Clamping device for a workpiece and a processing machine with such a clamping device

Abstract

Clamping device (S) for a workpiece (W), with a) a sleeve (2) for exerting a clamping force on the workpiece (W), which is displaceably guided and supported by a spring element (34), b) a self-locking feed device (3) for the spindle sleeve (2), c) an electric motor (6) for driving the feed device (3), d) an electrical measuring device (42) for continuously recording the actual value of the compression travel (344) of the spring element (34), and e) a control system (7) which e1) determines a first actual value of the clamping force of the sleeve (2) from the current of the motor (6) and stops the motor (6) when a target value of the clamping force is reached, and e2) determines a second actual value of the clamping force of the sleeve (2) from the actual value of the compression travel (344) achieved in this process and switches off the motor (6) if the value matches the target value and / or the first actual value of the clamping force of the sleeve (2).

Description

[0001] The invention relates to a clamping device for a workpiece and to a processing machine, in particular a lathe, equipped with a clamping device according to the invention.

[0002] A clamping device according to the invention serves to fix a workpiece in a fixed position, e.g., on a clamping table or the machine bed of a processing machine. This allows for the most precise machining of a workpiece using a program-controlled processing machine or machine tool, e.g., a lathe or milling machine.

[0003] DE 26 40 797 A1 discloses a clamping device known as a power-operated tailstock. This comprises a quill as a clamping device for axially fixing a workpiece. This quill is moved by an electric motor via a worm drive and a rack and pinion. When moving against a workpiece, the quill moves relative to the tailstock upon reaching the workpiece, and a spring assembly is tensioned. When a desired counterforce is reached, a limit switch is actuated by the quill extension, and the drive motor is switched off. A brake motor is used as the drive motor. The self-locking of the worm drive blocks the tailstock longitudinally and holds the workpiece. The spring assembly causes re-tensioning. The desired force of the quill against the workpiece is achieved by moving the limit switch carrier with the aid of a handwheel according to a scale on which the clamping force can be read.

[0004] Such a device has the disadvantage that it only provides an imprecise value for the quill force acting on the workpiece. Both pre-setting via the handwheel and triggering via the mechanical actuation of a limit switch are associated with great inaccuracies. In addition, the drive motor for securing a workpiece is no longer available when the switch-off occurs and there is no guarantee that the quill force required to act on the workpiece has actually been built up at the moment of switch-off. It is also assumed that the force achieved is sufficient for the entire duration of workpiece machining and that the re-clamping effect of the spring assembly is adequate. However, there is a risk of the quill force dropping unnoticed, even leading to complete failure of the workpiece clamping. In addition, an undesirable increase in the clamping effect can occur, e.g.This can happen unnoticed, for example, due to heating of the workpiece during processing.

[0005] A "tailstock control device" is known from JP 2007-61971 A1. This device features a spring-supported tailstock center, which is held on a reciprocating tailstock. The tailstock is moved toward a workpiece by a servomotor until the center rests against the workpiece and the servomotor torque exceeds a limit. At this moment, the motor's advance is stopped, and the reached tailstock position is recorded. This tailstock position is maintained by position control via the servomotor to ensure workpiece clamping.

[0006] This device has the particular disadvantage that the servo motor is constantly engaged to maintain the clamping of the workpiece via position control. This not only requires constant electrical power and causes continuous wear on the device's components, but also carries the risk that the clamping effect on the workpiece will be lost in the event of a power failure. The device has the further disadvantage that it is not possible to check the force ratios at the holding center from the tailstock position reached at the moment of transition from torque to position control of the motor.

[0007] The invention is based on the object of providing a clamping device which enables a desired clamping force to be reliably applied and monitored on a workpiece in a manner which is as energy-efficient as possible.

[0008] This object is achieved by the clamping device specified in claim 1 and the processing machine specified in claim 7 and equipped with a clamping device according to the invention. Advantageous further embodiments of the invention are specified in the respective subclaims.

[0009] A clamping device for a workpiece designed according to the invention has a sleeve for exerting a clamping force on the workpiece. The sleeve is slidably guided and supported by a spring element. A self-locking feed mechanism driven by an electric motor serves to move the sleeve.

[0010] To reliably apply and monitor a desired clamping force, the clamping device according to the invention is further equipped with an electrical measuring device for continuously recording the actual value of the compression travel of the spring element. In a first step, a controller determines a first actual value of the clamping force of the sleeve from the motor current and stops the motor when a predetermined target value of the clamping force is reached. In a second step, the controller then determines a second actual value of the clamping force of the sleeve from the actual value of the compression travel thus achieved and switches the motor off when the target value and / or the first actual value of the clamping force of the sleeve match.

