SMASIS A DESIGN OF AN ACTIVE TOOL HOLDING DEVICE

Transcription

1 Proceedings of the ASME 2012 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2012 September 19-21, 2012, Stone Mountain, Georgia, USA SMASIS A DESIGN OF AN ACTIVE TOOL HOLDING DEVICE Alexander L. Boldering Institute of Machine Tools and Production Technology Technische Universität Braunschweig Germany a.boldering@tu-braunschweig.de Annika Raatz Institute of Machine Tools and Production Technology Technische Universität Braunschweig Germany a.raatz@tu-braunschweig.de ABSTRACT The challenge of producing high performance, productive and precise machine tools must be accepted by the machine tool industry. These requirements can be reached by increasing the grinding machine parameters, such as the cut depth or the feed rate. Both will increase the grinding force during the grinding process, and can result in an excitation of the machine structure. Based on this excitation, the machine structure shows an undesired dynamic behavior which can result in machine vibrations. These vibrations can cause chatter marks on the workpiece surface, and if large enough can result in a stop in production. To minimize the chance of this situation, the machine user often chooses machine parameters which will not excite the machine structure. This is a disadvantage, because the machine design allows much more efficiency. To employ the complete efficiency of the design, this paper presents an approach which allows the active reduction of possible vibrations during a grinding process. INTRODUCTION The metalworking industry is similar to most of the production industry in that efficiency within the production will decrease production costs. With machine tools being one of the primary production methods within this industry, its efficiency plays a large part in the overall efficiency of the production process. To improve the efficiency of these tools, it is possible to increase the machine tool parameters. The adjustable parameters of a grinding machine, can for example be the depth of cut or the feed rate. The correlation between these two parameters is important for the dynamic behavior of the machine. If the relation between depth of cut and a high feed rate is unfavorably chosen, it is possible that the machine will begin to vibrate. The result of these vibration are chatter marks on the workpiece surface [1]. If the vibrations are large enough, the result is a defective product as well as require the machining to stop. To prevent the appearance of these effects, the machine parameters are set well below what the machine is capable of, limiting the machine efficiency. The approach presented in this paper deals with the integration of active elements to minimized unwanted vibration, allowing an improved machine parameter set to be used. The Priority Program 1156 Adaptronics for Machine Tools, which is supported by the DFG (German Research Foundation), addressed some approaches, which deals with the active manipulation of machines and results [2]. In Europe, active methods are categorized in the field of adaptronics. The term adaptronic comes from the combination of the adaptive structure and electronics components [3]. In [4], an adaptronic system is defined as a composite of an actuator, a sensor, electric control and a structure. These four elements can be found in the projects of the Priority Program The projects within this Priority Program presented good results which were very machine specific, limiting their extension and adaption in other machine tools. There are some problems which complicate the implementation and the acceptance of adaptronic elements into current or new machines. One problem for the machine tool industry is the high cost of the adaptronic elements which must be implemented into the machine tools. It is also 1 Copyright c 2012 by ASME

2 Machine tower Spindle box Grinding wheel Cross table y z x Grinding force [N] Grinding process : pendular grinding v = 30 m/s Grinding whell : 89 A 150 K 5 V c v w = 500 mm/s Material : Ck 45, HB 220 n s = 23.8 Hz Cooling lubricant : emulsion 3% a = 10 µm e 50 Processing time t c[s] FIGURE 1. Model of a surface grinding machine Amplitude 25 0 Frequency f [Hz] common that the adaptronic elements are added after machine tool construction. The approach of these projects was specialized towards one kind of machine structure and often the approaches cannot transfered for other structures. Some projects within the Priority Program considered adaptronic elements, which can be used or transferred into other or structurally identical machines. One of these elements was an active module, which was implemented in a grinding machine [5, 6]. The active module was developed using a construction kit, which enables the module to be built dependent on the application and the available design space within the machine structure. To connect the module with the structure, an easy interface was realized. In the Priority