamfer angle being smaller than the first chamfer
angle.
[0015] According to still yet another aspect of the invention, a
method of making a cutting tap comprises the steps of: [0016]
grinding a blank to form a threaded body portion at an axially
forward end of the cutting tap; [0017] grinding one or more flutes
in the threaded body portion to form cutting edges; [0018] grinding
the threaded body portion to form a first cutting thread and a
second cutting thread, the first cutting thread at a first distance
from the axially forward end of the cutting tap, and the second
cutting thread at a second distance from the axially forward end of
the cutting tap; and [0019] grinding a chamfer in the threaded body
portion such that a thickness of sections of material removed from
the second cutting thread is smaller than a thickness of sections
of material removed from the first cutting thread during a tapping
operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed, description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0021] FIG. 1 is an isometric view of a exemplary embodiment of a
spiral-fluted cutting tap of the invention;
[0022] FIG. 2 is a side view of an exemplary embodiment of a
straight-fluted cutting tap of the invention;
[0023] FIG. 3 is a side view showing the profile of the axial
forward portion fo the cutting tap of FIG. 2 including the
chamfered fluted section and the junction between the chamfered
fluted section and the constant diameter (or finishing) section of
the cutting tap;
[0024] FIG. 4 is an enlarged view of the left side (as viewed in
FIG. 3) of the side view of FIG. 3 illustrating an exemplary
embodiment in which the outer periphery of the cutting teeth of the
chamfered fluted section is formed with a radius R;
[0025] FIG. 5 is an enlarged view of the left side (as viewed in
FIG. 3) of the side view of FIG. 3 illustrating an alternate
exemplary embodiment in which the chamfered fluted section is
formed with at least two sections having different chamfer
angles;
[0026] FIG. 6 is a cross-sectional view of the upper flute taken
along line 6-6 of FIGS. 4 and 5;
[0027] FIG. 7 is a cross-sectional view of the upper flute taken
along line 7-7 of FIGS. 4 and 5;
[0028] FIG. 8 is a cross-sectional view of the upper flute taken
along line 8-8 of FIGS. 4 and 5;
[0029] FIG. 9 is a cross-sectional view of one flute of a prior art
cutting tap that has a straight cutting face;
[0030] FIG. 10 is a cross-sectional view of the upper flute taken
along line 10-10 of FIG. 9; and
[0031] FIG. 11 is an enlarged view of the left side (as viewed in
FIG. 9) of the side view of FIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Referring now to FIG. 1, a cutting tap 20 with spiral flutes
is shown according to an embodiment of the invention. The cutting
tap 20 has an elongate body 22 with an axial forward end 24 and an
axial rearward end 26. The cutting tap 20 has a cylindrical shank
portion (bracket 28) adjacent to the axial rearward end 26 and a
spiral-fluted portion (bracket 30) adjacent to the axial forward
end 24.
[0033] The cutting tap 20 is operatively connected to a machine
tool or the like at the cylindrical shank portion 28 thereof The
spiral-fluted portion 30 has a chamfered region beginning at the
axial forward end 24 and extending in an axial rearward direction
therefrom. The chamfered region joins a constant diameter (or
finishing) region that extends in the axial rearward direction
terminating at the juncture with the cylindrical shank portion
28.
[0034] In regard to specific tapping applications, spiral flute
taps with a right hand helix pull the chips out of the hole (right
hand thread) and are effective in blind holes. Left hand spiral
fluted taps direct the chip ahead of the tap (right hand thread)
and are effective in through holes.
[0035] Referring now to FIG. 2, there is shown a straight-fluted
cutting tap 40 according to an embodiment of the invention. The
straight-fluted cutting tap 40 has an elongate body 42 with an
axial forward end 44 and an axial rearward end 46. The
straight-fluted cutting tap 40 has a cylindrical shank portion
(bracket 52) adjacent to the axial rearward end 46 and a
straight-fluted portion (bracket 50) adjacent to the axis forward
end 44. In reference to a specific application, taps with straight
flutes are effective in materials such as cast iron that produce a
short chip.
