U.S. patent application number 11/490235 was filed with the patent office on 2006-11-16 for machine tool and bed structure thereof.
This patent application is currently assigned to TOYODA KOKI KABUSHIKI KAISHA. Invention is credited to Yutaka Inada, Hideki Iwai, Tomohisa Katou, Hiroaki Suzuki, Katsuhiko Takeuchi.
Application Number | 20060254399 11/490235 |
Document ID | / |
Family ID | 32044695 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060254399 |
Kind Code |
A1 |
Inada; Yutaka ; et
al. |
November 16, 2006 |
Machine tool and bed structure thereof
Abstract
A machine tool includes X-axis, Y-axis, and Z-axis moving units
for producing relative movements between a tool and a workpiece; a
C-axis drive unit for rotating the workpiece about a C-axis
parallel to the Z-axis; and a B-axis turning unit for turning the
tool about a B-axis parallel to the Y-axis. The tool is disposed in
such a manner that a machining point of the tool coincides with the
B-axis. The moving units, the drive unit, and the turning unit are
controlled in such a manner that a work point of the workpiece
coincides with the machining point of the tool. The bed is formed
through casting and has a hollow structure and a hole as cast; and
a cover is provided to cover the hole as cast in order to close the
interior of the bed.
Inventors: |
Inada; Yutaka; (Kariya-shi,
JP) ; Suzuki; Hiroaki; (Nagoya-shi, JP) ;
Iwai; Hideki; (Toyoake-shi, JP) ; Takeuchi;
Katsuhiko; (Anjo-shi, JP) ; Katou; Tomohisa;
(Anjo-shi, JP) |
Correspondence
Address: |
C. IRVIN MCCLELLAND;OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYODA KOKI KABUSHIKI
KAISHA
Kariya-shi
JP
|
Family ID: |
32044695 |
Appl. No.: |
11/490235 |
Filed: |
July 21, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10684547 |
Oct 15, 2003 |
7104169 |
|
|
11490235 |
Jul 21, 2006 |
|
|
|
Current U.S.
Class: |
82/149 |
Current CPC
Class: |
B23Q 11/128 20130101;
Y10T 408/91 20150115; Y10T 82/2566 20150115; B23Q 1/015 20130101;
Y10T 409/309576 20150115; B23Q 11/0003 20130101; B23B 3/06
20130101; B23B 3/24 20130101; Y10T 82/2552 20150115; B23Q 1/017
20130101 |
Class at
Publication: |
082/149 |
International
Class: |
B23B 17/00 20060101
B23B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2002 |
JP |
2002-302871 |
Oct 23, 2002 |
JP |
2002-308652 |
Claims
1-7. (canceled)
8. A bed structure for a machine tool, comprising: a bed formed
through casting, the bed having a hollow structure and at least one
hole as cast; and a cover for covering each said at least one hole
as cast in order to close the interior of the bed, wherein said
cover substantially prevents the passage of air.
9. A bed structure for a machine tool, comprising: a bed formed
through casting, the bed having a hollow structure and a hole as
cast; and a cover for covering the hole as cast in order to close
the interior of the bed, wherein a liquid is charged into the
interior of the bed.
10. A bed structure according to claim 9, wherein the liquid is
oil, or water containing a rust preventing agent.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a machine tool for
precision machining of a workpiece, and to a bed structure of such
a precision machine tool.
[0003] 2. Description of the Related Art
[0004] A conventional precision machine tool is disclosed in, for
example, Japanese Patent Application Laid-Open (kokai) No.
10-151534. As shown in FIG. 1A, in the disclosed machine tool, a
machining unit, which includes a tool T provided on a main spindle
204 having a horizontal rotation axis, is provided on a Z-axis unit
203 (movable along a horizontal Z-axis). The Z-axis unit 203 is
supported on an X-axis unit 202 (movable along a horizontal X-axis
perpendicular to the Z-axis), which is disposed on a bed 201.
[0005] A C-axis unit 207 having a horizontal C-axis is disposed in
opposition to the main spindle 204. The C-axis unit 207 supports a
workpiece W for rotation about a horizontal rotational axis. The
C-axis unit 207 is supported on a B-axis unit 209 (rotatable about
a vertical B-axis), which is supported on a Y-axis unit 210
(movable along a vertical Y-axis), which is disposed on the bed
201.
[0006] A point of the workplace W to be machined (hereinafter
referred to as a "work point") is moved or indexed to a
predetermined position by means of the C-axis unit 207, the B-axis
unit 209, and the Y-axis unit 210, whereas a machining point of a
tip end of the tool T is moved or indexed to a predetermined
position by means of the X-axis unit 202 and the Z-axis unit 203,
whereby the work point of the workpiece W is machined (cut or
ground) by the tool T at its machining point.
[0007] In the conventional machine tool, the position of the work
point of the workplace W, which is represented by "A" in FIG. 1A
(overall front view), is separated by a "distance Lbw" from the
B-axis. Therefore, if an error .alpha. is generated as shown in
FIG. 1B (partial plan view) when the B-axis unit 209 is rotated by
an angle .theta. from a position (indicated by broken lines) at
which the C-axis coincides with the Z-axis, in order to index the
work point, the work point deviates from its theoretical position
"A(.theta.)" to a position "A(.theta.+.alpha.)." When the tool T is
moved toward the position "A(.theta.)," which deviates from the
actual position "A(.theta.+.alpha.)," the tool T machines the
position "A(.theta.)," although the position to be machined at that
time is "A(.theta.+.alpha.)." Such an error becomes remarkable as
the "distance Lbw" increases. Further, in addition to the error
involved in position indexing, an error stemming from a positioning
deviation at the time of B-axis stoppage becomes remarkable as the
"distance Lbw" increases.
[0008] Moreover, in the conventional machine tool, as shown in FIG.
1C (partial front view), the position "A" of the work point is
separated by a "distance Lyw" from the Y-axis. Therefore, when a
ram 217 (movable member) of the Y-axis unit is vertically moved
from a position at which the position "A" of the work point
coincides with the tip end of the tool T, in order to machine the
work point A, vertical forces Fu and Fd stemming from machining
resistance are applied to the work point A. The ram 217 is held by
a nut 221 in screw-engagement with a ball screw 220. Stemming from
the "distance Lyw" and the "forces Fu and Fd," a moment is
generated (an unnecessary stress acts on the nut 221 in a direction
not coinciding with the Y-axis), whereby the ram 217 may incline as
shown on the right side in FIG. 1C. When an "error .beta." is
generated stemming from the inclination, the work point deviates
from its theoretical position "A" to a position "A(.beta.)." When
the tool T is held at a height corresponding to that of the
position "A," which deviates from the actual position "A(.beta.),"
the tool T machines the position "A," although the position to be
machined at that time is "A(.beta.)." Such an error becomes
remarkable as the "distance Lyw" increases.
[0009] Influence of these errors is at a level which can be ignored
in machine tools which perform ordinary machining. However, in
precision machine tools which perform machining with very high
accuracy on the order of several hundreds to several tens of
nanometers, influence of such errors is large, and such errors must
be suppressed.
[0010] Incidentally, a bed used in a precision machine tool such as
a grinding machine is generally formed by casting. In general, such
a bed is cast to have a hollow structure in such a manner that the
bed is reinforced by integrally formed ribs arranged in a grid
pattern. Further, a plurality of holes as cast (hereinafter
referred to as "cast holes") penetrate the side and bottom walls of
the bed. The reason why the bed is cast to have a hollow,
rib-reinforced structure is to reduce the weight of the bed and the
influence of long-term distortion of the material. The cast holes
cannot be eliminated, because they are essential for casting a bed
having a hollow, rib-reinforced structure.
[0011] In some cases, instead of a cast bed, a bed formed of stone
such as granite is used in a super-precision machine tool which
must machine optical components or the like with very high
machining accuracy. Such a bed formed of stone such as granite has
characteristics such that the bed exhibits smaller long-term
changes in material properties and a larger heat capacity as
compared with the case of cast beds, and generally has a solid
structure.
[0012] The conventional cast bed is prone to receive-Influence of
outside air temperature, because the inner structure of the bed is
exposed to the outside air through the cast holes, and the area of
contact with the outside air is larger than in a case of a bed
having a solid structure.
[0013] In general, when an object has a temperature difference with
respect to outside air temperature, the time from exposure to
outside air temperature until the object attains the same
temperature as the outside air temperature decreases as the ratio
of surface area S to volume V; i.e., S/V, increases. FIG. 14 shows
results of calculation for obtaining temperature changes of three
objects which have the same volume and the same temperature
difference with respect to outside air temperature, but have
different surface areas. These three objects are formed of the same
material (gray cast iron), and the calculation for each object was
performed for the case where the initial temperature is 25.degree.
C., and the ambient temperature is 20.degree. C. FIG. 14 shows that
a spherical object, having the smallest S/V value, takes the
longest time to attain the outside air temperature, and that the
time required to attain the outside air temperature decreases as
the S/V value increases. In other words, influence of outside air
temperature increases as the S/V value increases.
[0014] Since the conventional cast bed has a hollow, rib-reinforced
structure, the bed has an S/V value greater than that of a bed
having a solid structure. Therefore, the bed temperature is prone
to change as the outside air temperature changes, and affects
structures mounted on the bed; specifically, slide surfaces, the
tool spindle, and the workplace spindle, whereby an error is
produced in the positional relation between a workplace and a tool.
As a result, machining accuracy fluctuates in the course of
long-term machining.
[0015] The above-described problem exerts considerable influence
not only on a machine tool disposed in a place, such as an ordinary
plant, where the outside air temperature changes greatly, but also
on a machine tool, such as a super precision machine tool, which is
placed in a thermostatic room, whose interior temperature is
controlled to a set temperature .+-.1.degree. C. and which is
required to provide very high machining accuracy.
[0016] Meanwhile, the conventional bed formed of stone such as
granite has a larger heat capacity as compared with the case of
cast beds, and has a smaller area of contact with the outside air,
because it assumes the shape of a solid rectangular parallelepiped.
Therefore, the conventional bed formed of stone such as granite has
an advantage in that the temperature of the bed is unlikely to
follow changes in the outside air temperature, and the bed enables
machining with high accuracy. However, the granite is more
expensive than a casting, and the degree of freedom in design is
low, because machining of granite is difficult.
SUMMARY OF THE INVENTION
[0017] In view of the foregoing, a first object of the present
invention is to provide a machine tool which has a structure for
suppressing generation of errors, to thereby improve machining
accuracy.
[0018] A second object of the present invention is to inexpensively
provide a bed for a machine tool which realizes low thermal
displacement.
[0019] In order to achieve the first object, the present invention
provides a machine tool, comprising: an X-axis moving unit, a
Y-axis moving unit, and a Z-axis moving unit for producing relative
movements between a tool and a workpiece along the respective
directions of an X-axis, a Y-axis, and a Z-axis, which differ from
one another; a C-axis drive unit for rotating the workpiece about a
C-axis parallel to the Z-axis; and a B-axis turning unit for
turning the tool about a B-axis which is defined on the B-axis
turning unit and is parallel to the Y-axis. The tool is disposed in
such a manner that a machining point of the tool substantially
coincides with the B-axis. The moving units, the drive unit, and
the turning unit are controlled in such a manner that a work point
of the workpiece substantially coincides with the machining point
of the tool.
[0020] In the machine tool of the present invention, the position
of the tool is determined in such a manner that the machining point
of the tool substantially coincides with the B-axis. Therefore,
even when an error is generated in turning movement of the B-axis
turning unit, the position of the machining point can be maintained
on the B-axis, whereby an error in the position of the machining
point can be suppressed. This feature effectively suppress an index
error during B-axis turning, along with an error stemming from a
positioning deviation at the time of B-axis stoppage.
[0021] As described above, since the machine tool of the present
invention has a structure which hardly generates errors, machining
accuracy can be improved.
[0022] Preferably, the B-axis turning unit is disposed on the
Y-axis moving unit in such a manner that the B-axis substantially
coincides with a center axis of a movable member of the Y-axis
moving unit, the center axis extending along the Y-axis; and the
tool is disposed on the B-axis turning unit.
[0023] In this case, the machining point of the tool can be located
on the center axis of the movable member of the Y-axis moving unit.
Therefore, when machining is performed while the Y-axis moving unit
is driven to move the movable member along the Y-axis direction,
unnecessary stresses acting on drive means or the like can be
suppressed, whereby errors caused by inclination of the Y-axis
moving unit and the B-axis turning unit can be suppressed.
Moreover, since the B-axis turning unit carrying the tool is
disposed on the Y-axis moving unit whose error is suppressed, error
in the position of the machining point of the tool can be
suppressed further.
[0024] Preferably, the C-axis drive is disposed on the Z-axis
moving unit in such a manner that the C-axis substantially
coincides with a center axis of a movable member of the Z-axis
moving unit, the center axis extending along the Z-axis.
[0025] In this case, the work point of the workpiece can be located
in the vicinity of the center axis of the movable member of the
Z-axis moving unit. Therefore, when the workpiece held by the
C-axis drive unit is machined, while the workpiece is moved along
the Z-axis direction by means of the Z-axis moving unit in order to
be pressed against the tool, unnecessary stresses which act, for
example, on drive means for Z-axis drive due to influence of the
reaction of the pressing operation can be suppressed, whereby
errors caused by inclination of the Z-axis moving unit and the
C-axis turning unit can be suppressed.
[0026] Preferably, the machine tool has a bed having a horizontal
top surface and a vertical side surface, wherein the X-axis moving
unit is disposed on the horizontal top surface of the bed, the
Z-axis moving unit is disposed on the X-axis moving unit, and the
C-axis drive unit is disposed on the Z-axis moving unit, and
wherein the Y-axis moving unit is disposed on the vertical side
surface of the bed in such a manner that the Z-axis-direction
center axis of the movable member of the Z-axis moving unit
perpendicularly intersects the Y-axis-direction center axis of the
movable member of the Y-axis moving unit, the B-axis turning unit
is disposed on the Y-axis moving unit, and the tool is disposed on
the B-axis turning unit.
[0027] In this case, a bed having a complicated shape is not
required, and a bed having a substantially rectangular
parallelepiped shape can be used. Therefore, the accuracy of the
bed can be easily improved, and thus the individual moving units,
turning unit. etc. can be mounted on the bed with improved
positional accuracy.
[0028] In order to achieve the second object, the present invention
provides a bed structure for a machine tool, comprising: a bed
formed through casting, the bed having a hollow structure and a
hole as cast; and a cover for covering the hole as cast in order to
close the interior of the bed.
[0029] This structure decreases the area of a surface exposed to
the outside air, to thereby suppress total thermal displacement of
the bed.
[0030] Preferably, a liquid is charged into the interior of the
bed. In this case, since the heat capacity of the bed increases,
thermal displacement can be suppressed to a greater degree as
compared to the case where the hole as cast is merely closed.
Preferably, the liquid is oil, or water containing a rust
preventing agent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Various other objects, features and many of the attendant
advantages of the present invention will be readily appreciated as
the same becomes better understood by reference to the following
detailed description of the preferred embodiments when considered
in connection with the accompanying drawings, in which:
[0032] FIGS. 1A to 1C are views showing a conventional machine
tool;
[0033] FIG. 2A is a side view of a machine tool according a first
embodiment of the present invention;
[0034] FIG. 2B is a cross-sectional view of the machine tool taken
along line IIB-IIB in FIG. 2A;
[0035] FIG. 2C is an enlarged side view showing the positional
relation between a workpiece and a tool;
[0036] FIG. 3A is a plan view of the machine tool;
[0037] FIG. 3B is a cross-sectional view of the machine tool taken
along line IIIB-IIIB in FIG. 3A;
[0038] FIG. 3C is an enlarged plan view showing the positional
relation between the workpiece and the tool;
[0039] FIG. 4 is a perspective view of the machine tool;
[0040] FIGS. 5A to 5C are explanatory views showing suppression of
errors;
[0041] FIGS. 6A to 6C are explanatory views showing suppression of
errors;
[0042] FIG. 7 is a cross sectional view of the bed taken along line
VII-VII in FIG. 3A;
[0043] FIG. 8 is a cross sectional view of the bed taken along line
VIII-VIII in FIG. 2A;
[0044] FIG. 9 is an enlarged cross sectional view showing a hole as
cast in a side wall of the bed, closed by a cover;
[0045] FIG. 10 is an enlarged cross sectional view showing a hole
as cast in a bottom wall of the bed, closed by a cover;
[0046] FIG. 11 is a table showing volumes V, surface areas S,
ratios S/V, weights, and total heat capacities of modeled
conventional bed structures and molded bed structures of the
present invention;
[0047] FIG. 12 is a graph showing the results of measurement of the
interior temperature (room temperature) of a thermostatic room and
the interior temperature of the bed of the machine tool in a state
in which the cast holes in the side walls and bottom wall of the
bed are closed by means of covers;
[0048] FIG. 13 is a graph showing the results of measurement of the
interior temperature (room temperature) of a thermostatic room and
the liquid temperature of the bed of the machine tool in a state in
which the cast holes in the side walls and bottom wall of the bed
are closed by means of covers, and the interior of the bed is
filled with liquid; and
[0049] FIG. 14 is a graph showing results of calculation for
obtaining temperature changes of three objects which have the same
volume and the same temperature difference with respect to outside
air temperature, but have different surface areas.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] A machine tool according to a first embodiment of the
present invention will now be described with reference to the
drawings.
<Overall Structure>
[0051] The arrangement of individual moving units, a turning unit,
etc. of the machine tool will be described with reference to FIGS.
2A to 2C, FIGS. 3A to 3C, and FIG. 4; and the positional relation
among the center axes of movable members of the individual moving
units, the turning unit. etc. will be described with reference to
FIGS. 2A to 2C and FIGS. 3A to 3C. FIG. 2A shows a left-hand side
view of the machine tool; FIG. 2B is a cross-sectional view of the
machine tool taken along line IIB-IIB in FIG. 2A (a cross-sectional
view of a Y-axis moving unit 30); and FIG. 2C is an enlarged side
view showing the positional relation between a work point A of a
workpiece W and a machining point B of a tool T shown in FIG. 2A.
FIG. 3A shows a plan view of the machine tool; FIG. 3B is a
cross-sectional view of the machine tool taken along line IIIB-IIIB
in FIG. 3A (a cross-sectional view of a Z-axis moving unit 50); and
FIG. 3C is an enlarged side view showing the positional relation
between the work point A of the workpiece W and the machining point
B of the tool T shown in FIG. 3A.
[0052] The machine tool according to the present embodiment is a
super precision machine tool adapted to machine a workpiece, such
as a lens or a lens mold, having an axisymmetric shape or a free
curved surface with an accuracy of several hundreds to several tens
of nanometers.
[0053] Various tools may be used as the tool T. For example, as
shown in FIG. 5A, a grinding wheel supported and rotated by a drive
unit 10 may be used. Alternatively, as shown in FIG. 6A, a cutting
tool (turning tool) may be used. In the example shown in FIGS. 2A
to 4, the grinding wheel shown in FIG. 5A is used as the tool T. In
this case, the machining point B of the tool T is located on a
circumferential surface of the tool T, and the machining point B of
the tool T is brought into contact with the work point A of the
workpiece W to thereby grind the workpiece W. Notably, the position
of the work point A may be changed on the workpiece W.
[0054] The machine tool has a bed 1, which generally assumes the
shape of a rectangular parallelepiped. The bed 1 has a horizontal
top surface (extending along the X-axis and Z-axis directions in
FIGS. 2A and 3A), and vertical side surfaces (extending along the
Y-axis direction in FIGS. 2A and 3A). Since the bed 1 has a
rectangular parallelepiped shape, which is very simple, each
surface can easily be machined to have high accuracy (in terms of
the horizontalness of the horizontal surface, and the verticalness
of the vertical surfaces). Individual parts, etc., which affect
machining accuracy, can be accurately disposed on the corresponding
surfaces, and position-adjusted thereon. Thus, the machining
accuracy can be improved further.
[0055] An X-axis moving unit 60 is disposed on the top surface of
the bed 1 in order to produce relative movement between the work
point A of the workplace W and the machining point B of the tool T
along a horizontal direction (along the X-axis direction in FIGS.
2A to 4). As shown in FIG. 2A, the X-axis moving unit 60 includes a
guide mechanism (stationary member) 60a, a movable member 60b, and
a linear motor 60c. The movable member 60b is in slidable
engagement with the guide mechanism 60a, and is reciprocated along
the X-axis direction by means of the linear motor 60c.
[0056] In order to minimize position errors involved in linear
motion, the linear motor 60c is used as drive means of the X-axis
moving unit 60, instead of a motor of a rotary motion type.
Therefore, a mechanism for converting rotary motion to linear
motion becomes unnecessary, the movable member can be moved
directly along a straight path, and backlash is hardly generated,
whereby errors can be reduced further.
[0057] The center axis of the movable member 60b of the X-axis
moving unit 60, which axis extends along the X-axis direction, is
referred to as the X-axis center axis 60z (see FIG. 2A).
[0058] The above-mentioned Z-axis moving unit 50 is disposed on the
top surface of the X-axis moving unit 60 in order to produce a
relative movement between the work point A of the workpiece W and
the machining point B of the tool T along a horizontal direction
perpendicular to the X-axis (along the Z-axis direction in FIGS. 2A
to 4). As shown in FIG. 3B, the Z-axis moving unit 50 includes a
guide mechanism (stationary member) 50a, a movable member 50b, and
a linear motor 50c. The movable member 50b is in slidable
engagement with the guide mechanism 50a, and is reciprocated along
the Z-axis direction by means of the linear motor 50c.
[0059] For the same reason as mentioned in connection with the
X-axis moving unit 60, the linear motor 50c is used as drive means
of the Z-axis moving unit 50. The center axis of the movable member
50b of the Z-axis moving unit 50, which axis extends along the
Z-axis direction, is referred to as the Z-axls center axis 50z (see
FIG. 3B).
[0060] Notably, the distance between the X-axis center axis 60z and
the Z-axis center axis 50z is preferably reduced to a possible
extent so as to reduce errors.
[0061] A C-axis drive unit 40 is disposed at the point of
intersection between the Z-axis center axis 50z and a front end
face of the movable member 50b of the Z-axis moving unit 50. The
C-axis drive unit 40 supports the workpiece W and rotates the same
about a C-axis drive axis (C-axls; i.e., a horizontal direction
which coincides with the Z-axis direction in FIGS. 2A and 3A), in
order to produce a relative turn (rotation) between the work point
A of the workpiece W and the machining point B of the tool T about
the C-axis (in this case, the Z-axis).
[0062] The above-described Y-axis moving unit 30 is disposed on a
side surface of the bed 1 in order to produce relative movement
between the work point A of the workpiece W and the machining point
B of the tool T along the vertical direction (along the Y-axis
direction in FIGS. 2A to 4). As shown in FIG. 2B, the Y-axis moving
unit 30 includes a guide mechanism (stationary member) 30a, a
movable member 30b, and a linear motor 30c. The movable member 30b
is in slidable engagement with the guide mechanism 30a, and is
reciprocated along the Y-axis direction by means of the linear
motor 30c.
[0063] For the same reason as mentioned in connection with the
X-axis moving unit 60, the linear motor 30c is used as drive means
of the Y-axis moving unit 30. The center axis of the movable member
30b of the Y-axis moving unit 30, which axis extends along the
Y-axis direction, is referred to as the Y-axis center axis 30z (see
FIG. 2B).
[0064] Notably, a balance cylinder 80 is disposed under the Y-axis
moving unit 30 in order to support the movable member 30b, on which
is mounted a B-axis turning unit 20 carrying the tool T, with a
force substantially equal to the force of gravity. This
configuration reduces the load acting on the linear motor of the
Y-axis moving unit 30, so as to further reduce errors. Notably, the
center axis of the balance cylinder 80 extending along the Y-axis
direction is adjusted to coincide with the Y-axis center axis 30z
of the movable member 30b, to thereby prevent application of
off-axis forces.
[0065] The above-mentioned B-axis turning unit 20 is disposed on
the top surface of the Y-axis moving unit 30 in order to produce a
relative turn between the work point A of the workpiece W and the
machining point B of the tool T about a B-axis turning axis
(B-axis); i.e., a vertical direction which coincides with the
Y-axis direction in FIG. 2A). As shown in FIG. 3A, a B-axis
turntable 20b is provided on the B-axis turning unit 20, and is
turned about the B-axis. The tool T is fixedly disposed on the
B-axis turntable 20b, whereby the direction of the machining point
B of the tool T (orientation of the tool T within a horizontal
plane) can be changed or indexed.
[0066] The tool T is disposed on the B-axis turntable 20b of the
B-axis turning unit 20 in such a manner that the machining point B
of the tool T on the circumferential surface thereof coincides with
the B-axis turning axis (B-axis). Therefore, irrespective of
angular position of the B-axis turntable 20b, the machining point B
of the tool T remains on the B-axis turning axis (B-axis) with
substantially no deviation therefrom. Notably, the orientation of
the tool T at the machining point B changes in accordance with the
turn angle of the B-axis turntable 20b.
[0067] The B-axis turning unit 20 is disposed on the top surface of
the Y-axis moving unit 30 in such a manner that the Y-axis center
axis 30z coincides with the B-axis turning axis (B-axis). Further,
the C-axis drive unit 40 is disposed at the front end portion of
the Z-axis moving unit 50 in such a manner that the Z-axis center
axis 50z coincides with the C-axis drive axis (C-axis).
[0068] Notably, as shown in FIG. 4, the machine tool is equipped
with a microscope 90 for initial positioning of the work point A
and the machining point B, and a stroboscope 92 for assisting the
position checking by the microscope 90. Moreover, a fine adjustment
mechanism 12 is provided between the tool T and the B-axis
turntable 20b (table turned by the B-axis turning unit 20) in order
to attain a perfect match between the machining point B of the tool
T and the B-axis. A machine operator operates the fine adjustment
mechanism 12, while viewing the machining point B of the tool T by
use of the microscope 90, in such a manner that the machining point
B of the tool T coincides with the B-axis (in the example shown in
FIG. 4, the operator finely adjusts the position of the drive unit
10, which supports and drives the tool T).
[0069] Moreover, in FIG. 4, there is shown a shock-absorbing base
3, which precisely maintains the bed 1 in a horizontal posture with
respect to the floor surface, and absorbs vibrations from the floor
surface or the like.
<Suppression of Error in Turn Angle of B-Axis Turning Unit
(FIGS. 5A to 5C)>
[0070] Next, the reason why error in turn angle of the B-axis
turning unit 20 is suppressed will be described with reference to
FIGS. 5A to 5C. In the conventional machine tool shown in FIG. 1B,
because of the "distance Lbw" between the B-axis turning axis
(B-axis) and the work point A of the workpiece W, an error in turn
angle (error angle .alpha.) may affect the position of the work
point A. The error is at a level which can be ignored in a machine
tool which performs ordinary machining. However, in a precision
machine tool which performs machining with very high accuracy on
the order of several hundreds to several tens of nanometers,
influence of such error is large, and such error must be
suppressed.
[0071] Such a positional error can be reduced by reducing the
"distance Lbw" to a value near zero. However, since the work point
A of the workpiece W is set at different positions on the workplace
W, reducing the distance Lbw to a value near zero is considerably
difficult (even when the distance between the B-axis and a certain
work point is reduced to zero, the distance between the B-axis and
another work point does not become zero). In view of the foregoing,
in the present embodiment, instead of the work point A of the
workpiece W, the machining point B of the tool T is turned by means
of the B-axis turning unit 20 (because the machining point B of the
tool T maintains a constant position).
[0072] In order to reduce the distance between the B-axis turning
axis and the machining point B of the tool T to a value near zero,
the tool T is disposed as shown in FIG. 5B, whereby the machining
point B of the tool T coincides with the B-axis turning axis.
Therefore, even when an "error angle .alpha." is produced as shown
in FIG. 5C when the B-axis turning unit 20 is rotated by an angle
.theta. from a position (indicated by broken lines) at which the
C-axis is parallel to the tool T, an error is hardly generated in
the position "B" of the work point. As described above, the machine
tool according to the present invention can effectively suppress an
index error during B-axis turning, along with an error stemming
from a positioning deviation at the time of B-axis stoppage.
[0073] In the conventional machine tool shown in FIGS. 1A to 1C,
since the C-axis unit 207 is mounted on the B-axis unit 209, the
B-axis unit 209 is large and heavy. In contrast, in the present
embodiment, only the tool T and the drive unit 10 are mounted on
the B-axis turning unit 20, so that the B-axis turning unit 20 can
be reduced in size and weight.
<Suppression of Stress Generated Between Work Point of Workpiece
and Y-Axis Center Axis (FIGS. 6A to 6C)>
[0074] Next, the reason why stress generated between the work point
A of the workpiece W and the Y-axis center axis 30z is suppressed
will be described with reference to FIGS. 6A to 6c. Notably, in
FIGS. 6A to 6C, the fine adjustment mechanism 12 shown in FIG. 4 is
omitted.
[0075] In the conventional machine tool shown in FIG. 1C, because
of the "distance Lyw" between the work point A of the workpiece W
and the Y-axis drive axis, an unnecessary stress is generated, and
an "error angle .beta. may affect the position of the work point A.
The error is at a level which can be ignored in a machine tool
which performs ordinary machining. However, in a precision machine
tool which performs machining with very high accuracy on the order
of several hundreds to several tens of nanometers, influence of
such error is large, and such error must be suppressed.
[0076] Such an unnecessary stress can be suppressed by reducing the
distance Lyw" to a value near zero. In view of this, in the present
embodiment, the B-axis turning axis (i.e., the machining point B of
the tool T) is made coincident with the Y-axis center axis 30z in
order to make the work point A of the workpiece W coincident with
the Y-axis center axis 30z (reduce the distance therebetween to
substantially zero), whereby generation of the error angle .beta.
as shown in FIG. 1C is suppressed.
<Suppression of Stress Generated Between C-Axis Drive Axis and
Z-Axis Center Axis>.
[0077] Next, the reason why stress generated between the C-axis
drive axis and the Z-axis center axis 50z is suppressed will be
described. In the case where the C-axis drive axis and the Z-axis
center axis 50z are separated from each other, when the work point
A of the workpiece W is moved along the Z-axis direction by means
of the Z-axis moving unit 50 so as to press the work point A to the
machining point B of the tool T, a stress is generated in the
direction (in the example of FIG. 5B, the left direction along the
Z-axis) opposite the pressing direction (in the example of FIG. 5B,
the right direction along the Z-axis). In order to suppress
influence of the stress, the C-axis drive axis is made coincident
with the Z-axis center axis 50z. Even in a case where the work
point A of the workpiece W is not located on the C-axis drive axis,
the distance between the work point A and the C-axis drive axis can
be reduced (on average) to a possible extent, whereby generation of
errors stemming from unnecessary stress can be suppressed.
[0078] As described above, the machine tool of the present
invention is configured in such a manner that the X-axis center
axis 60z, the Z-axis center axis 50z, the C-axis drive axis
(C-axis), the Y-axis center axis 30z, the B-axis turning axis
(B-axis), and the machining point B of the tool T are located at
proper positions, whereby generation of errors is suppressed and
machining accuracy is improved.
[0079] The machine tool of the present invention is not limited to
the details, such as structure and shape, described in the
embodiment, and can be subjected to modification, addition, and
deletion without departing from the scope of the invention.
[0080] The type of the tool T and the machining direction of the
tool T are not limited to those described in the embodiment. For
example, the tool T shown in FIGS. 6A to 6C and having a
horizontally directed cutting edge (machining portion) may be
replaced with a tool T having a vertically directed cutting edge
(machining portion).
[0081] Further, although in the embodiment the X-axis, Y-axis, and
Z-axis are orthogonal coordinates, the X-axis, Y-axis, and Z-axis
are not necessarily required to intersect perpendicularly.
<Structure of Bed>
[0082] Next, the structure of the bed 1 will be described in detail
with reference to FIGS. 7 to 10.
[0083] The bed 1 is formed through casting of iron, and as shown in
FIGS. 7 and 8, has a hollow, rib-reinforced inner structure.
Specifically, ribs 10 are integrally formed in the interior of the
bed 1 in such a manner that the ribs 10 are arranged in a grid
pattern in order to reinforce the bed 1 and divide the interior of
the bed 1 into twelve chambers which have the same volume and are
arranged in a matrix of 2 (longitudinal direction).times.3
(transverse direction).times.2 (height direction). A through hole
11 is formed in each of the ribs 10 in order to connect adjacent
chambers.
[0084] Cast holes 102 are formed in the bottom wall of the bed 1
and in the side walls of the bed 1, except for the side wall to
which the Y-axis moving unit 30 is attached. Therefore, no cast
hole is formed in the top wall of the bed 1. The cast hole 102 is
provided in order to remove casting sand from the individual
chambers of the bed 1 after casting. In order to facilitate the
removal of casting sand, each chamber is provided with at least one
cast hole 102. The cast holes 102 formed in the side walls of the
bed 1 are closed by means of covers 103, and the cast holes 102
formed in the bottom wall of the bed 1 are closed by means of
covers 104, whereby the interior of the bed 1 is completely
closed.
[0085] The covers 103 for closing the cast holes 102 formed in the
side walls of the bed 1 have a diameter greater than that of the
cast holes 102, in order to completely cover the cast holes 102. A
hole for allowing passage of a bolt 107, which will be described
later, is formed in a central portion of each cover 103. Further,
as shown in FIG. 9, an annular groove is formed in a peripheral
portion of each cover 103 to extend through the entire
circumference, which portion comes into close contact with the bed
1; and an O-ring 105 is fitted into the groove in order to seal the
interior of the bed 1. The cover 103 is fixed to the bed by means
of a clamper 106 and the bolt 107. Specifically, the clamper 106
has a cruciform shape, and has a threaded hole at the center
thereof. The bolt 107 is passed through the cover 103 and is
screwed into the threaded hole of the clamper 106. When the bolt
107 is fastened or screwed into the threaded hole, the clamper 106
comes into close contact with the bed 1. Thus, the cover 103 comes
in close contact with the bed 1, and completely covers the cast
hole 102, to thereby prevent leakage of air from the interior of
the bed 1 and entry of outside air into the interior of the bed
1.
[0086] Meanwhile, the covers 104 for closing the cast holes 102
formed in the bottom wall of the bed 1 have a diameter greater than
that of the cast holes 102, in order to completely cover the cast
holes 102. Each cover 104 has a plurality of holes formed in a
peripheral portion thereof. Further, as shown in FIG. 10, an
annular groove is formed in a peripheral portion of each cover 104
to extend through the entire circumference, which portion comes
into close contact with the bed 1; and an O-ring 109 is fitted into
the groove. Bolts 108 are passed through the holes of the cover 104
and then screwed into unillustrated threaded portions of holes
formed in the bed 1 around the corresponding cast hole 102. When
the bolts 108 are fastened or screwed into the threaded portions,
the cover 104 comes into close contact with the bed 1, and
completely covers the cast hole 102, to thereby prevent leakage of
air from the interior of the bed 1 and entry of outside air into
the interior of the bed 1.
[0087] As described above, since the cast holes 102 formed in the
side wall and the bottom wall of the bed 1 are closed by means of
the covers 103 and 104, the interior of the bed 1 becomes a closed
space, and thus, the area of the surface exposed to the outside air
decreases, whereby the thermal displacement of the entire bed 1 can
be suppressed. As a result, accuracy during long-time machining can
be stabilized. Notably, reference numeral 112 denotes liquid
charging openings to be used in a second embodiment. The liquid
charging openings 112 are unnecessary in the first embodiment, and
are closed by means of plugs.
[0088] Next, the second embodiment will be described. In the second
embodiment, the cast holes 102 of the bed 1 are closed by use of
covers, and a liquid is charged into the interior of the bed 1.
Notably, the bed 1 according to the second embodiment is identical
in structure with the bed 1 according to the first embodiment. The
process of fabricating the bed 1 is identical with the process
employed in the first embodiment up to the point where the cast
holes 102 are closed by use of the covers 103 and 104.
Subsequently, a liquid is charged into the closed interior of the
bed 1. Since the O-rings 105 and 109 are fitted to the covers 103
and 104, respectively, the liquid does not leak through portions
where the covers 103 and 104 are in close contact with the bed
1.
[0089] The liquid to be charged into the interior of the bed 1 is
injected from the liquid charge openings 112 provided in the top
wall of the bed 1. In general, plugs are fitted into the liquid
charge openings 112 in order to prevent entry of outside air. The
plugs are removed from the liquid charge openings 112 before
injection of the liquid. After completion of injection of the
liquid, the plugs are again fitted to the liquid charge openings
112 in order to prevent entry of outside air and evaporation of the
liquid, which results in a reduction in the amount of the
liquid.
[0090] Water, by virtue of its large specific heat, is most
preferably used as the liquid charged into the interior of the bed
1. Moreover, a rust preventing agent is preferably added to water
in order to avoid rusting of the bed 1 made of cast iron.
Furthermore, ethylene glycol serving as an antifreezing fluid may
be added to water so as to prevent freezing of the water. Instead
of water, oil may be charged into the interior of the bed 1,
thereby providing rust prevention and antifreeze protection.
[0091] As described above, the cast holes 102 of the bed 1 are
closed by means of the covers 103 and 104 so that the interior of
the bed 1 becomes a closed space; and a liquid is charged into the
interior of the bed 1. Therefore, the thermal capacity of the
entire bed increases, and thermal displacement of the entire bed
can be suppressed to a greater extent as compared with a bed whose
cast holes are closed by means of covers, but whose interior is not
filled with liquid.
[0092] FIG. 11 shows a table which shows the relation among volume
V, surface area S, ratio S/V, weight, and total heat capacity of
modeled conventional bed structures and molded bed structures of
the present invention. The table of FIG. 11 shows data for six bed
structures; i.e., a cast-iron bed A having a cubic solid structure
(1 m.times.1 m.times.1 m); a cast-iron bed B having a cubic hollow
structure (1 m.times.1 m.times.1 m) whose interior is divided by
ribs (thickness; 50 mm) into 27 chambers arranged in a matrix of 3
(longitudinal direction).times.3 (transverse direction).times.3
(height direction): a granite bed C having a cubic solid structure
(1 m.times.1 m.times.1 m): a bed Bo identical with the hollow,
rib-reinforced, cast-iron bed B, except that cast holes are closed
by means of covers; a bed B1 identical with the bed Bo whose cast
holes are closed by means of covers, except that mineral oil is
charged into the interior of the bed; and a bed B2 identical with
the bed Bo whose cast holes are closed by means of covers, except
that water is charged into the interior of the bed.
[0093] First, the cast-iron bed A having a solid structure has
advantageous features, such as small surface area and large heat
capacity. However, as described above, the cast-iron bed A having a
solid structure is not preferable, from the viewpoint of weight and
influence of distortion caused by long-term changes. Therefore, a
conventional cast-iron bed is fabricated to have a hollow,
reinforced structure, as the bed B, to thereby remove about 70% of
the cast iron. When the hollow, cast-iron bed B is compared with
the granite bed C having a solid structure, the granite bed C has a
larger heat capacity and a smaller ratio (S/V) of surface area S to
volume V. Therefore, the granite bed C can be said to be a
structure which is less likely to follow changes in the outside air
temperature.
[0094] However, in the case of the bed Bo identical with the
hollow, rib-reinforced, cast-iron bed B, except that cast holes are
closed by means of covers, since the area of a surface in contact
with the outside air decreases by virtue of closure of the cast
holes by covers, as compared with the bed B the bed Bo has a
reduced ratio S/V, and is less likely to follow changes in the
outside air temperature. Moreover, in the case of the bed B1
identical with the bed Bo whose cast holes are closed by means of
covers, except that mineral oil is charged into the interior of the
bed, a heat capacity almost the same as that of the granite bed C
is obtained; and in the case of the bed B2 filled with water, a
heat capacity two times that of the granite bed C is obtained.
Therefore, these beds B1 and B2 are much less likely to follow
changes in the outside air temperature, or are unresponsive to
changes in the outside air temperature.
[0095] FIG. 12 is a graph showing the results of measurement of the
interior temperature (room temperature) of a thermostatic room and
the interior temperature of the bed 1 according to the first
embodiment in which the cast holes 102 in the side walls and bottom
wall of the bed 1 are closed by means of the covers 103 and 104.
The interior temperature of the thermostatic room is set to
20.degree. C. The interior temperature of the thermostatic room and
the interior temperature of the bed were measured by use of
platinum thermometer resistors. Measurement of the interior
temperature of the bed was performed by use of a platinum
thermometer resistor inserted into the liquid charging opening 112
of the bed 1. As can be seen from FIG. 12, when the machine tool is
started ({circle around (1)} in FIG. 12), the interior temperature
of the thermostatic room increases, because of heat generation of
the machine tool, and fluctuates because of disturbances such as
entry of a person into the thermostatic room and departure of the
person therefrom. When the machine tool is stopped ({circle around
(2)} in FIG. 12), the interior temperature of the thermostatic room
decreases to the vicinity of the set temperature, because no heat
is generated from the machine tool.
[0096] The interior temperature of the bed also increases when the
machine tool is started. However, the interior temperature of the
bed does not coincide with the interior temperature of the
thermostatic room, and slowly increases with the interior
temperature of the thermostatic room. Further, the graph
demonstrates that the interior temperature of the bed is hardly
influenced by changes in the interior temperature of the
thermostatic room.
[0097] In other words, since the interior of the bed 1 is
completely closed by closing the cast holes 102 in the side walls
and bottom wall of the bed 1 by means of the covers 103 and 104,
the interior temperature of the bed 1 becomes less likely to follow
changes in the interior temperature of the thermostatic room; i.e.,
becomes comparatively unresponsive to changes in the interior
temperature of the thermostatic room. As a result, although the
outside surfaces of the bed 1 receive the influence of changes in
the interior temperature of the thermostatic room, the inside
surfaces of the bed 1 hardly receive the influence of changes in
the interior temperature of the thermostatic room. Therefore, the
area of a surface of the bed which undergoes changes in the outside
air temperature decreases, and the thermal displacement of the
entire bed can be suppressed.
[0098] FIG. 13 is a graph showing the results of measurement of the
interior temperature (room temperature) of a thermostatic room and
the liquid temperature of the bed 1 according to the second
embodiment in which a liquid is charged into the interior of the
bed 1. The interior temperature of the thermostatic room is set to
20.degree. C. Water containing a rust preventing agent was used as
the liquid charged into the interior of the bed 1. The interior
temperature of the thermostatic room and the liquid temperature
were measured by use of platinum thermometer resistors. Measurement
of the liquid temperature was performed by use of a platinum
thermometer resistor inserted into the liquid charging opening 112
of the bed 1. As can be seen from FIG. 13, as in the case shown in
FIG. 12, when the machine tool is started ({circle around (1)} in
FIG. 13), the interior temperature of the thermostatic room
increases because of heat generation of the machine tool, and
fluctuates because of disturbances such as entry of a person into
the thermostatic room and departure of the person therefrom. When
the machine tool is stopped ({circle around (2)} in FIG. 13), the
interior temperature of the thermostatic room decreases to the
vicinity of the set temperature, because no heat is generated from
the machine tool.
[0099] The liquid temperature also increases when the machine tool
is started. However, the liquid temperature does not coincide with
the interior temperature of the thermostatic room, and slowly
increases with the interior temperature of the thermostatic room.
Further, the graph demonstrates that the liquid temperature is
hardly influenced by changes in the interior temperature of the
thermostatic room.
[0100] When the bed 1 according to the second embodiment is compare
with the bed 1 according to the first embodiment in which the cast
holes 102 in the side walls and bottom wall of the bed 1 are closed
by means of the covers 103 and 104, but no liquid is charged into
the interior of the bed 1, the internal temperature of the bed 1
according to the second embodiment filled with liquid becomes much
less likely to follow changes in the interior temperature of the
thermostatic room; i.e., becomes comparatively unresponsive to
changes in the interior temperature of the thermostatic room. In
other words, since the total heat capacity of the bed increases by
virtue of water containing a rust-preventing-agent and charged into
the interior of the bed 1, the bed 1 according to the second
embodiment can be said to become much less likely to follow changes
in the interior temperature of the thermostatic room, or to become
comparatively unresponsive to changes in the interior temperature
of the thermostatic room, as compared with the case where the cast
holes of the bed are merely closed by means of covers. As a result,
the thermal displacement of the entire bed can be suppressed to a
greater extent, as compared with the case where the cast holes of
the bed are merely closed by means of covers.
[0101] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the present invention may be practiced otherwise than as
specifically described herein.
* * * * *