JP2000005907A - Tool locking device for machine tool main spindle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tool locking mechanism of a machine tool main spindle which surely locks a tool to a tool holding part by enabling a drawbar to be energized toward the tool locking side with a very large energizing force, and which is advantageous in terms of manufacturing cost because of a simple structure including the main spindle. SOLUTION: An energizing mechanism 7 of a tool locking device 3 comprises a piston member 41 mounted in a cylinder hole 40 formed in the base end portion of a main spindle 1, in such a manner that the piston member 41 is movable over a predetermined stroke, the piston member 41 having a piston portion 42 fitted over a drawbar 6. The drawbar 6 is energized toward the tool locking side via a cylinder member 41 by compressed gas sealed in the gas actuation chamber 45 of the cylinder hole 40.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tool fixing device for a main shaft of a machine tool, and more particularly to a draw bar associated with a collet engaged with a base end of a tool, which is biased toward a tool fixing side by compressed gas. Concerning the configured.

[0002]

2. Description of the Related Art A main shaft of a machine tool that performs cutting or the like on a workpiece has a tapered tool holding portion formed at a tip end thereof, and a tool fixing device for releasably fixing a tool to the tool holding portion is provided. ing. A general tool fixing device includes a collet which engages with a base end of a tool mounted on a tool holding portion, a draw bar which is inserted in the main shaft and is linked to the collet, and a draw bar which is fixed to a tool fixing side (a main shaft of the main shaft). An urging mechanism that urges the drawbar toward the base end), a releasing mechanism that can move the drawbar to the fixing releasing side (the distal end side of the main shaft) against the urging force of the urging mechanism, and the like.

[0003] When the drawbar is driven by the release mechanism to the unlocking side, the collet also moves to the unlocking side and enters an engagement standby state (a state in which the tip of the collet divided into three or four is open). The tool can be attached to and detached from the tool holder of the spindle. In this state, when the tool is mounted on the tool holder, the base end is located inside the collet. Next, when the draw bar is driven to the tool fixed side by the urging force of the urging mechanism, the collet is also moved to the tool fixed side. To the base end of the tool, the tool is pressed and fixed by the tool holding portion by the urging force acting via the draw bar and the collet, and the main shaft and the tool are frictional forces acting on their contact portion. To rotate together.

A conventional urging mechanism mounts a piston member fixed to a base end of a draw bar to a cylinder hole formed in a base end portion of a main shaft so as to be movable by a predetermined stroke, and moves the piston member from the cylinder hole to a collet side. A large number (for example, 20 to 30) of coned disc springs are mounted in a stacked manner in the spring accommodating holes extending in the direction of the arrow, and the draw bar is urged toward the tool fixing side by the elastic force of these disc springs. In order to ensure the machining accuracy of the machine tool, the friction acting on the contact part between the fixed holding part and the tool so that the tool is firmly attracted to the tool holding part and fixed, and the main shaft and the tool keep rotating integrally with each other You need to increase your power.

[0005]

However, in the conventional tool fixing device, a large number of disc springs are laminated, and as a result of accumulating manufacturing errors of the large number of disc springs, a stable urging force as designed is exhibited. It is difficult to perform this operation, and the size of the disc spring that can be accommodated in the spring accommodating hole of the main shaft is limited. Therefore, it is difficult to urge the drawbar toward the tool fixing side with a stable and strong urging force. Therefore, it is difficult to strongly fix the tool to the tool holding portion, and it is also difficult to maintain integral rotation between the main shaft and the tool when an overload is applied to the tool. as a result,
There is a problem that it is difficult to sufficiently increase the processing accuracy of the machine tool. Further, since a large number of disc springs are mounted, the tool fixing device including the main shaft has a complicated structure, and the manufacturing cost is high.

SUMMARY OF THE INVENTION It is an object of the present invention to enable a draw bar to be urged toward a tool fixing side with a very large urging force, thereby securely fixing a tool to a tool holding portion. It is an object of the present invention to provide a tool fixing mechanism for a machine tool spindle which is advantageous to the above.

