Machine tool

Abstract

Provided is a machine tool capable of highly accurately estimating force for gripping a workpiece. The present invention comprises: a first main shaft 4 that has a collet chuck 48 and rotates; an air cylinder 32 that, by supplying air to a first pressure chamber 324 to move a piston 322 to one side, reduces the diameter of the collet chuck 48 and, by supplying air to a second pressure chamber 325 to move the piston 322 to the other side, expands the diameter of the collet chuck 48; a first pressure sensor 3261 that measures a first pressure, which is the pressure of the air supplied to the first pressure chamber 324 when the piston 322 is moved to said one side; a second pressure sensor 3271 that measures a second pressure, which is the pressure of air flowing out from the second pressure chamber 325 when the piston 322 is moved to said one side; and an estimation unit 23 that estimates the gripping force of the collet chuck 48 on the basis of the differential pressure between the first pressure which is measured by the first pressure sensor 3261 and the second pressure which is measured by the second pressure sensor 3271.

Description

machine tools

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

[0002] There is a known machine tool that machines a rod-shaped workpiece gripped by a spindle that is rotatable about its centerline. The spindle is rotatably supported by a headstock. The spindle is provided with a gripper that grips the workpiece by reducing its diameter and releases the grip by expanding its diameter. The gripper is operated by an actuator fixed to the headstock (see, for example, Patent Document 1). In the machine tool disclosed in Patent Document 1, the actuator drives a shifter to displace the jaw members, which then swings the chuck sleeve along the spindle centerline via the push sleeve, causing the gripper to grip or release the workpiece. A fluid pressure cylinder, such as an air cylinder or a hydraulic cylinder, may be used as the actuator.

[0003] A typical spindle is equipped with an adjustment nut as a means for adjusting the workpiece gripping force. This adjustment nut is attached to the spindle body so that it can rotate relative to the spindle body. Rotation displaces the position of the adjustment nut relative to the spindle body in the spindle centerline direction, thereby changing the workpiece gripping force. When machining a workpiece with a different diameter or material than the previous workpiece, a machine tool operator inserts the next workpiece into the gripping section during a so-called setup operation and rotates the adjustment nut relative to the spindle body to adjust the workpiece gripping force. The machine tool operator then manually operates a shifter lever to displace the shifter, estimates the gripping force of the gripping section from the force required for that operation, and repeats this relative rotation and estimation until the desired gripping force is achieved, at which point the adjustment nut is fixed to the spindle body.

[0004] Japanese Patent Application Laid-Open No. 2020-97075

[0005] However, conventional adjustment methods rely on the operator's intuition to estimate the gripping force, resulting in low accuracy. Furthermore, after adjustment during setup, the gripping force can change due to wear and thermal displacement of mechanisms such as the shifter and jaws between the fluid pressure cylinder and the gripping unit, deterioration of the fluid cylinder's rubber gasket, or variations in the workpiece diameter. Therefore, the gripping force of the workpiece during processing may differ from the gripping force estimated during adjustment.

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide a machine tool that can estimate the gripping force of a workpiece with high accuracy.

[0007] The machine tool of the present invention that solves the above problem is characterized by comprising: a spindle having a gripping portion that grips a workpiece by reducing its diameter and releases the grip of the workpiece by expanding its diameter, and that rotates around the spindle center line; a fluid pressure cylinder having a first pressure chamber and a second pressure chamber partitioned by a piston, and that supplies fluid to the first pressure chamber to move the piston to one side to reduce the diameter of the gripping portion, and supplies fluid to the second pressure chamber to move the piston to the other side to expand the diameter of the gripping portion; a first pressure sensor that measures a first pressure which is the pressure of the fluid supplied to the first pressure chamber when the piston is moved to the one side; a second pressure sensor that measures a second pressure which is the pressure of the fluid flowing out of the second pressure chamber when the piston is moved to the one side; and an estimation unit that estimates the gripping force of the gripping portion based on the differential pressure between the first pressure measured by the first pressure sensor and the second pressure measured by the second pressure sensor.

[0008] According to this machine tool, the gripping force is estimated based on the differential pressure, which is the driving force that actually operates the fluid pressure cylinder, so the gripping force of the workpiece can be estimated with high accuracy.

[0009] Here, the spindle may have a claw member that swings to open and close the gripper, and a shifter that swings the claw member by displacing along the spindle center line, and the fluid pressure cylinder may displace the shifter along the spindle center line by moving the piston to one side or the other side using fluid pressure. The spindle may also have an adjustment unit for adjusting the gripping force of the gripper on the workpiece. The estimation unit may estimate the gripping force of the gripper based on the differential pressure when the piston is moved to the one side. This machine tool may also include a conversion information storage unit that stores conversion information between the differential pressure and the gripping force.

[0010] In this machine tool, the estimation unit may estimate the gripping force based on the pressure difference a predetermined time before the piston reaches the one-side arrival point.

[0011] By doing so, it is only necessary to store the information on the differential pressure for the predetermined time period in the storage means, and therefore the storage area of ​​the storage means required for saving the information can be reduced.

