Working machinery

Abstract

To provide a machine tool that can determine the gripping force of the gripping part with high precision using an inexpensive configuration. [Solution] The system comprises a first spindle 4 having a collet chuck 41 for gripping a workpiece W1, a first headstock 3 that rotatably supports the first spindle 4, an air cylinder 33 that generates a driving force to change the state of the collet chuck 41 between a gripping state for gripping the workpiece W1 and a release state for releasing the workpiece W1, a non-rotating transmission member 34 that transmits the driving force received from the air cylinder 33, a rotating transmission member 45 that rotates together with the first spindle 4 and transmits the driving force received from the non-rotating transmission member 34 to the collet chuck 41, a strain detection means 46 attached to the non-rotating transmission member 34 that detects the strain of the non-rotating transmission member 34 that occurs when the driving force is transmitted from the air cylinder 33 to the collet chuck 41, and a derivation unit 23 that derives the gripping force of the collet chuck 41 based on the detection result of the strain detection means 46.

Description

Technical Field

[0001] The present invention relates to a machine tool for machining a workpiece held by a spindle.

Background Art

[0002] Machine tools for machining a bar-shaped workpiece held by a rotatable spindle are known (see, for example, Patent Document 1). The spindle of this machine tool is rotatably supported by a spindle headstock. The spindle is provided with a gripping portion that changes its state between a gripping state for gripping the workpiece and a gripping release state for releasing the gripping of the workpiece. The gripping portion is operated by an actuator fixed to the spindle headstock. In the machine tool of Patent Document 1, the shifter lever swings by driving the actuator, and the claw member inclines by the displacement of the shifter. Then, by moving the push sleeve and the chuck sleeve along the axis of the spindle due to the inclination of the claw member, the gripping portion grips or releases the workpiece.

[0003] Generally, the spindle is provided with an adjusting nut as an adjusting means for adjusting the gripping force of the workpiece. This adjusting nut is attached to the spindle body so as to be relatively rotatable with respect to the spindle body. By relatively rotating it, the position of the adjusting nut in the axial direction with respect to the spindle body is displaced, and the gripping force of the workpiece changes. When machining a workpiece having a different diameter or a different material from the workpiece that has been machined so far, the operator of the machine tool inserts the next workpiece to be machined into the gripping portion in a so-called setup operation and relatively rotates the adjusting nut with respect to the spindle body to adjust the gripping force of the workpiece. Then, the operator of the machine tool manually operates the shifter lever for displacing the shifter, estimates the gripping force of the gripping portion from the force required for the operation, and repeatedly performs the relative rotation and estimation of the adjusting nut until the desired gripping force is reached, and then fixes the adjusting nut to the spindle body at a position where it is considered to have reached the desired gripping force.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] However, conventional adjustment methods rely on the operator's intuition to estimate the gripping force, resulting in low accuracy in the estimation. Furthermore, after adjusting the gripping force, the force may change due to wear, thermal displacement, deterioration of rubber gaskets, and other changes over time and age-related factors in mechanisms such as shifters and claw components between the actuator and the gripping part. As a result, the gripping force of the gripping part after adjustment may differ from the gripping force estimated during adjustment, but this difference could not be detected. As a countermeasure, it is conceivable to attach measuring means such as load cells to the gripping part to detect the gripping force. However, obtaining detection results from measuring means attached to a rotating spindle requires expensive equipment such as slip rings. Moreover, because the spindle rotates at high speed, noise may be generated in the sliding parts between it and non-rotating parts such as slip rings, potentially reducing the reliability of the detection signal.

[0006] This invention has been made in view of the above-mentioned problems, and aims to provide a machine tool that can determine the gripping force of the gripping part with high precision using an inexpensive configuration. [Means for solving the problem]

[0007] The machine tool of the present invention, which solves the above problems, A spindle having a gripping part for gripping a workpiece, A headstock that rotatably supports the main spindle, An actuator mounted on the spindle head generates a driving force to change the state of the gripping portion between a gripping state in which the workpiece is gripped and a release state in which the workpiece is released. A non-rotational transmission member attached to the headstock and transmitting the driving force received from the actuator, A rotational transmission member that rotates together with the main shaft and transmits the driving force received from the non-rotating transmission member to the gripping portion, A strain detection means is attached to the non-rotating transmission member to detect the strain of the non-rotating transmission member that occurs when driving force is transmitted from the actuator to the gripping portion, The device is characterized by comprising a derivation unit that derives the gripping force of the gripping unit based on the detection result of the strain detection means.

[0008] This machine tool derives the gripping force of the gripping part based on the strain generated when the driving force is transmitted from the actuator to the gripping part, thus enabling highly accurate determination of the gripping force and eliminating variations due to the operator. Furthermore, when this machine tool performs continuous machining using an NC program, the gripping force of the gripping part can be determined for each workpiece gripping operation (state change operation from the release state to the gripped state) performed between machining operations, allowing for the recognition of changes over time or due to aging. In addition, since the strain detection means is attached to the non-rotating transmission member, the detection results of the strain detection means can be communicated using an inexpensive communication means, allowing this machine tool to be constructed at a lower cost compared to the case where the measuring means is attached to the rotating transmission member.

[0009] Here, the gripping portion may be a collet chuck. The non-rotating transmission member may transmit the driving force from the actuator to the rotating transmission member. The rotating transmission member may be attached to the main shaft. The derivation portion may derive the gripping force of the gripping portion in the gripping state based on the strain generated in the non-rotating transmission member during the state change from the release state to the gripping state.

[0010] In this machine tool, The rotation transmission member includes a claw member capable of changing its position between a first position in which the gripping portion is in the gripping state and a second position in which the gripping portion is in the release state, and a shifter capable of changing the position of the claw member between a first position in which the claw member is in the first position and a second position in which the claw member is in the second position by moving in the axial direction of the main shaft. The non-rotating transmission member has a shifter lever that moves the shifter in the axial direction, The strain detection means may be one that detects the strain of the shifter lever.

[0011] Since the shifter lever is located downstream in the power transmission path of the non-rotating transmission member, it is less susceptible to disturbances than members located upstream in the power transmission path. Therefore, the gripping force of the gripping part can be determined with greater precision.

[0012] Here, the claw member may change its orientation by rotating within a predetermined angular range. Furthermore, the claw member may be rotatable about a direction perpendicular to the axial direction of the main shaft (the direction of the rotation center axis). The shifter lever may swing about the oscillation center axis due to the driving force generated by the actuator.