Program are other, similar projects which deal with the integration of active elements within machine tools [2]. Another problem limiting the acceptance of adaptronic systems is the need of expertise in closed-loop control. To create and implement a close-loop control, for example a model-based controller, it is necessary to get information of the machine behavior. The behavior of the machine is usually identified by a vibration analysis or often by a modal analysis. Such an analysis requires not only an expert in these fields, but also a lot of measurement instrumentation. This has hampered the extention of adaptronic elements into industrial machine tools. To minimize the barrier to applied adaptronic elements, this paper presents an approach of an adaptronic element, which can be easily integrated in several machine tools which have a rotated tool holding device. This approach is aided by the DFG (Ra 1736/5). DYNAMIC BEHAVIOR OF THE TEST CARRIER The adaptronic element is developed and will be implemented for a surface grinding machine from the company Hauni- Blohm. The machine consists of a cross table and a horizontal spindle. The table executes the feed motion in x- and z-direction and the y-direction is realized by the raising and lowering of the spindle box (see Fig. 1). The dynamic behavior of the test carrier is important for the construct and implement of the adaptronic el- FIGURE 2. Unstable grinding process ements. This information allows the resulting system to actively influence the correct spectrum of the machine behavior. The behavior can be detected using different analysis, for example with the help of a modal analysis or the recording and analyze of the forces and displacements during the grinding process. Forces during an exemplary grinding process can be seen in Fig. 2, using the grinding parameters seen in the top portion of the picture. These are the workpiece speed of v w = 500mm/s and the cutting speed of v c = 30m/s. As illustrated in the upper graph of Fig. 2, the chosen parameters result in an unstable process and show the high dynamical behavior of the grinding force. The bottom graph presents the amplitude spectrum of the frequencies and two peaks stand out. The first peak occurs at 97 Hz and the second peak at 193 Hz. There are two small peaks at the frequencies of 285 Hz and 382 Hz. Earlier experiments have shown that some frequencies, for example the one at 382 Hz, can be found when a workpiece surface has chatter marks [7]. These frequencies, particular the first two significant peaks, will not changed by variations in the output speed during a grinding process. These characteristics indicate that the detected frequencies are the eigenfrequencies of the machine structure. These eigenfrequencies, especially the mode of motion of the frequencies are essential in understanding the machine behavior. To gain further insight into the motion of the machine, a modal analysis is performed. It was found that the first mode of motion is a rigid body motion of the spindle box. The spindle box rotates around the machine tower and the maxima of the vibration is located at the end of the spindle box (see Fig. 3). The mode of motion at 193 Hz is a flexure mode. At this mode, a maximum antinode is located at the middle of the spindle box. More detailed information of the machine behavior and more analysis of the grinding process can be found in [7]. Both modes of motion show a high oscillation amplitude at the end of the spindle box. 2 Copyright c 2012 by ASME

3 Mode# Hz, Undeformed Upper deflection Bottom deflection Grinding wheel adapter ø 203,2 mm Potential design space for the actuators Mode 1 (89 Hz) Grinding wheel adapter ø 155 mm Taper Mode 2 (192 Hz) FIGURE 3. Modes of motion of the surface grinding machine This is definitely a problem, because this is the place where the tool holding device is implemented. Another interesting characteristic of the mode of motion is the relatively small displacement of the table in contrast to the spindle box. Based on this characteristic, it seems to be useful to change the passive tool holding device into an active device. This active arrangement allows the direct interaction against increasing vibration, resulting in a reduction of chatter marks on the workpiece surface and ultimately preventing the process from being stopped. ACTUATOR SETUP The goal of this project is to construct an active tool holding device which will replace the current tool holding device (see Fig. 4). The different between these two devices is that the active device has implemented actuators which allow vibrations to be induced in the machine structure. The current device will be described in the next section. FIGURE 4. The current tool holding device The original tool holding device can carry two different size grinding wheels. The smaller having a diameter of 155 mm while the larger mm. Only the mm diameter is standardized [8] and the active tool holding device shall