Abstract
A tuneable clamping device (1) is envisaged which comprises a stationary part (2) which is fixed to a table of a machine tool (10) or being part of a table of a machine tool (10), and a moving table (3) on which the actual flexible workpiece (11) is clamped. The tuneable clamping device (1) achieves tuning between the standing part (2) and the machine tool table (10) in order to damp a dominant mode of the workpiece (11) by playing the role of a device that provides serial dynamic coupling or coupling through the process to dissipate indirectly kinetic energy related to the workpiece (11).
Claims
1. A tuneable clamping device (1) for suppressing vibrations of a clamped workpiece (11), comprising a stationary part (2) and a moving part (3) connected to the stationary part (2) and configured to move along a relative direction of motion (27), characterized in that the stationary part (2) and the moving table (3) are connected by means of a tuneable connection (19) having adjustable stiffness element (7) and adjustable damping element (20), and wherein accelerometers (5, 4) are connected to the stationary part (2) and the moving table (3), respectively.
2. The tuneable clamping device (1) of claim 1 characterized in that relative direction of motion (27) between the stationary part (2) and the moving table (3) is unidirectional and provided by a guiding element (21).
3. The tuneable clamping device (1) of claim 2, characterized in that the guiding element (21) is a roller-track.
4. The tuneable clamping device (1) of claim 1 characterized in that the tuneable connection (19) is further combined with a constant stiffness elastic element (6).
5. The tuneable clamping device (1) of claim 4 characterized in that the constant stiffness elastic element (6) is a plate made from steel.
6. The tuneable clamping device (1) of claim 2 characterized in that the relative direction of motion (27) between the stationary part (2) and the moving table (3) is determined by constant stiffness elastic elements (6) and guiding elements (21).
7. The tuneable clamping device (1) of claim 1 characterized in that the tuneable connection (19) comprises an adjustable stiffness element (7) formed as an “H” shaped spring with walls (7.1, 7.2) of different thickness, and a rotating mechanism (24).
8. The tuneable clamping device (1) of claim 1 characterized in that the tuneable connection (19) comprises an adjustable stiffness element (7) formed as a cantilever beam (51) with a ball screw of adjustable coupling point (52), and a rotating mechanism (24).
9. The tuneable clamping device (1) of claim 1 characterized in that the adjustable damping element (20) comprises a pair of magnets (9) and an electrically conducting plate (15) fixed to the stationary (2) and the moving (3) parts of the tuneable clamping device (1), respectively, with a variable overlapping area between them by adjustable relative position in a direction (29b).
10. The tuneable clamping device (1) of claim 1 characterized in that the adjustable damping element (20) comprises a magnet (9) and an electrically conducting plate (15) fixed to the stationary (2) and the moving (3) parts of the tuneable clamping device (1), respectively, with a variable gap between them by an adjustable relative position in a direction (29b).
11. The tuneable clamping device (1) of claim 9, characterized in that the magnet (9) is an electromagnet having a steel casing (53) and a coil (30) adapted to flow variable electric current therethrough.
12. The tuneable clamping device (1) of claim 1 characterized in that the adjustable stiffness element (7) of the tuneable connection (19) is replaceable.
13. The tuneable clamping device (1) of claim 1 characterized in that the accelerometers (4, 5), the adjustable stiffness element (7) and the adjustable damping element (20) of the tuneable connection (19) are connected to a control unit (18).
14. The tuneable clamping device (1) of claim 13, characterized in that an output signal of the control unit (18) is configured to indicate required stiffness and damping parameters determined from regenerative chatter frequencies.
15. The tuneable clamping device (1) of claim 13, characterized in that an output signal of the control unit (18) is configured to indicate required stiffness and damping parameters determined from regenerative chatter frequencies and their modulations.
16. The tuneable clamping device (1) of claim 10, characterized in that the magnet (9) is an electromagnet having a steel casing (53) and a coil (30) adapted to flow variable electric current therethrough.