[0036] Referring now to FIG. 3, there is shown the axial forward
portion of the straight-fluted portion 50 of the straight-fluted
cutting tap 40. There is a chamfered fluted section (bracket 54)
beginning at the axial forward end 44 and extending in an axial
rearward direction therefrom. Chamfered fluted section 54 extends
for a pre-selected distance shown by the dimension "X" in FIG. 3.
The chamfered fluted section 54 terminates at the junction with a
constant diameter (or finishing) fluted section (bracket 56). The
constant diameter fluted section 56 begins at the junction with the
chamfered fluted section 54 and extends in an axial rearward
direction until it terminates at the junction with the cylindrical
shank portion 52.
[0037] The chamfered fluted section 54 has a series of V-shaped
cutting threads where each cutting thread has a cutting edge. The
distal cutting thread 58 has a cutting edge 59 and is the most
axial forward cutting thread. Distal cutting thread 58 is adjacent
to cutting thread 62, which has a cutting edge 63. Cutting thread
62 is adjacent to cutting thread 66, which has a cutting edge 67.
Cutting thread 66 is adjacent to cutting thread 68, which has a
cutting edge 69. It will be appreciated that the constant diameter
(or finishing) fluted section 56 begins with the cutting thread 66
and extends in the axial rearward direction therefrom until its
junction with the cylindrical shank portion 52.
[0038] The chamfered cutting edge 59 of the distal cutting thread
58 is the strongest of the cutting threads because it is wider
than, and not as narrow as, the cutting edges of the other cutting
threads (for example, the cutting edges 63 and 67 of cutting
threads 62 and 66, respectively).
[0039] Reducing the thickness of the sections (thickness times the
width) of material removed by each cutting edge of chamfered fluted
section 54 can reduce the forces imposed on the weaker cutting
edges approaching the first full thread 70. One common way to
accomplish this is to lengthen the dimension "X" of chamfered
fluted section 54. But there are many applications, especially when
tapping blind holes, where the clearance at the bottom of the hole
is limited and therefore the dimension "X" of chamfered fluted
section 54 cannot be increased. It is desirable to reduce the
dimension "X" of the chamfered fluted section 54 even on taps for
through holes in order to keep the distance the tap must travel to
a minimum.
[0040] According to the principles of the invention, the cutting
tap 40 has greater resistance to chipping by reducing the forces
imposed on the relatively narrower cutting edges of the chamfered
fluted section 54 that approach and include the first full cutting
thread 66. In general, the principles of the invention are
accomplished by shaping the chamfered fluted section 54 such that
the thickness of the sections of material removed by the cutting
edges approaching the first full cutting thread 66 is smaller than
the thickness of the sections of material removed by the relatively
wider cutting edges of the most axial forward cutting threads of
the chamfered fluted section 54. Only the cutting tap 40 will he
discussed below for brevity, however it will be understood that the
principles of the invention can also be applied to the cutting tap
20.
[0041] The principles of the invention described above can be
accomplished by many different embodiments. Referring now to FIG.
4, one embodiment of the invention that accomplishes the principles
of the invention is to form the peripheral surface of the chamfered
fluted section 54 of the cutting tap 40 on a non-linear, curved
line. For example, the peripheral surface of the chamfered fluted
section 54 may be having a radius, R. The shape of the curved line
is such that the thickness T2 of the sections of material removed
by the cutting edge 67 approaching the first full cutting thread 66
is smaller than the thickness T3 of the sections of material
removed by the wider cutting edges 59, 63 of the most axial forward
cutting threads 58, 62. Therefore, the force on the cutting edges
approaching the first full cutting thread 66 will he reduced and
the likelihood of chipping reduced. The radius R of the curved line
may vary along the curve in order to accomplish the principles of
the invention.
[0042] FIG. 5 illustrates an alternative embodiment that also
accomplishes the principles of the invention. In this embodiment,
the chamfered fluted section 54 is formed with two or more sections
formed by straight lines (dashed lines) at different chamfer angles
with respect to the central longitudinal axis Z-Z of the cutting
tap 40 (i.e., the chamfer angles are linear). Specifically, the
chamfer angle of the last section of the chamfered fluted section
54 that approaches the first full cutting thread 66 is smaller than
the chamfer angle(s) of the one or more axial forward sections of
the chamfered fluted section 54. As shown in FIG. 5, for example,
the chamfered fluted section 54 is composed of two sections with
lengths L2 and L3 in which the last section with length L2 is
formed with a chamfer angle A3 that is smaller than chamfer angle
A4 of the more axial forward section with length L3. With this
construction, the sections of the material removed by the cutting
edges approaching the first full cutting thread 66 are smaller than
the thickness of sections of material removed by the wider cutting
edges on the entry part of the chamfered fluted section 54.
Therefore, the force on the cutting edges approaching the first
full cutting thread 66 is reduced, thereby reducing the likelihood
of chipping. It will be appreciated that the invention is not
limited by the number of sections of the chamfered fluted section
54 formed with different chamfer angles, and that the invention can
be practiced with two or more sections of different lengths and
chamfer angles, so long as the last section of the chamfered fluted
section 54 that approaches the first full cutting thread 66 has a
chamfer angle that is smaller than the chamfer angles of the more
axial forward sections.
[0043] FIG. 6 illustrates the cutting face for the cutting thread
58 in the axial forward portion of the chamfered portion 54. Here,
the cutting face 72 is straight and has an orientation to present a
positive cutting angle A1. Cutting angle A1 is the included angle
between the radial reference line G-G (Le., the line passing
through distal cutting edge 59 and the center 74 of the cutting
tap) and a line H-H that lies along the cutting face 72. The
cutting angle A1 is positive because the direction of inclination
of line H-H relative to line G-G is in the counterclockwise
direction as view in FIG. 6. Because the cutting edges are stronger
in the axial forward section of the chamfered portion 54, they can
utilize a positive cutting angle, which allows air an easier
cutting action.
[0044] FIG. 7 illustrates the cutting face at the cutting thread
62, which is located in a more axial rearward location than the
cutting thread 58. In FIG. 7, the cutting face 76 presents a convex
shape as defined by transition radius R1. The length of transition
radius R1 can vary between about five percent to about one hundred
percent of the diameter of the cutting tap. The cutting angle is
the included angle between the radius reference line and a line
(I-I) tangent to the cutting face at the distal cutting edge 63,
i.e., the axial forward termination of the convex cutting face 76.
Here, the cutting angle is zero degrees, and hence, only line I-I
is referenced because line I-I is coextensive with the radial
reference line. The convex cutting face 76 also has an axial
rearward termination 78. Line J-J is a line that is tangent to the
convex cutting face 76 at the axial rearward termination 76. Angle
A1 is the included angle between line I-I and line J-J and is equal
to the cutting angle A1 shown in FIG. 6.
[0045] In constant diameter or finishing section of the chamfer and
for threads past the chamber such as, for example, the threads 70
shown in FIG. 8, the edges of the chamfer or full threads are
weaker and prone to chipping. The cutting angle A2 is reduced
because the threads 70 are weaker than the more axial rearward
threads. Referring to thread 70, there is a convex-shaped cutting
face 80 that defines a cutting angle A2, which is the included
angle between the radial reference line (M-M) and a line (K-K)
tangent to the cutting face at the distal cutting edge 71. The
cutting angle A2 is negative because the inclination of the line
K-K relative to line M-M is in the clockwise direction as viewed in
FIG. 8. It will he appreciated that the negative cutting angle
compensates for the weaker thread 70 to optimize the overall
tapping operation of the cutting tap. The convex cutting face 80
also has an axial rearward termination 82. Line L-L is a line that
is tangent to the convex cutting face 80 at the axial rearward
termination 82. Angle A1 is the included angle between line L-L and
the line M-M and is equal to the cutting angle A1 shown in FIG.