carry this type of grinding wheel. Another condition is that the grinding machine structure will not need to be changed to accept the new device. With this limitation, the design space of the new device must be the same as the original. Based on the explained requirements an active tool holding device is constructed and several approaches are discussed in detail in the next chapters. To get more information about the behavior of the tool holding devices, especially of the active designs, the engineering software package ANSYS is used to calculated the flexibility of the different arrangements. First the tool holding was constructed in a CAD program and then imported in ANSYS. The simulations use a radial force of 150N, which is based on the measurement of a typical process force during a grinding process, as shown in Fig. 2. The radial force is the result of the bolt used to affix the mm grinding wheel to the holding device (see Fig. 5). The fixed bearing of the following simulation is placed at the contact point of the taper with the spindle. The original device is composed of one part (see Fig. 4), and the flexibility has a maximum deflection of µm at the largest diameter of the wheel mount, as shown in Fig. 5. Design Requirements Fig. 4 presents the current tool holding device as it is implemented within the grinding machine. The spindle of the grinding machine is connected with the tool holding using a taper. This is not standardized and is dependent on the machine manufacturer. This is a problem, because one important requirement of the new construction is to enable its interchangeability for different machine tools. To realize this, only one part of the new device must be changed to connect the device with another machine spindle. The Principle of the Actuator Setup The first step in reducing undesirable vibrations is to clarify which adequate actuator arrangement can reduce the relative distance between the tool and the workpiece. The vibrations are generated by the grinding force, which can be separated into the x- and y-directions. The force in x-direction (tangential force) is less important for the relative position of the workpiece to the grinding wheel, then the force in y-direction (radial force). This 3 Copyright c 2012 by ASME

4 Wheel head Wheel mount Inner actuator adapter Actuator Coupling Taper Spindle FIGURE 5. by ANSYS Deformation of the original tool holding device simulated Bearing Actuator Outer actuator adapter Bearing Tool Workpiece z y x radial force has a maximum of 150 N and the actuator of the active tool holding device shall reduce or counter this force. So it is necessary to implement these actuators in the potential design space, as shown in Fig. 4. There are two possible concepts to integrate the actuators in the device. The first is based on a rotated actuator setup and the second a fixed actuator setup. In the first setup the actuators are directly integrated between the rotated wheel mount and the rotated spindle. This arrangement is similar to the layout of a wheel with spokes. The implemented actuators, which are based on the piezoelectric effect, are fragile against torsion moments. The isolation of these moments can be realized by connecting the actuators to the base using revolute joints. These joints also allow the equalization of angle errors during an actuator adjustment. But this is at the same time a disadvantage, because the actuators cannot transfer the tangential force from the spindle to the wheel mount, and can only absorb these radial forces. To enable this transfer, a coupling between these two parts must be implemented in the active tool holding device. A positive characteristic of the coupling is the possibility to equalize axial offsets between the spindle and the wheel mount. Another disadvantage is the realization of the electric power supply of the rotated actuators, which typically need a voltage of up to 1000V. One alternative for the supply can be realized by a collector ring, but to guarantee the occupational safety and health at high voltage, a very complex construction is required. It must also be guaranteed that the power source has no contact with the dirty working environment and cooling lubricant during the grinding process. FIGURE 6. Schematic diagram of a fixed actuator setup The fixed actuator setup also has its advantages and disadvantages. One advantage is that it is simple to supply the electric power to the actuators. They will not rotate, and they can be directly connected with a power cable. A disadvantage is that a static integrated actuator can reduce only one force direction, for example a vertical integrated actuator can only reduce forces in the vertical direction. The IWF has applied for a patent on a fixed actuator setup [9]. This approach is followed within this paper and is explained in the next chapter. The Fixed Actuator Setup A schematic diagram of a fixed actuator setup is represented in Fig. 6. The active tool