Machine tool and method for controlling machine tool

Abstract

Provided are: a machine tool that is able to keep gripping a workpiece by a collet chuck even when the grip force of the collet chuck on the workpiece is weakened due to wear or the like of a bobbin or chuck jaws; and a control method for said machine tool. A bobbin 160 has an outer diameter (outer circumferential surface 161a, large-diameter outer circumferential surface 161b) that changes along the axial direction of a main shaft 110. The present invention has: a gripping-state detection unit 181 which detects the state of gripping by a collet chuck 140; and a drive control unit 182 that moves, on the basis of a detection result provided by the gripping-state detection unit 181, the bobbin 160 back and forth by controlling a servo motor 150 so that the chuck jaws 170 make contact with a prescribed portion in the outer diameter of the bobbin 160.

Description

Machine tool and machine tool control method

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

[0002] Conventionally, there has been known a machine tool that includes a collet chuck having a chuck sleeve that accommodates a collet, chuck jaws that move the chuck sleeve and the collet relative to one another 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 movable 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 jaws are supported so as to be swingable toward the outer peripheral surface of the spindle, and the bobbin is inserted between one end of the chuck jaws and the spindle to open and close the collet chuck and grip a workpiece (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2019-123038

[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 solves the problems of the prior art as described above, and the object of the present invention is to provide a machine tool and a method for controlling the machine tool that can maintain the grip 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.

[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 peripheral 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 degree of 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 above-mentioned problem is solved 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 based on the detection result by the gripping state detection unit to move the bobbin back and forth so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter.

[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 drive means being a motor and the gripping state detection unit detecting the gripping state based on the current value of the motor.

[0008] The invention according to 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 jaw in a direction intersecting with 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 in a continuous sequence 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 workpiece by the collet chuck has decreased.

[0011] The invention according to claim 6 provides a collet chuck including a chuck sleeve for accommodating a collet, chuck jaws for moving the chuck sleeve and the collet relative to each other to open and close the collet chuck, a spindle to which the collet chuck is attached, a bobbin fitted on 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 axial direction of the spindle, the chuck jaws are supported so as to be able to swing toward the outer peripheral surface of the spindle, and the bobbin is inserted between one end of the chuck jaws and the spindle, 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 degree of 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 solves the above-mentioned problems by comprising a gripping state detection step for detecting the gripping state by the collet chuck, and a bobbin position adjustment step for controlling 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 of the gripping state detection step.

[0012] The invention of claim 7 further solves the above-mentioned problems 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.

[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 main shaft, and is equipped with 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 back and forth so that the chuck jaws abut against 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 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 workpiece by the collet chuck decreases due to wear of the bobbin or 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 of claim 3, in addition to the effects achieved by the machine tool of the invention of 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 of claim 4, in addition to the effects achieved by the machine tool of the invention of claim 1, the supply of workpieces to the spindle is carried out successively by the material supply device, and the gripping state detection unit detects the gripping state when the workpiece is gripped 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 to maintain the closed state of the collet chuck, so that normal machining of the workpieces can be carried out continuously for long periods 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 notify that the gripping force of the collet chuck on the workpiece has decreased, so that the machine tool manager can know when an abnormality has occurred in the chuck jaws, etc., causing the chuck jaws to enter 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 a closed state, thereby shortening the time the machine tool is stopped from operating 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 that detects the gripping state of the collet chuck, and a bobbin position adjustment step that controls the drive means to move the bobbin forward and backward so that the chuck jaws abut against 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 grip of the workpiece by the collet chuck even if the gripping force of the workpiece by the collet chuck decreases due to wear of the bobbin or the chuck jaws, etc.

[0019] According to the machine tool control method of the invention of claim 7, there is provided a work supply step in which a material supply device supplies a workpiece to the spindle, and a gripping state detection step in which the gripping state is detected when the workpiece is gripped during continuous machining of the workpiece. This makes it possible to detect the gripping state while continuously machining products from the workpiece supplied from the material supply device to the spindle, and to maintain the collet chuck in a closed state, thereby enabling normal machining of the workpiece to be performed continuously for a long period of time.

[0020] FIG. 1 is a schematic diagram of a machine tool according to an embodiment of the present invention; FIG. 2 is an enlarged cross-sectional view of a main part when the collet chuck of the machine tool shown in FIG. 1 is in an open state; FIG. 3 is a configuration diagram of an opening control device for the machine tool shown in FIG. 1; FIG. 4 is a flowchart showing a procedure for maintaining the collet chuck in a closed state; FIG. 5 is an enlarged cross-sectional view of a main part when the chuck jaws of FIG. 2 abut against an outer peripheral surface and the collet chuck is in a closed state; FIG. 6 is an enlarged cross-sectional view of a main part showing the position of a chuck sleeve relative to the state of one end of the chuck jaws shown in FIG. 2; FIG. 7 is a schematic diagram showing the change over time in the value of current input to a servo motor; FIG. 8 is an enlarged cross-sectional view of a main part when the chuck jaws of the machine tool shown in FIG. 1 abut against a large diameter outer peripheral surface and the collet chuck is in a closed state; FIG. 9 is an enlarged cross-sectional view of a main part showing the position of a bobbin relative to the state of one end of the chuck jaws shown in FIG.

[0021] 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 on 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 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 that adjusts the gripping force of the collet chuck on a workpiece by adjusting it according to the outer diameter of the bobbin that abuts against the workpiece, 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 back and forth so that the chuck jaws abut against a portion of the bobbin with a predetermined diameter based on the detection result of the gripping state detection unit, and the specific embodiment of the machine tool can be any as long as it maintains the gripping of the workpiece by the collet chuck even if the gripping force of the workpiece by the collet chuck 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, wherein 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 toward the outer peripheral surface of the spindle, and the bobbin is inserted between one end side of the chuck jaws and the spindle, thereby operating 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 abut against the chuck jaws. A control method for a machine tool that adjusts the gripping force of a collet chuck on a workpiece by adjusting it according to the outer diameter of the bobbin includes a gripping state detection step that detects the gripping state of the collet chuck, and a bobbin position adjustment step that controls a 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 of the gripping state detection step, and the specific implementation can be any as long as the collet chuck maintains grip of the workpiece even if the gripping force of the collet chuck on the workpiece is reduced due to wear of the bobbin or chuck jaws, etc.

[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 is equipped with a spindle 110 that grips a workpiece W supplied from a material supply device 200, and a tool 120 that performs processing to produce a product from the workpiece W gripped by the spindle 110. The material supply device 200 is equipped with a material feed mechanism 210 that supplies the workpiece W to the spindle 110 of the automatic lathe 100, and a material feed mechanism 220 that supplies the workpiece W to the material feed mechanism 210.

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

[0025] The collet chuck 140 is configured such that a cylindrical collet 142 is housed within a cylindrical chuck sleeve 141 that is inserted so as to be able to move back and forth in the axial direction of the center axis C of the main shaft 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 of the front end of the chuck sleeve 141 is tapered. A plurality of axially extending slots 142a are formed circumferentially on the front end of the collet 142. The outer peripheral surface of the front end of the collet 142 is also tapered to engage with the tapered surface of the chuck sleeve 141. The front end surface of the collet 142 is positioned by the front end portion of the spindle 110. Therefore, the opening 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 headstock 130 side. The servo motor 150 is configured so that a servo motor body 151 controls the protrusion of a piston rod 152 so that the piston rod 152 can move forward and backward. The tip of a lever 133, which is pivotally supported on a lever swing shaft 132 of the headstock 130, is attached to the piston rod 152.

[0028] A bobbin 160 that can move axially forward and backward relative to the main shaft 110 is fitted onto the outside of the main shaft 110. The bobbin 160 is composed of an annular inner ring 161 into which the main shaft 110 is inserted, an annular outer ring 162 positioned outside the inner ring 161, and a spherical rolling element 163 that is arranged between the inner ring 161 and the outer ring 162.

[0029] 2, inner ring 161 has an outer peripheral surface 161a of a predetermined diameter that is substantially parallel to central axis BC of bobbin 160, a large-diameter outer peripheral surface 161b that is disposed forward of outer peripheral surface 161a, is substantially parallel to central axis BC, and has a larger diameter than outer peripheral surface 161a, and an inclined outer peripheral surface 161c that is disposed rearward of outer peripheral surface 161a and has a gradually decreasing diameter toward the rear end. That is, inner ring 161 has outer peripheral surface 161a and large-diameter outer peripheral surface 161b that have different outer diameters along the axial direction, which is the direction of central axis C of spindle 110.

[0030] The outer ring 162 has an annular key groove 162a on its outer periphery. This key groove 162a engages with a key 133a 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, which serves as a drive means for moving the bobbin 160 back and forth.

[0031] Chuck jaws 170, which open and close the collet chuck 140 by moving the chuck sleeve 141 and the collet 142 relative to each other, are supported by a support body integrally attached to the spindle 110 so as to be able to 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. Since the chuck sleeve 141 is pressed rearward by the coil spring 143, one end 172 of the chuck jaws 170 abuts against the inner ring 161 of the bobbin 160 or the outer circumferential surface 111 of the spindle 110 due to the biasing force of the coil spring 143, and the other end 173 abuts 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 in contact with the chuck sleeve 141, while the chuck sleeve 141 retreats relative to the collet 142. This increases the opening of the collet 142, and one end 172 of the chuck jaws 170 comes into contact with the outer peripheral surface of the spindle 110, placing the collet chuck 140 in an open state in which it releases the workpiece W.

[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 that is configured to alarm by sound, light, or message display 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 that serves as a drive control unit that controls the servo motor 150 .