[0013] In this machine tool, The strain detection means may detect strain in the area between the force-receiving portion of the shifter lever that receives the driving force from the actuator and the action portion on which the reaction force received by the shifter lever from the shifter acts when the driving force received by the shifter lever is transmitted to the shifter.

[0014] This allows for highly accurate detection of the distortion that occurs in the shifter lever when the driving force from the actuator is transmitted.

[0015] Furthermore, in this machine tool, The shifter lever is swingable about a swing center axis as a swing center, and swings to move the shifter in the axial direction. The strain detection means may detect the strain on the side surface of the shifter lever formed parallel to the swing center axis.

[0016] By doing so, the strain generated when the driving force from the actuator is transmitted can be detected with high accuracy by the strain detection means.

[0017] Also, in this machine tool, The shifter lever is swingable about a swing center axis as a swing center, and swings to move the shifter in the axial direction. The strain detection means may detect the strain at a site where the distance from the swing center axis is shorter than the distances from the force receiving site and the acting site.

[0018] The strain generated in the shifter lever when the driving force from the actuator is transmitted is larger near the swing center axis than near the force receiving site or the acting site. Therefore, by shortening the distance from the swing center axis, the strain can be detected with high accuracy.

[0019] Here, the swing center axis may be located between the force receiving site and the acting site.

[0020] Furthermore, in this machine tool, The derivation unit may derive the gripping force of the gripping part based on the maximum value of the strain of the non-rotational transmission member in the state change from the gripping release state to the gripping state detected by the strain detection means.

[0021] Since there is a correlation between the maximum value of the strain detected by the strain detection means and the gripping force of the gripping part, the gripping force of the gripping part can be derived with high accuracy based on the maximum value of the strain.

Advantages of the Invention

[0022] According to the present invention, it is possible to provide a machine tool capable of deriving the gripping force of the gripping portion with high accuracy at a low cost.

Brief Description of the Drawings

[0023] [Figure 1] It is a plan view which shows simply the internal structure of the NC lathe concerning this embodiment. [Figure 2] It is a perspective sectional view which cut | disconnected a part of the 1st headstock and the 1st spindle shown in FIG. 1. [Figure 3] It is a sectional view seen from above which cut | disconnected the 1st headstock and the 1st spindle shown in FIG. 1 with the horizontal plane which passes along the axis line of the 1st spindle. [Figure 4] It is an enlarged view of the 1st spindle and the rotation transmission member shown in FIG. 3. [Figure 5] It is a perspective sectional view which cut | disconnected a part of the 1st headstock shown in FIG. 1 and shows a part of a non - rotation transmission member and a rotation transmission member. [Figure 6] It is a perspective sectional view which cut | disconnected a part of the shifter lever shown in FIG. 5 along the extending direction of the shifter lever. [Figure 7] It is a figure which looked at the shifter lever shown in FIG. 5 from the rear end side of the spindle. [Figure 8] It is a control block diagram of the NC lathe shown in FIG. 1. [Figure 9] It is a graph which shows an example of the distortion of the shifter lever which a distortion detection means detects when the 1st spindle shown in FIG. 1 changes from the gripping release state to the gripping state. [Figure 10] It is a flowchart which shows the automatic adjustment operation of the gripping force in the 1st spindle shown in FIG. 1. [Figure 11] (a) is a schematic diagram which shows the positional relationship between the force which the shifter lever shown in FIG. 5 receives and the swing center, and (b) is a schematic diagram which shows the positional relationship between the force which the shifter lever of a modification and the shifter lever of a modification receive and the swing center.

Embodiments for Carrying Out the Invention

[0024] Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, the present invention will be described using an example in which it is applied to a Swiss-type NC (Numerical Control) lathe. That is, the NC lathe described below corresponds to an example of a machine tool.

[0025] Figure 1 is a simplified plan view showing the internal configuration of the NC lathe 1 according to this embodiment.

[0026] As shown in Figure 1, the NC lathe 1 contains a control device 2, a first headstock 3, a first spindle 4, a guide bush 5, a first tool post 6, a second headstock 7, a second spindle 8, and a second tool post 9. The control device 2 is a computer that operates the first headstock 3, first spindle 4, first tool post 6, second headstock 7, second spindle 8, and second tool post 9 according to an NC program. In addition to operation using an NC program, the NC lathe 1 can also be operated by directly inputting commands to the control device 2 from the operation unit 11 (see Figure 8).

[0027] The first headstock 3 moves in the Z1 axis direction together with the first spindle 4 in response to a signal from the control device 2. The Z1 axis direction is horizontal, and in Figure 1, it is the left-right direction. The first spindle 4 is rotatably supported on the first headstock 3. A first spindle motor 31 (see Figure 2) is also provided between the first headstock 3 and the first spindle 4. When the first spindle motor 31 rotates in response to a signal from the control device 2, the first spindle 4 rotates around the first axis CL1. This first axis CL1 is an example of an axis. The direction of the first axis CL1 coincides with the Z1 axis direction.

[0028] The first spindle 4 changes state between a gripping state in which it grips the long, rod-shaped workpiece W1 inserted inside it, and a release state in which it releases the workpiece W1. This first spindle 4 is an example of a spindle. The workpiece W1 gripped by the first spindle 4 rotates together with the first spindle 4 around the first axis CL1 as the first spindle 4 rotates. The configuration of the first headstock 3 and the first spindle 4 will be described in detail later.

[0029] The guide bush 5 is fixed inside the NC lathe 1. The guide bush 5 slidably supports the tip portion of the workpiece W1 protruding from the first spindle 4 in the Z1 axis direction. The portion of the guide bush 5 that supports the workpiece W1 is rotatable around the first axis CL1 in synchronization with the first spindle 4. In other words, the first axis CL1 is also the axis of rotation of the portion of the workpiece W1 supported by the guide bush 5. The presence of the guide bush 5 suppresses the deflection of the workpiece W1 during machining, allowing for high-precision machining of particularly long and slender workpieces W1.

[0030] The first tool post 6 is movable in the X1 axis direction, which is perpendicular to the Z1 axis direction and oriented horizontally, and in the Y1 axis direction, which is oriented vertically. The first tool post 6 moves in the X1 axis direction and Y1 axis direction in response to signals from the control device 2. In Figure 1, the vertical direction is the X1 axis direction, and the direction perpendicular to the plane of the paper is the Y1 axis direction. The first spindle tool T1 for machining the workpiece W1 is mounted on the first tool post 6. Figure 1 shows the first spindle tool T1 mounted on the first tool post 6. Multiple types of first spindle tools T1, including cutting tools for outer diameter machining and cutting tools, are mounted on the first tool post 6 in a row along the Y1 axis direction. When the first tool post 6 moves in the Y1 axis direction, any first spindle tool T1 is selected from these multiple types of first spindle tools T1. Then, as the first tool post 6 moves in the X1 axis direction, the selected first spindle tool T1 cuts into the workpiece W1 held by the first spindle 4, machining the workpiece W1.

[0031] The second headstock 7 moves together with the second spindle 8 in the X2 axis direction and the Z2 axis direction in response to signals from the control device 2. The X2 axis direction is the same direction as the X1 axis direction described above, and the Z2 axis direction is the same direction as the Z1 axis direction described above. The second headstock 7 is equipped with a second spindle motor (not shown), such as a built-in motor. When this second spindle motor receives a signal from the control device 2 and rotates, the second spindle 8 rotates around the second axis CL2. This second axis CL2 is also an example of an axis. The direction of the second axis CL2 coincides with the Z2 axis direction.

[0032] The cut workpiece W2, which has been cut by a cutting tool after machining using the first spindle 4, is transferred to the second spindle 8. The second spindle 8 changes state between a gripping state, in which it holds the cut workpiece W2 transferred from the first spindle 4, and a released state, in which it releases the cut workpiece W2. This second spindle 8 is also an example of a spindle. The cut workpiece W2 held by the second spindle 8 rotates together with the second spindle 8 around the second axis CL2 as the second spindle 8 rotates.

[0033] The second tool post 9 moves in the Y2 axis direction in response to a signal from the control device 2. This Y2 axis direction is the same direction as the Y1 axis direction described above, i.e., perpendicular. Multiple second spindle tools T2 for machining the cut workpiece W2 held by the second spindle 8 are mounted on the second tool post 9. Figure 1 shows the second spindle tools T2 mounted on the second tool post 9. Multiple types of second spindle tools T2, such as drills and end mills, are mounted on the second tool post 9. Although not shown in Figure 1, the second spindle tools T2 are mounted in both the X2 axis direction and the Y2 axis direction. By moving the second headstock 7 in the X2 axis direction and the second tool post 9 in the Y2 axis direction, any second spindle tool T2 is selected from these multiple types of second spindle tools T2. Then, when the second headstock 7 moves in the Z2 axis direction, the cut end portion of the cut workpiece W2 held by the second spindle 8 is machined.

[0034] Figure 2 is a perspective cross-sectional view showing a portion of the first headstock 3 and first spindle 4 shown in Figure 1. In Figure 2, the first headstock 3 and first spindle 4 are shown with the 90-degree portion on the upper front side of Figure 2 cut out. Note that, in the drawings used to describe this embodiment, hatching indicating cross-sections is not used, except for the cross-sectional view of the shifter lever 341 in Figure 6, which will be described later.

[0035] As shown in Figure 2, the first headstock 3 comprises a first headstock body 30, a first spindle motor 31, and a bearing 32. The first headstock body 30 is the base member of the first headstock 3 and is a movable body that moves in the Z1 axis direction by a Z1 axis motor (not shown). The first spindle motor 31 is a built-in motor provided in the first headstock body 30. The bearing 32 rotatably supports the first spindle 4. As described above, by rotationally driving the first spindle motor 31, the first spindle 4 rotates around the first axis CL1 via the bearing 32.

[0036] Furthermore, an air cylinder 33 is attached to the first headstock 3. In addition, a non-rotating transmission member 34 is attached to the first headstock 3. This non-rotating transmission member 34 includes a piston rod 340, a shifter lever 341, a lever shaft 342, a connecting pin 343, an operating shaft 344, and an operating bearing 345. The configurations of the air cylinder 33 and the non-rotating transmission member 34 will be described in detail later.

[0037] The first spindle 4 comprises a spindle body 40, a collet chuck 41, and a spindle cap 42. A rotational transmission member 45 is also attached to the first spindle. The rotational transmission member 45 comprises a shifter 452, a jaw member 453, a push sleeve 454, a coil spring 455, a chuck sleeve 456, and an adjustment nut 457. This rotational transmission member 45 transmits the driving force received from the non-rotational transmission member 34 to the collet chuck 41, causing the collet chuck 41 to contract or expand in diameter. The spindle body 40 is the base member of the first spindle 4 and is a cylindrical body extending in the Z1 axial direction. The collet chuck 41, spindle cap 42, and rotational transmission member 45 rotate together with the spindle body 40 around the first axis CL1 as the center of rotation.

[0038] Figure 3 is a cross-sectional view of the first headstock 3 and first spindle 4 shown in Figure 1, taken from above by cutting them through a horizontal plane passing through the first axis CL1. Figure 4 is an enlarged view of the first spindle 4 and rotation transmission member 45 shown in Figure 3. In Figures 3 and 4, the first headstock 3 and first spindle 4 in the released gripping state are shown by solid lines. In Figures 3 and 4, the left side of the figure is the rear end of the first spindle 4, and the right side of the figure is the front end of the first spindle 4. The direction toward the front end and the direction toward the rear end coincide with the Z1 axis direction.

[0039] As shown in Figure 4, the collet chuck 41 is configured to expand and contract in the radial direction perpendicular to the first axis CL1. This collet chuck 41 is an example of a gripping part. The outer circumferential surface 41a of the tip of the collet chuck 41 is a tapered surface whose diameter increases towards the tip. The collet chuck 41 has three continuous slots formed at 120° intervals in the circumferential direction, extending from the tip to a hole formed near the center in the Z1 axis direction. These holes and slots allow the collet chuck 41, especially the tip side, to expand and contract in the radial direction. Figures 3 and 4 show the collet chuck 41 when expanded with solid lines. Figure 4 shows the collet chuck 41 when contracted with dashed lines. When the collet chuck 41 is contracted, the first spindle 4 is gripped, and when the collet chuck 41 is expanded, the first spindle 4 is released from gripping.

[0040] The spindle cap 42 is located at the very front of the first spindle 4. The spindle cap 42 is bowl-shaped with a circular through-hole formed in the center that penetrates in the Z1 axis direction. The tip surface of the collet chuck 41 contacts the bottom of the bowl-shaped part of the spindle cap 42, thereby restricting the movement of the collet chuck 41 toward the tip. The collet chuck 41 is constantly pressed against the spindle cap 42 by a coil spring 455. The spindle cap 42 is fixed to the spindle body 40 by a female thread formed on the inside of its bowl-shaped rim engaging with a male thread formed on the tip of the spindle body 40.

[0041] The shifter 452 is located on the outside of the spindle body 40. The shifter 452 is generally cylindrical and is attached to the spindle body 40 so as to be slidable in the Z1 axis direction by sliding contact between its inner circumferential surface and the outer circumferential surface of the spindle body 40. As shown in Figure 4, a cam surface 452a and a groove 452b are formed on the outer circumferential surface of the shifter 452. The cam surface 452a is composed of a small-diameter surface 452a1 with the smallest and constant outer diameter, a changing surface 452a2 with a gradually increasing outer diameter, and a large-diameter surface 452a3 with the largest and constant outer diameter, in order from the rear end side of the first spindle 4. This shifter 452 slides in the Z1 axis direction by the driving force of an air cylinder 33 (see Figure 2) whose operation is controlled by a control device 2 (see Figure 1). Figures 3 and 4 show the position of the shifter 452 as far forward as possible on the first spindle 4 with solid lines. Furthermore, Figure 4 shows the shifter 452 in its most rearmost position relative to the first spindle 4, indicated by a dashed line. The position of the shifter 452 shown by this dashed line is the first position, and the position of the shifter 452 shown by the solid line is the second position.

[0042] Two claw members 453 are attached to the spindle body 40. The claw members 453 are rotatable within a predetermined angular range around the claw shaft 4531. The tip portion of the claw member 453 has a claw tip portion 453b which acts as a cam follower that contacts the cam surface 452a of the shifter 452. As the shifter 452 slides, the claw tip portion 453b moves closer to or further away from the first axis CL1 along the cam surface 452a, causing the claw member 453 to rotate around the claw shaft 4531 as the pivot point. The claw member 453 has a sleeve pressing portion 453a which contacts the rear end of the pressing sleeve 454.

[0043] The push sleeve 454 is a cylindrical body positioned inside the spindle body 40, and its tip contacts the rear end portion of the chuck sleeve 456. The push sleeve 454 and the chuck sleeve 456 move in the opposite direction to the shifter 452 as the shifter 452 moves in the Z1 axis direction. Specifically, as the shifter 452 slides toward the rear end of the first spindle 4, the tip portion 453b of the jaw member 453 is pushed up by the cam surface 452a of the shifter 452 and moves away from the first axis CL1. As a result, the upper jaw member 453 in Figure 4 rotates counterclockwise around the jaw shaft 4531 as the center of rotation, and the lower jaw member 453 in Figure 4 rotates clockwise, and the push sleeve 454 and the chuck sleeve 456 are pushed by the sleeve push portion 453a of the jaw member 453 and move toward the front end of the first spindle 4. Figure 4 shows the position of the claw tip portion 453b of the claw member 453 relative to the first axis CL1, indicated by a dashed line. The position of the claw member 453 shown by this dashed line is the first position, and the position of the claw member 453 shown by the solid line is the second position.

[0044] As the jaw tip portion 453b is pushed up against the cam surface 452a of the shifter 452, the moment the contact point between the jaw tip portion 453b and the cam surface 452a reaches the boundary point between the changing surface 452a2 and the large-diameter surface 452a3, the collet chuck 41 becomes most contracted and the first spindle 4 enters a gripping state. At that moment, the force transmitted from the air cylinder 33 (see Figure 3) to the collet chuck 41 is greatest. When the workpiece W1 (see Figure 1) is inside the collet chuck 41, this force correlates with the gripping force of the collet chuck 41 on the workpiece W1, and the greater this force, the greater the gripping force. Furthermore, when the contact point between the jaw tip portion 453b and the cam surface 452a is beyond the boundary point between the changing surface 452a2 and the large-diameter surface 452a3 and on the large-diameter surface 452a3, the first spindle 4 maintains its gripping state.

[0045] Conversely, as the shifter 452 slides toward the tip of the first spindle 4, the tip portion 453b of the jaw member 453 moves toward the first axis CL1 along the cam surface 452a of the shifter 452. As a result, the upper jaw member 453 in Figure 4 rotates clockwise around the jaw shaft 4531 as the center of rotation, and the lower jaw member 453 in Figure 4 rotates counterclockwise, causing the push sleeve 454 and chuck sleeve 456 to move toward the rear end of the first spindle 4. The collet chuck 41 then expands in diameter. In Figure 4, the solid lines show the upper jaw member 453 rotating clockwise and the lower jaw member 453 rotating counterclockwise.

[0046] The coil spring 455 constantly pushes the chuck sleeve 456 toward the rear end and the collet chuck 41 toward the front end. As a result, the push sleeve 454 is also pushed toward the rear end via the chuck sleeve 456, and the rear end of the push sleeve 454 pushes the sleeve push portion 453a toward the rear end.

[0047] The inner circumferential surface 456a of the tip of the chuck sleeve 456 is a tapered surface whose diameter increases towards the tip. In the gripping release state shown by the solid line in Figures 3 and 4, the chuck sleeve 456 is located towards the rear end, so there is almost no force from the inner circumferential surface 456a of the tip of the chuck sleeve 456 pushing radially inward against the outer circumferential surface 41a of the tip of the collet chuck 41, and the collet chuck 41 expands in diameter at the tip due to its own elasticity. On the other hand, in the gripping state shown by the dashed line in Figures 3 and 4, the chuck sleeve 456 is located towards the tip, so the inner circumferential surface 456a of the tip of the chuck sleeve 456 pushes radially inward against the outer circumferential surface 41a of the tip of the collet chuck 41, causing the tip of the collet chuck 41 to contract in diameter.

[0048] The adjustment nut 457 is located at the rear end of the first spindle 4. This adjustment nut 457 is an example of an adjustment mechanism. A female thread is formed on the inside of the adjustment nut 457, and this female thread engages with a male thread formed at the rear end of the spindle body 40. By rotating the adjustment nut 457 in the tightening direction, the jaw member 453, push sleeve 454, and chuck sleeve 456 move toward the front end of the first spindle 4 together with the adjustment nut 457. As a result, the inner diameter of the collet chuck 41 in the gripping state becomes relatively smaller, and the gripping force of the first spindle 4 on the workpiece W1 increases. On the other hand, by rotating the adjustment nut 457 in the loosening direction, the jaw member 453, push sleeve 454, and chuck sleeve 456 move toward the rear end of the first spindle 4 together with the adjustment nut 457. As a result, the inner diameter of the collet chuck 41 in the gripping state becomes relatively larger, and the gripping force of the first spindle 4 on the workpiece W1 decreases.

[0049] The adjustment nut 457 is C-shaped with a notch that runs radially when viewed from the rear end to the front end of the first spindle 4, and the width of the notch can be changed by a fixing screw (not shown). After the adjustment of the gripping force using the adjustment nut 457 is completed, the adjustment nut 457 is fixed to the spindle body 40 in the adjusted position by narrowing the notch with the fixing screw. The first spindle head 3 is also fitted with a nut rotation prevention device (not shown) that prevents the rotation of the adjustment nut 457. By loosening the screw that narrows the notch of the adjustment nut 457 and preventing the rotation of the adjustment nut 457 with the nut rotation prevention device, the adjustment nut 457 can be moved to the desired rotation angle position relative to the first spindle 4 by rotating the first spindle 4 at a specified angle. In other words, the gripping force of the workpiece W1 of the first spindle 4 can be automatically adjusted by preventing rotation by the nut rotation prevention device and rotating the first spindle 4. Furthermore, the gripping force of the workpiece W1 of the first spindle 4 may be adjusted using an adjustment means other than the adjustment nut 457.

[0050] As shown in Figure 3, the air cylinder 33 has a cylinder tube 321 and a piston 322. This air cylinder 33 is an example of an actuator. Alternatively, a hydraulic cylinder or motor may be used instead of the air cylinder 33. The cylinder tube 321 is the housing of the air cylinder 33 and is cylindrical in shape. The piston 322 is positioned inside the cylinder tube 321, in contact with its inner circumferential surface, and is movable to one side and the other. This piston 322 is driven by the air supplied to the air cylinder 33 and moves between the rear end and the front end within the cylinder tube 321. Figure 3 shows the piston 322 in its rearmost position.

[0051] Figure 5 is a perspective cross-sectional view showing a portion of the first headstock 3 shown in Figure 1, cut in half to reveal the non-rotating transmission member 34 and a portion of the rotating transmission member 45. Figure 6 is a perspective cross-sectional view showing a portion of the shifter lever 341 shown in Figure 5, cut along the direction of extension of the shifter lever 341. In Figure 6, the cut surface of the shifter lever 341 is shown with hatching.

[0052] As described above, the non-rotating transmission member 34 includes a piston rod 340, a shifter lever 341, a lever shaft 342, a connecting pin 343, an operating shaft 344 (see Figure 6), and an operating bearing 345 (see Figure 6). The piston rod 340 is rod-shaped with one end fixed to the piston 322 (see Figure 3). As shown in Figure 5, the other end of the piston rod 340 is connected to the shifter lever 341 via the connecting pin 343. The piston rod 340 is driven by the piston 322 and changes state between an extended state, where it extends towards the rear end of the cylinder tube 321, and a retracted state, where it retracts towards the front end. That is, the driving force of the air cylinder 33 is transmitted to the piston rod 340 and acts as the forward and backward movement of the piston rod 340.

[0053] The shifter lever 341 is pivotable around a lever shaft 342 fixed to the first headstock body 30. That is, the axis of rotation of the shifter lever 341 is the axis of rotation of the lever shaft 342. A connecting pin 343 is fixed to one end of the shifter lever 341. This connecting pin 343 rotatably connects one end of the shifter lever 341 to the other end of the piston rod 340. As a result, the driving force of the air cylinder 33 is transmitted to the shifter lever 341 via the connecting pin 343. The contact point of the shifter lever 341 with the connecting pin 343 becomes the force-receiving point where the shifter lever 341 receives the driving force from the air cylinder 33.

[0054] As shown in Figure 6, an operating shaft 344 is fixed to the other end of the shifter lever 341, and an operating bearing 345 is attached to the operating shaft 344. This operating bearing 345 fits into a groove 452b of the shifter 452. As a result, the driving force transmitted from the air cylinder 33 to the shifter lever 341 via the connecting pin 343 is transmitted to the shifter 452. That is, by driving the air cylinder 33, the shifter lever 341 oscillates, and this oscillation causes the shifter 452 to slide in the Z1 axis direction. The centerline of the operating shaft 344 and the centerline of the operating bearing 345 coincide. When the shifter lever 341 transmits the driving force to the shifter 452, a reaction force acts on the shifter lever 341 from the shifter 452 via the operating bearing 345 and the operating shaft 344. The contact point of the shifter lever 341 with the operating shaft 344 becomes the operating point. The distance from the axis centerline of the lever shaft 342, which is the pivot centerline of the shifter lever 341, to the axis centerline of the connecting pin 343 is longer than the distance from the axis centerline of the lever shaft 342 to the axis centerline of the working shaft 344. Therefore, the shifter lever 341 corresponds to an example of a power assist mechanism (lever mechanism) that increases the driving force transmitted from the piston rod 340 and transmits it to the shifter 452. The non-rotating transmission member 34 and the rotating transmission member 45 described above constitute a driving force transmission mechanism that transmits the driving force from the air cylinder 33 to the collet chuck 41 (see Figure 3).

[0055] Figure 7 is a view of the shifter lever 341 shown in Figure 5, seen from the rear end of the spindle. In Figure 7, the first spindle 4, piston rod 340, shifter 452 and push sleeve 454, and air cylinder 33 are shown by dashed lines.

[0056] As shown in Figure 7, the shifter lever 341 is roughly U-shaped when viewed from the rear end. The aforementioned operating shaft 344 and operating bearing 345 are arranged symmetrically in the horizontal direction on the other ends of the shifter lever 341, one above the other. A strain detection means 46 is attached to the rear end side of the shifter lever 341 (the front side in Figure 7). The strain detection means 46 may also be attached to the front end side of the shifter lever 341 (the back side in Figure 7).

[0057] The strain detection means 46 is a so-called strain gauge that detects the strain generated in the shifter lever 341 when the driving force of the air cylinder 33 is transmitted from the air cylinder 33 to the collet chuck 41. A signal line 461 is connected to the strain detection means 46. The strain signal detected by the strain detection means 46 is transmitted to the control device 2 (see Figure 1) through the signal line 461.

[0058] The strain detection means 46 detects the strain of the shifter lever 341 that occurs at the part where it is attached. The strain detection means 46 is attached to a part that coincides with the axial centerline of the lever shaft 342 when viewed in the Z1 axis direction (the direction perpendicular to the plane of the paper in Figure 7). It is preferable to position the strain detection means 46 between the contact point between the shifter lever 341 and the connecting pin 343, which is the force-receiving part where the shifter lever 341 receives the driving force, and the contact point between the shifter lever 341 and the acting shaft 344, which is the acting part where the reaction force from the shifter 452 acts on the shifter lever 341. By positioning the strain detection means 46 between the force-receiving part and the acting part, the strain that occurs in the shifter lever 341 when the shifter lever 341 receives the driving force from the air cylinder 33 can be detected with high accuracy. Furthermore, in a configuration like this embodiment, where the axial centerline of the lever shaft 342 is located between the force-receiving part and the action part, it is more preferable to mount the strain detection means 46 at a location where the distance from the axial centerline of the lever shaft 342 is shorter than the distance from the force-receiving part and the action part, and even more preferable to mount it at a location that coincides with the axial centerline of the lever shaft 342. This is because the greatest strain occurs near the axial centerline of the lever shaft 342 when the shifter lever 341 receives driving force from the air cylinder 33, and by shortening the distance from the axial centerline of the lever shaft 342, the strain detection means 46 can detect the strain with higher accuracy.

[0059] Furthermore, it is preferable to attach the strain detection means 46 to the side of the shifter lever 341 that is formed parallel to the axial centerline of the lever shaft 342. At the center of the shifter lever 341 in the thickness direction (Z1 axis direction), there is a neutral surface that does not expand or contract even when the shifter lever 341 receives driving force from the air cylinder 33. The greatest strain occurs on the side away from this neutral surface, and by attaching the strain detection means 46 to that side, the strain can be detected with high accuracy. In other words, it can be said that it is preferable to attach the strain detection means 46 to the side of the shifter lever 341 that is parallel to the neutral surface.

[0060] Figure 8 is a control block diagram of the NC lathe 1 shown in Figure 1. Although Figure 8 only shows control configurations that are particularly relevant to this embodiment, the control device 2 performs control for all components of the NC lathe 1.

[0061] As shown in Figure 8, the NC lathe 1 is equipped with an operation unit 11 and a display unit 12 as an interface with the operator. The operation unit 11 is an input device for operating the NC lathe 1. The operation unit 11 consists of a number of buttons, keys, etc. that accept input operations from the operator of the NC lathe 1. The operation unit 11 may also be a touch panel integrated with the display unit 12. The display unit 12 is a display that shows various information related to the NC lathe 1, such as the NC program, various setting values, error details, and the derivation results of the derivation unit 23, which will be described later.

[0062] The control device 2 includes a timer 21, a storage means 22, and a derivation unit 23. The timer 21 is used to measure the elapsed time from the start of measurement. For example, the timer 21 measures the elapsed time from when the power to the NC lathe 1 is turned on, or the elapsed time from when a predetermined operation is started.

[0063] The memory means 22 stores control programs, NC programs, and various other information, and is composed of non-volatile memory and volatile memory. The memory means 22 also includes a strain-gripping force information storage unit 221, an adjustment nut-gripping force information storage unit 222, and a maximum strain storage unit 223.

[0064] The strain-gripping force information storage unit 221 stores the correlation between the magnitude of strain detected by the strain detection means 46 and the gripping force of the collet chuck 41. The adjustment nut-gripping force information storage unit 222 stores the correlation between the rotation angle of the adjustment nut 457 and the gripping force of the collet chuck 41. The maximum strain storage unit 223 stores the maximum strain, which is the maximum value of strain detected by the strain detection means 46 from the time a gripping command (closing command) for the collet chuck 41 is issued until a predetermined time has elapsed. The predetermined time is set to be sufficient time from the time a gripping command for the collet chuck 41 is issued to the first spindle 4 in the released state until the state changes to the gripped state. Whether or not the predetermined time has elapsed is measured by the timer 21. This predetermined time is set by the manufacturer of the NC lathe 1, but it may be changeable by operator input. Alternatively, instead of a predetermined time, a sensor may be provided in the air cylinder 33 shown in Figure 3 to detect when the piston 322 or piston rod 340 reaches the stroke end, and the time may be defined as the period until a signal indicating the arrival of the piston or piston rod is transmitted to that sensor.

[0065] Figure 9 is a graph showing an example of the strain of the shifter lever 341 detected by the strain detection means 46 when the first spindle 4 shown in Figure 1 changes state from a released state to a gripped state. The operation of deriving the gripping force of the collet chuck 41 will also be explained below using Figure 8, with reference to Figures 4 and 9.

[0066] When a gripping command is issued to the first spindle 4, which is in the release state, the control device 2 starts driving the air cylinder 33. The driving force of the air cylinder 33 is then transmitted to the shifter lever 341 (see Figure 3), causing the shifter lever 341 to swing and the shifter 452 to move towards the rear end. The tip portion 453b of the claw member 453 is then pushed up against the cam surface 452a of the shifter 452. At this time, the shifter lever 341 is subjected to a reaction force from the shifter 452 due to the resistance of the shifter 452's movement and a driving force from the air cylinder 33, causing distortion in the shifter lever 341. The distortion detected by the distortion detection means 46 increases as shown in Figure 9. The shifter lever 341 experiences maximum strain when the contact point between the jaw tip 453b and the cam surface 452a reaches the boundary point between the change surface 452a2 and the large-diameter surface 452a3, and the strain of the shifter lever 341 decreases as the jaw tip 453b crosses that boundary point. As described above, the collet chuck 41 becomes most compact at the moment the contact point between the jaw tip 453b and the cam surface 452a reaches that boundary point, and the first spindle 4 enters a gripping state.

[0067] The control device 2 receives the strain detected by the strain detection means 46 and performs the following processing, for example, every tens of microseconds. After a grip command is issued to the first spindle 4, which is in a released grip state, the control device 2 stores the first strain detected in the maximum strain storage unit 223. From then on, the control device 2 compares the strain stored in the maximum strain storage unit 223 with the strain detected by the strain detection means 46. If the current strain is greater, the control device 2 overwrites the strain value stored in the maximum strain storage unit 223. On the other hand, if the strain detected by the strain detection means 46 is smaller than the strain stored in the maximum strain storage unit 223, the strain value stored in the maximum strain storage unit 223 remains unchanged. This process is repeated for a predetermined time after a grip command is issued to the first spindle 4, so that the maximum strain is stored in the maximum strain storage unit 223. Furthermore, if the strain detected by the strain detection means 46 is smaller than the strain stored in the maximum strain memory unit 223, the control device 2 may stop processing without waiting for a predetermined time to elapse, as it is highly likely that the strain stored in the maximum strain memory unit 223 at that time is the maximum strain. This reduces the load on the control device 2.

[0068] Once the maximum strain is stored in the maximum strain memory unit 223, the derivation unit 23 derives the gripping force of the collet chuck 41 (see Figure 1) based on the maximum strain stored in the maximum strain memory unit 223, using the relationship between strain and gripping force stored in the strain-gripping force information memory unit 221. The derivation unit 23 then displays the derived gripping force value on the display unit 12. This operation of the derivation unit 23 is performed not only during setup but also during continuous machining, where the same operation is repeated using an NC program, each time the first spindle 4 is changed to the gripping state. This state change is usually performed between machining operations in continuous machining. However, the operation of the derivation unit 23 may be performed only during setup or only during continuous machining. Furthermore, the operator may be able to select whether or not to perform the operation of the derivation unit 23 during setup and during continuous machining.

[0069] Next, we will explain the operation when the gripping force is automatically adjusted during the setup process.

[0070] Figure 10 is a flowchart illustrating the automatic adjustment operation of the gripping force in the first spindle 4 shown in Figure 1. Note that this automatic adjustment operation shown in Figure 10 is performed under control by the control device 2.

[0071] Prior to the automatic adjustment operation, the operator of the NC lathe 1 inputs the desired gripping force to the control device 2 using the control unit 11. The operator of the NC lathe 1 also uses the control unit 11 to release the grip of the first spindle 4 and inserts the workpiece W1 into the collet chuck 41. Then, the operator loosens the fixing screw (not shown) that secures the adjustment nut 457 to the first spindle 4 so that the adjustment nut 457 can rotate relative to the first spindle 4. After these preparatory operations are completed, the automatic gripping force adjustment operation is performed.

[0072] As shown in Figure 10, when automatic adjustment is started, the control device 2 activates a nut rotation prevention device (not shown) and then rotates the first spindle 4 to return the adjustment nut 457 to its initial position. This initial position is the position where the gripping force of the collet chuck 41 is weakest. Next, the adjustment nut-gripping force information storage unit 222 extracts the rotation angle of the adjustment nut 457 that results in the desired gripping force, which was input in advance, from the correlation between the rotation angle of the adjustment nut 457 and the gripping force of the collet chuck 41, and rotates the first spindle 4 and the adjustment nut 457 relative to each other by that rotation angle (step S11). Alternatively, the current rotation angle of the adjustment nut 457 may be stored in the storage means 22, and the adjustment nut 457 may be rotated by the difference between the current rotation angle and the rotation angle that results in the desired gripping force.

[0073] Next, the control device 2 issues a gripping command to the first spindle 4 and changes the state of the first spindle 4 from a released state to a gripped state while executing the above process of storing the strain detected by the strain detection means 46 in the maximum strain storage unit 223 (step S12).

[0074] Once the state change is complete, the derivation unit 23 derives the gripping force of the workpiece W1 by the collet chuck 41 from the relationship between strain and gripping force stored in the strain-gripping force information storage unit 221, based on the maximum strain obtained in the state change performed in step S12 (step S13).

[0075] Then, the derivation unit 23 determines whether the derived gripping force matches the desired gripping force entered by the operator (step S14). If it is determined in step S14 that they do not match, the adjustment nut-gripping force information storage unit 222 calculates the additional rotation angle of the adjustment nut 457 necessary to eliminate the difference between the gripping force derived in step S13 and the desired gripping force, based on the correlation between the rotation angle of the adjustment nut 457 and the gripping force of the collet chuck 41 stored in the adjustment nut-gripping force information storage unit 222 (step S15).

[0076] Then, after releasing the grip of the first spindle 4, the control device 2 rotates the first spindle 4 and the adjustment nut 457 relative to each other by the additional rotation angle calculated in step S15 (step S16). Once step S16 is complete, the device returns to step S12 and changes the state of the first spindle 4 back to the gripped state while executing the above operation to be stored in the maximum strain memory unit 223.

[0077] On the other hand, if it is determined in step S14 that they match, the nut rotation prevention device is deactivated and the automatic adjustment operation ends. After that, the operator of the NC lathe 1 tightens a fixing screw (not shown) to prevent the adjustment nut 457 from rotating relative to the first spindle 4 in order to maintain the desired gripping force.

[0078] As described above, the NC lathe 1 of this embodiment derives the gripping force of the collet chuck 41 based on the detection result of the strain detection means 46 attached to the non-rotating transmission member 34. Therefore, compared to the case where the strain detection means 46 is attached to the rotating transmission member 45, the detection result of the strain detection means 46 can be transmitted to the control device 2 with a less expensive configuration. Furthermore, since the gripping force of the collet chuck 41 is derived based on the detection result of the strain detection means 46, variations due to the operator do not occur, and the gripping force of the collet chuck 41 can be derived with high accuracy. Moreover, since the gripping force can be derived each time the workpiece W1 is gripped, not only during setup but also during the execution of continuous machining using an NC program, it is possible to recognize changes in the gripping force due to changes over time or aging.

[0079] Furthermore, since the strain detection means 46 is attached to the shifter lever 341 located downstream of the driving force transmission path in the non-rotating transmission member 34, the influence of disturbances such as play between parts and frictional resistance is reduced compared to when it is attached to a part upstream of the transmission path. As a result, the gripping force of the collet chuck 41 can be determined with higher accuracy.

[0080] Furthermore, by deriving the gripping force of the collet chuck 41 based on the maximum strain, which is the maximum value of the strain detected by the strain detection means 46, the influence of variations and noise during each state change from the released state to the gripped state can be suppressed, and the gripping force of the collet chuck 41 can be derived with higher accuracy.

[0081] The above describes the configuration for deriving the gripping force of the collet chuck 41 in the NC lathe 1 of this embodiment based on the configuration of the first headstock 3 and the first spindle 4. The drive force transmission mechanism from the drive source to the chuck of the second headstock 7 and the second spindle 8 has the same configuration as that of the first headstock 3 and the first spindle 4. Therefore, although the explanation will be omitted, it is also possible to derive the gripping force of the chuck with high accuracy using an inexpensive configuration based on the detection result of the strain detection means attached to the non-rotating transmission member for the second headstock 7 and the second spindle 8. In the case of the second spindle 8, the cut workpiece W2 is an example of a workpiece.

[0082] Next, we will further explain the shifter lever 341 and its modified forms, which have been described so far. In the following explanation, components with the same names as those described so far will be given the same symbols as those used previously, and redundant explanations may be omitted.

[0083] Figure 11(a) is a schematic diagram showing the positional relationship between the force acting on the shifter lever 341 shown in Figure 5 and the pivot center, and Figure 11(b) is a schematic diagram showing a modified shifter lever 341 and the positional relationship between the force acting on the modified shifter lever 341 and the pivot center.

[0084] As shown in Figure 11(a), in this embodiment, the shifter lever 341 has a force-receiving portion where the shifter lever 341 receives driving force from the air cylinder 33 (see Figure 3) via a connecting pin 343, and an action portion where the shifter lever 341 receives a reaction force from the shifter 452 (see Figure 3) via an action shaft 344, with the lever shaft 342 and the pivot axis, which is the axial centerline of the lever shaft 342, located between them. As described above, the strain detection means 46 is attached to a portion that coincides with the pivot axis when orthographically projected in the Z1 axis direction (left-right direction in Figure 11). By positioning the strain detection means 46 between the force-receiving portion and the action portion, the strain generated in the shifter lever 341 when it is subjected to an external force can be detected with high accuracy. Furthermore, by positioning the strain detection means 46 on the side of the shifter lever 341, close to the pivoting central axis, strain can be detected with even higher accuracy. However, in the configuration of this embodiment, the strain detection means 46 is attached to a portion that coincides with the pivoting central axis when projected orthogonally in the Z1 axis direction, thus enabling strain detection with even higher accuracy. The force-receiving portion and the force-acting portion shown in Figures 11(a) and 11(b) are the portions where the shifter lever 341 receives force when changing the state of the first spindle 4 from a released state to a gripped state.

[0085] As shown in Figure 11(b), in the modified shifter lever 341, the lever shaft 342 and the pivot axis are located outside the force-receiving part and the action part, rather than between them. In this modified example as well, the strain detection means 46 is mounted on the side of the shifter lever 341, between the force-receiving part and the action part. In this modified shifter lever 341 as well, a large amount of strain occurs between the force-receiving part and the action part when changing the state of the first spindle 4 from the release state to the gripping state. For this reason, the mounting position of the strain detection means 46 can be anywhere between the force-receiving part and the action part, but the strain is particularly large on the side of the shifter lever 341, at a point equidistant from both the force-receiving part and the action part, so in this modified example, the strain detection means 46 is mounted at that point.

[0086] The present invention is not limited to the embodiments and modifications described above, and can be modified in various ways within the scope of the claims. For example, although the description of this embodiment shows an example of applying the present invention to a so-called Swiss-type NC lathe 1 equipped with a guide bush 5, the present invention may also be applied to other machine tools such as lathes or machining centers that do not have a guide bush 5. Furthermore, the second headstock 7, second spindle 8, and second tool post 9 may be omitted. In addition, the strain detection means 46 may be attached anywhere on the non-rotating transmission member 34. However, as described above, by attaching the strain detection means 46 to the shifter lever 341, the gripping force of the collet chuck 41 can be determined with higher accuracy.

[0087] Furthermore, even if a constituent element is included only in the description of each of the modified examples described above, that constituent element may be applied to other modified examples. [Explanation of Symbols]

[0088] 1 NC Lathe (Machine Tool) 4 1st spindle (main spindle) 3 1st headstock (headstock) 23 Derivation part 33. Air cylinder (actuator) 34 Non-rotating transmission member 41. Collet chuck (gripping part) 45 Rotation transmission member 46. ​​Strain detection means W1 Work

Claims

1. A spindle having a gripping part for gripping a workpiece, A headstock that rotatably supports the main spindle, An actuator mounted on the spindle head generates a driving force to change the state of the gripping portion between a gripping state in which the workpiece is gripped and a release state in which the workpiece is released. A non-rotational transmission member attached to the headstock and transmitting the driving force received from the actuator, A rotational transmission member that rotates together with the main shaft and transmits the driving force received from the non-rotating transmission member to the gripping portion, A strain detection means is attached to the non-rotating transmission member to detect the strain of the non-rotating transmission member that occurs when driving force is transmitted from the actuator to the gripping portion, A machine tool characterized by comprising a derivation unit that derives the gripping force of the gripping unit based on the detection result of the strain detection means.

2. The rotation transmission member includes a claw member capable of changing its position between a first position in which the gripping portion is in the gripping state and a second position in which the gripping portion is in the release state, and a shifter capable of changing the position of the claw member between a first position in which the claw member is in the first position and a second position in which the claw member is in the second position by moving in the axial direction of the main shaft. The non-rotating transmission member has a shifter lever that moves the shifter in the axial direction, The machine tool according to claim 1, characterized in that the strain detection means detects the strain of the shifter lever.

3. The machine tool according to claim 2, characterized in that the strain detection means detects strain in the area between the force-receiving portion where the shifter lever receives the driving force from the actuator and the action portion where the reaction force received by the shifter lever from the shifter acts when the driving force received by the shifter is transmitted to the shifter.

4. The shifter lever is capable of pivoting around the pivot axis, and by pivoting, it moves the shifter in the direction of the axis. The machine tool according to claim 3, characterized in that the strain detection means detects the strain on the side surface of the shifter lever, which is formed parallel to the pivot axis.

5. The shifter lever is capable of pivoting around the pivot axis, and by pivoting, it moves the shifter in the direction of the axis. The machine tool according to claim 3, characterized in that the strain detection means detects strain in a part where the distance from the pivoting center axis is shorter than the distance from the force-receiving part and the action part.

6. The machine tool according to any one of claims 1 to 5, characterized in that the derivation unit derives the gripping force of the gripping unit based on the maximum value of the strain of the non-rotation transmission member during the state change from the release state to the gripping state detected by the strain detection means.

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