holding device is connected to the machine spindle using a friction based taper, and is normally secured using a nut. One or more rolling bearings are then used to connect the taper to the inner actuator adapter. This assembly is fixed with the wheel head, so the adapter will not rotate. The inner actuator adapter carries the actuators, which are connect by a screw. The actuators are also connected with the outer actuator adapter, which, as the inner adapter, do not rotate. Both adapters can perform a relative movement relative to one another through a positive or negative expansion of the piezo actuators. To carry this movement up to the wheel mount, the outer actuator adapter is combined with the wheel mount with rolling bearings. The rotary motion of the spindle will be transferred to the wheel 4 Copyright c 2012 by ASME

5 Wheel mount Outer actuator adapter Parallel pin Inner actuator adapter Force sensor Taper Preloaded Piezo actuator FIGURE 7. Sectional view of an actuator adapter and a tool holding device mount by a qualified coupling. During a grinding process vibration will occur and can cause a relative displacement between the workpiece and the grinding wheel. To overcome this offset, the coupling must allow a radial offset between the spindle/taper, and the wheel mount. To minimize this displacement, both actuator adapters will move to each other and a radial offset will occur between the spindle and the wheel mount. The actuators are connected with a force transducer to form one unit (see Fig. 7). This sensor is necessary for the system identification and can also measure the force of the actuator as well as the forces of the grinding process during the process. In addition to giving an insight into the forces of the grinding process, this information can be used within a feedback control strategy. The unit of both elements can be implemented in the potential design space (see Fig. 4) in different variations. One configuration is to integrate the actuators above and below the taper. The second is to embed the actuators left and right the taper. The Different Actuator Arrangements The advantage of the previously presented arrangement is that the actuators are directly integrated within the tool holding device and allow the forces to be distributed. One requirement of the construction is that the installation space of the active device is similar to the space of the original design. The distance between the bearing surface of the taper and the inner ring of the bearing surface of the wheel mount amounts to approximately 80 mm. From this size, the size of the force transducer, circa 16mm must be subtract. Also a part to fix the actuator-sensor unit (size = 8mm) at the outer actuator adapter must be considered. As a result, the potential size of the implemented actuator can be up to circa 56mm. The overall design space is very limited, and only a few actuators can realize these requirements. To meet these requirements a preloaded actuator, such as a piezo stack actuator, can generate the required push and pull forces. The chosen piezo stack actuators offer a closed-loop travel of 15 µm, which is not very large with respect to the bearing clearance. The active tool holding devices does not share the same fixed bearings as the original device. One fixed bearing position is at the contact point of the taper with the spindle, the same as the original. Another fixed bearing position is at the tool holding/machine grinding adapter, because this adapter is fixed with the wheel head. Fig. 8 illustrates the simulation of the flexibility of both the described arrangements. The arrangement which places the actuator at the top and bottom of the taper has a maximum flexibility of µm at the largest diameter of the wheel mount. A second arrangement of the actuators is possible, in which the actuators are positioned on the left and right side of the taper. This layout allows for a little more design space for the actuators, which are attached between the taper and the wheel mount. The base diameter of each actuator is 25mm, and requires a drillhole length of 88mm. From this length, the sensor size (15mm) and the fixing part of the actuator-sensor unit (8mm) must be subtracted. Altogether an actuator size of up to 65mm is possible, and have an achievable closed loop-travel of 30 µm. In the arrangement where the actuators are placed on the left and right side of the taper, the resulting construction has a maximum flexibility of µm. This is also at the largest diameter of the wheel mount, and is shown in Fig Copyright c 2012 by ASME