[0007]

According to a first aspect of the present invention, there is provided a tool fixing device for a machine tool main spindle, wherein a tool mounted on a tapered tool holding portion of the main spindle of the machine tool is fixed to a base end of the tool. A collet to be engaged, a drawbar linked to the collet, a biasing means for biasing the drawbar toward the tool fixing side, and a release capable of driving the drawbar to the fixing release side against the biasing force of the biasing means. Means for fixing the tool to the tool holding portion so that the tool can be unlocked. In the fixed tool device, the urging means is capable of moving a predetermined stroke to a cylinder hole formed in a base end portion of the main shaft and a cylinder hole. A piston member having a piston portion mounted on the drawbar and externally fitted to the drawbar and moving integrally with the drawbar; and a compressed gas sealed in a gas working chamber on the collet side with respect to the piston portion of the cylinder hole. It is provided with.

When a tool is mounted on the tool holding portion with the draw bar being driven to the fixing releasing side by the releasing means, the base end thereof is in an engagement standby state (a state in which the distal end portion of the collet divided into three or four is open). ) Located inside the collet,
Next, when the draw bar is driven together with the piston member to the tool fixed side by the biasing force of the compressed gas sealed in the gas working chamber, the collet also moves to the tool fixed side and engages with the base end of the tool. Then, the tool is pressed and fixed to the tool holding part by the urging force acting via the piston member, the draw bar, and the collet.

In this tool fixing device for a machine tool spindle, the drawbar can be urged toward the tool fixing side with a very large and stable urging force due to the compressed gas sealed in the gas working chamber. Can be strongly pressed and fixed to the fixed holding section. Here, it is possible to reliably seal the space between the gas working chamber and the outside, and to configure the gas working chamber so that the compressed gas hardly leaks to the outside. Also, even if the compressed gas in the gas working chamber leaks to the outside and the gas pressure is reduced by repeatedly using the tool fixing device for a long time, the biasing force for biasing the draw bar by filling the gas working chamber with the compressed gas. Can be restored to the initial large value.

According to a second aspect of the present invention, there is provided a tool fixing device for a machine tool spindle according to the first aspect of the invention, wherein the gas working chamber is formed in a large volume working chamber that is longer in the axial direction than the movement stroke of the draw bar. Is what you do. Therefore,
The ratio of the compressed gas leaked to the outside relative to the compressed gas in the gas working chamber, that is, the rate of decrease in the gas pressure in the gas working chamber can be reduced. Therefore, it is possible to prevent the gas pressure in the gas working chamber from rapidly decreasing. Other claim 1
It has the same function as.

According to a third aspect of the present invention, there is provided a tool fixing device for a machine tool spindle according to the first or second aspect of the present invention, further comprising a gas pressure detecting means for detecting a physical quantity related to a gas pressure of the compressed gas sealed in the gas working chamber. The gas working chamber is provided with a gas filling port with a check valve for filling compressed gas. Therefore, when the gas pressure in the gas working chamber detected by the gas pressure detecting means is low, the gas working chamber can be filled with the compressed gas from the gas filling port with the check valve. Other operations are the same as those of the first or second aspect.

According to a fourth aspect of the present invention, there is provided a tool fixing device for a machine tool main spindle, wherein the gas pressure detecting means includes a gap sensor for detecting a change in the radius of the main spindle outside the gas working chamber, and a temperature of the main spindle. And a temperature sensor for detecting the temperature. Therefore, even while the main shaft is rotating, the gas pressure in the gas working chamber can be detected based on the change in the radius of the main shaft detected by the gap sensor and the temperature of the main shaft detected by the temperature sensor. Other effects are the same as those of the third aspect.

According to a fifth aspect of the present invention, there is provided a tool fixing device for a machine tool spindle for fixing a tool mounted on a tapered tool holding portion of a machine tool spindle, the collet engaging with a base end of the tool. A drawbar associated with the collet, a biasing means for biasing the drawbar toward the tool fixing side, and a release means capable of driving the drawbar to the fixing release side against the biasing force of the biasing means, the tool holding unit comprising: In a tool fixing device for releasably fixing a tool to the tool, the urging means has a cylinder hole formed at a base end portion of the main shaft, and a piston portion movably mounted in the cylinder hole by a predetermined stroke. And a piston member externally fitted to the drawbar and moving integrally with the drawbar, and an annular metallic bellows mounted in the working chamber on the collet side with respect to the piston portion of the cylinder hole, and the compressed gas is provided. The entrance to the piston member is obtained by a bellows which urges the tool stationary.

After the tool is mounted on the tool holding portion with the draw bar being driven to the unlocking side by the unlocking means, the draw bar is integrated with the piston member to the tool fixing side together with the piston member by the urging force of the compressed gas sealed in the bellows. When the collet is driven, the collet also moves to the tool fixed side and engages with the base end of the tool, and the tool is pressed by the tool holding portion by the urging force acting through the piston member, the draw bar, and the collet. Fixed. This tool fixing device for a machine tool spindle has substantially the same effect as the first aspect, but can prevent the compressed gas in the bellows from leaking to the outside as much as possible.

According to a sixth aspect of the present invention, there is provided a tool fixing device for a machine tool spindle according to the fifth aspect of the present invention, further comprising a gas pressure detecting means for detecting a physical quantity related to a gas pressure of the compressed gas sealed in the bellows. A gas filling port with a check valve for filling compressed gas is provided. Therefore, when the gas pressure in the gas working chamber detected by the gas pressure detecting means is low, the bellows can be filled with the compressed gas from the gas filling port with the check valve. Other operations are the same as those of the fifth aspect.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. The present embodiment is an example of a case where the present invention is applied to a tool fixing device that releasably fixes a tool to a tool holding portion at a tip portion of a main shaft of a machine tool. As shown in FIGS. 1 to 3, a tapered tool holding portion 2 is formed at a front end portion of a main shaft 1 of a machine tool rotatably supported via a plurality of bearings 1a. In addition, a tool fixing device 3 for fixing the tool 4 to the tool holding unit 2 so as to be able to release the fixing is provided.

The tool fixing device 3 includes a collet 5 which engages with the engaged portion 22 at the base end of the tool 4 mounted on the tool holding portion 2.
And a draw bar 6 linked to the collet 5, an urging mechanism 7 for urging the draw bar 6 toward the tool fixing side (the base end side of the main spindle 1), and a draw bar 6 fixed against the urging force of the urging mechanism 7. The release mechanism 8 and the like can be driven to the release side (the tip side of the main shaft 1).

The main shaft 1 has a tool holding portion 2, a housing hole 10 for housing the collet 5, an insertion hole 11 through which the draw bar 6 is inserted, and a cylinder hole 40 in series from the distal end to the proximal end. The main shaft 1 is provided with a sleeve 1
The base member 3 is externally screwed and fixed, and a cover member 17 is fixed to the base end of the main shaft 1 with a plurality of bolts 17a. A plurality of holes 14 are formed between the distal end portion of the sleeve 13 and the main shaft 1, and a base end of a pressing member 15 having a distal end protruding toward the distal end of the main shaft 1 is slidably inserted into each of the holes 14. The pressing member 15 is fitted and urged toward the distal end by a plurality of leaf springs 16 mounted in the holes 14. However, the hole 14 may be formed in a ring shape, and the plurality of leaf springs 16 may be integrated in a ring shape.

The tool 4 includes a tapered shaft portion 20 having a diameter reduced toward the base end, an engaged portion 22 provided on the base end of the tapered shaft 20 via a small-diameter shaft 21, and a tapered shaft portion. 20 has a large-diameter disk portion 23 provided on the distal end side, and when the tool 4 is mounted on the tool holding portion 2, the disk portion 23 is pressed against the distal ends of the plurality of pressing members 15. Then, the tapered shaft portion 20 comes into surface contact with the tool holding portion 2, and the engaged portion 22 is inserted into the accommodation hole 10.