6.
[0046] The movement of the center point of the transition radius R1
relative to the distal cutting edge allows a smooth transition from
the positive cutting angle A1 in the axial forward section of the
chamfered fluted section 54 to the negative cutting angle A2. The
geometry of the cutting face as defined by the radial inward
progressive movement of the center point of the constant radius
(R1) relative to the distal cutting edge results in cutting angles
that are in between the positive cutting angle A1 and the negative
cutting angle A2. Therefore, the cutting face geometry of the
inventive cutting tap is optimized to allow effective cutting
angles where needed on the forward entry part of the chamfer, and
chip resistant cutting edges on later finishing portions of the
chamfer and threads axial rearward of the chamfer. In regard to the
cutting action of the cutting tap 40, the cutting tap 40 generates
an internal screw thread form by a succession of cutting edges on
the chamfered section of the tap. Material is removed from the wall
of the hole until the final thread form is obtained with the first
full thread on the constant diameter fluted section 56. This
progressive formation of an internal thread is shown in FIG. 6 by
superimposing the sections of material removed by each of the four
flutes.
[0047] In regard to ranges of the cutting angles, the cutting tap
40 made from cemented carbide can be effectively used when angle A1
is within the range of about 5 degrees negative to about 15 degrees
positive and the angle A2 is within the range of about 0 degrees to
about 25 degrees negative. The size of the radius R1 controls the
transition from the cutting angle A1 to the cutting angle A2 by
forming a chord between A1 and A2 that ranges in width from about 0
percent to about 80 percent of the thread height. An exemplary
chord N of a length P is shown in FIG. 6.
[0048] It should be appreciated that the balance of the cutting tap
flute leading to the cutting face of the cutting tap 40 can take
any shape used in current practice as long as the radius of the
flute is tangent to the line defined by angle A1.
[0049] Another option is to form the tap such that this profile
remains constant along both the chamfer and the body of the tap
past the chamfer. In this case, the cutting face angle at the
cutting edges will be A2 along the entire length. As the chip is
formed starting at the cutting edge and flows across the cutting
face, it will be first opposed by a low cutting angle A2 that
transitions through the radius R1 to a higher cutting angle A1.
[0050] In regard to the manufacture of the cutting tap, the cutting
tap is manufactured from a cylindrical blank composed of high-speed
steel or sintered tungsten carbide, frequently referred to as a
substrate. The blank has a diameter that is sized larger than the
finished dimensions of the cutting tap and is cut to length.
[0051] The first step in processing the substrate is to grind the
blank to precision cylindrical tolerances by methods, such as
cylindrical traverse grinding on centers or be centerless infeed
grinding methods. During this step, a cylindrical shank is ground
to size at the axially rearward end of the tap and the major
diameter of a threaded body portion is formed at the axially
forward end of the tap. Additionally during this process, or as a
consequence of an additional process, an optional neck portion may
be created with a cylindrical surface, and a bevel between the
cylindrical shank and the neck portion. Additionally, an optional
bevel may be ground on the ends of the taps by cylindrical
grinding. In general, the shank diameter is approximately equal to
the nominal thread diameter, but the shank diameter may be smaller
than the nominal thread diameter for large diameter taps, and
alternatively larger for small diameter taps. An option may be the
grinding of a square as part of the shank at the extreme axially
rearward end of the tap, as shown in FIG. 2.
[0052] In the next step, one or more flutes are ground so as to
provide cutting edges, in combination with the chamfer. The flutes
may be straight or helical, either right or left hand in any
combination with either right or left hand threads. As shown in
FIG. 10, the cutting angle A1 may be between about 20 degrees
negative for use in very hard materials to about 20 degrees
positive for very ductile materials.
[0053] Alternatively, the flute may be formed with a varying
cutting face angle along the length of the chamfer, as shown in
FIGS. 6-8. The shape of the grinding wheel is formed so as to
provide a cutting face with the selected cutting angles A1 and A2,
with A1 and A2 tangent to radius R1, where A1 is more positive than
A2. The balance of the flute may be shaped according to current
art, as long as A1 is tangent to a radius leading to the balance of
the flute. The complete form may be ground in one or two steps. For
example, the flute may be ground in two steps by first grinding the
flute according to current art, and then grinding the invented
cutting face in a following operation. Alternatively, the wheel may
he shaped so as to generate the complete form in one operation.
[0054] In the next step, the threaded body portion is ground to
form the V-shaped thread flank surfaces, along with minor and major
diameters, on the helix. Subsequently, the shape of a threaded
cutting chamfer portion is formed by grinding. The V-shaped thread
flank surfaces and major diameter replicate the internal screw
thread that is generated during tapping.
[0055] The cutting chamfer portion is ground with a taper so as to
allow entry in the hole to be tapped. The chamfer may be ground
either to form the chamfer on a curved line as shown in FIG. 4, or
by forming a chamfer with two or more sections formed by straight
lines at angles to the axis of the tap such that the chamfer angle
of the last section that approaches the first full cutting thread
is smaller than the chamfer angle of the first section, as shown in
FIG. 5. By either method, the sections of material removed by the
cutting edges approaching the first full cutting thread are smaller
than the thickness of sections of material removed on the entry
part of the chamfer.
[0056] The length of the chamfer may be as small as one (1) thread
pitch for tapping blind holes to as long as fifteen (15) thread
pitches when tapping very hard materials. The number of chamfer
sections each with a different angle (FIG. 5) will depend on the
overall length of the chamfer and will increase in number as the
overall chamfer length increases.
[0057] After the chamfer is ground, the effective cutting edge
angle is A1 with the first entry portion of the chamfer and
gradually progresses to cutting angle A2 in later finishing
portions of the chamfer. This combination will reduce the
likelihood of chipping by not only reducing the force on the
cutting edges approaching the first full cutting thread, but also
by increasing the strength of the same edges by reducing the
cutting face angle.
[0058] After grinding, the tap may be honed with abrasive media or
abrasive brushes so as to form a small radius on the cutting edges
and other sharp corners. The resulting radius may be between about
0 microns and about 100 microns. This honing further increases the
strength of these edges.
[0059] As a final step in the process, the tap may be optionally
coated with a wear resistant layer (not shown) of metal nitrides,
carbides, carbonitride, borides and/or oxides, wherein the metal is
chosen from one or more of the following: aluminum, silicon and the
transition metals from Groups IVa, Va and VIa of the Periodic
Chart. This layer is deposited as a single monolayer or in multiple
layers, including alternating layers. Low friction layers can also
be deposited on top of these wear resistant layers.
[0060] As can be appreciated, the invention provides a cutting tap
that allows for the use of a cemented carbide cutting tap that is
not prone to chipping. The use of a cemented carbide cutting tap
possesses a number of advantages as compared to a tap made of
"high-speed" steel. For example, the cemented carbide cutting tap
results in an improvement of the dimensional accuracy with respect
to the size and shape of the threads as compared to high speed
steel cutting taps. In addition, a cemented carbide cutting tap
results in an increase in the useful tool life of the cutting tap
as compared to high speed steel cutting taps. Further, a cemented
carbide cutting tap increases the production speed for internal
screw threads as compared to a high speed steel cutting tap.
[0061] The documents, patents and patent applications referred to
herein are hereby incorporated by reference.
[0062] While the invention has been specifically described in
connection with certain specific embodiments thereof, it is to be
understood that this is by way of illustration and not of
limitation, and the scope of the appended claims should be
construed as broadly as the prior art will permit.
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