[0011] The clamping device according to the invention offers a number of advantages. A first advantage is that the continuous evaluation of the current of the electric motor, which serves to drive the sleeve feed device, enables a precise determination of a first actual value of the sleeve clamping force by means of the control. This makes it possible to interrupt the further feed of the motor at the moment in which the currently detected actual value has reached the size of a predetermined target value for the clamping force to be exerted by the sleeve in a desired manner. The rotary movement of the motor can thus be stopped precisely when a desired clamping force value is reached. The value of the clamping force applied at this moment is therefore precisely known and is initially precisely maintained by the motor because it is only stopped but not yet completely switched off, i.e. in particular its torque control is still active.

[0012] This first actual clamping force value can be derived, for example, from the actual motor current. The control system uses this to determine the torque generated by the motor and, taking into account the known value of the gear ratio of the self-locking feed mechanism, the current clamping force exerted by the sleeve on the workpiece. For this purpose, characteristic curves can advantageously be stored in the control system, which contain the relationships between motor current and motor torque, as well as the gear ratio and clamping force of the feed mechanism.

[0013] A further advantage of the invention is that the additional continuous detection of the actual value of the compression travel of the spring element according to the invention enables a check of the actual clamping force value reached at the moment the electric motor stops and exerted by the sleeve on the workpiece.

[0014] According to the invention, the actual achievement of a desired clamping force setpoint is verified by comparing the second actual value of the clamping force of the sleeve, derived from the measurement of the compression travel, with the first actual value of the clamping force of the sleeve, derived from the measurement of the motor current, at the moment the motor stops. Since at this moment the first actual value of the clamping force derived from the motor current has assumed the size of the respectively specified setpoint value of the clamping force of the sleeve, verification can also be carried out according to the invention in the same way by comparing the second actual value of the clamping force at the moment the motor stops with the setpoint value of the clamping force.

[0015] This verification can also be described as a two-channel safety check. It can detect potential measurement errors caused, for example, by contamination- and aging-related changes in internal friction in and between the components of the clamping device, i.e., the electric motor, the self-locking feed mechanism, the spindle sleeve, and possibly other intermediate components, such as a transmission gear between the motor and feed mechanism.

[0016] However, if the first and second actual values differ, the motor is not shut down for safety reasons. Secure clamping of the workpiece, which is necessary for error-free and accident-free machining, cannot be assumed. The workpiece continues to be held active by the motor's torque control. A machine operator can then investigate the cause of the problem and, if necessary, repeat the clamping process.

[0017] This second actual clamping force value can be derived, for example, from the actual value of the spring element's compression travel. The control system then uses this value, taking into account known spring characteristics, to determine a value for the current clamping force exerted by the sleeve on the workpiece. For this purpose, a spring characteristic curve can also advantageously be stored in the control system, which contains the relationship between the compression travel and the resulting spring force.

[0018] A further advantage of the invention is that if the result of the two-channel safety check is positive, the control system can switch off the motor. The further maintenance of the clamping force exerted by the sleeve on the workpiece can then be safely left to the self-locking mechanical properties of the feed device. The workpiece is held passively and securely, and the electric motor is completely switched off, i.e. the torque control is stopped and the power supply is interrupted. This has the advantageous effect that the clamping force exerted by the sleeve on a workpiece is reliably maintained even in the event of an accidental or deliberate interruption of the motor's power supply. According to the invention, the clamping force is maintained solely by the self-locking feed device, without the need for further motor support.This saves electrical energy and reduces wear on mechanical engine components. Furthermore, the engine, in particular, does not require additional brakes.

[0019] According to the invention, the feed mechanism is designed to be self-locking. This prevents the spindle sleeve from shifting independently in the sense of yielding, i.e., an undesired retreat from the contact point on the workpiece, when the feed mechanism is at rest—i.e., in the event of a failure or shutdown of the pressure force exerted by the motor. For this purpose, the feed mechanism can have an internal resistance, particularly caused by friction. The feed mechanism is thus particularly inhibited against axial shortening and maintains approximately the same axial length even without the action of the motor.

[0020] The motor is preferably shut down when the second actual value of the quill's clamping force matches the target value and / or the first actual value of the quill's clamping force as closely as possible. In practice, tolerance-related deviations considered normal for the respective application may also be permitted. Thus, deviations of the second actual value within a tolerance range of, for example, up to +15% from the target value or the first actual value may still be considered consistent within the meaning of the invention.

[0021] According to a further embodiment of the invention, the clamping device has a base body in which the sleeve is axially guided and is displaceable, i.e., extendable and retractable, for its initial extension. Preferably, a clamping force is exerted on a workpiece placed in front of the sleeve by advancing the sleeve. For this purpose, the sleeve can be moved relative to the base body, in particular extended from the base body, via a feed device that preferably also acts axially on the sleeve. Furthermore, the feed device itself can preferably be variable in length for this purpose.

[0022] This engages axially at one end of the sleeve and is axially supported on the base body via a spring element, preferably an axial compression spring. To generate a clamping force, the feed device is driven axially to the sleeve by the electric motor. The sleeve is preferably extended and placed against the workpiece. In addition, the spring element is at least partially elastically compressed during this process. The resulting shortening is referred to as the compression travel. The elastic rebound force generated in this way is transferred via the feed device to the sleeve and as a clamping force to a workpiece.