The collet 5 is a divided body divided into three or four, and is an elastic body which is deformed from the engaged state of FIG. 1 to the standby state of FIG. 2, and is provided with a shaft hole 30 through which the draw bar 6 is inserted. And a large-diameter hole 31 provided on the tip side of the shaft hole 30 and having a larger diameter than the shaft hole 30. The large-diameter hole 31 is fixed to the distal end portion of the draw bar 6 and the locking portion 33 is fitted therein. When the draw bar 6 moves to the base end side from the state of FIG. 2, a step at the boundary between the shaft hole 30 and the large-diameter hole 31 is formed. The locking portion 33 is locked to the portion 32, and the collet 5 also moves to the base end side. When the draw bar 6 moves to the distal end from the state shown in FIG. 1, the collet 5 deforms from the engaged state to the engagement standby state, and moves to the distal end through the deformation.

The urging mechanism 7 includes a cylinder hole 40 formed at a base end portion of the main shaft 1 and a piston portion 42 and a piston portion 42 which are fitted on the draw bar 6 so as to be movable by a predetermined stroke. The gas working chamber 45 has a piston member 41 having cylindrical portions 43 and 44 extending from the 42 to the distal end side and the proximal end side, and the collet 5 side of the piston portion 41 of the cylinder hole 40. The gas working chamber 45 is formed as a large-volume working chamber that is longer in the axial direction than the movement stroke of the draw bar 6.
5 is filled with a compressed gas of a predetermined pressure (for example, compressed nitrogen gas of 100 kg / cm ² G). The gas working chamber 45 is sealed by O-rings 54 and 55 and the like.

The distal end of the cylindrical portion 43 is movably fitted in a hole 50 connected to the cylinder hole 40 of the main shaft 1, and the cylindrical portion 44 is movably fitted in a hole 51 formed in the lid member 17. . A shaft member 53 facing the outside of the main shaft 1 is fixed to the base end of the draw bar 6, and the driving force of the hydraulic cylinder 67 of the release mechanism 8 is input from the shaft member 53. The shaft member 53 is formed to have a larger diameter than the draw bar 6, and the base end of the cylindrical portion 44 is locked by the shaft member 53, so that the urging force of the compressed gas in the gas working chamber 45 is transmitted to the draw bar 6.

A gas passage 60 communicating with the gas working chamber 45 is formed in the main shaft 1, and a gas filling port 61 with a check valve for filling the gas working chamber 45 with a compressed gas is provided at an end of the gas passage 60. Have been. Also, the gas working chamber 45
Gas pressure detecting means including a gap sensor 65 for detecting the gas pressure in the inside and a temperature sensor 66 (for example, an infrared sensor) is provided. Gap sensor 65 and temperature sensor 66
Is fixedly provided so as to face an outer peripheral surface of a portion of the main shaft 1 corresponding to the gas working chamber 45, and the gap sensor 6 is provided.
5, the change in the radius of the main shaft 1 and the temperature sensor 66
Gas working chamber 45 based on the temperature of the spindle 1 obtained by
The gas pressure inside can be measured.

The release mechanism 8 has a hydraulic cylinder 67 fixedly provided on the base end side of the main shaft 1 and driven and controlled by a drive control section including a hydraulic supply source.
A shaft member 53 at the base end of the draw bar 6 is pushed by the distal end of the draw bar 7a, and the draw bar 6 is driven to the unlocking side.

The operation and effect of the tool fixing device 3 will be described. 1 from the state of FIG.
Is driven to the unlocking side, as shown in FIG. 2, the collet 5 moves to the unlocking side and waits for the engagement (3 or 4).
In this state, the divided collet 5 is in an open state, and the tool 4 can be attached to and detached from the tool holder 2.

In this state, when the tool 4 is mounted on the tool holder 2, the engaged portion 22 of the tool 4 is located inside the collet 5, and then the piston rod 67a of the hydraulic cylinder 67 of the release mechanism 8 is moved. When retracted, the urging force of the compressed gas sealed in the gas working chamber 45 causes the draw bar 6 to move integrally with the piston member 41 to the tool fixing side together with the piston member 41, and FIG.
As shown in the figure, the collet 5 also moves to the tool fixing side and engages with the engaged portion 22 of the tool 4, and the tool 4
And the tool bar 2 is pressed and fixed by the urging force acting via the draw bar 6 and the collet 5.

According to the tool fixing device 3, the draw bar 6 can be urged toward the tool fixing side with a very large urging force by the compressed gas sealed in the gas working chamber 45 of the urging mechanism 7. Become. Even if the compressed gas in the gas working chamber 45 leaks to the outside and the gas pressure drops due to repeated use of the tool fixing device 3 for a long time, the compressed gas is filled into the gas working chamber 45 from the gas filling port 61 with a check valve. By doing so, the urging force for urging the drawbar 6 can be restored to the initial large value.

Therefore, since the tool 4 can be strongly pressed and fixed to the tool holding portion 2, the fixed holding portion 2 and the tool 4
The frictional force acting between the main shaft 1 and the tapered portion 29 becomes extremely large, and even if an overload is applied to the tool 4, the integrated rotation of the main spindle 1 and the tool 4 can be maintained. Can be enhanced. In addition, the tool fixing device 3 including the main shaft 1 can be configured so that the manufacturing cost can be reduced.

Since the gas working chamber 45 is formed as a large volume working chamber which is longer in the axial direction than the movement stroke of the draw bar 6, the ratio of the compressed gas leaked to the outside to the compressed gas in the gas working chamber 45, that is, the gas working The rate of decrease in the gas pressure in the chamber 45 can be reduced. Therefore, it is possible to prevent the gas pressure in the gas working chamber 45 from rapidly decreasing. Since the gap sensor 65 for detecting a change in the radius of the main shaft 1 and the temperature sensor 66 for detecting the temperature of the main shaft 1 are provided outside the gas working chamber 45, even when the main shaft 1 is rotating, the gap sensor 65 is used. The gas pressure in the gas working chamber 45 can be detected based on the detected change in the radius of the main shaft and the temperature of the main shaft detected by the temperature sensor 66.

Next, another embodiment of the above embodiment will be described with reference to FIG. In another embodiment, the urging mechanism 7 of the above embodiment is changed, and the same components as those of the above embodiment are denoted by the same reference numerals and description thereof is omitted.

The biasing mechanism 7A of the tool fixing device 3A has a gas operating chamber 45 fitted with an annular stainless steel bellows 70 through which the cylindrical portion 43 of the piston member 41 is inserted. It is enclosed. The cylindrical member 44A of the piston member 41, the hole 51A of the lid member 17, and the shaft member 53A fixed to the base end of the draw bar 6 are formed to have a slightly larger diameter as compared with the above-described embodiment. A gas passage 71 communicating with the inside of the bellows 70 is formed in the bellows 70 and the cylindrical portion 44A, and a gas filling port 72 with a check valve for filling the bellows 70 with a compressed gas is provided at an end of the gas passage 71. .

In order to prevent the compressed gas from leaking from the bellows 70 and the gas passage 71, the base end surface of the bellows 70 and the distal end surface of the piston portion 42 are strongly bonded. A seal 74 is provided. A sealing plug 73 that closes the gas filling port 72 so as to be openable and closable is screwed to the cylindrical portion 44A. Bellows 7
0 in the gas working chamber 45 and then the gas filling port 72
The bellows 70 can be easily assembled because the compressed gas can be filled from the inside.

