U.S. patent application number 11/926929 was filed with the patent office on 2008-05-29 for machining apparatus.
This patent application is currently assigned to FANUC LTD. Invention is credited to Kenzo Ebihara, Tomohiko Kawai, Takayuki Oda.
Application Number | 20080125015 11/926929 |
Document ID | / |
Family ID | 39092660 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080125015 |
Kind Code |
A1 |
Kawai; Tomohiko ; et
al. |
May 29, 2008 |
MACHINING APPARATUS
Abstract
A machining apparatus capable of obscuring periodic machining
irregularities. A workpiece W is mounted on a workpiece mounting
surface of a worktable, and a tool T is supported by an elastic
member (leaf spring) that is attached to a movable unit. A
piezoelectric element is provided on the back of the leaf spring,
and a driving voltage with irregular frequency and amplitude is
applied to the piezoelectric element by a white noise generator
during machining. Thus, a relative vibration displacement in a
cutting-depth direction (vertical direction) is induced between the
tool T and the workpiece W. In the case of a lathe apparatus, a
relative vibration displacement is applied in the direction
(horizontal direction) of a rotary axis of a workpiece fixture.
Inventors: |
Kawai; Tomohiko;
(Minamitsuru-gun, JP) ; Ebihara; Kenzo;
(Minamitsuru-gun, JP) ; Oda; Takayuki;
(Minamitsuru-gun, JP) |
Correspondence
Address: |
LOWE HAUPTMAN HAM & BERNER, LLP
1700 DIAGONAL ROAD, SUITE 300
ALEXANDRIA
VA
22314
US
|
Assignee: |
FANUC LTD
Minamitsuru-gun
JP
|
Family ID: |
39092660 |
Appl. No.: |
11/926929 |
Filed: |
October 29, 2007 |
Current U.S.
Class: |
451/11 |
Current CPC
Class: |
B23B 2260/108 20130101;
B23B 29/125 20130101; B23B 5/00 20130101 |
Class at
Publication: |
451/11 |
International
Class: |
B24B 49/10 20060101
B24B049/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2006 |
JP |
2006-318957 |
Claims
1. A machining apparatus comprising: a tool for performing
machining on a surface of a workpiece; a relative motion applying
mechanism that applies relative motion to said tool with respect to
the workpiece in three-dimensional directions including a
machining-depth direction in which said tool moves closer to and
away from the surface of the workpiece; and a
vibratory-displacement applying mechanism that applies vibratory
displacements with frequency and amplitude thereof varying
irregularly to one of said tool and the workpiece in the
machining-depth direction during the machining of the workpiece by
said tool.
2. A machining apparatus according to claim 1, wherein said
vibratory-displacement applying mechanism comprises a piezoelectric
element that makes vibratory displacements proportional to a ripple
voltage having irregular frequency and amplitude.
3. A machining apparatus according to claim 2, wherein the ripple
voltage is generated by using a white noise generator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a machining apparatus for
machining a surface of a workpiece with use of a tool, and more
specifically, to a technique for improving a machining apparatus
capable of relatively three-dimensionally moving a tool with
respect to a workpiece so that a periodic change of machined
surface accuracy, if any, is obscured on a machined surface.
[0003] 2. Description of the Related Art
[0004] It is well-known that precision machining is performed by
using a machining apparatus that can relatively three-dimensionally
move a tool with respect to a workpiece. As a typical example of
this precision machining, there is machining of a mold for a
precision optical component, such as a light guide plate used in a
liquid crystal display, DVD pickup lens, etc. Such machining
requires uniformity in surface accuracy as well as a very high
surface accuracy of nanometer order. In the machining of a mold
(mold part) used in the manufacture of a light guide plate, for
example, the required machining accuracy for a machined surface
generally ranges from about 1 to 40 nm, as shown in FIG. 1. It is
undesirable, however, that the machining accuracy should vary
depending on the position on the machined surface.
[0005] In actual machining, however, it is not seldom that
non-negligible variation in surface accuracy develops with
periodicity at the location of manifestation. If a surface accuracy
of less than 10 nm is aimed at in the machining of the mold part
shown in FIG. 1, for example, belt-shaped regions of which the
surface accuracy is reduced to 10 to 20 nm may be periodically
generated, as shown in FIG. 2. The shape of the belt-shaped regions
with the reduced surface accuracy varies according to the type,
shape, etc. of machining. In the case of, for example, lathe
machining, as mentioned later, only concentric and radial shapes
can be obtained, and they share the "periodicity at the location of
manifestation" in common.
[0006] It is to be understood that the reduction of the machining
accuracy caused with the periodicity at the location of
manifestation is undesirable. If the workpiece to be machined is an
optical component or a mold (or mold part) used in the manufacture
of the optical component, in particular, it is not rare that local
unevenness in optical properties can be recognized macroscopically.
Even if the local unevenness cannot be recognized macroscopically,
moreover, it can be distinctly observed through a microscope in
many cases. In any case, the performance of an apparatus that uses
this optical component is lowered.
[0007] According to the aforesaid example (FIG. 2), the reduction
of the surface accuracy itself is only more than 10 nm. Owing to
regular development, however, it is impossible to fulfill the
optical properties that are required by the light guide plate, in
many cases. FIG. 1 shows the light guide plate manufactured using
the mold part with the surface accuracy of its entire machined
surface ranging from 10 to 20 nm. FIG. 2 shows a light guide plate
manufactured using a mold part that includes alternately developed
regions with a surface accuracy of less than 10 nm and a surface
accuracy of 10 to 20 nm. Comparison between these light guide
plates indicates that the latter is more liable to cause problems
on optical properties than the former.
[0008] In many cases, the aforementioned periodic machining
irregularities may be supposed to be caused by fine vibration of a
damper or the like that is used in installing a spindle for
machining or a machining apparatus. Although the problems can be
solved by using a high-performance spindle and damper whose
vibrations are on the nanometer level, therefore, these elements
are very expensive. In some cases, moreover, the cause of the
periodic vibration cannot be located and easily coped with.
[0009] Accordingly, there is a demand for a problem-solving
technique that dispenses with the use of the expensive spindle or
damper and is also applicable to the case where the cause of the
periodic vibration cannot be located. According to an embodiment of
the present invention, as mentioned later, fine irregular
displacement is artificially caused between workpiece and tool by
means of a piezoelectric element. Disclosed in JP 2005-7519A, on
the other hand, is a technique for cutting a workpiece by utilizing
a piezoelectric element. Thus, this disclosed technique shares the
"utilization of the piezoelectric element for the generation of the
fine displacement between workpiece and tool" in common with the
present invention.
[0010] However, the technique disclosed in JP 2005-7519A is not
intended to solve the problems of the aforementioned periodic
machining irregularities. Further, this document contains no
description of application of a voltage with irregular frequency
and amplitude to the piezoelectric element, which is described in
connection with the embodiment of the present invention.
SUMMARY OF THE INVENTION
[0011] The present invention provides a technique that solves those
problems caused by the periodic machining irregularities without
preparing any expensive high-performance spindle or damper, and is
applicable to the case where the cause of periodic vibration cannot
be specified.
[0012] A machining apparatus of the present invention comprises: a
tool for performing machining on a surface of a workpiece; a
relative motion applying mechanism that applies relative motion to
the tool with respect to the workpiece in three-dimensional
directions including a machining-depth direction in which the tool
moves closer to and away from the surface of the workpiece; and a
vibratory-displacement applying mechanism that applies vibratory
displacements with frequency and amplitude thereof varying
irregularly to one of the tool and the workpiece in the
machining-depth direction during the machining of the workpiece by
the tool.
[0013] The vibratory-displacement applying mechanism may comprise a
piezoelectric element that makes vibratory displacements
proportional to a ripple voltage having irregular frequency and
amplitude. The ripple voltage may be generated by using a white
noise generator.
[0014] If a regular vibration attributable to disturbance during
machining exists between the tool and the workpiece so as to cause
periodic machining irregularities that reflect the vibration,
according to the present invention, a relative displacement in the
direction of cut of by the tool is applied as a vibration
displacement with irregularly variable frequency and amplitude
between the tool and the workpiece. Therefore, the periodic
machining irregularities are obscured by cutting corresponding to
the vibration displacement, so that the uniformity of the machining
accuracy can be improved.
[0015] If the workpiece to be machined is an optical component or a
mold (or mold part) used in the manufacture of the optical
component, in particular, bad influences on its optical properties
can be suppressed and obscured. Further, the present invention is
economically advantageous because it requires no preparation of any
expensive, high-performance spindle or damper. The invention also
has an advantage of being applicable to the case where the cause of
periodic vibration cannot be located and removed even with use of
an expensive, high-performance spindle or damper.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a view showing a situation in which the machining
accuracy for a machined surface is substantially uniform;
[0017] FIG. 2 is a view illustrating belt-shaped regions
periodically generated such that the machining accuracy for the
machined surface is reduced;
[0018] FIGS. 3a and 3b illustrate a machining apparatus according
to one embodiment of the present invention, in which FIG. 3a is a
sectional view showing an outline of the machining apparatus and
FIG. 3b is a view illustrating the way a white noise generator is
used to apply vibration voltage to a piezoelectric element in the
machining apparatus;.
[0019] FIG. 4 is a diagram showing an example of the waveform of
the vibration voltage applied to the piezoelectric element;
[0020] FIG. 5 illustrates an external view of a lathe apparatus to
which the invention is applied;
[0021] FIGS. 6a and 6b illustrate an example of a machined surface
obtained by machining without applying the invention to the lathe
apparatus shown in FIG. 5, in which FIG. 6a is a front view and
FIG. 6b is a sectional view taken along line A-A of FIG. 6a;
[0022] FIGS. 7a and 7b illustrate another example of the machined
surface obtained by machining without applying the invention to the
lathe apparatus shown in FIG. 5, in which FIG. 7a is a front view
and FIG. 7b is a sectional view taken along line B-B of FIG.
7a;
[0023] FIGS. 8a and 8b illustrate another example of the machined
surface obtained by machining as a structure based on the invention
applied to the lathe apparatus shown in FIG. 5, in which FIG. 8a is
a front view and FIG. 8b is a sectional view taken along line C-C
of FIG. 8a; and
[0024] FIG. 9 is a view illustrating an example of a structure for
applying a vibration displacement to a workpiece.
DETAILED DESCRIPTION
[0025] FIGS. 3a and 3b are views for illustrating a machining
apparatus according to one embodiment of the present invention.
Referring first to FIG 3a that shows an outline of the machining
apparatus with a workpiece mounted therein, numeral 10 denotes a
worktable. A workpiece W is mounted on its workpiece mounting
surface 11. A tool T for machining the workpiece W is supported on
a movable unit 21 by an elastic member 22 that is attached to the
movable unit 21, and can move relatively to the workpiece W with a
three-dimensional degree of freedom.
[0026] In the case of this embodiment, a relative motion along a
direction of depth of cutting by the tool T is achieved by
ascent/descent motion of the worktable 10 by a lift mechanism (not
shown). In FIG. 3a, the direction of the ascent/descent motion
(i.e., the direction of the cutting depth by the tool T) is
described as "first direction". The first direction can also be
referred to as "direction in which the tool T is moved closer
to/away from the workpiece W".
[0027] The remaining two degrees of freedom of movement are given
to the movable unit 21. Specifically, the movable unit 21 is
configured to move in a second direction perpendicular to (or
inclined at a predetermined angle to, in some cases) the first
direction and a third direction (not shown) equivalent to a
direction traversing the drawing plane of FIG. 3a (typically,
perpendicular to both the first and second directions) by a
two-dimensional drive mechanism (not shown) that is provided on a
fixed unit 20.
[0028] As is generally known, the shape into which the workpiece W
is machined by means of the tool T is usually specified by a
machining program, and the necessary movements in the three
directions (first to third directions) for the machining are
executed by operations of servo axes that cover the movements in
the individual directions.
[0029] By way of example, the machining apparatus described herein
is used in machining such that a large number of fine grooves that
traverse in the horizontal direction of FIG. 1 (corresponding to
the second direction) are cut at fine intervals (corresponding to
repeated movement pitches in the third direction) on the upper
surface of a mold part shown in FIG. 1. FIG. 2 illustrates a
phenomenon that low-accuracy regions periodically develop like
belts along the horizontal direction of FIG. 2 (corresponding to
the second direction) during the machining.
[0030] In order to obscure this phenomenon, according to the
present embodiment, a piezoelectric element 30 is provided in the
manner shown in FIG. 3a. As shown in FIG. 3b, the piezoelectric
element 30 is connected to a white noise generator 41 in a
controller (CNC) 40 so that it can be driven by a driving voltage
with irregular frequency and amplitude generated by the generator
41. ON/OFF operation for the output of the white noise generator
41, maximum amplitude, etc., can be controlled by a command from a
CPU (not shown) in the CNC 40.
[0031] In the present embodiment, as shown in FIG. 3a, a leaf
spring with a U-shaped profile is used as the elastic member 22 and
it is fixed in the illustrated posture to the movable unit 21. A
gap is defined between the movable unit and the back surface of the
leaf spring 22 (surface opposite from the surface on which the tool
T is mounted), and the piezoelectric element 30 is located so as to
fill the gap. The piezoelectric element 30 is mounted on the
movable unit 21, while its operating surface (which is deformed in
the first direction in response to applied voltage) is mechanically
coupled to the back surface of the leaf spring 22 by, for example,
adhesive bonding.
[0032] If the driving voltage with irregular frequency and
amplitude is output from the white noise generator 41 in the CNC 40
the moment the machining is started, therefore, the piezoelectric
element 30 is actuated to realize machining such that the cutting
depth in the machined surface of the workpiece W varies depending
on vibration of the tool T, thereby adding indentations of a fine
height (depth). The addition of these indentations serves to
obscure periodic reduction (unevenness) of the machining
accuracy.
[0033] As is generally known, the piezoelectric element 30 is an
element that causes a deformation that is substantially
proportional to the applied driving voltage (piezoelectric applied
voltage) based on a piezo effect. If the applied voltage and a
deformation (displacement) caused thereby are V(t) and d(t),
respectively, d(t) is
d(t)=kV(t) (1)
where t is time.
[0034] In the above equation (1), k is a proportionality constant,
which may, for example, be given by k=10 nm/volt [0 volt=V(t)=10
volts], among various other available values.
[0035] As seen from the above proportional relationship, a
"vibration waveform with irregularly variable frequency and
amplitude" should only be used as the applied voltage V(t) in order
to realize a "relative displacement in the first direction between
the tool T and the workpiece W is a vibration displacement with
irregularly variable frequency and amplitude". It is known that the
white noise generator can be utilized as a source of such a
vibration waveform, and this generator is used in the present
embodiment (see FIG. 3b).
[0036] FIG. 4 shows an example of the "vibration waveform with
irregularly variable frequency and amplitude". If a driving voltage
with the vibration waveform shown in FIG. 4 is applied to the
piezoelectric element, a vibration displacement with a similar
waveform can be obtained. The amplitude of the vibration
displacement can be determined according to the aforesaid equation
(1). If the piezoelectric element 30 has the aforementioned value
of k and if an irregular displacement with a maximum amplitude of
about 20 nm is required, the applied voltage V(t) should only be
selected having the waveform shown in FIG. 4 and the maximum
amplitude of about 2 volts.
[0037] In general, the amplitude of the vibration displacement to
be applied between the tool and the workpiece depends on the degree
of unevenness in accuracy that is developed when no vibration is
applied. If the unevenness in accuracy that is developed when no
vibration is applied is large, the amplitude of the vibration
displacement should also be made large. If the unevenness in
accuracy developed when no vibration is applied is small, the
amplitude of the vibration displacement may also be small. This is
to be understood in the light of the concept of the present
invention to prevent the development of machining irregularities by
making them submerged in fine indentations in the machined surface
obtained by the vibration displacement.
[0038] Based on the same concept, the frequency of the applied
voltage V(t) should preferably be sufficiently higher (but shorter
in period) than the frequency of disturbance. Main causes of
disturbance include vibrations of machines and machine locations,
tool chatter, etc. For quantitative conditions for the
"irregularity" of the frequency of the applied voltage V(t), this
frequency should preferably be changed within a range of about 2 to
100 times that of the disturbance.
[0039] It is only by way of example that the direction in which the
"relative vibration displacement between the tool and the
workpiece" is applied (i.e., the first direction of the
cutting-depth direction) is a vertical direction (gravitational
direction) in the case of the machining apparatus shown in FIGS. 3a
and 3b. In some cases, for example, the direction in which the
"relative vibration displacement between the tool and the
workpiece" is applied may be a horizontal direction (perpendicular
to the gravitational direction). Machining using a lath shown in
FIG. 5 is one such typical example. FIG. 5 illustrates an external
view of a lathe apparatus in which the workpiece W is mounted on a
workpiece fixture in the conventional manner and is rotated as it
is machined by means of the tool T.
[0040] In the lathe apparatus of this type, the cutting-depth
direction in which the tool T is moved closer to/away from the
workpiece W is the direction of a rotary axis of the workpiece
fixture. A movement in the cutting-depth direction is achieved by
relative movement of the workpiece fixture with respect to a
machining head for supporting a tool T along the direction of the
rotary axis of the workpiece fixture. The machining head supports
the tool T on its top side surface and is mounted on an axis for
movement in a horizontal direction perpendicular to the direction
of the rotary axis of the workpiece fixture, so that a distance
between the tool T and the rotary axis of the workpiece fixture
changes by the movement. Adjusting screws #1 and #2 serve to adjust
the attitude of the tool T. During machining operation, the screws
are tightened to keep the attitude of the tool T fixed.
[0041] According to the present invention, the relative vibration
displacement along the direction of the rotary axis of the
workpiece fixture is applied between the tool T and the workpiece
W. To attain this, it is necessary only that the tool T be mounted
on an elastic member (leaf spring, not shown) that is coupled to
the piezoelectric element in the manner shown in FIG. 3a and
provided on a tool supporting portion on the top side surface of
the machining head. In the case where the present invention is
applied to the lathe apparatus shown in FIG. 5, however, the
horizontal direction (direction of the rotary axis of the workpiece
fixture) corresponds to the "first direction" (direction of cut by
the tool T) shown in FIG. 3a, so that the respective attitudes of
the piezoelectric element and the elastic member set in position
are different by 90 degrees, as compared with the case of FIG.
3a.
[0042] The properties, drive modes, etc., of the piezoelectric
element 30 have been described before. As described with reference
to FIG. 3b, the piezoelectric element is connected to the white
noise generator in the controller (CNC) of the machining apparatus
(lathe apparatus in this case) so that it is driven by the driving
voltage with irregular frequency and amplitude generated by the
white noise generator. The ON/OFF operation for the output of the
white noise generator, maximum amplitude, etc., can be controlled
by the command from the CPU (not shown) in the CNC.
[0043] If the voltage of the vibration waveform with irregular
frequency and amplitude is applied the moment the machining is
started, the "relative vibration displacement in the first
direction (horizontal direction in this case) between the tool T
and the workpiece W" is induced. Based on the aforementioned
proportional relationship between the applied voltage V(t) and the
displacement d(t), the frequency of the relative vibration
displacement is substantially equal to the frequency of the applied
voltage waveform including its transition. Further, the vibration
displacement of the piezoelectric element on the operating surface
along the first direction, which is induced by the relationship
given by the aforesaid equation (1), is transmitted to the elastic
member (leaf spring), whereupon the tool T that is attached to the
elastic member vibrates.
[0044] As mentioned before, machining is realized such that the
depth of cut in the machined surface of the workpiece W finely
varies depending on the vibration of the tool T, thereby adding
indentations of a fine height (depth). The addition of these
indentations serves to obscure periodic reduction (unevenness) of
the machining accuracy. Since the amplitude of the vibration
displacement of the piezoelectric element on the operating surface
is usually reduced according to the elasticity of the elastic
member (leaf spring), the mass of the tool T, etc., the amplitude
of the applied voltage should only be determined in consideration
of it.
[0045] FIGS. 6a and 6b and FIGS. 7a and 7b show examples of
machined surfaces for cases where machining is performed with the
tool T fixed to the machining head (or without applying the
aforesaid irregular vibration to the supporting structure) without
applying the present invention to the lathe apparatus shown in FIG.
5. FIG. 6b is a sectional view taken along line A-A of FIG. 6a, and
FIG. 7b is a taken along line B-B of FIG. 7a.
[0046] In the examples shown in FIGS. 6a and 6b and FIGS. 7a and
7b, machining irregularity portions a1 to a3 and b1 to b3 develop
as projections of which the depth of cut is deficient by about Ha
or Hb. Ha or Hb may also be said to be the height of the
projections. As mentioned before, the distribution patterns of the
machining irregularity portions a1 to a3 and b1 to b3 viewed in the
frontal direction of the machined surfaces have some periodicity.
In the example shown in FIGS. 6a and 6b, the pattern is shaped like
concentric circles (with periodicity based on repetition along the
radial direction). In the example shown in FIGS. 7a and 7b, the
pattern is radial (with periodicity based on repetition along the
circumferential direction).
[0047] In contrast with the examples described above, the machining
irregularities may possibly emerge as excessively cut portions.
However, the machining irregularities (deficiency or surplus in the
depth of cut) are scanty in any event and are about several nm in
many cases. If the present invention is applied to the lathe
apparatus shown in FIG. 5 so that the relative vibration
displacement along the direction of the rotary axis of the
workpiece fixture is given to the tool T, as mentioned before,
therefore, machining irregularities such as the ones shown in FIGS.
6a and 6b and FIGS. 7a and 7b can be obscured to obtain a machined
surface shown in FIGS. 8a and 8b, for example.
[0048] Specifically, the depth of cut by the tool T is increased or
reduced by a margin corresponding to the vibration of the
piezoelectric element during machining operation, and innumerable
fine indentations such as the ones shown in FIG. 8b are formed.
These indentations can obscure the periodic machining
irregularities. Preferably, the amplitude of the vibration voltage
applied to the piezoelectric element should be set so as to obtain
the vibration displacement that ensures fulfillment of this
obscuring effect.
[0049] Preferably, in general, the maximum amplitude of the
displacement obtained should range from about two to ten times the
size (e.g., Ha or Hb) of the machining irregularities. In the case
where Ha or Hb=about 10 nm is given for the piezoelectric element
that has the aforementioned properties, the machining
irregularities with Ha or Hb=about 10 nm can be obscured by
irregular indentations with Hc (maximum indentation height)=about
20 to 60 nm shown in FIG. 8b if the amplitude of V(t) is set to
about 2 to 5 volts, for example.
[0050] Although the tool is vibrated in order to apply the relative
vibration displacement along the cutting depth direction between
the tool and the workpiece according to the above description, the
workpiece may be vibrated instead. As shown in FIG. 9, for example,
a worktable 52 may be provided on a leaf spring 51, which has the
same shape as the aforementioned leaf spring 22 and is coupled to a
piezoelectric element 30 for vibration. If this is done, the same
vibration as aforesaid can be applied to the workpiece W with the
same effect (machining irregularity obscuring effect).
* * * * *