[0036] Next, the gripping of the workpiece W during continuous machining by the automatic lathe 100 configured as described above 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 the collet chuck 140 is input to the 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 section 182, and causes the piston rod 152 to move the bobbin 160 backward until one end 172 of the chuck jaws 170 abuts against the outer circumferential surface 161 a 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 spindle 110 and the chuck jaws 170 via the inclined outer circumferential surface 161 c.

[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 claws 170 moves from the outer surface 111 of the main shaft 110 onto the outer 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 unit 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 worn (solid line), and one end 172 of the chuck jaws 170 abuts against the outer peripheral surface 161 a of the bobbin 160, the peak current value Ip decreases (dotted line) as shown in FIG. 7 when the one end 172 of the chuck jaws 170 approaches the central axis BC of the bobbin 160 compared to when no wear has occurred (dashed line).

[0041] Therefore, when the gripping state detection unit 181 determines that the peak current value Ip of the servo motor 150 has decreased 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 in the state where one end 172 of the chuck jaws 170 abuts against the outer peripheral surface 151a of the bobbin 160 (dotted line) can be ensured, as shown in Fig. 9, so the collet chuck 140 is in a closed state, 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 forward or backward based on the detection result by the gripping state detection unit 181 so that the chuck jaws 170 abut against a portion of the bobbin 160 with a predetermined diameter.

[0043] With the machine tool consisting of the automatic lathe 100 of the present invention and the machine tool control method described above, the gripping state detection unit 181 can easily detect when the chuck jaws 170 or bobbin 160 have worn out, reducing the gripping force of the workpiece W and making it impossible to sufficiently 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, etc. 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, making it less likely for uneven wear to occur on the bobbin 160 and preventing the collet chuck 140 from poorly gripping 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 processing 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 processing of the workpiece W for a long period of time without temporarily stopping the processing of the product to adjust the gripping force.

[0047] By providing the alarm means 190, the manager of the automatic lathe 100 can be aware of a decrease in the gripping force of the collet chuck 140, so that preparations 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 automatic lathe 100 is stopped from operating in order to replace the chuck jaws 170.

[0048] The outer periphery of the bobbin 160 may have three or more steps, or may have a continuous inclined surface with an outer diameter varying in distance from the central axis BC. The gripping state detection unit 181 of the control device 180 may 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.

[0049] DESCRIPTION OF SYMBOLS 100 Automatic lathe 110 Spindle 111 Outer circumferential surface 120 Tool 130 Headstock 131 Bearing 132 Lever swing shaft 133 Lever 133a Key 140 Collet chuck 141 Chuck sleeve 142 Collet 142a Slot 143 Coil spring (elastic member) 150 Servo motor (driving means) 151 Servo motor body 152 Piston rod 160 Bobbin 161 Inner ring 161a Outer circumferential surface 161b Large diameter outer circumferential surface 161c Inclined outer circumferential surface 162 Outer ring 162a Keyway 163 Rolling element DESCRIPTION OF SYMBOLS 170: Chuck jaws 171: Oscillating 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: Workpiece C: Center axis of spindle BC: Center axis of deep groove ball bearing Ip: Peak current value

Claims

1. A machine tool comprising 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 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 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, the bobbin is inserted between one end side of the chuck jaws and the spindle to open and close the collet chuck to grip a workpiece, and the opening degree of the chuck jaws is adjusted in accordance with the outer diameter of the bobbin that comes into contact with the chuck jaws, thereby adjusting the gripping force of the collet chuck on the workpiece, 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 machine tool according to claim 1, wherein the drive means is a motor, and the gripping state detection unit detects the gripping state based on a current value of the motor.

3. A machine tool according to claim 1, characterized in that the gripping state detection unit detects the position of one end of the chuck jaws in a direction intersecting the axial direction.

4. A machine tool as described in claim 1, characterized in that the workpiece is supplied to the spindle sequentially and continuously by a material supply device, and the gripping state detection unit detects the gripping state when gripping the workpiece during continuous machining of the workpiece.

5. A machine tool as claimed in any one of claims 1 to 4, further comprising an alarm means for notifying that the gripping force of the collet chuck on the workpiece has decreased.

6. A method for controlling a machine tool comprising 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 to each other, a spindle to which the collet chuck is attached, a bobbin fitted onto the spindle so as to be freely advanced and retreated, 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 spindle, the chuck jaws are supported so as to be freely swingable 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 adjusted in accordance with the outer diameter of the bobbin that comes into contact with the chuck jaws to adjust the gripping force of the workpiece by the collet chuck, comprising: a gripping state detection step for 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 method for controlling a machine tool as described in claim 6, further comprising a work supply step in which a material supply device supplies the workpiece to the spindle, and wherein the gripping state detection step detects the gripping state when the workpiece is gripped during continuous machining of the workpiece.

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