After the tool 4 is mounted on the tool holder 2 of the main spindle 1 with the collet 5 moved to the unlocking side, the draw bar 6 is moved together with the piston member 41 by the urging force of the compressed gas sealed in the bellows 70. When integrally driven to the tool fixing side, the collet 5 also moves to the tool fixing side and engages with the base end of the tool, and the tool 4 is pressed and fixed by the tool holding portion 2 by the urging force. According to the tool fixing device 3A, substantially the same operation and effect as those of the above embodiment can be obtained, but since the compressed gas is sealed in the bellows 70, the compressed gas can be prevented from leaking to the outside as much as possible.

The release mechanism 8, the gap sensor 65, and the temperature sensor 66 of the above embodiment are not shown. However, the gap sensor 65 is fixedly provided on the base end side of the main shaft 1, and is configured to detect a change in the length of the main shaft and to detect the gas pressure in the bellows 70.

Although not shown, in the main embodiment, instead of the gas working chamber 45 of the above-described embodiment, the gas working chamber 45 is smaller than the gas working chamber 45 in the axial direction (however, it is longer than the movement stroke of the draw bar 6). A gas working chamber may be provided. In this case, it is desirable to improve the sealing performance of the gas working chamber 45 so that the compressed gas does not leak from the gas working chamber. In another embodiment, a working chamber that is shorter in the axial direction than the gas working chamber 45 may be provided, and a bellows may be provided in the working chamber in the axial direction.

In another embodiment, a bellows 70 filled with compressed gas may be assembled in the gas working chamber 45.
In this case, the gas passage 71 of the piston member 41, the gas filling port 72 with a check valve, the sealing plug 73, the seal 74
Since the piston member can be omitted, the piston member has a very simple structure, and the compressed gas of the bellows 70 can be reliably prevented from leaking to the outside.

[0037]

According to the tool fixing device for a machine tool spindle according to the first aspect, the drawbar can be urged toward the tool fixing side with a very large and stable urging force caused by the compressed gas sealed in the gas working chamber. The tool can be strongly pressed and fixed to the fixed holding part of the main spindle, and the frictional force acting on the fixed holding part, the tool and the contact part becomes very large. The rotation of the tool and the tool can be maintained integrally, the machining accuracy of the machine tool can be reliably increased, and the tool fixing device including the main shaft has a simple structure, so that the manufacturing cost can be reduced.

According to the tool fixing device for a machine tool spindle of the second aspect, the same effect as that of the first aspect is obtained, but the gas working chamber is formed in a large volume working chamber that is longer in the axial direction than the movement stroke of the draw bar. Therefore, in the case of compressed gas leaked to the outside of the compressed gas sealed in the gas working chamber,
That is, the rate of decrease in the gas pressure in the gas working chamber can be reduced, and the gas pressure in the gas working chamber can be prevented from rapidly decreasing.

According to the third aspect of the present invention, the same effect as in the first or second aspect is obtained, but a physical quantity related to the gas pressure of the compressed gas sealed in the gas working chamber is detected. Since the gas pressure detecting means is provided, and the gas working chamber is provided with a gas filling port with a check valve for filling the compressed gas, when the gas pressure detected by the gas pressure detecting means is reduced, The gas working chamber can be filled with the compressed gas from the gas filling port with the check valve.

According to the tool fixing device for a machine tool spindle of the fourth aspect, the same effect as that of the third aspect is obtained, but the gas pressure detecting means detects a change in the radius of the spindle outside the gas working chamber. Since the sensor and the temperature sensor for detecting the temperature of the main shaft are included, even while the main shaft is rotating, based on the change in the radius of the main shaft detected by the gap sensor and the temperature of the main shaft detected by the temperature sensor, The gas pressure of the compressed gas in the gas working chamber can be detected.

According to the fifth aspect of the present invention, similarly to the first aspect, the draw bar is urged toward the tool fixing side with a very large urging force stabilized by the compressed gas sealed in the bellows. The tool can be strongly pressed and fixed to the fixed holding part of the spindle, the frictional force acting on the fixed holding part, the tool and the contact part becomes very large, and the tool is overloaded. In addition to maintaining the integral rotation of the spindle and the tool, the machining accuracy of the machine tool can be reliably increased, and the compressed gas is sealed in the bellows to minimize the leakage of the compressed gas to the outside. Can be.