[0023] The axially movable guide of the quill in a base body offers further advantages. For example, the base body allows at least one mounting of the quill on a supporting structure. Depending on the design, the entire clamping device can also be advantageously fixed to a supporting structure via the base body. Supporting structures can include, for example, the machine bed of a lathe, milling machine, or machine tool, a tailstock that can be longitudinally displaced on a machine bed, and a steady rest.

[0024] In a further preferred embodiment, the base body can be designed essentially as a hollow cylinder, within which the sleeve is guided for axial displacement. This design enables the application of particularly high clamping forces without the risk of bending the sleeve.

[0025] According to a further advantageous embodiment of the invention, the feed device comprises a drive shaft and a threaded nut. Both engage with each other via a self-locking thread. The position of the threaded nut relative to the drive shaft and thus the distance between a shaft end or shaft shoulder and the threaded nut can be changed by rotating one of the two bodies. Thus, the axial effective length of a feed device consisting of a drive shaft and a threaded nut can be changed by rotating one of the bodies.

[0026] The thread between the drive shaft and the threaded nut is designed to be self-locking, preventing any rotation of the bodies relative to each other due to axial compressive forces. This type of feed device is advantageously easy to manufacture, features reliable self-locking, and has a long service life.

[0027] In a further advantageous embodiment of the invention, the drive shaft is connected to the motor and the threaded nut is held fixedly against rotation on the sleeve. In this embodiment, the rotation can preferably be brought about by the electric motor acting directly on the axis of the drive shaft. A change in the length of the feed device is brought about by a rotation of the drive shaft, which is stationary except for the compression travel of the spring element, via the motor and the movement of the threaded nut, which is fixedly connected to the sleeve in a rotationally fixed manner. When the threaded nut and sleeve return, particularly inside a preferably hollow body-shaped base body, the drive shaft and sleeve overlap. This advantageously prevents the drive shaft from projecting when the sleeve is retracted.Such an embodiment of the clamping device according to the invention is particularly compact since the feed device has an approximately constant axial length regardless of its respective logical length.

[0028] In a further advantageous embodiment of the invention, the feed device is supported on the base body via a rotationally fixed sleeve and the spring element, and a signal transmitter of the electrical displacement measuring device, in particular the signal head of a displacement measuring device based on an inductive measuring principle, is held on the sleeve. In this embodiment, an additional sleeve is arranged between the feed device and the spring element. When the spring element is compressed, an axial displacement of the sleeve also occurs. The sleeve is designed to be rotationally fixed, so that almost no additional rotational movement occurs during its axial displacement. This facilitates the scanning of the compression travel by the electrical displacement measuring device.A sensor of the electrical travel measuring device is attached to the sleeve and moves approximately in a straight line along the compression travel during elastic compression of the spring element, without any additional rotational movement. Thus, the compression travel can be determined along just one component of motion.

[0029] In a particularly advantageous further embodiment of the invention, the self-locking threads between the drive shaft and the threaded nut are designed as trapezoidal threads. If these threads have a pitch suitable for self-locking, a useful internal jamming can occur between the threads.

[0030] According to a further embodiment of the invention, at least one axial compression spring serves as a spring element. When the feed device is driven in such a way that a small compression travel and a low elastic spring force occur on the spring element, a low clamping force is exerted on a workpiece by the sleeve. When the feed device is driven in such a way that a high compression travel and a high elastic spring force occur on the spring element, a high clamping force is exerted on a workpiece by the sleeve. When an axial compression spring is used as the spring element, an approximately linear relationship advantageously occurs between the compression travel and the resulting spring force. When an axial compression spring is advantageously used, the clamping force exerted by the clamping device on a workpiece is approximately proportional to the compression travel brought about by the feed device and detected by the electrical measuring device.

[0031] Machine tools are particularly suitable for being equipped with at least one clamping device according to the invention. For example, the tailstock, particularly on a lathe, can be equipped with a clamping device designed according to the invention as an additional component without requiring any further structural modifications. Furthermore, a steady rest, for example, can be equipped with three clamping devices according to the invention, which are arranged radially at intervals of 120° around a receiving opening.

[0032] This makes it possible, according to a particularly advantageous further embodiment of the invention, for the control system to release the processing machine by switching off the electric drive motor for the self-locking feed device. Since the successful establishment of a clamping state of the workpiece via the sleeve of the clamping device is double-checked at this moment, safe machining of the workpiece is now possible using the operating resources of the respective machine tool. These resources can thus be activated.

[0033] For example, in the case of a lathe as an example processing machine, the rotation of the main spindle, slide drives and program-controlled machining of a workpiece held in the main spindle can be enabled.

[0034] In a particularly advantageous further embodiment of the invention, the clamping device according to the invention offers the particular further advantage that a clamping force exerted on a workpiece can not only be maintained independently without auxiliary energy, but can also be permanently monitored in this state by means of the displacement measuring device.

[0035] For this purpose, the control system can record the compression travel of the spring element at the moment the feed drive motor is switched off, i.e., the value of the compression travel when a desired workpiece clamping position is reached, and save it as a target value. Continuous recording of the actual value of the compression travel while the feed motor is switched off allows for a constant comparison with the target value. If the first deviation is detected, the control system generates an error message.

[0036] It is advantageous to configure the magnitude of this deviation so that the error message is generated before a safety-critical change in the workpiece clamping occurs. This can, for example, trigger a manual check of the workpiece clamping status by an operator without requiring an emergency stop of the entire machining device. In practice, deviations within a tolerance range of -5% to +10% around the actual value of the compression travel reached at the moment the motor is shut down can trigger such an error message.

[0037] This makes it possible to reliably observe and monitor the clamping status of a workpiece both during operation of the processing machine and after a temporary interruption of work.

[0038] The error message informs a machine operator during ongoing machining that an undesirable change in the clamping state of the workpiece has occurred. This can be caused by a decrease in the desired clamping force, e.g., due to loosening of the tool clamping devices, or by an increase in the clamping force, e.g., due to undesirable thermal expansion during workpiece machining.

[0039] In practice, however, it is also possible for an operator to interrupt workpiece machining, leave the location of the workpiece machining, and upon returning there, obtain reliable information about the current status of the workpiece clamping. If the operator finds the processing machine without an error message and in a state that is still released by the control system of the clamping device, he can assume that the workpiece clamping is being properly maintained by the clamping device according to the invention. Due to the inventive shutdown of the drive motor for the feed device of the clamping device, a processing machine equipped with this device is also in a safe operating state. Upon returning to his workplace, an operator can resume workpiece machining by switching the operating resources back on, e.g.a lathe immediately, provided that the clamping device according to the invention reports that the workpiece clamping remains unchanged. Otherwise, the operator will investigate an existing error message and search for and eliminate the cause, in particular any possible loosening of the workpiece. The invention thus makes it possible to leave a clamping device according to the invention acting on a workpiece, as well as a processing machine equipped therewith that has been temporarily stopped manually, unattended for an extended period of time, even when the workpiece is clamped, e.g., overnight or over a weekend.

[0040] In a further advantageous embodiment of the invention, the controller issues an active stop command for the processing machine if the actual value of the compression travel differs from the target value by more than a second deviation, which is greater than the first deviation.

[0041] If the processing machine is in operation, this leads to an immediate, forced shutdown and an interruption of the ongoing machining process, even at the risk of an unfinished workpiece being irreversibly damaged. In such a case, the risk of a workpiece clamping failure is considered so safety-relevant that continued operation can not be permitted. The risk of damage to the processing machine and possible injury to operators is avoided. In practice, deviations that lie, for example, within an extended tolerance range of + 15% around the actual value of the compression travel reached at the moment the motor is switched off can trigger an emergency shutdown.

[0042] Machining machines equipped with a tailstock are particularly suitable for being equipped with at least one clamping device according to the invention. The tailstock itself can be used to attach the clamping device to the machine.

[0043] The invention and further advantageous embodiments thereof are explained in more detail below with reference to the briefly described figures. Fig. 1 an exemplary advantageous embodiment of a clamping device according to the invention in a side schematic sectional view, Fig. 2 a detail of the sectional view of Fig. 1 in the area of the self-locking feed device, wherein a spring element preferably designed as an axial compression spring is not yet axially elastically compressed and thus no clamping force is yet generated by the clamping device according to the invention, and Fig. 3 the exemplary detail of Fig. 1 in a state in which the spring element is axially elastically compressed, so that a clamping force is generated by the clamping device according to the invention.

[0044] The present figures are schematic diagrams and serve to clearly explain the invention.

[0045] Fig. 1 shows an exemplary embodiment of a clamping device S according to the invention for a workpiece W. This is advantageously equipped with a base body 1, which is particularly advantageously designed as a hollow body. In the example of Fig. 1, the base body 1 comprises a sleeve tube 11 with a receiving channel 111 for a sleeve 2 and a support tube 12, which serves in particular to accommodate a self-locking feed device 3. Sleeve tube 11 and support tube 12 are advantageously designed as hollow bodies, e.g. as hollow cylinders, and merge into one another lying on a common central axis 9. In the example of Fig. 1, the support tube 12 advantageously has a flange 122 at a first tube end 121, via which the sleeve tube 11 is held axially at a second tube end 113.

[0046] The quill 2 is used to exert a clamping force on a workpiece W. This is Fig. 1 is only schematically indicated on the left and is held by the clamping device according to the invention, for example, in a workpiece holder (not shown) of a processing machine. This fixation makes it possible for the tools of a processing machine to perform various types of machining on the workpiece safely and precisely, e.g., drilling, milling, surface machining, and much more.

[0047] The sleeve 2 is particularly advantageously guided axially displaceably in the receiving channel 111 of the sleeve tube 11 of the base body 1 along a central axis 9 and has a preferably tubular sleeve body 21 with a central channel 22. In the example of the Fig. 1, an additional conical receptacle 25 is advantageously provided at its first end 23, in which a centering point 8 is held. The sleeve 2 is initially applied to the workpiece W via this receptacle without exerting any clamping force.

[0048] The clamping device S according to the invention further comprises a self-locking feed device 3 for the sleeve 2. This enables the sleeve 2 to be moved in the base body 1 along the central axis 9. This allows the sleeve 2 to be moved out, for example, at the first tube end 112 of the sleeve tube 11 and to be applied to the workpiece W. To release the workpiece, the sleeve 2 can be retracted back into the sleeve tube 11 in the opposite direction via the feed device 3. According to the invention, the self-locking feed device 3 is supported by a spring element, which in the example of the Fig. 1 is designed as an axial compression spring 34 supported in the support tube 12 of the base body 1.

[0049] According to the invention, the feed device 3 is inhibited against axial self-adjustment. The respective state of the feed device, in particular a current extended position, cannot be changed even by forces acting along the central axis 9 and occurring when a clamping force is exerted by the sleeve 2 on the workpiece W.

[0050] In a particularly advantageous and in the example of Fig. In the embodiment already shown in Figure 1, the self-locking feed device 3 has a drive shaft 31 located in the central axis 9 of the sleeve 2 and a threaded nut 32. These are advantageously mutually engaged via a self-locking internal thread 321 on the threaded nut 32 and a self-locking external thread 312 on the drive shaft 31. Self-locking represents a state in which the feed device 3 is prevented from self-adjusting due to internal friction between the threads 321 and 312, provided an axial compressive force acts on the feed device.

[0051] A change in the resulting axial length of the feed device 3 and thus an axial extension or retraction of the sleeve 2 from or into the base body 1 is achieved in the embodiment of the Fig. 1 caused by a rotation of the stationary drive shaft 31 and a resulting forward or reverse movement of the threaded nut 32 on the stationary drive shaft 31. The intermeshing threads 321 and 312 are in Fig. 1 symbolically represented by a thick line.

[0052] In the example of Fig. 1, the drive shaft 31 advantageously has a first sleeve section 311 with the self-locking thread 312 for the threaded nut 32, a second bearing and support section 313 for guidance in the base body 1 and a third drive section 317 in the support tube 12. The threaded nut 32 is advantageously held in a rotationally fixed manner at the second sleeve end 24. The sleeve 2 can thus be moved out of the base body 1 by rotating the drive shaft 31 along the central axis 9 in order to exert a clamping force on the workpiece W, in the example of Fig. 1 to the left, and to release the workpiece W can be retracted into the base body 1, in the example of Fig. 1 to the right. To prevent undesired rotational movement of the sleeve during such retraction and extension movements, means for preventing rotation may advantageously be provided, e.g., guides between the hollow sleeve tube 11 of the base body 1 and the sleeve body 21.

[0053] In the example of Fig. 1, the advantageous embodiment of a clamping device S according to the invention shown comprises a sleeve 33 and a sleeve bearing 35. In particular, the drive shaft 31 and the axial compression spring 34 of the feed device 3 are supported on the base body 1. An advantageous structure of sleeve 33 and sleeve bearing 35 is shown in the exemplary detailed view of Fig. 2 is explained in more detail.

[0054] According to the invention, the clamping device S has an electric motor 6 for driving the self-locking feed device 3. In the example of the Fig. 1 advantageously connected via a gear 5 to the drive section 317 of the drive shaft 31. The gear 5 is held at the second end 128 of the support tube 12. Gear 5 and motor 6 are in the example of Fig. 1 only shown schematically.

[0055] According to the invention, the tensioning device S further comprises an electrical measuring device for continuously detecting the actual value of the compression travel of the spring element, in particular in the form of a preferably absolute travel measuring device.

[0056] This makes it possible, in particular, to record the deflection of the axial compression spring 34 when a predetermined clamping force is applied to the workpiece and the subsequent interruption of the further rotation of the motor 6 acting on the spindle sleeve 2. For this purpose, the Fig. 1 an electrical measuring device 42 for automatically recording the actual value of the spring deflection 344 is provided, the electrical measuring signal of which is transmitted to the control system via an electrical connection 72. Such a spring deflection is in the example of Fig. 3 shown.

[0057] In the example of Fig. 1, an additional mechanical measuring device 41 for the compression travel is advantageously provided. This additionally allows the machine operator a simple, visual, and quick reading of the current value of the compression travel.

[0058] Advantageous structures of an electrical measuring device 42 and a possible additional mechanical measuring device 41 are shown by way of example in Fig. 2 explained.

[0059] According to the invention, the clamping device S also has a control 7 which, as already explained in detail above, initially stops the electric motor 6 when a predetermined clamping force is reached and then switches it off after successful verification.

[0060] In the example of Fig. 1, the controller 7 is connected to the motor 6 via a first electrical connection 71 and to the measuring device 42 via a second electrical connection 72. An application-dependent value of the clamping force can be specified to the controller 7, e.g. in the form of a force, torque or current value for the motor 6. To apply the clamping force to the workpiece W, the controller 7 activates the motor 6. The drive of the feed device 3 is carried out via the gear 5, ie in the Fig. 1 to 3, advantageously by a rotation of the drive shaft 31 about the central axis 9. This results in an axial displacement of the non-rotatable threaded nut 32 on the drive shaft 31 and the entrained sleeve 2 along the central axis 9.

[0061] When the spindle sleeve 2 is placed on the workpiece W, ie in the example of Fig. 1 the workpiece W is reached by the centering point 8, no further displacement of the sleeve 2 in the direction of the workpiece W is possible. To build up a predetermined clamping force, the feed device 3 is further driven by the motor 6. This results in the drive shaft 31 deflecting back in the direction of the gear 5. In the example of the Fig. 1, the right end of the third section 317 of the drive shaft 31 deflects to the right into the interior of the gearbox 5. This further results in elastic compression of the spring element 34 and the buildup of a rebound force. This is then transferred as a clamping force to the workpiece W via the feed device 3 and the sleeve 2.

[0062] According to the invention, the motor 6 is first stopped by the controller 7 after reaching a predetermined clamping force and is switched off after a successful verification of the value of the clamping force.

[0063] According to the invention, the achievement of a predetermined target value of the clamping force is detected directly in the controller 7 by evaluating the current consumed by the motor 6, and in a first step, the further rotation of the electric motor 6 is stopped while torque control is still active. In addition, the resulting compression travel 344 of the axial compression spring 34 is recorded via the electrical measuring device 42 and transmitted to the controller 7 via an electrical signal connection 72 for further processing. According to the invention, the controller derives a second actual value of the clamping force from this, which, by comparing it with the target value and / or the first actual value of the clamping force, enables verification of the existing clamping force. If a match is found, the motor 6 is switched off by the controller 7 in a second step.The clamping force is then maintained without loss by the mechanical self-locking of the feed device 3.

[0064] In a particularly advantageous further embodiment of the invention, the controller 7 stores the compression travel 344 reached when the motor 6 is switched off as the target value, continuously records the actual value of the compression travel, and outputs an error message if the actual value and the target value differ by more than a first deviation. The invention thus offers the further advantage of monitoring the maintenance of the clamping force built up by means of mechanical self-locking by monitoring the compression travel of the spring element. For this purpose, the drive motor for the feed device can be completely switched off, thus saving electrical energy and preventing wear. Only the continued power supply to the electrical measuring device and the controller needs to be maintained.

[0065] In a further advantageous embodiment of the invention, the controller 7 advantageously provides an electrical signal when the actual value of the compression travel of the spring element diverges from the stored target value by more than a second deviation, which is greater than the first deviation. This signal can be used, for example, as a stop command to stop a machine tool which is equipped with at least one clamping device according to the invention and to interrupt further machining of a workpiece. As a second tolerance value, a deviation can be specified, for example, above which reliable clamping of a workpiece is no longer guaranteed. A stop command issued when the actual value deviates by more than the second tolerance value can be evaluated by devices in which a clamping device S according to the invention is used, e.g. in machine tools, to the effect that the device is put into operation, e.g.B. a lathe, is prevented, or a device in operation is stopped. Such a stop command can be used, for example, to quickly shut down a machine tool equipped with a clamping device according to the invention. Such a case can occur if mechanical mountings have become loose during machining, e.g., a tailstock with the clamping device according to the invention unexpectedly deflects, and there is a risk of the workpiece falling out.

[0066] In Fig. 1 further shows, by way of example, a tailstock R which is equipped with an advantageous clamping device S designed according to the invention. For this purpose, the clamping device S is held on a base R1 which, for example, is mounted on a Fig. 1, a machine tool bed (not shown in detail). An example of a lathe or the steady rest of a machine tool equipped with a clamping device S according to the invention is also not shown in the figures for reasons of clarity.

[0067] In Fig. 2 is a detailed view of the particularly advantageous clamping device S according to the embodiment of Fig. 1 in the area of the second bearing and support section 313 of the drive shaft 31 of the feed device 3 with sleeve 33, sleeve bearing 35, axial compression spring 34, and position measuring device 41. The clamping device S is shown in a relaxed state, so that the axial compression spring 34 is not elastically compressed and no clamping force is exerted on a workpiece.

[0068] Drive shaft 31 and sleeve 33 are advantageously arranged centrally to one another, so that their central axes lie on the central axis 9 of the clamping device S. The drive shaft 31 has a circumferential support ring 314 between the first sleeve section 311 with the self-locking external thread 312 and the second bearing and support section 313, against which the sleeve 33 rests with its first end face 332 via the support bearing 351. The sleeve 33 is advantageously designed to be non-rotatable. The support bearing 351 is thus designed as a combined radial-axial bearing. The sleeve 33 is further mounted in the second bearing and support section 313 on the drive shaft 31 via a radial bearing 352, which rests on the radial shoulder 335 of the sleeve 33. For axial fixation of the sleeve 33 on the drive shaft 31, the sleeve has a threaded shoulder 315 with a shaft nut 316.The sleeve 33 further has a circumferential sleeve ring 331 on its outer surface in the region of its first end face 332. This ring projects into the radial groove 124 on the inner surface 123 of the support tube 12, so that the sleeve 33 is supported on the flange 122 at the first tube end 121 of the support tube 12. The sleeve ring 331 thus functions as a stop for the sleeve 33 in the direction of the sleeve sleeve.

[0069] The electrical measuring device 42 advantageously has a signal head 422 and a position sensor 424. The signal head 422 is advantageously held on the sleeve 33 via a retaining pin 421. The sleeve 33 has the receiving bore 334 for this purpose. The retaining pin 421 extends through the second axial slot 127 in the support tube 12. When a clamping force and elastic compression of the axial compression spring 34 are applied, the sleeve 33, retaining pin 421, and signal head 422 are carried along by the deflection of the axial compression spring 34 and displaced by the same distance along the central axis 9. The position sensor 424 is held firmly on the support tube 12 of the base body 1 via the retaining bracket 423. When the signal head 422 is deflected, it is guided along the position sensor 424 to detect the deflection. The position sensor 424 can be read by the controller via the second electrical connection 72.The position sensor 424 advantageously has an inductive displacement sensor 424 segmented along the suspension travel. The segments are lined up along the central axis 9 and can be individually identified by the control system. When the signal head 422 passes the sensor, an induction occurs in the individual segments. Based on the degree of induction in the segments, the control system can then determine in which segment the signal head 422 is currently located. The suspension travel can then be derived from the position of the signal head 422 on the displacement sensor 424.

[0070] In the example of Fig. 2 and Fig. 3, an additional mechanical measuring device 41 for the compression travel is advantageously provided. This device comprises a dial gauge 411, which is held on the support tube 12 of the base body 1. The dial gauge 411 has a shaft 413 arranged radially to the sleeve 33 and with a stylus 414 displaceably arranged therein. A radial deflection of the stylus 414 causes a change in the display 412 of the dial gauge 411. The display 412 can, for example, represent an absolute compression travel or a relative scale of the compression travel or the applicable clamping force. Furthermore, an inclined surface 416 is advantageously provided for deflecting the stylus 414 when the axial compression spring 34 compresses. The inclined surface 416 is arranged on the wedge 415 and coupled to the feed device 3 via the sleeve 33.Upon axial displacement of the feed device 3, the inclined surface 416 converts the axial displacement into a radial deflection of the stylus 414 and a change in the reading 412 of the dial gauge 411. The wedge 415 with the inclined surface 416 is advantageously held on the sleeve 33. For this purpose, the sleeve 33 has the receiving slot 333 into which the wedge 415 is inserted. The wedge 415 and the shaft 413 of the dial gauge 411 protrude into a first axial slot 126 in the support tube 12.

[0071] In the exemplary embodiment, the axial compression spring 34 is advantageously designed as a package comprising a first, second, and third pair of disc springs 341, 342, 343, which are arranged centrally to the central axis 9 on the drive shaft 31. The axial compression spring 34 is supported on the base body 1 on the second end face 336 of the sleeve 33 and on a support shoulder 125 on the inner surface 123 of the support tube 12. The support shoulder 125 is arranged at the transition between the second bearing and support section 313 and the third drive section 317 of the drive shaft 31.

[0072] Fig. 3 shows a further detailed view of the exemplary embodiment of a clamping device S according to the invention in the region of the second bearing and support section 313 of the drive shaft 31 of the feed device 3. The clamping device S is shown in a clamped state, so that the axial compression spring 34 is elastically compressed and a clamping force is exerted on a workpiece.

[0073] The sleeve with the threaded nut of the feed device 3 rests against a workpiece outside the detailed view shown. The drive shaft 31 of the feed device 3 is further driven by the motor, so that the drive shaft 31 is displaced along the central axis 9 in the direction of the third bearing and support section 317. This displacement is transmitted via the support ring 314 of the drive shaft 31 and the support bearing 351 of the sleeve bearing 35 to the sleeve 33, so that the latter is also displaced. The circumferential sleeve ring 331 in the area of the first end face 332 of the sleeve 33 no longer rests against the flange 122 at the first tube end 121 in the radial groove 124 of the support tube 12. The displacement is also transmitted to the axial compression spring 34, so that it is elastically compressed. The displacement path thus corresponds to the compression travel 344 of the axial compression spring 34.This is shown in the present figure with dashed lines in the area of the signal head 422.

[0074] Due to the displacement of the sleeve 33, the retaining pin 421 with the signal head 422 of the electrical measuring device 42 is displaced in the second axial slot 127 of the support tube 12 along the central axis 9 by the spring deflection 344. The signal head 422 is guided along the position sensor 424, so that the spring deflection 344 can be detected or monitored by the control system using the position sensor 424.

[0075] Furthermore, due to displacement of the sleeve 33, the wedge 415 with inclined surface 416 of the mechanical measuring device 41 is displaced in the first axial slot 126 of the support tube 12 along the central axis 9 by the spring deflection 344.

[0076] The stylus 414 of the dial gauge 411 is deflected so that its display 412 shows, for example, a value for the spring deflection 344. If necessary, the display can also be calibrated to show the value of a clamping force.

[0077] Due to the self-locking of the threads of the drive shaft 31 and the threaded nut of the feed device 3, the clamping device S remains in the clamped position shown without any further energy supply. To reduce the clamping force, in the present embodiment, the drive shaft 31 is driven in the opposite direction until the clamping device S is again in the position shown in Fig. 1 and Fig. 2. When the drive shaft 31 is further driven in the opposite direction, the sleeve 2 is retracted into the base body 1. List of reference symbols W workpiece S clamping device 1 base body, in particular a hollow body 11 quill tube 111 Recording channel 112, 113 first, second pipe end 12 support tube 121 first pipe end 122 flange 123 interior surface 124 Radial groove 125 support heel 126, 127 first, second axial slot 128 second pipe end 2 quills 21 quill body 22 Central channel 23, 24 first, second quill end 25 cone holder 3 self-locking feed device 31 Drive shaft 311 first section, especially quill section 312 external thread, self-locking 313 second section, especially storage and support section 314 support ring 315 threaded shoulder 316 shaft nut 317 third section, especially drive section 32 threaded nut 321 internal thread, self-locking 33 sleeve 331 circumferential sleeve ring 332 first frontal surface 333 Recording slot 334 mounting hole 335 radial heel 336 second frontal surface 34 Spring element, in particular an axial compression spring 344 travel 341, 342, 343 Disc springs, first, second, third pair 35 Sleeve storage 351 Support bearings, especially radial-axial bearings 352 radial bearings 42 electrical measuring device for the compression travel, travel measuring device 421 retaining pin 422 Signal devices, in particular signal heads 423 Holding bracket 424 displacement sensor, in particular position sensor 41 mechanical measuring device for spring travel 411 Dial gauge, especially precision indicator 412 ad 413 shaft 414 Stylus with stylus tip 415 Wedge 416 Inclined surface 5 gearboxes 6 electric motor 7 Control 71 electrical connection to motor 6, measurement of actual motor current 72 electrical connection to electrical sensor 42 8 Centering point 9 Central axis R tailstock with clamping device R1 socket

Claims

[1] Clamping device (S) for a workpiece (W), with a) a sleeve (2) for exerting a clamping force on the workpiece (W), which is displaceably guided and supported by a spring element (34), b) a self-locking feed device (3) for the spindle sleeve (2), c) an electric motor (6) for driving the feed device (3), d) an electrical measuring device (42) for continuously recording the actual value of the compression travel (344) of the spring element (34), and e) a control (7) which e1) determines a first actual value of the clamping force of the sleeve (2) from the current of the motor (6) and stops the motor (6) when a target value of the clamping force is reached, and e2) determines a second actual value of the clamping force of the sleeve (2) from the actual value of the compression travel (344) achieved in this process and switches off the motor (6) if the value matches the target value and / or the first actual value of the clamping force of the sleeve (2). [2] Clamping device (S) according to claim 1, with a base body (1), in particular a hollow body, in which the sleeve (2) is guided axially displaceably. [3] Clamping device (S) according to claim 1 or 2, wherein the feed device (3) comprises a drive shaft (31) in the central axis (9) of the sleeve (2) and a threaded nut (32), wherein the drive shaft (31) and the threaded nut (32) are engaged via self-locking threads (312, 321). [4] Clamping device (S) according to claim 3, wherein the drive shaft (31) is connected to the motor (6) and the threaded nut (32) is held non-rotatably on the sleeve (2). [5] Clamping device (S) according to claim 3 or 4, with trapezoidal threads as self-locking threads (312, 321). [6] Clamping device (S) according to one of the preceding claims, with at least one axial compression spring as spring element (34). [7] Processing machine, in particular a lathe, with at least one clamping device (S) according to one of the preceding claims, wherein the control (7) releases the processing machine when the electric motor (6) for driving the feed device (3) is switched off. [8] Processing machine according to claim 7, wherein the control (7) continuously compares the actual value of the compression travel (344) with the actual value of the compression travel (344) reached at the moment of switching off the motor (6) and generates an error message when a first deviation occurs. [9] Processing machine according to claim 8, wherein the control (7) continuously compares the actual value of the compression travel (344) with the actual value reached and, upon occurrence of a second deviation which is greater than the first deviation, stops the processing machine. [10] Processing machine according to claim 7, 8 or 9, with a tailstock (R) for holding the clamping device (S).

Images