[0012] Here, the fluid pressure cylinder may have an end sensor that detects when the piston reaches a destination point on the one side. The destination point may be a stroke end at which the piston rod has moved furthest to the one side. The machine tool may include a differential pressure information storage unit that sequentially stores the differential pressure. The estimation unit may acquire information on the differential pressure from the differential pressure information storage unit before the predetermined time. The estimation unit may estimate the gripping force based on the differential pressure after the piston, which was located on the other side, started to move to the one side. In other words, the estimation unit may estimate the gripping force based on the differential pressure after fluid started to be supplied to the first pressure chamber.

[0013] In this machine tool, the estimation unit may estimate the gripping force based on a maximum value of the differential pressure within a predetermined time range before the piston reaches the one-side arrival point.

[0014] This allows the gripping force to be estimated more accurately.

[0015] Here, the estimation unit may acquire information about the differential pressure within the predetermined time range from the differential pressure information storage unit.

[0016] In this machine tool, the estimation unit may be configured to determine whether the differential pressure is within a predetermined range, and if the differential pressure exceeds the predetermined range, to determine that a gripping force abnormality has occurred.

[0017] In this embodiment, the operator of the machine tool can easily recognize the abnormality in the gripping force.

[0018] According to the present invention, it is possible to provide a machine tool that can estimate the gripping force of a workpiece with high accuracy.

[0019] FIG. 1 is a plan view showing a simplified internal configuration of an NC lathe according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of the first headstock and first spindle shown in FIG. 1 cut along a horizontal plane passing through the center line of the first spindle and viewed from above. FIG. 3 is an enlarged view of the first spindle shown in FIG. 2. FIG. 4 is a perspective view of the first headstock and first spindle shown in FIG. 1. FIG. 5 is a cross-sectional view similar to FIG. 2, showing the first headstock and first spindle in a gripping state. FIG. 6 is a pneumatic circuit diagram for changing the state of the first spindle shown in FIG. 1 between a gripping state and a released state. FIG. 7 is a control block diagram of the NC lathe shown in FIG. 1. FIG. 8 is a graph showing a differential pressure generated when the first spindle shown in FIG. 1 changes state from the released state to the gripping state.

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In this embodiment, the present invention will be described by taking an example in which the present invention is applied to a Swiss-type NC (Numerical Control) lathe.

[0021] FIG. 1 is a plan view showing a simplified internal configuration of an NC lathe 1 according to this embodiment.

[0022] 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 rest 6, a second headstock 7, a second spindle 8, and a second tool rest 9. The control device 2 is a computer that operates the first headstock 3, the first spindle 4, the first tool rest 6, the second headstock 7, the second spindle 8, and the second tool rest 9 in accordance with an NC program. In addition to operating using an NC program, the NC lathe 1 can also be operated by directly inputting commands to the control device 2 from an operation unit 11 (see Figure 7).

[0023] 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, i.e., the left-right direction in FIG. 1. The first spindle 4 is rotatably mounted on the first headstock 3. A first spindle motor 31 (see FIG. 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 about the first spindle center line CL1. The direction of this first spindle center line CL1 coincides with the Z1-axis direction.

[0024] The first spindle 4 changes state between a gripping state in which it grips a long rod-shaped workpiece W1 inserted therein and a release state in which it releases the grip of the workpiece W1 in response to a signal from the control device 2. This first spindle 4 corresponds to an example of a spindle. When the first spindle motor 31 (see FIG. 2) receives a signal from the control device 2 and rotates, the workpiece W1 gripped by the first spindle 4 rotates together with the first spindle 4 about the first spindle center line CL1. The configurations of the first headstock 3 and the first spindle 4 will be described in detail later.

[0025] The guide bush 5 is fixed to a leg that serves as the base of the NC lathe 1. The guide bush 5 supports the tip portion of the workpiece W1, which is inserted into the first spindle 4 and whose tip protrudes from the first spindle 4, so that it can slide freely in the Z1 axis direction. The portion of this guide bush 5 that supports the workpiece W1 can rotate about the first spindle center line CL1 in synchronization with the first spindle 4. In other words, the first spindle center line CL1 is also the rotation center line of the portion of the workpiece W1 that is supported by the guide bush 5. The presence of the guide bush 5 suppresses deflection of the workpiece W1 during machining, allowing for high-precision machining of particularly long and slender workpieces W1.

[0026] The first tool rest 6 is movable in the X1-axis direction, which is perpendicular to the Z1-axis direction and faces horizontally, and in the Y1-axis direction, which faces vertically. The first tool rest 6 moves in the X1-axis direction and the Y1-axis direction in response to signals from the control device 2. In FIG. 1 , the up-down direction is the X1-axis direction, and the direction perpendicular to the paper surface is the Y1-axis direction. A first spindle tool T1 for machining a workpiece W1 is attached to the first tool rest 6. FIG. 1 shows the first spindle tool T1 attached to the first tool rest 6. Multiple types of first spindle tools T1, including a cutting tool for outer diameter machining and a cutting tool for cut-off machining, are attached to the first tool rest 6 in the Y1-axis direction. By moving the first tool rest 6 in the Y1-axis direction, an arbitrary first spindle tool T1 is selected from the multiple types of first spindle tools T1. Then, the first tool rest 6 moves in the X1 axis direction, causing the selected first spindle tool T1 to cut into the workpiece W1 held by the first spindle 4 and machine the workpiece W1.

[0027] The second headstock 7 moves in the X2-axis direction and the Z2-axis direction together with the second spindle 8 in response to a signal from the control device 2. The X2-axis direction is the same as the X1-axis direction described above, and the Z2-axis direction is the same as the Z1-axis direction described above. The second headstock 7 is provided with a second spindle motor (not shown), such as a built-in motor. When the second spindle motor receives a signal from the control device 2 and rotates, the second spindle 8 rotates about the second spindle center line CL2. The direction of the second spindle center line CL2 coincides with the Z2-axis direction.

[0028] After machining using the first spindle 4 is completed, the cut workpiece W2 cut by a cutting-off tool is delivered to the second spindle 8. The second spindle 8 changes state between a gripping state in which it grips the cut workpiece W2 delivered from the first spindle 4 and a release state in which it releases its grip on the cut workpiece W2. This second spindle 8 also corresponds to an example of a spindle. When the second spindle motor receives a signal from the control device 2 and rotates, the workpiece W2 gripped by the second spindle 8 rotates together with the second spindle 8 about the second spindle center line CL2.

[0029] The second tool rest 9 moves in the Y2-axis direction in response to a signal from the control device 2. This Y2-axis direction is the same as the Y1-axis direction described above, i.e., vertical. A plurality of second spindle tools T2 for machining the cut workpiece W2 gripped by the second spindle 8 are attached to the second tool rest 9. FIG. 1 shows the second spindle tools T2 attached to the second tool rest 9. A plurality of types of second spindle tools T2, such as drills and end mills, are attached to the second tool rest 9. Although not shown in FIG. 1 , the second spindle tools T2 are attached in rows not only in the X2-axis direction but also in the Y2-axis direction. An arbitrary second spindle tool T2 is selected from the plurality of types of second spindle tools T2 by moving the second headstock 7 in the X2-axis direction and the second tool rest 9 in the Y2-axis direction. Then, by moving the second headstock 7 in the Z2-axis direction, the cut end portion of the cut workpiece W2 gripped by the second spindle 8 is machined.

[0030] Fig. 2 is a cross-sectional view of the first headstock 3 and the first spindle 4 shown in Fig. 1, taken along a horizontal plane passing through the first spindle center line CL1, as viewed from above. Fig. 3 is an enlarged view of the first spindle 4 shown in Fig. 2. Figs. 2 and 3 show the first headstock 3 and the first spindle 4 in an ungriped state. Note that Figs. 2 and 3 do not have hatching indicating a cross section.

[0031] 2, the first spindle 4 is generally cylindrical and has an axis aligned with the first spindle center line CL1. The first spindle 4 is rotatably supported on the first headstock 3 by bearings 39. The first spindle 4 has a spindle body 41, a shifter 42, a pawl member 43, an adjusting nut 44, a pressing sleeve 45, a chuck sleeve 46, a coil spring 47, a collet chuck 48, and a spindle cap 49. The collet chuck 48 corresponds to an example of a gripping portion.

[0032] The spindle body 41 is a cylindrical body extending in the Z1-axis direction and serves as the base of the first spindle 4. The shifter 42 is disposed at the rear end of the first spindle 4. The shifter 42 is generally cylindrical and is attached to the spindle body 41 so that its inner peripheral surface is in sliding contact with the outer peripheral surface of the spindle body 41, allowing it to slide in the Z1-axis direction. Note that in FIGS. 2 and 3 , the left side of the drawings corresponds to the rear end of the first spindle 4. As shown in FIG. 3 , a cam surface 42a and a groove 42b are formed on the outer peripheral surface of the shifter 42. The cam surface 42a is composed of, in order from the rear end of the first spindle 4, a small-diameter surface 42a1 with the smallest and constant outer diameter, a varying surface 42a2 with a gradually increasing outer diameter, and a large-diameter surface 42a3 with the largest and constant outer diameter. The shifter 42 slides in the Z1-axis direction using an air cylinder 32 (see FIG. 2 ), the operation of which is controlled by the control device 2 (see FIG. 1 ). 2 and 3 show the shifter 42 positioned at its initial position closest to the tip of the first main shaft 4. In FIGS. 2 and 3, the right side of the drawings corresponds to the tip of the first main shaft 4.

[0033] Two pawl members 43 are attached to the spindle body 41 so as to be swingable around a pawl shaft 431. A pawl tip portion 43b that acts as a cam follower and contacts a cam surface 42a of the shifter 42 is formed at the tip of each pawl member 43. As the shifter 42 slides, the pawl tip portion 43b moves along the cam surface 42a toward or away from the first spindle center line CL1, causing the pawl member 43 to swing around the pawl shaft 431. A sleeve pressing portion 43a is formed on each pawl member 43, and the sleeve pressing portion 43a contacts the rear end of the pressing sleeve 45.

[0034] The adjusting nut 44 is threadedly coupled to the spindle body 41 at the rearmost portion of the first spindle 4. This adjusting nut 44 corresponds to an example of an adjusting means. A female thread is formed on the inside of the adjusting nut 44, and it engages with a male thread formed on the rear end portion of the spindle body 41. By rotating the adjusting nut 44 in the tightening direction, the claw members 43, the push sleeve 45, and the chuck sleeve 46 move together with the adjusting nut 44 toward the tip end of the first spindle 4. This relatively reduces the inner diameter of the collet chuck 48 in the gripping state, thereby increasing the gripping force of the workpiece W1 of the first spindle 4. On the other hand, by rotating the adjusting nut 44 in the loosening direction, the claw members 43, the push sleeve 45, and the chuck sleeve 46 move together with the adjusting nut 44 toward the rear end of the first spindle 4. This relatively increases the inner diameter of the collet chuck 48 in the gripping state, thereby decreasing the gripping force of the workpiece W1 of the first spindle 4. That is, the adjusting nut 44 adjusts the gripping force of the first spindle 4 on the workpiece W1. The adjusting nut 44 is C-shaped with a notch along the radial direction when viewed from the rear end side to the front end side of the first spindle 4, and the width of the notch can be changed with a screw (not shown). After the gripping force adjustment using the adjusting nut 44 is complete, the notch is narrowed with the screw, and the adjusting nut 44 is fixed to the spindle body 41 at the adjusted position. Note that the gripping force of the first spindle 4 on the workpiece W1 may be adjusted using an adjustment means other than the adjusting nut 44.

[0035] The push sleeve 45 is a cylindrical body disposed inside the spindle body 41, and its tip contacts the rear end portion of the chuck sleeve 46. Movement of the shifter 42 in the Z1 axis direction causes the push sleeve 45 and the chuck sleeve 46 to move in the opposite direction to the shifter 42. Specifically, as the shifter 42 slides toward the rear end of the first spindle 4, the claw tip portions 43b of the claw members 43 are pushed up by the cam surfaces 42a of the shifter 42 and move in a direction away from the first spindle center line CL1. As a result, the upper claw member 43 in FIG. 3 swings counterclockwise around the claw shaft 431 as the swing center, and the lower claw member 43 in FIG. 3 swings clockwise, causing the push sleeve 45 and the chuck sleeve 46 to be pushed by the sleeve pushing portions 43a of the claw members 43 and move toward the tip side of the first spindle 4. As the claw tip portions 43b are pushed up by the cam surface 42a of the shifter 42, the collet chuck 48 is in its most closed state and the first spindle 4 is in a gripping state at the moment when the contact point between the claw tip portions 43b and the cam surface 42a reaches the boundary point between the transition surface 42a2 and the large-diameter surface 42a3. When the workpiece W1 (see FIG. 1) is in the collet chuck 48, the force of the air cylinder 32 (see FIG. 2) at that moment is substantially proportional to the gripping force for gripping the workpiece W1. The force of the air cylinder 32 at this time is based on the pressure difference between the pressure of the air supplied to the first pressure chamber 324 (see FIG. 2) and the pressure of the air discharged from the second pressure chamber 325 (see FIG. 2), so the above-mentioned instantaneous pressure difference is substantially proportional to the gripping force for the workpiece W1. Hereinafter, the differential pressure at the moment when the contact point between the hook tip portion 43b and the cam surface 42a reaches the boundary point between the transition surface 42a2 and the large diameter surface 42a3 may be referred to as the effective differential pressure.

[0036] Conversely, as the shifter 42 slides toward the tip end of the first main spindle 4, the claw tip portions 43b of the claw members 43 move along the cam surfaces 42a of the shifter 42 in a direction approaching the first main spindle center line CL1. As a result, the upper claw member 43 in Figure 3 swings clockwise around the claw shaft 431 as the swing center, and the lower claw member 43 in Figure 3 swings counterclockwise, and the push sleeve 45 and the chuck sleeve 46 move toward the rear end of the first main spindle 4.

[0037] The coil spring 47 constantly presses the chuck sleeve 46 toward the rear end and constantly presses the collet chuck 48 toward the front end, thereby pressing the push sleeve 45 toward the rear end via the chuck sleeve 46, and the rear end of the push sleeve 45 presses the sleeve pressing portion 43 a toward the rear end.

[0038] The inner peripheral surface 46a of the tip of the chuck sleeve 46 is a tapered surface whose diameter increases toward the tip. The collet chuck 48 is configured to be able to expand and contract in the radial direction perpendicular to the first spindle centerline CL1. The outer peripheral surface 48a of the tip of the collet chuck 48 is a tapered surface whose diameter increases toward the tip. The collet chuck 48 has three continuous slots spaced circumferentially at 120° intervals from the tip to a hole formed near the center in the Z1 axis direction. These slots allow the collet chuck 48, particularly the tip side, to expand and contract in the radial direction. In the released state shown in FIGS. 2 and 3 , the chuck sleeve 46 is located at the rear end, so there is almost no force that presses the inner peripheral surface 46a of the tip of the chuck sleeve 46 radially inward against the outer peripheral surface 48a of the tip, and the collet chuck 48 expands in the radial direction due to its own elasticity.

[0039] Spindle cap 49 is bowl-shaped with a circular cap through-hole formed in its center in the Z1-axis direction. The tip surface of collet chuck 48 comes into contact with the part of spindle cap 49 that corresponds to the bottom of the bowl, thereby restricting movement of collet chuck 48 toward the tip. Collet chuck 48 is constantly pressed against spindle cap 49 by being pressed by coil spring 47. Spindle cap 49 is fixed to spindle body 41 by engaging a female thread formed on the inside of the bowl-shaped edge with a male thread formed at the tip of spindle body 41.

[0040] 2, the first headstock 3 has a first spindle motor 31, an air cylinder 32, and a shifter lever 33. The first spindle motor 31 is a built-in motor provided in the first headstock 3. As described above, by driving and rotating the first spindle motor 31, the first spindle 4 supported by the first headstock 3 via the bearing 39 rotates about the first spindle center line CL1.

[0041] The air cylinder 32 has a cylinder tube 321, a piston 322, and a piston rod 323. The air cylinder 32 corresponds to an example of a fluid pressure cylinder. The air supplied to the air cylinder 32 corresponds to an example of a fluid. Note that a fluid pressure cylinder such as a hydraulic cylinder may be used instead of the air cylinder 32. The cylinder tube 321 is a cylindrical housing of the air cylinder 32. The piston 322 is disposed within the cylinder tube 321 in contact with the inner circumferential surface of the cylinder tube 321 and is movable between one side and the other. The piston 322 divides the interior of the cylinder tube 321 into a first pressure chamber 324 and a second pressure chamber 325.

[0042] The piston rod 323 has a rod shape and one end is fixed to the piston 322. The other end of the piston rod 323 is connected to the shifter lever 33.

[0043] The air cylinder 32 is provided with a first port 326 that is connected to the first pressure chamber 324 and supplies air to the first pressure chamber 324, and a second port 327 that is connected to the second pressure chamber 325 and supplies air to the second pressure chamber 325. By supplying air to the first pressure chamber 324 and exhausting the air from the second pressure chamber 325, the piston 322 moves to one side, and the extension length of the piston rod 323 that has advanced from the cylinder tube 321 to the other side becomes shorter. Furthermore, by supplying air to the second pressure chamber 325 and exhausting the air from the first pressure chamber 324, the piston 322 moves to the other side, and the extension length of the piston rod 323 that has advanced from the cylinder tube 321 to the other side becomes longer. Figure 2 shows the piston 322 having moved to the other side to the furthest extent, thereby increasing the extension length of the piston rod 323.

[0044] The shifter lever 33 is freely swingable around a lever shaft 331 as the swing center. One end of the shifter lever 33 is rotatably connected to the other end of the piston rod 323 by a connecting pin 332. The other end of the shifter lever 33 is fitted into a groove 42b of the shifter 42. Driving the air cylinder 32 causes the shifter lever 33 to swing, and this swing causes the shifter 42 to slide in the Z1-axis direction. The air cylinder 32, shifter lever 33, shifter 42, claw member 43, push sleeve 45, chuck sleeve 46, coil spring 47, and collet chuck 48 described above form a collet opening / closing mechanism that expands and contracts the collet chuck 48.

[0045] Fig. 4 is a perspective view of the first headstock 3 and the first spindle 4 shown in Fig. 1. Fig. 4 also shows the first headstock 3 and the first spindle 4 in the released state.

[0046] As shown in FIG. 4 , a first pressure sensor 3261 is provided in the first port 326 of the air cylinder 32 to measure the pressure of air supplied to or exhausted from the first pressure chamber 324 (see FIG. 2 ). Hereinafter, the pressure of air measured by the first pressure sensor 3261 may be referred to as the “first pressure.” Furthermore, a second pressure sensor 3271 is provided in the second port 327 of the air cylinder 32 to measure the pressure of air supplied to or exhausted from the second pressure chamber 325 (see FIG. 2 ). Hereinafter, the pressure of air measured by the second pressure sensor 3271 may be referred to as the “second pressure.” Supplying air to the first pressure chamber 324 through the first port 326 moves the piston 322 (see FIG. 2 ) to one side, causing the diameter of the collet chuck 48 (see FIG. 2 ) to contract, and the state of the first spindle 4 changes from the released state shown in FIGS. 2 to 4 to the gripped state.

[0047] Fig. 5 is a cross-sectional view similar to Fig. 2, showing the first headstock 3 and the first spindle 4 in a gripping state. Note that in Fig. 5, hatching indicating a cross section is not used.

[0048] As described above, when air is supplied to first pressure chamber 324, the air that had been in second pressure chamber 325 is pushed out and exhausted through second port 327, causing piston 322 to move to one side. As a result, as shown in Fig. 5, the extension length of piston rod 323 shortens, shifter 42 slides toward the rear end of first spindle 4, claw tip portion 43b moves away from first spindle center line CL1, and push sleeve 45 and chuck sleeve 46 move toward the tip of first spindle 4. Then, the tapered surface of inner peripheral surface 46a at the tip of chuck sleeve 46 and the tapered surface of outer peripheral surface 48a at the tip of collet chuck 48 radially reduce the diameter of the tip side of collet chuck 48 against the elasticity of collet chuck 48 itself, and first spindle 4 enters a gripping state in which it grips workpiece W1 (see Fig. 1).

[0049] Conversely, when air is supplied to second pressure chamber 325, the air previously in first pressure chamber 324 is pushed out and exhausted through first port 326, causing piston 322 to move to the other side. As a result, as shown in FIG. 2, the extension length of piston rod 323 increases, causing shifter 42 to slide toward the tip of first spindle 4. Then, due to the restoring force of coil spring 47, push sleeve 45 and chuck sleeve 46 move toward the rear end of first spindle 4, causing claw member 43 to swing and claw tip portions 43b to approach first spindle center line CL1. Then, the force with which inner peripheral surface 46a of the tip of chuck sleeve 46 presses radially inward against outer peripheral surface 48a of the tip of collet chuck 48 is reduced or eliminated, causing the elasticity of collet chuck 48 to expand the diameter of the tip side of collet chuck 48 in the radial direction, and first spindle 4 enters a release state in which it releases its grip on workpiece W1 (see FIG. 1).

[0050] FIG. 6 is a diagram of a pneumatic circuit for changing the state of the first main spindle 4 shown in FIG. 1 between the gripped state and the released state.

[0051] As shown in FIG. 6 , the air cylinder 32 has an end sensor 328. The end sensor 328 detects when the piston 322 has moved to one side to the fullest extent and transmits an arrival signal to the control device 2 (see FIG. 1 ). Air for driving the air cylinder 32 is supplied from a pressure source 34 installed in a factory or the like where the NC lathe 1 (see FIG. 1 ) is installed. This pressure source 34 is, for example, an air compressor. The air supplied from the pressure source 34 is supplied to one of the first pressure chamber 324 and the second pressure chamber 325 after being switched by a switching valve 35, which is switched by the control device 2. FIG. 6 shows air being supplied to the second pressure chamber 325.

[0052] The switching valve 35 switches the supply destination of air from the pressure source 34 in response to a switching signal from the control device 2, and also switches the discharge destination of air exhausted from the air cylinder 32 to one of the first exhaust port 36A and the second exhaust port 36B. In this embodiment, when air is supplied from the pressure source 34 to the first pressure chamber 324 and the piston 322 moves to one side, the air that was in the second pressure chamber 325 is exhausted from the second exhaust port 36B. Conversely, when air is supplied from the pressure source 34 to the second pressure chamber 325 and the piston 322 moves to the other side, the air that was in the first pressure chamber 324 is exhausted from the first exhaust port 36A. Note that the first exhaust port 36A and the second exhaust port 36B are each provided with a silencer to reduce exhaust noise.

[0053] As described above, the first pressure sensor 3261 measures the air pressure, which is the pressure of the fluid in the first port 326 located immediately before the first pressure chamber 324. The second pressure sensor 3271 measures the air pressure, which is the pressure of the fluid in the second port 327 located immediately before the second pressure chamber 325. However, the first pressure sensor 3261 may be placed anywhere as long as it can measure the air pressure between the switching valve 35 and the first pressure chamber 324. The second pressure sensor 3271 may be placed anywhere as long as it can measure the air pressure between the switching valve 35 and the second pressure chamber 325.

[0054] FIG. 7 is a control block diagram showing the control configuration of the NC lathe 1 shown in FIG. 1 according to the present invention.

[0055] As shown in Figure 7, the NC lathe 1 is equipped with an operation unit 11 and a display unit 12 as interfaces with an operator. The operation unit 11 is an input device for operating the NC lathe 1. The operation unit 11 is made up of a plurality of buttons, keys, etc. that accept input operations by the operator of the NC lathe 1. The operation unit 11 may be a touch panel integrated with the display unit 12. The display unit 12 is a display that displays various information related to the NC lathe 1, such as the NC program, various setting values, error contents, and estimation results by an estimation unit 23 (described later).

[0056] The control device 2 has a timer 21, a storage means 22, and an estimation unit 23. The timer 21 is used to measure the passage of time. For example, the timer 21 measures the elapsed time from when the NC lathe 1 is powered on or the elapsed time from when a predetermined operation is started.

[0057] The storage means 22 stores the control program, the NC program, various information, etc. and is composed of a non-volatile memory and a volatile memory. The storage means 22 has a conversion information storage section 221, a differential pressure information storage section 222, and a set time storage section 223.

[0058] The conversion information storage unit 221 shown in FIG. 7 is a functional portion that stores the relationship between the gripping force of the first spindle 4 on the workpiece W1 and the above-mentioned effective differential pressure.

[0059] The differential pressure information storage unit 222 is a functional part that stores information on the differential pressure between the first pressure measured by the first pressure sensor 3261 and the second pressure measured by the second pressure sensor 3271, together with information on the elapsed time from a predetermined timing. Note that the differential pressure information storage unit 222 may store information on the first pressure and information on the second pressure, instead of the differential pressure information, together with information on the elapsed time from the predetermined timing.

[0060] 8 is a graph showing the differential pressure that occurs when the first spindle 4 shown in FIG. 1 changes from the released state to the gripped state. This FIG. 8 shows the differential pressure as a function of the elapsed time from when a command to switch from the released state to the gripped state is issued in the setup operation or NC program, with 0 msec as the time. The differential pressure information storage unit 222 sequentially stores the differential pressure information shown in this graph as numerical information together with time information. The operation of estimating the gripping force will be described below using FIG. 7 and with reference to FIG. 8.

[0061] The set time memory unit 223 stores the time when an effective differential pressure is expected to be generated when the first main spindle 4 is changed from the released state to the gripped state by driving the air cylinder 32 (see FIG. 2). Specifically, the set time memory unit 223 stores a time S1 (see FIG. 8) indicating how long before the effective differential pressure is generated, based on the arrival signal generated when the end sensor 328 detects that the piston 322 (see FIG. 2) has moved to the farthest side. This time S1 corresponds to an example of a predetermined time. If the shape of the cam surface 42a is the same, the time S1 is approximately constant regardless of the magnitude of the gripping force. This is because, after the effective differential pressure is generated, the claw tip portion 43b (see FIG. 3) contacts the large diameter surface 42a3 (see FIG. 3), which does not change in diameter, and therefore the inertial force acts strongly on the movement of the shifter 42 toward the rear end, causing the piston 322 to move to the farthest side in an approximately constant time. In this embodiment, the time S1 is set to 50 msec. This time S1 is set by the manufacturer of the NC lathe 1 through operational tests, but may be changeable by input from the operator.

[0062] When a switching command indicating a state change to the gripping state is issued, the control device 2 switches the switching valve 35 to supply air to the first pressure chamber 324 and exhaust air from the second pressure chamber 325. The control device 2 then calculates a differential pressure by subtracting the second pressure measured by the second pressure sensor 3271 from the first pressure measured by the first pressure sensor 3261, and sequentially stores the resulting differential pressure information in the differential pressure information storage unit 222 in association with information on the elapsed time since the issuance of the switching command. Figure 8 shows the stored differential pressure information as well as the output signal from the end sensor 328. The signal indicated by the rising line among the output signals from the end sensor 328 is the arrival signal. While Figure 8 displays differential pressure information from the issuance of the switching command until the reception of the arrival signal, differential pressure information from the time of storage prior to time S1 may be erased or overwritten from the differential pressure information storage unit 222. This reduces the capacity of the storage unit 22. As shown in Figure 8, the differential pressure gradually increases from the time the switching valve 35 is switched. Then, when the hook tip portion 43b reaches the boundary between the transition surface 42a2 and the large diameter surface 42a3, the effective differential pressure reaches its maximum value up to that point. The effective differential pressure then temporarily decreases as a reaction to the hook tip portion 43b crossing the boundary, and then begins to rise again. In other words, the effective differential pressure can be considered a maximal value, which is a temporary peak of the differential pressure that occurs when the hook tip portion 43b reaches the boundary between the transition surface 42a2 and the large diameter surface 42a3. Thereafter, when the control device 2 receives the arrival signal, the differential pressure reaches a value slightly exceeding the maximum value (maximum value) described above.

[0063] When the control device 2 receives the arrival signal, the estimation unit 23 obtains the differential pressure at a time point a time S1 before that time, which is stored in the set time memory unit 223, from the differential pressure information memory unit 222. Then, the estimation unit 23 regards the differential pressure as the effective differential pressure and estimates the gripping force of the first spindle 4 based on the relationship between the gripping force and the effective differential pressure stored in the conversion information memory unit 221. The estimation unit 23 also determines whether the effective differential pressure is within a predetermined range, and if it is not within the predetermined range, determines that a gripping force abnormality exists. The predetermined range of the effective differential pressure, which is the criterion for determining a gripping force abnormality, is a numerical range input by the operator using the operation unit 11 during setup work. However, this predetermined range may also be set as a percentage, such as ±30%, of the effective differential pressure adjusted during setup work. The predetermined range may also be set in terms of gripping force, which is essentially the same as the predetermined range of the effective differential pressure.

[0064] The estimation unit 23 displays the estimated grip force value and whether or not the grip force is abnormal on the display unit 12. However, the estimation unit 23 may only display the grip force value. In this case, the determination of a grip force abnormality may be omitted. Furthermore, the estimation unit 23 may display the fact that the grip force is abnormal only when the grip force is abnormal. The operation of this estimation unit 23 is executed each time the state changes to the gripping state during setup work and machining. However, the operation of the estimation unit 23 may be executed only during setup work or only during machining. Furthermore, the operator may be able to select whether or not to execute the operation of the estimation unit 23 during setup work and during machining. In addition, the control device 2 may issue an alarm and stop the operation of the NC lathe 1 when the estimation unit 23 determines that the grip force is abnormal.

[0065] According to the NC lathe 1 of the present embodiment described above, the gripping force of the collet chuck 48 is estimated based on the differential pressure between the first and second pressures, which are the driving force of the air cylinder 32, the driving source that contracts the collet chuck 48 and grips the first spindle 4. This allows for highly accurate estimation of the gripping force of the workpiece W1. Furthermore, the gripping force can be estimated each time the workpiece W1 is gripped, not only during setup but also during machining, such as continuous machining using an NC program. This allows for the detection of gripping force abnormalities during machining, such as a decrease in gripping force due to wear of the components constituting the collet opening / closing mechanism from the air cylinder 32 to the collet chuck 48 and their surrounding components. By detecting gripping force abnormalities during machining, production defects can be reduced, improving the productivity of the NC lathe 1. Additionally, the present invention can be applied to existing NC lathes 1 by installing the first pressure sensor 3261 and the second pressure sensor 3271 and adding or rewriting the control program of the control device 2.

[0066] Furthermore, when the gripping force is set to be weak and the pressure difference changes slowly, or when disturbances occur, it may be difficult to recognize the maximum value of the pressure difference that appears when the hook tip portion 43b reaches the boundary between the transition surface 42a2 and the large diameter surface 42a3. However, by regarding the pressure difference before the time S1 from the arrival signal as the effective pressure difference, the gripping force can be easily estimated even when the maximum value is difficult to recognize. Moreover, because there is no need to extract the maximum value, the amount of calculation is small, and the burden on the control device 2 is light.

[0067] Furthermore, since the gripping force abnormality is determined and displayed on the display unit 12, the operator can easily recognize whether or not the gripping force is abnormal.

[0068] The collet opening / closing mechanism of the NC lathe 1 of this embodiment has been described above based on the configuration of the first headstock 3 and the first spindle 4. However, the second headstock 7 and the second spindle 8 are also provided with collet opening / closing mechanisms configured similarly to the first headstock 3 and the first spindle 4. Therefore, although not described further, the estimating unit 23 can estimate the gripping force of the cut workpiece W2 with high accuracy for the second spindle 8, as with the first spindle 4. Furthermore, since the second spindle 8 often grips the machined portion of the workpiece W1 machined by the first spindle 4 and the first tool rest 6, estimating the gripping force of the cut workpiece W2 can also detect machining defects in the gripped portion of the cut workpiece W2 gripped by the second spindle 8. In other words, if the gripping force of the cut workpiece W2 significantly differs from the desired gripping force, it can be determined that there is a machining defect in the gripped portion. In addition, for the second spindle 8, the cut workpiece W2 corresponds to an example of a workpiece.

[0069] Next, a description will be given of a modified example of the estimation unit 23. In the following description, components having the same names as components described so far will be assigned the same reference numerals as used so far, and duplicate descriptions may be omitted.

[0070] The estimation unit 23 of this modified example differs from the previous embodiment in that it estimates the gripping force using the maximum value of the differential pressure within a predetermined time range as the effective differential pressure. The set time memory 223 of this modified example stores a time range S2 (see FIG. 8 ) in which the effective differential pressure may occur before the arrival signal reception time, which is the time when the end sensor 328 detects that the piston 322 has moved to one side. The estimation unit 23 acquires differential pressure information within the time range S2 from the differential pressure information memory 222 and estimates the gripping force based on the maximum differential pressure within the time range S2. In this modified example, the time range S2 is set to extend from 150 msec before the arrival signal to 20 msec before the arrival signal, excluding the increase in differential pressure immediately before the arrival signal. The time range S2 may also be the entire range from when the command to switch to the gripping state is issued to 20 msec after the arrival signal. Although this time range S2 is set by the manufacturer of the NC lathe 1, it may also be changeable by the operator.

[0071] In this modified example, the load on the control device 2 increases compared to the previous embodiment because it requires the task of extracting the maximum value in the time range S2. Also, compared to the previous embodiment, it is necessary to store differential pressures for a longer period of time in the differential pressure information storage unit 222, so a large-capacity storage means 22 is provided. However, in addition to the effects of the previous embodiment, the effect of being able to estimate the gripping force more accurately by obtaining the maximum value in the time range S2 is also achieved.

[0072] The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the claims. For example, in the description of the present embodiment, an example of applying the present invention to a so-called Swiss-type NC lathe 1 equipped with a guide bush 5 has been shown. However, the present invention may also be applied to other machine tools, such as lathes and machining centers, that do not have a guide bush 5. The second headstock 7 and the second tool rest 9 may also be omitted. Furthermore, the conversion information storage unit 221 may store information on the relationship between gripping force and effective differential pressure for each characteristic of the workpiece W1, such as the material and shape. In this case, the estimation unit 23 may estimate the gripping force by selecting information corresponding to the characteristics of the workpiece W1 to be used from the information on the relationship between gripping force and effective differential pressure stored in the conversion information storage unit 221.

[0073] It should be noted that 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.

[0074] DESCRIPTION OF SYMBOLS 1 NC lathe (machine tool) 4 First spindle (spindle) 23 Estimation unit 32 Air cylinder (fluid pressure cylinder) 48 Collet chuck (gripping unit) 322 Piston 324 First pressure chamber 325 Second pressure chamber 3261 First pressure sensor 3271 Second pressure sensor W1 Workpiece

Claims

1. A machine tool comprising: a spindle having a gripping portion which grips a workpiece by reducing its diameter and releases its grip of the workpiece by expanding its diameter, the spindle rotating about a center line of the spindle; a fluid pressure cylinder having a first pressure chamber and a second pressure chamber partitioned by a piston, the piston being moved to one side by supplying a fluid to the first pressure chamber to reduce the diameter of the gripping portion, and the piston being moved to the other side by supplying a fluid to the second pressure chamber to expand the diameter of the gripping portion; a first pressure sensor which measures a first pressure, which is the pressure of the fluid supplied to the first pressure chamber when the piston is moved to the one side; a second pressure sensor which measures a second pressure, which is the pressure of the fluid flowing out of the second pressure chamber when the piston is moved to the one side; and an estimation unit which estimates the gripping force of the gripping portion based on the differential pressure between the first pressure measured by the first pressure sensor and the second pressure measured by the second pressure sensor.

2. A machine tool according to claim 1, characterized in that the estimation unit estimates the gripping force based on the pressure difference a predetermined time before the piston reaches the destination point on one side.

3. A machine tool as described in claim 1, characterized in that the estimation unit estimates the gripping force based on the maximum value of the differential pressure within a specified time range prior to the piston reaching the destination point on one side.

4. A machine tool as claimed in any one of claims 1 to 3, wherein the estimation unit determines whether the differential pressure is within a predetermined range, and if it exceeds the predetermined range, determines that a gripping force abnormality has occurred.

Images