According to the tool fixing device for a machine tool spindle of the sixth aspect, the same effect as the fifth aspect is obtained, but a gas pressure detecting means for detecting a physical quantity related to the gas pressure of the compressed gas sealed in the bellows. The bellows is provided with a gas filling port with a check valve for filling the compressed gas, so if the gas pressure detected by the gas pressure detecting means is low, the gas filling with a check valve is provided. The gas working chamber can be filled with compressed gas through the mouth.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a spindle and a tool fixing device of a machine tool according to an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of the spindle and a tool fixing device (tool removal state).

FIG. 3 is an enlarged sectional view of a main part of the spindle and the tool fixing device.

FIG. 4 is an enlarged longitudinal sectional view of a main part of a tool fixing device according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Main shaft 2 Tool holding part 3, 3A Tool fixing device 4 Tool 5 Collet 6 Drawbar 7, 7A Urging mechanism 8 Release mechanism 40 Cylinder hole 41 Piston member 42 Piston part 45 Gas working chamber 61, 72 Gas filling port 65 Gap sensor 66 Temperature sensor 70 bellows

Claims (6)

[Claims] To fix a tool mounted on a tapered tool holding portion of a main shaft of a machine tool, a collet engaged with a base end of the tool, a draw bar associated with the collet, and a draw bar are provided. An urging means for urging the tool to the fixing side, and a releasing means for driving the draw bar to the fixing releasing side against the urging force of the urging means are provided, and the tool is releasably fixed to the tool holding portion. The urging means has a cylinder hole formed at a base end side portion of the main shaft, a piston portion attached to the cylinder hole so as to be movable by a predetermined stroke, and is externally fitted to a draw bar to A tool fixing device for a machine tool spindle, comprising: a piston member that moves integrally with the piston; and a compressed gas sealed in a gas working chamber on the collet side with respect to the piston portion of the cylinder hole. 2. The tool fixing device according to claim 1, wherein the gas working chamber is formed in a large-sized working chamber that is longer in the axial direction than a movement stroke of the draw bar. 3. A gas filling port with a check valve for filling the gas working chamber with the compressed gas, the gas working chamber being provided with gas pressure detecting means for detecting a physical quantity related to the gas pressure of the compressed gas sealed in the gas working chamber. The tool fixing device for a machine tool spindle according to claim 1 or 2, further comprising: 4. The apparatus according to claim 3, wherein said gas pressure detecting means includes a gap sensor for detecting a change in radius of the main shaft outside the gas working chamber, and a temperature sensor for detecting a temperature of the main shaft. A tool fixing device for a machine tool spindle according to the above. 5. A collet engaged with a base end of a tool mounted on a tapered tool holding portion of a main shaft of a machine tool, a draw bar linked to the collet, and a draw bar connected to the collet. An urging means for urging the tool to the fixing side, and a releasing means for driving the draw bar to the fixing releasing side against the urging force of the urging means are provided, and the tool is releasably fixed to the tool holding portion. The urging means has a cylinder hole formed at a base end side portion of the main shaft, a piston portion attached to the cylinder hole so as to be movable by a predetermined stroke, and is externally fitted to a draw bar to A piston member that moves integrally with the piston member, and a ring-shaped metallic bellows mounted in the working chamber on the collet side with respect to the piston portion of the cylinder hole. Tool fixing device for machine tools proximal main shaft, characterized in that a bellows which urges constant side. 6. A gas pressure detecting means for detecting a physical quantity related to a gas pressure of the compressed gas sealed in the bellows, and a gas filling port with a check valve for filling the bellows with the compressed gas is provided. The tool fixing device for a machine tool spindle according to claim 5, characterized in that: