Machine tool and control method of machine tool

Abstract

To provide a machine tool and a control method of a machine tool which even if force for gripping a work-piece by a collet chuck is dropped due to abrasion or the like of a bobbin and of a chuck claw, can maintain a state where the work-piece is gripped by the collect chuck.SOLUTION: A machine tool comprises: a gripped state detecting part 181 whose bobbin 160 has external diameters with different diameters (an outer peripheral surface 161a and a large-diameter outer peripheral surface 161b) along an axial direction of a spindle 110 and which detects a state where a work-piece is gripped by a collet chuck 140; and a driving control part 182 that controls a servo motor 150 so that a chuck claw 170 contacts a predetermined portion of the outer diameters of the bobbin 160, on the basis of a detected result by the gripped state detecting part 181, so as to move the bobbin 160 back and forth.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a machine tool having a collet chuck and a method for controlling the machine tool. [Background technology]

[0002] Conventionally, there is known a machine tool which includes a collet chuck having a chuck sleeve that accommodates a collet, chuck claws that open and close the collet chuck by moving the chuck sleeve and the collet relative to one another, a spindle on which the collet chuck is attached, a bobbin that is fitted onto the spindle so as to be able to move back and forth, and a drive means for moving the bobbin back and forth, wherein the bobbin has an outer diameter that varies along the axis of the spindle, the chuck claws are supported so as to be able to swing freely toward the outer circumferential surface of the spindle, and the bobbin is inserted between one end of the chuck claws and the spindle to open and close the collet chuck and grip a workpiece (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-123038 A Summary of the Invention [Problem to be solved by the invention]

[0004] The machine tool described in Patent Document 1 has a problem in that the gripping force of the collet chuck on the workpiece may decrease due to wear of the bobbin or chuck jaws, etc.

[0005] Therefore, the present invention is intended to solve the problems of the prior art as described above. Specifically, the object of the present invention is to provide a machine tool and a control method for the machine tool that can maintain the gripping of a workpiece by a collet chuck even when the gripping force of the collet chuck on the workpiece is reduced due to wear of the bobbin or chuck jaws, etc. [Means for solving the problem]

[0006] The invention according to claim 1 comprises a collet chuck having a chuck sleeve that accommodates a collet, chuck jaws that move the chuck sleeve and the collet relatively to open and close the collet chuck, a main shaft to which the collet chuck is attached, a bobbin that is fitted onto the main shaft so as to be able to move back and forth, and a drive means for moving the bobbin back and forth, wherein the bobbin has an outer diameter that varies along the axial direction of the main shaft, the chuck jaws are supported so as to be able to swing toward the outer circumferential surface of the main shaft, and the bobbin is inserted between one end side of the chuck jaws and the main shaft. In a machine tool in which the collet chuck is opened and closed to grip a workpiece and the opening degree of the chuck jaws is adjusted according to the outer diameter of the bobbin that abuts against the chuck jaws to adjust the gripping force of the workpiece by the collet chuck, the machine tool solves the above-mentioned problems by having a gripping state detection unit that detects the gripping state of the collet chuck, and a drive control unit that controls the drive means to move the bobbin forward and backward so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result by the gripping state detection unit.

[0007] The invention of claim 2 further solves the above-mentioned problem by, in addition to the configuration of the machine tool described in claim 1, the driving means being a motor and the gripping state detection unit detecting the gripping state based on a current value of the motor.

[0008] The invention of claim 3 further solves the above-mentioned problem by, in addition to the configuration of the machine tool described in claim 1, having the gripping state detection unit detect the position of one end of the chuck jaws in a direction intersecting the axial direction.

[0009] The invention of claim 4 further solves the above-mentioned problem by, in addition to the configuration of the machine tool described in claim 1, supplying the workpiece to the spindle sequentially and continuously by a material supply device, and by the gripping state detection unit detecting the gripping state when gripping the workpiece during continuous machining of the workpiece.

[0010] The invention of claim 5 further solves the above-mentioned problem by providing, in addition to the configuration of the machine tool described in any one of claims 1 to 4, an alarm means for notifying that the gripping force of the collet chuck on the workpiece has decreased.

[0011] The invention according to claim 6 is a collet chuck having a chuck sleeve for accommodating a collet, chuck jaws for opening and closing the collet chuck by moving the chuck sleeve and the collet relatively, a main shaft to which the collet chuck is attached, a bobbin fitted on the main shaft so as to be able to move back and forth, and a drive means for moving the bobbin back and forth, wherein the bobbin has an outer diameter that varies along the axial direction of the main shaft, the chuck jaws are supported so as to be able to swing toward the outer circumferential surface of the main shaft, and the bobbin is inserted between one end side of the chuck jaws and the main shaft, thereby opening and closing the collet chuck. In a control method for a machine tool in which a chuck is opened and closed to grip a workpiece and the opening of the chuck jaws is adjusted according to the outer diameter of the bobbin that abuts against the chuck jaws to adjust the gripping force of the workpiece by the collet chuck, the method includes a gripping state detection step for detecting the gripping state by the collet chuck, and a bobbin position adjustment step for controlling the driving means to move the bobbin forward and backward so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result in the gripping state detection step, thereby solving the above-mentioned problems.

[0012] The invention of claim 7 further solves the above-mentioned problem by including, in addition to the configuration of the machine tool control method described in claim 6, a work supply step in which a material supply device supplies the workpiece to the spindle, and the gripping state detection step detects the gripping state when the workpiece is gripped during continuous machining of the workpiece. Effect of the Invention

[0013] According to the machine tool of the invention of claim 1, the bobbin has an outer diameter that varies along the axial direction of the spindle, and the machine tool has a gripping state detection unit that detects the gripping state of the collet chuck, and a drive control unit that controls the drive means to move the bobbin forward and backward so that the chuck jaws abut a predetermined portion of the outer diameter of the bobbin based on the detection result by the gripping state detection unit, so that it is possible to easily detect a decrease in the gripping force of the workpiece due to wear of the bobbin or the chuck jaws and adjust the opening of the collet of the collet chuck, and therefore it is possible to maintain grip of the workpiece by the collet chuck even if the gripping force of the collet chuck of the workpiece decreases due to wear of the bobbin or the chuck jaws, etc.

[0014] According to the machine tool of the invention of claim 2, in addition to the effects achieved by the machine tool of the invention of claim 1, the drive means is a motor, and the gripping state detection unit detects the gripping state based on the current value of the motor, so that the gripping state detection unit can easily detect the gripping state of the collet chuck.

[0015] According to the machine tool of the invention as claimed in claim 3, in addition to the effects achieved by the machine tool of the invention as claimed in claim 1, the gripping state detection unit can easily detect the gripping state of the collet chuck by detecting the position of one end of the chuck jaws in a direction intersecting the axial direction.

[0016] According to the machine tool of the invention as claimed in claim 4, in addition to the effects achieved by the machine tool of the invention as claimed in claim 1, the supply of workpieces to the spindle is performed continuously in sequence by the material supply device, and the gripping state detection unit detects the gripping state when gripping the workpiece during continuous machining of the workpiece. This makes it possible to detect the gripping state while continuously machining the workpieces supplied from the material supply device to the spindle into products, and maintain the closed state of the collet chuck, thereby making it possible to continuously machine the workpieces normally for a long period of time.

[0017] According to the machine tool of the invention of claim 5, in addition to the effects achieved by the machine tool of the invention of any one of claims 1 to 4, an alarm means is provided to alarm when the gripping force of the collet chuck on the workpiece has decreased, allowing the manager of the machine tool to know when an abnormality has occurred in the chuck jaws, etc., resulting in a half-closed state in which the workpiece cannot be sufficiently gripped. This makes it possible to avoid the half-closed state and prepare to replace the chuck jaws while the collet chuck is in the closed state, thereby shortening the time the machine tool is stopped to replace the chuck jaws.

