JP2002172504A - Workpiece holder for machine tool - Google Patents

Abstract

PROBLEM TO BE SOLVED: To miniaturize a driving gear of a hold mechanism holding a workpiece of a lathe, moreover hold the workpiece highly accurately even in cutting work. SOLUTION: A hybrid hydraulically-driven unit 10 integrally formed with a hydraulic cylinder 11, piston pump 12, motor 13, etc., is provided in a chuck inside, a clamp arm 2a, 2b is associated to operate by a reciprocating motion of a draw bar. A chuck control part 20, receiving a detection signal from a pressure sensor 17 detecting an oil pressure in an oil feed pipe 18b and a linear scale 16 detecting a position of a piston 11b in a cylinder, controls the direction of rotation and the speed of the motor 13 through a motor drive part 21. By suitably changing a moving speed of the clamp arm 2a, 2b, shortening of clamp action can be attained, and by controlling the flow of oil so as to fixedly maintain an oil pressure during the clamping of a workpiece W, its vibration can be suppressed, and high work accuracy can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine tool such as an automatic lathe for holding a workpiece (work).

[0002]

2. Description of the Related Art Automatic lathes hold workpieces.
A chuck mechanism that detachably grips the work and a tailstock mechanism that presses and holds the work at a position away from the gripping part when the work is long are provided at positions substantially opposite to each other. Normally, the chuck mechanism has a configuration in which a plurality of gripping pieces move in conjunction with each other so as to sandwich the work. On the other hand, the tailstock mechanism has a configuration in which the tailstock portion is movable in the extending direction of the rotation axis of the work, and the tip of the tailstock portion presses the work on the rotation shaft.

[0003] In a conventional lathe apparatus, a hydraulic drive device is used as a drive source for the movement of the chucking piece and the movement of the tailstock. As a chuck mechanism using a conventional hydraulic drive device as a drive source, for example, Japanese Unexamined Patent Application Publication No.
An apparatus described in JP-A-354605 is known. This device includes a hydraulic cylinder for opening and closing the gripping pieces of the chuck, a tank storing oil, and a pump for sucking oil from the tank and forcing it to the hydraulic cylinder, and operates the pump when performing the gripping operation. And suck the oil from the tank,
Oil is pumped to a hydraulic cylinder to operate a piston in the cylinder. According to such a hydraulic drive device, there is an advantage that a strong operating force can be applied, and the work can be reliably held.

[0004]

However, such a work holding device using a hydraulic drive device has the following problems. (1) Since a large-capacity oil storage tank and a pipe for connecting the tank to the main unit are required, the size of the device itself becomes large and the cost becomes high. In addition, since the amount of oil storage is large, the procedure for installation such as notification to the fire department as fire hazardous materials is also troublesome. (2) The working environment may be deteriorated due to oil leaks from tanks and pipes. (3) Since it is difficult to finely control the gripping force and the tail pressing force of the work, it becomes a bottleneck when performing high-precision cutting or the like. In particular, in recent years, the accuracy of cutting tools such as cutting tools and their driving parts has been significantly improved, and the accuracy of the work holding device, which has not been a problem so far, is becoming a major problem. Such an improvement in accuracy is difficult. (4) Oil leakage cannot be ignored, and the response of the operation of the gripping piece and the tailstock to the operation of the hydraulic pump is not good. Therefore, the hydraulic pump should be operated almost always during gripping or tailstock of the work. And energy saving is difficult. Also, the noise when cutting is not performed is large. (5) Since the moving speed of the gripping piece and the tailstock cannot be finely controlled, it is necessary to move the gripping piece and the tailstock at such a speed that the work is not damaged at the time of contact. It is difficult to reduce the time required.

[0005] The present invention has been made to solve these various problems, and its main object is to reduce the size and energy consumption of the device,
An object of the present invention is to provide a work holding device for a machine tool that contributes to improvement of machining accuracy and is useful for improvement of work efficiency.

[0006]

Means for Solving the Problems and Effects of the Invention A first work holding apparatus for a machine tool according to the present invention, which has been made to solve the above problems, is a machine tool for cutting a work. In a work holding device for holding a work,
It has a plurality of gripping pieces for sandwiching and holding the work, and a drive unit for moving the gripping pieces. The drive unit has: a) two outflow / inlet ports, and a suction / inlet port according to the rotation direction of the shaft.
A) a piston pump for changing the discharge direction; b) a hydraulic cylinder connected to the two outflow ports of the piston pump at both ends via respective pipe lines to operate the gripping pieces according to the reciprocation of the rod; c) A motor for rotating and driving the shaft of the piston pump; d) position information corresponding to the moving direction of the gripping piece and corresponding to the position of the rod of the hydraulic cylinder and / or information corresponding to the hydraulic pressure in the pipeline. And control means for controlling the rotation direction and the rotation speed of the motor based on the above.

In the first work holding device for machine tools according to the present invention, when performing a gripping operation for gripping a work, the control means controls the motor so as to rotate at a predetermined direction and at a predetermined speed. Then, the piston pump operates to discharge the oil sucked from one outflow port to another outflow port. As a result, oil flows out from one end of the hydraulic cylinder, while oil flows in from the other end, and the rod advances (or retreats) with a change in the amount of oil, and the gripping piece approaches the work. Move in the direction to grip the work. When releasing the gripping of the work, the control means controls the motor to rotate in a direction opposite to the previous direction, whereby the rod retreats (or advances) and the gripping piece moves in a direction away from the work.

According to a second aspect of the present invention, there is provided a work holding apparatus for cutting a work, wherein the work holding apparatus holds the work. A tailstock that presses part of the tailstock, and a drive unit that moves the tailstock in a direction approaching the work or in the opposite direction, the drive unit includes: a) two outflow ports, Suction depending on the rotation direction of
A) a piston pump that changes the discharge direction; b) a hydraulic cylinder that is connected to the two outflow ports of the piston pump at both ends via respective pipelines and operates the tailstock in accordance with the reciprocation of the rod; c. A) a motor for rotating and driving the shaft of the piston pump; d) a position corresponding to a moving direction of the tailstock and a position information corresponding to a position of a rod of the hydraulic cylinder and / or a hydraulic pressure in the pipeline. Control means for controlling the rotation direction and the rotation speed of the motor based on the obtained information.

In the second machine tool holding device according to the present invention, similarly to the first machine tool work holding device, the control means controls the rotation direction and the rotation speed of the motor, thereby controlling the center. The pressing portion can be moved in a direction approaching the workpiece and in a direction opposite to the direction.

In the first or second machine tool work holding device, a pressure detecting means for detecting a hydraulic pressure is provided in a part of an oil flow path including the pipe, and the control means includes a pressure detecting means for detecting the pressure. The rotation direction and the rotation speed of the motor may be controlled using the detection pressure of the means. In particular, if the rotation direction and the rotation speed of the motor are controlled so that the detected pressure is maintained at a predetermined value during the gripping operation and the tailstock operation, the vibration of the gripping pieces and the tailstock portion can be suppressed.

According to the workpiece holding device for a machine tool according to the present invention, the moving speed of the gripping pieces and the tailstock can be arbitrarily changed by adjusting the rotation speed of the motor.
Therefore, the work can be securely held without giving a strong impact to the work by reducing the speed at which the gripping piece or the tailstock contacts the work. In addition, the gripping piece and tailstock can be moved at a high speed while the gripping piece and tailstock are away from the work, so that the time required for gripping and tailstock can be shortened, and processing work can be shortened. Efficiency can be improved.

Further, by appropriately controlling the rotation of the motor when the work is being gripped or tailstocked, the operating pressure such as the gripping pressure or the tailstock pressure can be changed. These working pressures are changed according to the type of the process and the like, so that reliable gripping or tailstock can be performed without damaging the work. Furthermore, since vibration and blurring of the work generated at the time of cutting can be suppressed,
Processing with higher precision than before becomes possible.

In addition, since a large hydraulic tank, which is required in the conventional apparatus of this type, is not required, the machine tool itself can be reduced in size and space can be saved, and the hydraulic tank and piping are not required. Costs can be reduced. Furthermore, in the present invention, since the responsiveness of the gripping and tailstock operation to the operation of the motor is good, it is not necessary to always operate the motor even when gripping or tailstocking the work, and it is possible to reduce the power consumption. Energy saving can also be achieved.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A lathe according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of a main part of the lathe device of the present embodiment.

In this lathe apparatus, the headstock 1 and the tailstock 3 are provided to face each other with a predetermined center distance. The headstock 1 is provided with a chuck 2 for gripping the work W so as to be rotatable about an axis C by a motor (not shown). The chuck 2 has a plurality of gripping pieces (only the reference numerals 2a and 2b are shown in this figure, but usually three or more gripping pieces are provided). , M2 direction), the workpiece W is gripped so as to sandwich it, or the grip is released. In other words, the gripping of the workpiece in the lathe apparatus is of the external fastening (outside gripping) type, but the opposite is true in the case of the lining (inner gripping).

A tailstock 3 has a spire-shaped tailstock 4 on an axis C.
The tailstock 4 is configured to reciprocate along the axis C. The tailstock 4 is not used when the work W has a size that can be securely gripped only by the chuck 2, but when the work W has a long shape such as the work W ′,
The tailstock 4 is brought into contact with the work W ′ and pressed with an appropriate pressure. The tailstock 4 is rotatable about an axis C by a bearing (not shown), and when the work W ′ is rotated by the rotation of the chuck 2, the work W ′ is rotated together with the work W ′, thereby suppressing rotational shake of the work W ′.

A tool post 5 is disposed above the axis C. The tool rest 5 is provided with a cutting tool such as a cutting tool which can move forward and backward with respect to the work W and can reciprocate in the direction of the axis C so as to perform various types of machining such as cutting and polishing on the work W. Has become.

In the present apparatus, the holding pieces 2a, 2a
The moving operation of the b and the moving operation of the tailstock 4 are achieved by the hybrid hydraulic drive unit 10 and the electric circuit that controls the unit 10.

FIG. 2 is a configuration diagram of a drive unit for moving the gripping pieces 2a, 2b of the chuck 2. Hybrid hydraulic drive unit 10 built in non-rotating headstock 1
Are hydraulic cylinder 11, piston pump 12, motor 1
3 and so on. On the other hand, the hybrid hydraulic drive unit 10
The control circuit connected only to the electric signal lines includes a chuck control unit 20 and a motor drive unit 21.

The end of a draw bar 2d for transmitting an operating force to a gripping piece driving mechanism 2c for moving the gripping pieces 2a, 2b in an interlocking manner toward the inner peripheral side or the outer peripheral side is connected to a rod 11a of a single rod type hydraulic cylinder 11. The gripping piece 2 is connected with the piston 11b fitted and inserted into the cylinder.
a and 2b reciprocate in the directions m1 and m2, respectively. For the hydraulic cylinder 11, the rod 11
a is rotatable about a central axis with respect to a cylindrical cylinder main body, and includes a draw bar 2d and a rod 11d.
a rotates integrally with the chuck 2, and the hybrid hydraulic drive unit 10 including the oil feed pipe described later and the control circuit are fixed. In addition, when the draw bar 2d rotates integrally with the chuck 2, a connection method that does not rotate the rod 11a may be adopted.

The oil outlets at both ends of the hydraulic cylinder 11 are:
Two discharge ports 12 of a two-way discharge type piston pump 12
a and 12b are connected to each other through oil feed pipes 18a and 18b, and the whole forms a substantially closed flow path. The piston pump 12 is a fixed displacement piston pump,
By rotating the 2c in the normal rotation direction, oil is sucked from the first discharge port 12a side and discharged to the second discharge port 12b side. When the shaft 12c is rotated in the reverse direction, the direction of oil suction / discharge is reversed.

A hydraulic shock absorber comprising a switching valve 14 and a small-capacity oil storage tank 15 is provided between two oil feed pipes 18a and 18b connecting the hydraulic cylinder 11 and the piston pump 12. In the above configuration, since the hydraulic cylinder 11 is a single-rod type, when the piston 11b moves by a certain length, the working flow in the cylinders on both sides of the piston 11b must be slightly unbalanced. Absent. On the other hand, since the suction-side flow rate and the discharge-side flow rate in the piston pump 12 are the same, if a completely closed flow path is used, a part of the flow path runs out of oil or conversely becomes excessive. May cause problems. Therefore, the switching valve 14 is switched according to the liquid feeding direction of the piston pump 12 to supply the insufficient oil from the oil storage tank 15, or conversely, to supply the excess oil to the oil storage tank 1.
By recovering to 5, the unbalance in the amount of oil supply is eliminated.

The shaft 12c of the piston pump 12 is
3 rotary shaft 13a. Here, both shafts 12
c and 13a are connected to each other so that the same number of rotations can be obtained.
May be reduced and transmitted to the shaft 12c of the piston pump 12.

The hydraulic cylinder 11 has a linear scale 16
And detects the position of the piston 11b and outputs a detection signal corresponding thereto to the chuck control unit 20. Further, a pressure sensor 17 for detecting the oil pressure in the oil feed pipe 18b is provided, and a detection signal thereof is also input to the chuck control unit 20. Note that the linear scale 16 may recognize the moving position by another method such as detecting the amount of advance of the rod 11a. Further, the position of the piston 11b may be indirectly detected by using a rotational position detecting means such as a pulse encoder built in the motor 13 instead of the linear scale 16.

The substance of the chuck control unit 20 provided outside the hybrid hydraulic drive unit 10 is CPU, RO
M, a microcomputer including a RAM, etc., receives a position detection signal or a hydraulic pressure detection signal in accordance with a control program, executes predetermined arithmetic processing, sends a control signal to the motor drive unit 21, and sends a switching control signal to the switching valve 14. Send.
The main controller 22 controls the entire lathe apparatus and sends an instruction signal related to the chuck opening / closing operation to the chuck controller 20. The operation instructing unit 23 is for inputting and setting various parameters required for the operation of the lathe device, and for instructing the start of the operation and the like.

Next, an operation of holding the workpiece W by the chuck 2 and an operation of releasing the workpiece W will be described with reference to the flowchart of FIG. In the initial state where the workpiece W is not gripped,
The gripping pieces 2a, 2b are located at the position farthest from the axis C in the chuck 2. First, the operator sets the opening / closing speed of the gripping pieces 2a, 2b by inputting a numerical value or selecting one of predetermined options in the operation instruction unit 23 (step S11), and instructs the start of the chucking operation (step S12). .

The chuck control unit 20, which has received this instruction via the main control unit 22, controls the motor drive unit 21 so that the motor 13 rotates in the normal rotation direction at a predetermined rotation speed corresponding to the set opening / closing speed. Control is performed (step S13). When the rotation shaft 13a of the motor 13 rotates, the piston pump 12 operates to suck oil from the first discharge port 12a and discharge it to the second discharge port 12b. As a result, oil flows in the oil feed pipes 18a and 18b in the direction of the arrow in FIG. 2, and the rod 11a of the hydraulic cylinder 11 moves backward. Then, the gripping pieces 2a and 2b move in the direction of the axis C, that is, in the direction approaching the work W in conjunction with the gripping piece driving mechanism 2c via the draw bar 2d.

When the gripping pieces 2a and 2b come into contact with the work W, the retraction of the rod 11a is restricted, and it is detected from the position detection signal by the linear scale 16 that the gripping position of the work W has been reached (step S14). Y ").
Further, since the detection pressure of the pressure sensor 17 rapidly rises due to the stop of the movement of the piston 11b, the arrival of the workpiece W at the gripping position is detected by detecting this pressure change. Therefore, when it is determined that the grip position has been reached using one or both of these detection signals, the chuck control unit 20 reduces the rotation speed of the motor 13 or temporarily stops the rotation of the motor 13. Is controlled (step S15).

Thus, the work W is held between the gripping pieces 2a and 2b. Thereafter, the chuck control unit 20 monitors the pressure detected by the pressure sensor 17 and determines whether or not there is a change in the hydraulic pressure (step S16). If there is a change in the oil pressure, the rotation of the motor 13 is adjusted so that the oil flows in a direction to reduce the change in the oil pressure (step S17). After the workpiece W is gripped by the gripping pieces 2a and 2b, the chuck 2 is rotated and subjected to cutting by a cutting tool or the like, so that the gripping pieces 2a and 2b slightly move by the load. Since the oil pressure changes in accordance with the movement, by adjusting the oil flow so as to keep this oil pressure constant, the grip pieces 2a, 2
The vibration of b can be suppressed.

Next, it is determined whether or not an instruction to release the chuck has been issued (step S18). If there has been no instruction to release the chuck, the process returns to step S16 to continue the vibration suppression control of the gripping pieces 2a and 2b. When predetermined processing is completed and the operator instructs release of the chuck, the chuck controller 20 receives the instruction and sets the motor 13 so that the motor 13 rotates in the reverse direction at a predetermined rotation speed corresponding to the set opening / closing speed. The drive unit 21 is controlled (Step S19). As a result, the oil flows in the direction opposite to that of the previous rotation of the motor 13,
The rod 11a of the hydraulic cylinder 11 advances, and the gripping piece 2
a, 2b move in a direction away from the workpiece W. Then, when it is detected from the position detection signal by the linear scale 16 that the motor 13 has reached the initial position ("Y" in step S20), the motor 13 is stopped (step S2).
1).

Of course, not only the above-described automatic gripping operation of the workpiece W, but also the gripping pieces 2a and 2b can be stopped or temporarily stopped at arbitrary positions. Work W
By appropriately setting the target hydraulic control value at the time of gripping, it is possible to adjust gripping strength such as strong gripping or weak gripping according to the type of the work W. Further, by setting the size and the like of the work W in advance, the grip pieces 2a, 2b
Is moved at a high speed, and the gripping piece 2
The workpiece W can also be gripped without giving a large impact by lowering the moving speed of a and 2b. In this way, the gripping operation time can be reduced.

FIG. 3 is a structural view of a drive unit for moving the tailstock unit 4. Since the basic configuration of this driving unit is the same as that of the above-described chuck driving unit, the same or corresponding parts are denoted by the same reference numerals and description thereof will be omitted. In this configuration, the tailstock 4 is connected to the rod 11a of the single rod type hydraulic cylinder 11, and the tailstock 4 is moved to the axis C with the advance and retreat of the piston 11b inserted into the cylinder body.
Reciprocate in the m3 direction along the axis. The reciprocating motion of the rod 11a may be transmitted to the tailstock 4 through an appropriate mechanism such as a speed reduction mechanism.

The tailstock operation and its release operation will be described with reference to the flowchart of FIG. This tailstock operation is almost the same as the above-described operation of gripping the work W by the gripping pieces.
That is, in an initial state in which the work W is not pressed, the tailstock 4 is located at a position farthest from the chuck 2. The operator first sets the advance / retreat movement speed of the tailstock 4 by inputting a numerical value or selecting one of predetermined options in the operation instruction unit 23 (step S31), and the tailstock operation, that is, the advance operation of the tailstock unit 4 is performed. (Step S32).

The tailstock control unit 24, which has received the instruction via the main control unit 22, operates the motor drive unit 2 so that the motor 13 rotates in the normal rotation direction at a predetermined rotation speed corresponding to the advance / retreat speed.
5 is controlled (step S33). Accordingly, the piston pump 12 sucks oil from the second discharge port 12b and
Since the fluid is discharged to the discharge port 12a, the rod 11a of the hydraulic cylinder 11 advances, and the tailstock 4 moves along the axis C in a direction approaching the work W '. When the tailstock 4 comes into contact with the workpiece W ', the advance of the rod 11a is restricted. Therefore, it is detected that the tailstock position has been reached by the position detection signal from the linear scale 16 or the pressure signal detected by the pressure sensor 17 (step S34). "Y"), motor 1
The motor drive unit 25 is controlled so as to reduce the rotation speed or stop the rotation temporarily (step S35).

Thus, the work W ′ is pressed by the tailstock 4. Thereafter, the tailstock control unit 24 monitors the detected pressure, and if there is a change in the hydraulic pressure (“Y” in step S36), the rotation of the motor 13 is controlled so that the oil flows in a direction to reduce the change in the hydraulic pressure. Adjust (Step S3
7). This suppresses fluctuations in tailstock pressure while the work W 'is undergoing cutting or the like, and can stably hold the work W'.

When there is an instruction to release the tailstock (step S
("Y" at 38)), the tailstock control unit 24 controls the motor drive unit 25 so that the motor 13 rotates in the reverse direction at a predetermined rotation speed corresponding to the advance / retreat speed (step S39).
As a result, the rod 11a of the hydraulic cylinder 11 retreats, and the tailstock 4 moves in a direction away from the work W '. When it is detected from the position detection signal by the linear scale 16 that the linear position has reached the initial position ("Y" in step S40), the motor 13 is stopped (step S41).

Of course, the automatic work W 'as described above
The tailstock 4 can be stopped or temporarily stopped at an arbitrary position, in addition to the pressing operation of. Further, by making it possible to appropriately set the target hydraulic control value when the work W ′ is pressed, it is possible to adjust the strength of the press such as the strong press or the weak press according to the type of the work W ′. it can. Further, by preliminarily setting the size of the work W ′ in the direction of the axis C, the tailstock 4 is moved at a high speed until the work W ′ is approached, and the tailstock is brought close to the work W ′. It is also possible to reduce the moving speed of the part 4 and press the work W 'without giving a large impact. In this way, the tailstock operation time can be reduced.

The above embodiment is merely an example, and it is apparent that modifications and modifications can be made as appropriate within the scope of the present invention. For example, in the description of the above-described embodiment, the work grip is of the externally tightened (outside grip) type. it is obvious. In the above embodiment, the single rod type hydraulic cylinder is adopted. However, a double rod type hydraulic cylinder may be adopted and only one of the rods may be used. According to such a configuration, at least Since there is no imbalance between the oil inflow and the oil outflow in the cylinder, the hydraulic shock absorber can be eliminated.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a main part of an embodiment of a lathe provided with a work holding device according to the present invention.

FIG. 2 is a configuration diagram of a drive unit that moves a chuck gripping piece in the lathe device of the present embodiment.

FIG. 3 is a configuration diagram of a driving unit that moves a tailstock in the lathe device of the present embodiment.

FIG. 4 is an operation flowchart of a driving unit shown in FIG. 2;

FIG. 5 is an operation flowchart of a driving unit shown in FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Headstock 2 ... Chuck 2a, 2b ... Grab piece 2c ... Grab piece drive mechanism 2d ... Drawbar 3 ... Tailstock 4 ... Tailstock 5 ... Turret 10 ... Hybrid hydraulic drive unit 11 ... Hydraulic cylinder 11a ... Rod 11b ... Piston 12 ... Piston pump 12a, 12b ... Discharge port 12c ... Shaft 13 ... Motor 13a ... Rotary shaft 14 ... Switching valve 15 ... Oil storage tank 16 ... Linear scale 17 ... Pressure sensor 18a, 18b ... Oil feeding pipe 20 ... Chuck control unit 21 , 25: motor drive unit 22: main control unit 23: operation instruction unit 24: tailstock control unit W, W ': work

Claims (3)

[Claims] 1. A work holding device for holding a work in a machine tool for cutting a work, the work holding device comprising: a plurality of gripping pieces for sandwiching and holding the work; and a driving unit for moving the gripping piece; The drive has: a) two outflow ports, and suction and suction depending on the rotation direction of the shaft.
A) a piston pump for changing the discharge direction; b) a hydraulic cylinder connected to the two outflow ports of the piston pump at both ends via respective pipe lines to operate the gripping pieces according to the reciprocation of the rod; c) A motor for rotating and driving the shaft of the piston pump; d) position information corresponding to the moving direction of the gripping piece and corresponding to the position of the rod of the hydraulic cylinder and / or information corresponding to the hydraulic pressure in the pipeline. Control means for controlling a rotation direction and a rotation speed of the motor based on the following. 2. A work holding device for holding a work in a machine tool for cutting a work, wherein a tail stock portion for pressing a part of the work and a tail stock portion in a direction approaching the work or vice versa. A driving unit for moving in the direction,
The drive has: a) two outflow ports, and suction and suction depending on the rotation direction of the shaft.
A) a piston pump for changing the discharge direction; b) a hydraulic cylinder for connecting the two outflow ports of the piston pump with both ends via respective pipe lines and operating the tailstock in accordance with the reciprocation of the rod; c) A) a motor for rotating and driving the shaft of the piston pump; d) a position corresponding to a moving direction of the tailstock and a position information corresponding to a position of a rod of the hydraulic cylinder and / or a hydraulic pressure in the pipeline. Control means for controlling the rotation direction and the rotation speed of the motor based on the obtained information. 3. A pressure detecting means for detecting a hydraulic pressure in a part of an oil flow path including the pipeline, wherein the control means uses a detected pressure of the pressure detecting means to detect a rotation direction and a rotation speed of the motor. The workpiece holding device for a machine tool according to claim 1, wherein: