U.S. patent application number 10/732061 was filed with the patent office on 2004-06-24 for screw vibration assisted tapping device.
Invention is credited to Zhang, Bi.
Application Number | 20040121848 10/732061 |
Document ID | / |
Family ID | 32595089 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040121848 |
Kind Code |
A1 |
Zhang, Bi |
June 24, 2004 |
Screw vibration assisted tapping device
Abstract
A vibration assisted tapping device includes an elastic frame
for translating axial vibration to a vibration having both axial
and torsional components. The elastic frame has flexural members
that connect an upper plate to a lower plate with a vibratory
actuator preloaded therebetween. The flexural members are inclined
relative to an axis of the frame in a direction calculated to
result in translated vibration substantially aligned with the lead
angle of the thread being cut. Also disclosed is an automated
system for applying different vibration patterns to the
tap/workpiece interface during tapping and for recording the
tapping torque associated with each vibration pattern. The
disclosed system permits identification of the vibration frequency
and amplitude that results in the greatest reduction in tapping
torque. The automated system may be configured as an adaptive
machine tool to first identify and then apply the optimum vibration
pattern.
Inventors: |
Zhang, Bi; (Storrs,
CT) |
Correspondence
Address: |
ALIX YALE & RISTAS LLP
750 MAIN STREET
SUITE 1400
HARTFORD
CT
06103
US
|
Family ID: |
32595089 |
Appl. No.: |
10/732061 |
Filed: |
December 10, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60432837 |
Dec 12, 2002 |
|
|
|
Current U.S.
Class: |
470/96 |
Current CPC
Class: |
B23G 1/46 20130101; B23B
29/125 20130101; B23G 1/04 20130101 |
Class at
Publication: |
470/096 |
International
Class: |
B21K 001/64 |
Claims
What is claimed is:
1. A screw-vibration assisted tapping device for tapping threads
having a lead angle, said vibration assisted tapping device
comprising: an upper plate for supporting a workpiece or tap; a
lower plate axially spaced from said upper plate; a vibratory
actuator fixed to one of said upper or lower plates and arranged to
exert a reciprocal axial vibration on the other of said upper or
lower plate; and a plurality of flexural members connecting said
upper plate to said lower plate, each said flexural member inclined
relative to a longitudinal axis at an angle equal to or greater
than the lead angle of the threads to be tapped, wherein the
reciprocal axial vibration is translated into a screw-vibration
having axial and torsional elements.
2. The screw-vibration assisted tapping device of claim 1, wherein
said upper plate, lower plate and plurality of flexural members are
formed from a single piece of metal.
3. The screw-vibration assisted tapping device of claim 1, wherein
said vibratory actuator is a piezoelectric actuator.
4. The screw-vibration assisted tapping device of claim 1, wherein
said vibration has a frequency of between approximately 400 Hz and
approximately 800 Hz.
5. The screw-vibration assisted tapping device of claim 1, wherein
said vibratory actuator has a first axial stiffness and said upper
plate, lower plate and flexural members are connected to form an
assembly with a second axial stiffness of approximately equal to
said first axial stiffness.
6. The screw-vibration assisted tapping device of claim 1, wherein
the plurality of flexural members comprise six or more flexural
members integrally connecting the upper plate to the lower
plate.
7. The screw-vibration assisted tapping device of claim 6, wherein
the flexural members are arranged around the periphery of said
upper and lower plates and the vibratory actuator is arranged along
the axis of the device.
8. The screw-vibration assisted tapping device of claim 6, wherein
said flexural members are angled such that the motion induced in
the plate supporting the workpiece or tap is substantially aligned
with a helix defined by the thread to be cut.
9. An adaptive machine tool comprising: a vibration assisted
cutting tool, said vibration assisted cutting tool having a
vibration pattern comprising a frequency and an amplitude that are
variable in response to a driving signal; a sensor for measuring
the specific energy of material removal by the vibration assisted
cutting tool; and a computer programmed to apply a plurality of
driving signals to said vibration assisted cutting tool to produce
a plurality of vibration patterns with different frequencies and/or
amplitudes, said computer further programmed to record the specific
energy of material removal for each of said plurality of vibration
patterns, wherein said computer is programmed to identify the
vibration pattern that has the smallest specific energy of material
removal and to apply that vibration pattern to the vibration
assisted cutting tool.
10. A method for optimizing the vibration assistance applied to a
tapping operation comprising: applying a plurality of vibration
patterns to a tap/workpiece interface of the tapping operation,
each vibration pattern comprising a vibration frequency, a
vibration amplitude and a vibration direction, said plurality of
vibration patterns comprising a stepwise decrease or increase in
the vibration frequency and/or vibration amplitude over a range of
vibration directions centered on a lead angle of the thread being
tapped; recording torque data comprising a tapping torque
associated with each said vibration pattern; and using the torque
data to identify the optimum vibration frequency and vibration
amplitude for the operation by identifying the vibration pattern or
patterns associated with the lowest tapping torque.
11. A screw-vibration assisted device for performing drilling,
wrenching or other machining process having a lead angle, said
vibration assisted device comprising: an upper plate for supporting
a workpiece; a lower plate axially spaced from said upper plate; a
vibratory actuator fixed to one of said upper or lower plates and
arranged to exert a reciprocal axial vibration on the other of said
upper or lower plate; and a plurality of flexural members
connecting said upper plate to said lower plate, each said flexural
member inclined relative to a longitudinal axis at an angle equal
to or greater than the lead angle appropriate for the process to be
performed, wherein the reciprocal axial vibration is translated
into a screw-vibration having axial and torsional elements.
12. The screw-vibration assisted tapping device of claim 1, wherein
said vibratory actuator is selected from the group comprising
ultrasonic drives, magnetostrictive drives, electrostrictive drives
and mechanical drives.
13. The screw-vibration assisted device of claim 11, wherein said
vibratory actuator is selected from the group comprising ultrasonic
drives, hydraulic drives, magnetostrictive drives, electrostrictive
drives and mechanical drives.
14. The screw-vibration assisted tapping device of claim 1, wherein
the plurality of flexural members comprise more than three flexural
members integrally connecting the upper plate to the lower
plate.
15. The screw-vibration assisted device of claim 11, wherein the
plurality of flexural members comprise more than three flexural
members integrally connecting the upper plate to the lower
plate.
16. The adaptive machine tool of claim 9 wherein said vibration
assisted cutting tool is incorporated into a workpiece support
element such as a work table or a work surface.
17. The adaptive machine tool of claim 9 wherein said vibration
assisted cutting tool is incorporated into a holder element such as
a tool holder, chuck or collet.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to the field of vibration-assisted
machining processes and more particularly to methods and devices
for vibration-assisted tapping.
[0003] 2. Description of the Related Art
[0004] Among the machining processes practiced in modern
manufacturing, threading is one of the most time-consuming and,
therefore, often results in a production bottleneck. Meanwhile,
demand for high quality, high accuracy threaded holes, specifically
those with a small diameter and a long depth has increased. Current
threading technologies cannot fully satisfy this demand, which
creates a need for an efficient and inexpensive threading
technology.
[0005] It is known that the application of vibration to the tap or
workpiece during tapping can reduce the force required for tapping
and increase the useful life of the tap. For example, ultrasonic
vibration has been successfully applied to assist various machining
processes for many years. Various experiments have tested axial
vibration-assisted tapping. Axial vibration-assisted tapping
experiments have generally concluded that, while tapping torque was
reduced and tap life extended, the accuracy of the threads were
unacceptably reduced when compared to threads tapped without
vibration assistance. Further experiments have tested torsional
vibration-assisted tapping, finding that tapping torque is reduced,
tap life is extended and thread accuracy is improved relative to
threads tapped without vibration assistance.
[0006] Ultrasonic (>20 KHz) screw-vibration has been applied to
assist the tapping process on pure aluminum, and found that chip
thickness and tapping torque were reduced, while the surface
integrity and accuracy of the threads were significantly improved
(Suzuki et al. (1991)). Shamoto et al (1994 & 1996) applied a
synchronized two-directional vibration to the machining of copper
to form an eliptical vibration cutting pattern that reduces chip
thickness, cutting forces and the specific energy of material
removal while significantly improving the surface integrity of the
machined surface.
[0007] The results of vibration assisted machining and the above
discussed experiments indicate that application of vibration to the
tapping process may reduce tapping torque, lengthen tool life and
improve the quality of the thread produced. There remains a need in
the art for an empirically proven, reliable and inexpensive
vibration-assistance device that can be incorporated into existing
thread-tapping equipment. Preferably, such a vibration-assistance
device would permit the resulting vibration to be tailored to a
particular tapping process and will require little or no
modification of the existing thread-tapping equipment.
SUMMARY OF THE INVENTION
[0008] The torque required for tapping includes the torque required
for material removal as well as relief face friction due to spring
back (elastic recovery) of the machined surface. Vibration tapping
can reduce tapping torque mostly because the workpiece surface is
repeatedly cut, each successive cut removing a further portion of
the material bearing on the relief face of the tap. The more times
the workpiece surface is cut, the smaller the frictional force
should be.
[0009] Repeated cutting is a function of vibration direction,
amplitude and frequency. Vibration above a certain frequency tends
to reduce the amplitude of cutting face oscillation, thereby
reducing the overlapping length of repeated cutting. Axial
vibration adversely affects the tapping process by increasing
thread error. Further axial vibration fails to improve the number
of cutting times because the tap cutting takes place primarily in
an axial direction with a small torsional element. Intuitively,
torsional vibration should have a beneficial effect on thread
tapping by increasing repeated cutting, as experimentation has
proved. However, torsional cutting does not account for the axial
component of a helix defined by the thread being cut.
[0010] Briefly stated, an exemplary embodiment of a
vibration-assisted tapping device comprises an elastic frame
surrounding a source of axial vibration. The frame includes an
upper plate and lower plate connected by angled flexural members.
The axial vibrator is fixed to one of the upper or lower plates and
biased against the other of the upper or lower plates to apply its
axial vibratory force to the elastic frame. The flexural members
are inclined relative to a longitudinal axis of the device at an
angle equal to or greater than the lead angle of the thread being
tapped.
[0011] An exemplary embodiment of the frame is machined from a
single piece of spring steel. A plurality of flexural members is
arranged around the periphery of the frame to connect the upper
plate to the lower plate. A piezoelectric vibratory actuator is
aligned with the central axis of the frame and biased or preloaded
relative to the frame such that the axial vibratory motion is
efficiently transmitted to the frame. Axial stretching of the frame
by the actuator is translated by the inclined flexural members into
relative movement between the upper and lower plates having axial
and torsional components. The configuration of the frame, i.e., the
angle of the flexural members, the stiffness and elasticity of the
frame, the frequency and amplitude of the vibration applied,
combine to produce a relative movement between the upper and lower
plates of the frame. This relative movement may be substantially
aligned with a helix defined by the thread being tapped.
[0012] A driver circuit operatively connected to the vibratory
actuator allows the frequency of the vibration as well as the
effective force, and hence the amplitude of the vibration to be
varied. A torque sensor is arranged to measure the torque required
for tapping over a range of vibration frequencies and amplitudes.
This arrangement allows for testing to determine the most effective
combination of vibration frequency and amplitude for a particular
tapping process.
[0013] An object of the invention is to provide a new and improved
vibration assisted tapping device that is inexpensive,
technologically simple and improves the speed and accuracy of
thread tapping.
[0014] Another object of the present invention is to provide a new
and improved vibration-assisted tapping device that produces a
screw vibratory motion substantially aligned with a helix defined
by the thread being tapped.
[0015] A further object of the present invention is to provide a
new and improved vibration-assisted tapping device and a method for
matching the vibration assistance to a particular thread tapping
process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] These and other objects, features and advantages of the
invention will become readily apparent to those skilled in the art
upon reading the description of the exemplary embodiments, in
conjunction with the accompanying drawings, in which:
[0017] FIG. 1 is an elevational exterior view of an exemplary frame
for translating axial vibration into vibration having an axial and
torsional component in accordance with a first aspect of the
present invention;
[0018] FIG. 2 is an exemplary tapping arrangement incorporating an
alternative exemplary frame supporting a workpiece;
[0019] FIG. 3 is an exterior view of a vibration-assisted tap
incorporating an alternative exemplary frame in accordance with an
aspect of the present invention;
[0020] FIG. 4 illustrates an exemplary screw thread for which a
tapping operation may be carried out by a vibration-assisted
tapping device in accordance with the present invention;
[0021] FIGS. 4.2.1-4.2.16 graphically illustrate tapping torque
ranges as a function of driving frequency in Hz and driving
voltage, while associated Tables 4.2.1-4.2.16 illustrate the
experimental parameters and resulting data for the corresponding
Figures;
[0022] FIG. 5 is a sectional view through a workpiece holder for
use in conjunction with the vibration-assisted tap holder shown in
FIG. 3; and
[0023] FIG. 6 is a functional block diagram of a system for
applying the experimental parameters of Tables 4.2.1-4.2.16 to
particular tapping operations and recording the resulting tapping
torque data.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] With reference to FIG. 1, an aspect of the present invention
relates to a frame 10 configured to translate an applied axial
vibration into vibration having axial and torsional components. The
exemplary frame 10 of FIG. 1 comprises a top plate 12 connected to
a bottom plate 16 by a plurality of angled flexural members 14. The
exemplary frame 10 is machined from a single piece of spring steel,
although alternative materials and methods of construction may
occur to one of skill in the art. The bottom plate 16 is in the
form of a ring integrally connected to the bottom of the flexural
members 14. This bottom plate configuration is a result of
machining the frame 10 from a single piece of steel.
[0025] The exemplary frame 10 incorporates six flexural members 14
connecting the upper plate 12 to the lower plate 16. Each of the
six flexural members 14 is inclined relative to an axis A of the
frame 10. The frame 10 and its angled flexural members 14 are
configured such that, when exposed to that portion of the axial
vibration which seeks to push the upper and lower plates apart,
this axial spreading is translated into relative movement between
the upper and lower plates having both an axial and torsional
component. In the context of this application, this movement will
be referred to as "screw vibration" and is illustrated as
reciprocating movement generally along a path illustrated by arrow
40. It can be seen from FIG. 1 that arrow 40 defines a path having
both an axial component and a torsional component. The incline
angle .O slashed.s, elasticity and stiffness of the flexural
members 14 are calculated to produce a screw vibration 40 having an
angle relative to a plane normal to the frame axis A that is
substantially equal to or greater than the lead angle .O
slashed..sub.L of the thread being cut.
[0026] FIG. 4 illustrates an exemplary screw 30 having a
single-lead thread 32. The illustrated single-lead thread 32
defines a helix around a central axis B. The thread 32 has a lead
angle .O slashed..sub.L relative to a plane P normal to the axis B
of the helix defined by the thread 32. According to an aspect of
the present invention, the frame 10 is configured to translate
axial vibration into screw vibration substantially aligned with the
lead angle of the thread being cut. Experimentally, a flexural
member angle .O slashed..sub.S relative to the frame axis A equal
to or greater than the lead angle .O slashed..sub.L of the thread
being cut has proven to produce screw vibration which produces a
greater reduction in tapping torque than an equivalent torsional
vibration. Screw vibration along path 40 is more closely aligned
with the helical configuration of the thread being cut, thereby
improving the accuracy of the resulting threads.
[0027] The inventive frame may be incorporated into a work surface
such as that of a work table and used to support the workpiece as
shown in FIG. 2 or it may be incorporated into a tool holder, chuck
or collet such as a tap holder 60 as shown in FIG. 3. The frame 10a
illustrated in FIG. 2 incorporates four flexural members 14
connecting the top plate 12 to the bottom plate 16. In the
exemplary configuration shown in FIG. 2, a piezoelectric vibratory
actuator 50 is arranged along the axis of the frame. The
actuator/frame assembly is configured such that the actuator 50 is
compressed or preloaded between the upper and lower plates 12, 16.
Each of the actuator 50 and frame 10a have an axial stiffness. In
accordance with an aspect of the invention, the axial stiffness of
the actuator 50 is approximately equal to the axial stiffness of
the frame 10a. This relationship has proven to result in an
efficient translation of actuator vibration into screw vibration by
the frame 10a.
[0028] FIG. 3 shows an exemplary vibration-assisted tap holder 60
incorporating a frame 10b and vibratory actuator 50. It will be
noted that the position of the upper and lower plates 12, 16 of the
frame 10b are reversed with respect to the orientation of the upper
and lower plates shown in FIG. 2. This frame 10b is reversed so
that the resulting screw vibration path 40 is substantially aligned
with the path of thread cutting. In FIG. 3, clockwise rotation of
the tap 62 to cut a conventional right hand thread is reinforced by
screw vibration along path 40 produced by the inventive frame 10b
and vibratory actuator 50. It is understood that a left-hand thread
would be tapped with a vibration-assisted tap holder with flexural
members inclined at an angle of--.O slashed..sub.S relative to axis
A. The opposite helical thread configuration of the left-hand
thread would require a corresponding opposite angular inclination
of the flexural members 14.
[0029] In FIG. 2, it is the workpiece to which the screw vibration
is being applied. The illustrated orientation of frame 10a and
vibratory actuator 50 produces a screw vibration along a path 40
substantially aligned with the path of thread cutting made by the
tap 62. It will be understood that the configuration illustrated in
FIG. 2 is practical only if the workpiece 64 is small enough to
vibrate. It will also be understood that larger workpieces and/or
larger tap diameters will have greater tapping torque levels and
greater masses to be vibrated and will likely require a vibratory
actuator capable of producing a larger force to produce
satisfactory results.
[0030] FIGS. 2, 3, 5 and 6 illustrate a basic configuration for an
adaptive vibration assisted tapping device in accordance with
several aspects of the present invention. In FIG. 2, the workpiece
holder 80 is mounted to a torque sensor 70 arranged to measure the
tapping torque applied to the workpiece 64. FIG. 5 illustrates an
alternative workpiece holder 80a for use in conjunction with the
vibration assisted tap holder of FIG. 3. The workpiece holder 80a
is also supported on a torque sensor 70. Of course, a torque sensor
may also be incorporated into a vibration assisted tap holder such
as that illustrated in FIG. 3. The piezoelectric vibratory
actuators 50 of FIGS. 2 and 3 are responsive to oscillating signals
that may take various forms such as a square wave, sine wave, or
the like. The amplitude (in volts) and frequency of the driving
signal determine the force and frequency of the vibration produced
by the actuators.
[0031] FIG. 6 illustrates a system for automating the application
of different vibration patterns to a tapping operation and for
collecting torque data associated with each vibration pattern. A
computer 100 controls the piezoelectric driver 130 and receives
torque sensor readings from a charge amplifier 120 via an interface
box 110. The computer 100 may be programmed to cycle through a
range of vibration frequencies and driving voltages and record the
resulting tapping torque for each frequency/driving voltage point.
The resulting data can be used to determine the most effective
vibration pattern for a particular tapping operation. This
arrangement might be incorporated into a machine tool for the
purpose of producing an adaptive machine tool. When the adaptive
machine tool has cycled through the available range of frequencies
and driving voltages, it may be programmed to return to the
frequency/driving voltage combination that produced the greatest
tapping torque reduction. Alternatively, the system of FIG. 6 might
be used to configure a tapping machine to perform many
substantially identical tapping operations. The tapping machine
could be configured to produce vibration assistance at the
frequency and amplitude that was determined experimentally to
provide the greatest tapping torque reduction.
[0032] Tables 4.2.1-4.2.16 and related FIGS. 4.2.1-4.2.16 are
constructed from data gathered from a series of tapping operations
by a system such as that illustrated in FIG. 6. Tables 4.2.1-4.2.16
illustrate the tapping torque in Newton centimeters (N-cms) for a
given driving frequency (200 Hz, 400 Hz, 600 Hz, and 800 Hz) and
driving voltage (0V, 2V, 4V, 6V, 8V, and 10V) for four tap
diameters (A=4-40, B=6-32, C=8-32, and D=10-32) and four materials
(Aluminum Alloy 1100, Aluminum Alloy 6061, Stainless Steel 304 and
Carbon Steel 1018) at a hole depth of {fraction (3/8)} inch and a
spindle speed of 80 rpm. Corresponding FIGS. 4.2.1-4.2.16
graphically illustrate the experimental results in terms of tapping
torque ranges as a function of driving frequency in Hz and driving
voltage. For the piezoelectric actuator used, the most effective
vibratory frequency is generally in the range between approximately
400 Hz and 800 Hz. In other words, a driving frequency in this
range produces the greatest reduction in tapping torque for a given
drive voltage.
[0033] The experimental results suggest that tapping operations
involving different materials, tap sizes, thread types and tap
rotational speeds are likely to require a different vibration
frequency and/or amplitude to achieve the maximum available tapping
torque reduction. An arrangement such as that disclosed which
allows the tapping torque to be measured over a range of vibration
frequencies and amplitudes will permit selection of the most
effective combination for a given tapping operation. Generally
speaking, the experimental results suggest that vibration
frequencies above approximately 1000 Hz become less effective
because the amplitude of the resulting screw vibration goes down,
reducing the overlapping length of repeated cutting. Also generally
speaking, larger taps require larger driving voltages to produce
effective vibration amplitudes due to the increased mass of the tap
and the relatively higher tapping torque.
[0034] The illustrated embodiments incorporate a piezoelectric
vibratory actuator although other actuators will occur to one of
skill in the art and may be applicable to arrangements in
accordance with the present invention.
[0035] A screw vibration assisted tapping device in accordance with
the present invention is advantageously compact and includes no
moving parts. The simple configuration can be produced using known
and well-established manufacturing techniques. The resulting
assembly is extremely rugged and should have a long service life
when incorporated into tapping equipment. The materials and
configuration of the frame may be selected to produce a screw
vibration tailored for the threads being tapped.
[0036] While exemplary embodiments of the invention have been shown
and described for purposes of illustration, the foregoing
descriptions should not be deemed a limitation of the invention
herein. Accordingly, various modification, adaptations and
alternatives may occur to one skilled in the art without departing
from the spirit and the scope of the present invention.
* * * * *