6 FIGURE 8. Flexibility of the first actuator setup FIGURE 9. Flexibility of the second actuator setup THE ACTIVE TOOL HOLDING DEVICE With the advantage of the larger closed-loop travel, the second arrangement of the fixed actuator setup will be realized. This setup has a higher flexibility than the first arrangement, also a very strong tilting of the inner actuator adapter to the outer actuator adapter can be detected. To reduce both of these properties, two parallel pins are integrated between the inner and outer actuator adapter (see Fig. 10). With this extension the maximum flexibility is µm with a radial force of 150 N, allowing the tilting of the adapters to be countered. A simplified version of the constructed active tool holding device is presented in Fig. 7. The green parts of the device are the elements which rotate. The direction of their possible movement is illustrated by the arrows. The interlock nut regulates the friction connection between the taper and the spindle and is not shown in the figure (see Fig. 11). The MONOLASTIC coupling, from the company KTR, is implemented on the interlock nut and transfers the rotary motion via the mass-balance gear holder to the wheel mount. The coupling can tolerate a maximal displacement of K r = 0.6mm at 2200rpm. This is sufficiently to counter the movement of the closed-loop travel of the piezo stack actuators and possible axial distance between the taper and wheel mount. The parts that are not in red are stationary. The inner actuator adapter is fixed via the tool holding/grinding machine adapter to the wheel head of the grinding machine. The inner and outer actuator adapters are only connected by the actuator-sensor unit. To prevent actuator damage, the actuators must be secured to avoid torsional moments. These moments can be evoked by the rotated system or the possibility of a relative rotation of the outer actuator adapter to the inner actuator adapter. Such a situation can arise if one actuator is extended more than the other. The two parallel pins prevent these torsional moments. A relative rotation around the x-axis of the outer actuator adapter to the inner adapter is also be prohibited. The tool holding/grinding machine adapter fixes the inner actuator adapter via three connection elements (see Fig. 11) to the wheel head. The connection elements can be added with a minimal amount of modifications at the wheel head. One requirement of the design is the transferability of the active tool holding device into another machine. If the desired machine 6 Copyright c 2012 by ASME

7 FIGURE 10. Wheel head Connecting element Grinding wheel Maas-balance gear MONOLASTIC-Coupling Tool holding/grinding wheel adapter FIGURE 11. Flexibility of the realized actuator setup The connected active tool holding device structure has a different taper, it is possible to change the taper of the active device as well as the connection elements to accommodate the new wheel head. CONCLUSION An active tool holding device has been designed to reduce vibrations during a grinding process. This device is explained with a discussion of possible actuator arrangements. Resulting from this discussion, the best concept is chosen and analyzed. Using the simulation software ANSYS the design with the actuators on the left and right side of the taper is evaluated. This concept allows the implementation of piezo stack actuators with an closed-loop travel up to 30 µm. The actuators are combined with actuator adapters and the adapters does not rotate. This arrangement allows a simplified realization of the power supply for the piezo stack actuators. One of the main advantages of this new design is that only a few parts need to be altered to allow its integration into different machines. These are the connection elements, which must be modified for each machine structure. If the machine structure is very different to the original one and the connecting elements cannot placed in a similar position as presented in the realized arrangement, the tool holding/grinding wheel adapter must be modified. The simulation was used to show that the original tool holding device has a deformation of up to µm when a real force of 150N is applied to the wheel mount. The active tool holding device has a maximum deformation of µm by the same radial force. This deformation does not have a critical dimension, and so the presented construction of the active tool holding allow the minimization of possible vibrations of the grinding wheel. To minimize the vibration with the actual device, some steps are necessary and will be the focus of future work. The next step is the implementation of the active device within the grinding machine. After this, an identification tool will be developed which allows the identification of self-excited and externally excited vibrations. For the identification an excitation of the machine structure is necessary, which can be realized using an external actuator. Based on the results of the identification, an adequate controller will be developed. Both, the identification and the generating of the controller, shall be realized by an automatically approach. ACKNOWLEDGMENT The authors gratefully acknowledge the funding of the reported work by the German Research Foundation (DFG) within the project Ra 1736/5. REFERENCES [1] Insaki, I., Kerpuschewski, B., and Lee, H.-S., