METHOD FOR DETERMINING THE CLAMPING FORCE

Abstract

A method for determining the clamping force of a force-actuated, rotationally drivable clamping device of a machine tool. An object is clamped with an actuation force F. Model-based calculation of the clamping force F.sub.calc to be expected is performed based on the actuation force F and the operating parameters of the machine tool in a control unit of the machine tool. Performing clocked measurement of the clamping force F.sub.real acting upon the object in the clamping device within the time span of a measurement interval I. Wireless transming the measured value F.sub.real to the control unit of the machine tool, and comparison of the measured value F.sub.real with the calculated clamping force F.sub.calc. The measurement interval is adapted if the comparison reveals a deviation which reaches a threshold value.

Claims

1. A method for determining a clamping force of a force-actuated, rotationally drivable clamping device of a machine tool, the method comprising: clamping an object with an actuation force; model-based calculating the clamping force to be expected based on the actuation force and operating parameters of the machine tool in a control unit of the machine tool; clocked measuring the clamping force acting upon the object in the clamping device within a time span of a measurement interval; wirelessly transmitting the measured value to the control unit of the machine tool; comparing the measured value with a calculated clamping force; and adapting the measurement interval if the comparison reveals a deviation which reaches a threshold value.

2. The method according to claim 1, wherein the measurement interval is lengthened if the threshold value is not reached, and/or the measurement interface is shortened if the threshold value is exceeded.

3. The method according to claim 1, wherein a size of the change of the measurement interval is proportionate to a distance of the measured value from the threshold value.

4. The method according to claim 1, wherein the threshold value for a deviation where the measured value is less than the calculated clamping force is less than/equal to the threshold value for a deviation where the measured value is greater than calculated clamping force.

5. The method according to claim 1, wherein the deformation sensitivity of the object to be clamped is taken into account in the definition of the threshold value for a deviation where measured value is greater than calculated clamping force.

6. The method according to claim 1, wherein the threshold value lies at a deviation of less than/equal to 2%, in particular a deviation of 1.5% and preferably a deviation of 0.5%.

7. The method according to claim 1, wherein a sensor is present for detecting the actuation force, whose measured data is transmitted in a wirebound manner to the control unit.

8. The method according to claim 1, wherein the clamping device is formed by a chuck, and the sensor for measuring the clamping force is assigned to one of the clamping jaws.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0016] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein the sole FIGURE shows a schematic representation of the procedures for determining the clamping force with the necessary safety level.

DETAILED DESCRIPTION

[0017] The FIGURE shows a machine tool 1, to which a chuck is assigned as a clamping device, in the illustrated exemplary embodiment namely a three-jaw chuck having three clamping jaws 3, which are arranged evenly over the circumference and adjustable radially with respect to the chuck axis, whose adjustment in the illustrated exemplary embodiment takes place via an electrical drive 4, via which an actuation force F is provided. This actuation force is converted by mechanical transmission in the force transmission chain to a clamping force, which is applied by clamping jaws 3 onto workpiece 5, which in the illustrated exemplary embodiment is a sleeve held under external tension. The clamping force stemming from actuation force F is further influenced by the centrifugal force during the rotational actuation of the clamping device, by the design and quality of the functional surfaces of clamping jaws 3 as well as by the wear and lubrication condition thereof. From clamping force F. a model-based calculation of clamping force F.sub.calc to be expected based on the actuation force F and the operating parameters of machine tool 1 then takes place in a control unit 6 of machine tool 1. Moreover, a clocked measurement of clamping force F.sub.real acting upon the object in the clamping device takes place within the time span of a measurement interval, as does the wireless transmission of measured value F.sub.real to control unit 6 of machine tool 1, in which a comparison of measured value F.sub.real with calculated clamping force F.sub.calc occurs.

[0018] An adaptation of the measurement interval takes place if the comparison results in a deviation which reaches a threshold value, the measurement interval being lengthened if the threshold value is not reached, and the measurement interval is shortened if the threshold value is exceeded, so that the radio signal for the wireless transmission of measured value F.sub.real must be transmitted less often upon a good correspondence, whereby the lifespan of the rechargeable battery integrated into clamping jaw 3 for providing the necessary electrical energy is significantly prolonged. During the transmission pauses between the transmission of measured values F.sub.real, the clamping force is calculated according to a model 7, based on actuation force F and additional measured variables, such as rotational speed 8 of the machine spindle, mass 10 of the jaws, accumulated number of cycles 11 and other definitive parameters 9, such as wear and lubrication condition.

[0019] It should be noted that the threshold value for a deviation where F.sub.real is less than F.sub.calc does not have to be identical to the threshold value for a deviation where F.sub.real is greater than F.sub.calc and, in particular, may be selected to be lower. The deformation sensitivity of workpiece 5 clamped or to be clamped may also be taken into account in defining the threshold value. Finally, it should be noted that actuation force F may not only be derived from the generated actuation force but may also be controlled with the aid of a sensor, whose measured data is transmitted in a wirebound manner to control unit 6. Two different sensors are thus available, which transmit their measured data to control unit 6 on different paths for the redundant control of the clamping state.

[0020] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.