Description
DESCRIPTION OF THE DRAWINGS
Brief Description of the Drawings
[0037] FIG. 1 shows the sketch of the tuneable clamping device during machining in a machine tool.
[0038] FIG. 2 shows the sketch of the variable stiffness H-shaped spring with walls of different thickness.
[0039] FIG. 3 shows the sketch of the variable stiffness cantilever beam spring.
[0040] FIG. 4 shows three different solutions for the variable damping element.
[0041] FIG. 5 shows one possible embodiment of the tuneable clamping device.
[0042] FIG. 6 shows different vertical, horizontal, built-in and mountable arrangements of the tuneable clamping device in a machine tool.
[0043] FIG. 7 shows a possible solution for the tuning capabilities of the controller of the tuneable clamping device.
DESCRIPTION OF EMBODIMENTS
[0044] A possible embodiment for a tuneable clamping device 1 is proposed in FIG. 1. The device 1 that holds a workpiece 11 comprises a stationary part 2 containing a stationary base 2a and an outer casing 2b, a moving table 3 and a tuneable connection 19 between the stationary and the moving parts. The main stiffness of the moving table 3 is given by a constant stiffness elastic element 6 and the unidirectional free movement of the moving table 3 along a relative direction of motion 27 is provided by a guide element 21. Accelerometer 4 is attached to the table 3 while another accelerometer 5 is attached to the stationary base 2a. The stationary base 2a can act as an interface between a machine tool 10 and the tuneable clamping device 1, or it can be the part of the machine tool 10. This way a tool 12 of the machine tool 10, the workpiece 11 flexible in direction 26, the moving table 3, the tuneable connection 19, the stationary part 2 and the machine tool 10 form a closed, serial force flow, resulting in a serial dynamical system. The tuneable connection 19 comprises an adjustable stiffness element 7 e.g. a spring and a contactless, preferably eddy current based adjustable damping element 20 comprising magnets 9 and electrically conducting plates 15. Thus, the stiffness of the connection between the stationary base 2a and the moving table 3 is the combined result of the constant stiffness elastic element 6 with fixed stiffness and the adjustable stiffness element 7. An accelerometer 4 is attached to the moving table 3 and another accelerometer 5 is attached to the stationary part 2 which are able to pick up vibrations of the system in order to enable the calculation of the required stiffness and damping parameters with a controller unit 18. Controller unit 18 can adjust the adjustable stiffness element 7 and the adjustable damping element 20 by actuators.
[0045] A possible embodiment is proposed for the adjustable stiffness element 7 in FIG. 2, in which the anisotropic elastic element is an H-shaped spring. The moving table 3 and the stationary part 2 are connected each other via this adjustable stiffness flexible element 7 in parallel with the constant stiffness elastic elements 6. The adjustable stiffness flexible element 7 has walls 7.1, 7.2 with different thicknesses in two perpendicular directions as shown in FIG. 2, designed such that the stiffness of it is anisotropic. The motion of the moving table 3 is constrained to be unidirectional (along the relative direction of motion 27) by the guiding element 21, thus changing the orientation of the adjustable stiffness element 7 results a change in the stiffness of the connection in the relative direction of the motion 27. An electric motor or other actuator device is able to change the angular position of the adjustable stiffness element 7 directly or through a transmission.
[0046] In a further possible embodiment shown in FIG. 3, the adjustable stiffness element 7 is formed as a cantilever beam 51, a supporting point 52 of which can be changed by means of an electric motor 8 by changing the translational position 28b of the supporting point 52 by means of a drive system 24, which can be a ball screw 51 or any other transmission mechanism. The unidirectional motion of the moving table 3 relative to the stationary part 2 in the relative direction of motion 27 is still provided by guiding elements 21. Guiding elements 21 can be realized e.g. as a roller-track. In both embodiments according to FIG. 2 and FIG. 3 varying stiffness can be achieved by an actuator setting an angular position 28 of a shaft.
[0047] In the following, three possible embodiments are proposed for controlling the damping intensity of the adjustable damping element 20. Embodiment shown in FIG. 4 i) a magnet 9 consisting a permanent magnet 54 and a steel cover 53 and an electrically conducting plate 15 are arranged that way that the gap between the magnet 9 and the plate 15 can be adjusted by the relative position in the direction 29b. The magnet 9 and the plate 15 are fixed to parts of the device 1 in a displaceable manner relative to each other, so the electrically conducting plate 15 moves in the magnetic field induced by the permanent magnet 54 resulting in eddy currents and energy dissipation therein. The change in the gap in direction 29b influences the intensity of the eddy currents in the plate 15, and results a variable intensity damping depending on the relative position of the magnet 9 and the plate 15 in direction 29b.
[0048] In another embodiment shown in FIG. 4 ii) the adjustable damping element 20 comprises an electrically conducting plate 15 and a pair of magnets 9 consisting a permanent magnet 54 and a steel cover 53. The magnets 9 and the plate 15 are fixed to parts of the device 1 in a displaceable manner relative motion to each other, so the electrically conducting plate 15 moves in the magnetic field induced by the permanent magnets 54 resulting eddy currents and energy dissipation therein. The position of the plate 15 can be adjusted in the depicted direction 29b relative to the magnets 9, so the overlapping area between the magnets 9 and the plate 15 changes, resulting in the desired variable damping ratio.
[0049] In a third embodiment shown in FIG. 4 iii the magnet 9 is an electromagnet having a steel cover 53 and a coil 30 with a controlled current flowing in the coil 30, next to which the electrically conducting plate 15 is arranged. The damping ratio of the adjustable damping element 20 can be changed by adjusting the electric current flowing through the 30 coil of the electromagnet. The change in electric current changes the intensity of the magnetic field, which changes the intensity of the eddy currents, i.e. the damping ratio of the adjustable damping element 20.
[0050] A possible embodiment of the tuneable clamping device 1 is shown in FIG. 5. Said device 1 comprises a base 2a and an outer casing 2b, which together form a stationary part 2, and a moving table 3, and a tuneable connection including constant stiffness elastic elements 6, which in turn play the role of the guiding element 21, a rotatable, H-shaped, anisotropic stiffness adjustable stiffness element 7 and a damping element 20 comprising a pair of magnets 9 and an electrically conducting plate 15. The adjustable stiffness element 7 can be rotated by the electromotor 8 through the transmission 24, and the actual angular position 28 can be read from an encoder 16. The damping ratio of the damping element can be adjusted by an electric motor 22 such that the position of the magnets 9 relative to the plate 15 can be varied in direction 29b by a ball screw 25. The plate 15 is connected to the moving table 3, while the magnets 9 are connected to the stationary part 2, thus the relative motion of the moving table 3 causes the relative motion of the plate 15 relative to the magnets 9. This relative motion results in eddy currents with intensity proportional to the relative velocity and the overlapping area of the plate 15 and the magnets 9. An accelerometer 4 is attached to the moving table 3, while another accelerometer 5 is attached to the stationary part 2 in order to be able to measure the vibration accelerations as an input for the optimal stiffness and damping calculations.
[0051] A possible embodiment of the tuneable clamping device 1 has constant stiffness elastic element 6 with rectangular shape plate made from steel, having at least 0.12 mm transversal displacement measured in the middle of the top edge under a transversal unit force acting at the middle of the top edge, when it is clamped along its longest edge. The replaceable H-shaped adjustable stiffness element 7 has at least a ratio of eight between its minimum and maximum operational stiffness in order to have the range for proper operation and reaching at least 2000 N/mm in the minimum stiffness operational direction. This makes the tuneable clamping device 1 to operate in unloaded frequency range 100-500 Hz. In the adjustable damping element 20 corresponding to FIG. 4 ii) multiple permanent magnets 9 are arranged along the electrically conducting plate 15 on both sides, generating perpendicular magnetic flux crossing the plate 15, whereas the neighboring magnets 9 and the magnets 9 on the opposite side of the plate 15 are arranged with alternating poles, resulting in alternating flux directions through the plate 15. The relative direction of motion 27 of the electrically conducting plate 15 relative to the magnets 9 is the same as the direction of the main vibration mode of the tuneable clamping device 1. The constant stiffness elastic element 6, the conducting plate 15 and the double row arrangement of magnets 9 are placed pared in parallel in symmetric arrangement to the axis of the rotation of the adjustable stiffness element 7 formed as a H-shaped spring.
[0052] The tuneable clamping device 1 can be oriented in various directions relative to the main spindle of the machine tool 10. FIG. 6 shows 4 different arrangements which are possible but not exclusive examples for the orientations. In FIG. 6 i) the device 1 is a built-in part of the machine tool 10 with the relative motion of the moving table 3 in one of the directions of the horizontal plane. The workpiece 11 is attached to the device 1 with any method common in machine tools. In FIG. 6 ii) the device 1 is not part of the machine tool 10, but it is attached to the machine tool table, whereas the relative motion of the moving table 3 is possible in one of the directions of the horizontal plane. In FIG. 6 iii) and FIG. 6 iv) the device 1 is a built-in part of the machine tool 10 or it is attached thereto, and the relative motion of the moving table 3 with respect to the stationary part 2 is possible in one of the directions in the vertical plane.
[0053] The tuneable clamping device 1 allows to determine optimal stiffness and damping parameters according to said optimal serial dynamic coupling or said coupling through machining process or any other preferred method. FIG. 7 shows a possible embodiment for the control algorithm implemented by the control unit 18. It may be possible to implement any other preferred method in order to optimize the dynamics of the serial dynamic system in order to attenuate the vibration amplitude of the workpiece 11. The control unit 18 acquires a table acceleration signal 31 and a base acceleration signal 32 using the table accelerometer 4 and the base accelerometer 5. In this case, the signals 31, 32 are preprocessed in a pre-signal processing unit 33. Depending on the selected operation mode of the control unit 18, whether the machining process is under operation, the control unit 18 is capable of non-operational control 39 and operational control 50. In non-operational case the control unit 18 determines the dominant mode in the direction 26 of the workpiece 11 by using external excitation with e.g., a simple hammer. It applies filters for non-operational modes 40 and spectrum analysis 34. Afterwards, it can detect the dominant mode of the workpiece 11 with dominant mode detection unit 35. The range feasibility analysis 36 decides if the tuning is possible. If not, non-operational tuning 39 is not possible, if yes, the parameters are determined for reaching said optimal serial dynamic coupling by a non-operational tuning unit 37, while parameter adjustment are carried out in step 38. In the operational control case 50 operational filter is used before spectrum analysis 34. By removing stationary solution 43 with the known spindle speed signals 42, the dominant chatter frequency is determined in step 44 and their modulations are calculated in step 45. Decision is reached based on the tuning feasibility in step 46 and said optimal serial dynamic coupling or said coupling through machining process is achieved in step 47. The stiffness and damping are adjusted in step 38 according to step 47. The quality of tuning is determined in step 48, which can initiate a new iteration cycle or accept the tuning in step 49.
INDUSTRIAL APPLICABILITY
[0054] The proposed tuneable clamping device 1 is applicable for machine tools 10 (FIG. 6) wherein the part of the machine tool 10 is playing the role of their main workpiece table FIG. 6 i) or workpiece column FIG. 6 iv), or wherein separate device fixed on the main workpiece table FIG. 6 ii) or workpiece column FIG. 6 iii). In all configurations the tuneable clamping device 1 can eliminate the effect of an essential vibration mode of the clamped workpiece 11 in the direction 26. The tuneable clamping device 1 according to the invention can continuously follows by its variable tuning the variable nature of the dominant mode of the workpiece 11 with flexible part during machining operation. The proposed invention can significantly increase the material removal rate especially around flexible segments of the workpiece 11 clamped on.