[0018] According to the machine tool control method of the invention of claim 6, the bobbin has an outer diameter that varies along the axial direction of the spindle, and includes a gripping state detection step of detecting the gripping state of the collet chuck, and a bobbin position adjustment step of controlling the drive means to move the bobbin forward and backward so that the chuck jaws abut a predetermined portion of the outer diameter of the bobbin based on the detection result of the gripping state detection step, so that it is possible to easily detect a decrease in the gripping force of the workpiece due to wear of the bobbin or the chuck jaws and adjust the opening of the collet of the collet chuck, and therefore it is possible to maintain gripping of the workpiece by the collet chuck even if the gripping force of the collet chuck of the workpiece decreases due to wear of the bobbin or the chuck jaws, etc.

[0019] According to the control method of the machine tool according to claim 7, it includes a workpiece supply step in which a material supply device supplies a workpiece to the main shaft, and a gripping state detection step detects the gripping state when gripping the workpiece during continuous machining of the workpiece. By detecting the gripping state while continuously performing the machining of the product from the workpiece supplied from the material supply device to the main shaft, it becomes possible to maintain the closed state of the collet chuck, so that normal machining of the workpiece can be continuously performed for a long time.

Brief Description of the Drawings

[0020] [Figure 1] Schematic diagram of the device of the machine tool which is an embodiment of the present invention. [Diagram 2] Enlarged cross-sectional view of the main part when the collet chuck of the processing machine shown in FIG. 1 is in the open state. [Diagram 3] Configuration diagram of the opening degree control device of the processing machine shown in FIG. 1. [Figure 4] Flow chart showing the procedure for maintaining the collet chuck in the closed state. [Diagram 5] Enlarged cross-sectional view of the main part when the chuck jaw shown in FIG. 2 abuts on the outer peripheral surface and the collet chuck is in the closed state. [Figure 6] Enlarged cross-sectional view of the main part showing the position of the chuck sleeve with respect to the state of one end of the chuck jaw shown in FIG. 2. [Figure 7] Schematic diagram showing the time change of the current value input to the servo motor. [Figure 8] Enlarged cross-sectional view of the main part when the chuck jaw abuts on the large-diameter outer peripheral surface and the collet chuck is in the closed state in the processing machine shown in FIG. 1. [Figure 9] Enlarged cross-sectional view of the main part showing the position of the bobbin with respect to the state of one end of the chuck jaw shown in FIG. 2.

Embodiments for Carrying Out the Invention

[0021] The present invention includes a collet chuck having a chuck sleeve that accommodates a collet, chuck jaws that open and close the collet chuck by moving the chuck sleeve and the collet relatively, a spindle to which the collet chuck is attached, a bobbin that is fitted onto the spindle so as to be able to move back and forth, and a drive means for moving the bobbin back and forth, the bobbin having an outer diameter that varies along the axial direction of the spindle, the chuck jaws are supported so as to be able to swing toward the outer circumferential surface of the spindle, and the bobbin is inserted between one end side of the chuck jaws and the spindle to operate the opening and closing of the collet chuck to grip a workpiece, and the opening degree of the chuck jaws is controlled by the chuck jaws. In a machine tool in which the gripping force of the collet chuck on the workpiece is adjusted according to the outer diameter of the bobbin that abuts against the collet chuck, the machine tool has a gripping state detection unit that detects the gripping state of the collet chuck, and a drive control unit that controls the drive means to move the bobbin forward and backward so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result by the gripping state detection unit, and the specific embodiment of the invention is not limited as long as it maintains gripping of the workpiece by the collet chuck even if the gripping force of the collet chuck on the workpiece is reduced due to wear of the bobbin or the chuck jaws, etc. Alternatively, the present invention provides a collet chuck having a chuck sleeve that accommodates a collet, chuck jaws that move the chuck sleeve and the collet relatively to open and close the collet chuck, a spindle to which the collet chuck is attached, a bobbin that is fitted onto the spindle so as to be able to move back and forth, and a drive means for moving the bobbin back and forth, the bobbin having an outer diameter that varies along the axial direction of the spindle, the chuck jaws being supported so as to be able to swing toward the outer circumferential surface of the spindle, and the bobbin being inserted between one end side of the chuck jaws and the spindle to operate the opening and closing of the collet chuck to grip a workpiece, and the opening of the chuck jaws is adjusted so that the chuck jaws come into contact with the chuck jaws. In a control method for a machine tool in which the gripping force of a collet chuck on a workpiece is adjusted according to the outer diameter of the bobbin, the method includes a gripping state detection step for detecting the gripping state of the collet chuck, and a bobbin position adjustment step for controlling a driving means to move the bobbin forward and backward so that the chuck jaws abut a portion of the bobbin with a specified diameter based on the detection result of the gripping state detection step, and the specific embodiment may be any one as long as it maintains grip of the workpiece by the collet chuck even if the gripping force of the collet chuck on the workpiece is reduced due to wear of the bobbin or the chuck jaws, etc. EXAMPLES

[0022] As shown in FIG. 1, an automatic lathe 100, which is a machine tool, is supplied with a long workpiece W by a material supply device 200.

[0023] The automatic lathe 100 includes a spindle 110 that holds a workpiece W supplied from a material supply device 200, and a tool 120 that performs machining to produce a product from the workpiece W held by the spindle 110. The material supplying device 200 includes a material feeding mechanism 210 that supplies the workpiece W to the spindle 110 of the automatic lathe 100 , and a material supplying mechanism 220 that supplies the workpiece W to the material feeding mechanism 210 .

[0024] As shown in Fig. 2, the automatic lathe 100 includes a spindle base 130 that rotatably supports a spindle 110 via a bearing 131, and a collet chuck 140 attached to the spindle 110.

[0025] The collet chuck 140 is configured such that a cylindrical collet 142 is accommodated in a cylindrical chuck sleeve 141 that is inserted into the spindle 110 so as to be movable forward and backward in the axial direction of the central axis C of the spindle 110, and a coil spring (elastic member) 143 is provided between the chuck sleeve 141 and the rear end of the collet 142.

[0026] The inner peripheral surface on the front end side of the chuck sleeve 141 is a tapered surface. A plurality of slits 142a extending in the axial direction are formed in the circumferential direction on the front end side of the collet 142. Also, the outer peripheral surface on the front end side of the collet 142 is a tapered surface that engages with the tapered surface of the chuck sleeve 141. And the front end surface of the collet 142 is positioned at the front end portion of the spindle 110. Therefore, the opening degree of the collet 142 is adjusted by the forward and backward movement of the chuck sleeve 141.

[0027] A servo motor 150 is provided on the spindle base 130 side. The servo motor 150 is configured such that the servo motor body 151 controls the forward and backward protrusion of a piston rod 152 in the front and rear directions. The tip of a lever 133 pivotally supported by a lever swing shaft 132 of the spindle base 130 is attached to the piston rod 152.

[0028] A bobbin 160 that can move forward and backward in the axial direction with respect to the spindle 110 is externally fitted to the spindle 110. The bobbin 160 is composed of an annular inner ring 161 that inserts the spindle 110, an annular outer ring 162 located outside the inner ring 161, and spherical rolling elements 163 disposed between the inner ring 161 and the outer ring 162.

[0029] As shown in FIG. 2, the inner ring 161 has an outer peripheral surface 161a of a predetermined diameter that is approximately parallel to the central axis BC of the bobbin 160, a large-diameter outer peripheral surface 161b that is located forward of the outer peripheral surface 161a, approximately parallel to the central axis BC, and has a larger diameter than the outer peripheral surface 161a, and an inclined outer peripheral surface 161c that is located rearward of the outer peripheral surface 161a and has a gradually smaller diameter toward the rear end. That is, the inner ring 161 has an outer circumferential surface 161a and a large-diameter outer circumferential surface 161b, which have different outer diameters along the axial direction that is the direction of the center axis C of the main shaft 110.

[0030] The outer ring 162 has an annular key groove 162a on its outer periphery. The key groove 162 a is engaged with a key 133 a of the lever 133 . Therefore, the bobbin 160 moves back and forth in the axial direction of the central axis C of the main shaft 110 via the lever 133 in response to the movement of a piston rod 152 of a servo motor 150 serving as a drive means for moving the bobbin 160 back and forth.

[0031] A support body integrally attached to the spindle 110 supports a chuck claw 170, which opens and closes the collet chuck 140 by moving the chuck sleeve 141 and the collet 142 relative to one another, so that the chuck claw 170 can swing freely toward the outer circumferential surface of the spindle 110 by a swing shaft 171 that intersects with the axial direction of the central axis C of the spindle 110. Because the chuck sleeve 141 is pressed backward by the coil spring 143, the chuck claws 170 have one end 172 abutting against the inner ring 161 of the bobbin 160 or the outer peripheral surface 111 of the main shaft 110 due to the biasing force of the coil spring 143, and the other end 173 abutting against the rear end of the chuck sleeve 141 of the collet chuck 140.

[0032] As one end 172 of the chuck jaws 170 swings in a direction away from the peripheral surface of the spindle 110, the other end 173 of the chuck jaws 170 presses the chuck sleeve 141 forward against the biasing force of the coil spring 143, and the chuck sleeve 141 of the collet chuck 140 advances relative to the collet 142. As a result, the opening of the collet 142 becomes smaller, and the collet chuck 140 enters a closed state in which it grips the workpiece W.

[0033] When the swinging of the chuck jaws 170 is released from the closed state of the collet chuck 140 , the urging force of the coil spring 143 causes the other end 173 of the chuck jaws 170 to follow the chuck sleeve 141 in contact with it, while the chuck sleeve 141 retreats relative to the collet 142 . As a result, the opening of collet 142 becomes larger, one end 172 of chuck jaws 170 comes into contact with the outer circumferential surface of spindle 110, and collet chuck 140 enters an open state in which workpiece W is released.

[0034] As shown in FIG. 1, the automatic lathe 100 is equipped with a control device 180 that controls the opening and closing of the collet chuck 140 and also controls the driving of the material supply device 200, and an alarm means 190 configured to alarm by sound, light, displaying a message on the display of the automatic lathe 100, etc.

[0035] As shown in FIG. 3, the control device 180 has a gripping state detection unit 181 that detects the gripping state of the collet chuck 140, and a servo motor drive control unit 182 as a drive control unit that controls the servo motor 150.

[0036] Next, the gripping of the workpiece W by the automatic lathe 100 configured as above during continuous machining of the workpiece W will be described. When a long rod-shaped workpiece W is supplied from the material supply device 200 to the rear end of the spindle 110 by the control device 180, the workpiece W is inserted into the open collet chuck 140 and protrudes a predetermined length from the tip of the spindle 110 (step SP10; workpiece supply step). In this state, as shown in FIG. 4, an external command to close collet chuck 140 is input to control device 180 (step SP11).

[0037] When an external command is input to the control device 180, the control device 180 drives the servo motor 150 via the servo motor drive control unit 182, and causes the bobbin 160 to move backward by the piston rod 152 until one end 172 of the chuck jaws 170 abuts against the outer peripheral surface 161a of the inner ring 161 of the bobbin 160, as shown in FIG. 5 (step SP12). At this time, the bobbin 160 is smoothly inserted between the main shaft 110 and the chuck jaws 170 via the inclined outer peripheral surface 161c.

[0038] After executing step SP12, the gripping state detection unit 181 of the control device 180 detects the peak current value Ip of the servo motor main body 151 when one end 172 of the chuck jaws 170 moves from the outer peripheral surface 111 of the spindle 110 onto the outer peripheral surface 161a of the bobbin 160 as the current value of the servo motor 150, and determines the gripping state of the workpiece W by the collet chuck 140 (step SP13; gripping state detection step).

[0039] If the gripping state detection unit 181 determines that the peak current value Ip of the servo motor 150 is equal to a predetermined peak current value (YES in step SP13), it determines that the collet chuck 140 is in a closed state gripping the workpiece W, and the control device 180 drives the spindle 110 to rotate relative to the headstock 130 and the outer ring 162 of the bobbin 160, thereby starting processing of the workpiece W.

[0040] Here, for example, as shown in FIG. 6, when the chuck jaws 170 or the bobbin 160 are in a worn state (solid line) and one end 172 of the chuck jaws 170 abuts against the outer peripheral surface 161a of the bobbin 160, as the one end 172 of the chuck jaws 170 approaches the central axis BC of the bobbin 160, the peak current value Ip decreases (dotted line) as shown in FIG. 7, compared to a state in which the above-mentioned wear does not occur (dashed line).

[0041] Therefore, when the gripping state detection unit 181 determines that the peak current value Ip of the servo motor 150 falls below the predetermined peak current value, the gripping force of the collet chuck 140 on the workpiece W has decreased, and the collet chuck 140 is in a half-closed state (NO in step SP13), the notification means 190 notifies the half-closed state (step SP14).

[0042] Then, the servo motor drive control unit 182 drives the servo motor 150 to move the bobbin 160 backward by the piston rod 152 so that one end 172 of the chuck jaws 170 abuts against the large diameter outer peripheral surface 161b of the bobbin 160, as shown in FIG. 8 (step SP15; bobbin position adjustment step). As a result, the same amount of pressing as when one end 172 of the chuck jaws 170 abuts against the outer peripheral surface 151a of the bobbin 160 (dotted line) as shown in Figure 9 can be secured, so the collet chuck 140 closes and processing of the workpiece W continues. That is, the servo motor drive control unit 182 controls the servo motor 150 to move the bobbin 160 back and forth so that the chuck jaws 170 abut against a portion of the bobbin 160 having a predetermined diameter based on the detection result by the gripping state detection unit 181 .

[0043] With the machine tool consisting of the automatic lathe 100 of the present invention and the control method for the machine tool described above, the gripping state detection unit 181 can easily detect when the chuck jaws 170 or the bobbin 160 wears out, reducing the gripping force of the workpiece W and making it impossible to adequately grip the workpiece W, and the opening of the collet 142 of the collet chuck 140 can be adjusted, so that the collet chuck 140 can maintain gripping of the workpiece W even when the above-mentioned wear or the like occurs.

[0044] The bobbin 160 functions as a deep groove ball bearing due to the rolling elements 163, and when one end 172 of the chuck jaws 170 abuts against the inner ring 161 of the bobbin 160, the force applied to the inner ring 161 of the bobbin 160 acts in the radial direction of the bobbin 160 rather than in the thrust direction of the bobbin 160. This makes it less likely for uneven wear to occur on the bobbin 160, and prevents the collet chuck 140 from failing to grip the workpiece due to wear between the chuck jaws 170 and the bobbin 160.

[0045] The gripping state detection unit 181 of the control device 180 can easily detect the gripping state of the collet chuck 140 with a simple structure by detecting the current value of the servo motor 150, such as the peak current value Ip.

[0046] While the material supply device 200 is continuously machining products from the workpiece W supplied to the spindle 110, the gripping state detection unit 181 detects the gripping state of the collet chuck 140 when gripping the workpiece W, thereby making it possible to continuously perform normal machining of the workpiece W for a long period of time without temporarily stopping the machining of the product to adjust the gripping force.

[0047] By providing the notification means 190, the manager of the automatic lathe 100 can become aware of a decrease in the gripping force of the collet chuck 140, so that preparation for replacing the chuck jaws 170 can be made while the collet chuck 140 is in a closed state with one end 172 of the chuck jaws 170 abutting against the large diameter outer surface 161b of the bobbin 160, thereby shortening the time that the operation of the automatic lathe 100 is stopped to replace the chuck jaws 170.

[0048] The outer periphery of the bobbin 160 may have three or more steps, or may have an outer diameter that varies in distance from the central axis BC as a continuous inclined surface. The gripping state detection unit 181 of the control device 180 can also be configured to detect the position of one end 172 of the chuck jaws 170 in a direction intersecting the axial direction, for example, the position in a direction perpendicular to the central axis BC of the bobbin 160, using a sensor or the like. [Explanation of symbols]

[0049] 100 ··· Automatic lathe 110 ··· Spindle 111 ... Outer surface 120... Tools 130... Headstock 131 Bearing 132 Lever swing shaft 133 Lever 133a Key 140 ··· Collet chuck 141 ··· Zipper sleeve 142 Colette 142a ..... Sliding 143 Coil spring (elastic member) 150 ··· Servo motor (drive means) 151 Servo motor body 152 Piston rod 160 ··· Bobbin 161... Inner circle 161a ... Outer surface 161b: Large diameter outer periphery 161c... Inclined outer surface 162 Outer ring 162a Keyway 163 ... rolling element 170 ··· Chuck jaws 171 Swing shaft 172 ... one end 173 ...other end 180 Control device 181 Gripping state detection unit 182 Servo motor drive control unit (drive control unit) 190 Notification means 200 ... Material supply device 210 Material feed mechanism 220 ... Material supply mechanism W ··· Work C: Central axis of the spindle BC: Central axis of deep groove ball bearing Ip: Peak current value

Claims

1. a collet chuck having a chuck sleeve for accommodating a collet, chuck claws for opening and closing the collet chuck by moving the chuck sleeve and the collet relatively, a main shaft on which the collet chuck is attached, a bobbin fitted on the main shaft so as to be movable back and forth, and a drive means for moving the bobbin back and forth, a machine tool in which the bobbin has an outer diameter that varies along the axial direction of the spindle, the chuck jaws are supported so as to be able to swing freely toward an outer peripheral surface of the spindle, the bobbin is inserted between one end side of the chuck jaws and the spindle, thereby opening and closing the collet chuck to grip a workpiece, and the opening degree of the chuck jaws is adjusted according to the outer diameter of the bobbin that comes into contact with the chuck jaws, thereby adjusting the gripping force of the workpiece by the collet chuck, a gripping state detection unit that detects the gripping state of the collet chuck, and a drive control unit that controls the drive means to move the bobbin forward and backward so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result by the gripping state detection unit.

2. The driving means is a motor, 2. The machine tool according to claim 1, wherein the gripping state detection unit detects the gripping state based on a current value of the motor.

3. 2. The machine tool according to claim 1, wherein the gripping state detection unit detects the position of one end of the chuck jaws in a direction intersecting with the axial direction.

4. The workpiece is supplied to the spindle in sequence and continuously by a material supply device; 2. The machine tool according to claim 1, wherein the gripping state detection unit detects a gripping state when gripping the workpiece during continuous machining of the workpiece.

5. 5. The machine tool according to claim 1, further comprising an alarm means for notifying that a gripping force of the collet chuck for the workpiece has decreased.

6. a collet chuck having a chuck sleeve for accommodating a collet, chuck claws for opening and closing the collet chuck by moving the chuck sleeve and the collet relatively, a main shaft on which the collet chuck is attached, a bobbin fitted on the main shaft so as to be movable back and forth, and a drive means for moving the bobbin back and forth, a bobbin having an outer diameter that varies along an axial direction of the spindle, the chuck jaws being supported so as to be able to swing freely toward an outer peripheral surface of the spindle, the bobbin being inserted between one end side of the chuck jaws and the spindle, thereby operating the collet chuck to open and close and grip a workpiece, and an opening degree of the chuck jaws being adjusted in accordance with the outer diameter of the bobbin that comes into contact with the chuck jaws, thereby adjusting a gripping force of the workpiece by the collet chuck, a gripping state detection step of detecting a gripping state by the collet chuck; and a bobbin position adjustment step of controlling the driving means to move the bobbin forward and backward so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result in the gripping state detection step.

7. A work supply step in which a material supply device supplies the work to the spindle, 7. The method for controlling a machine tool according to claim 6, wherein the gripping state detection step detects a gripping state when the workpiece is gripped during continuous machining of the workpiece.

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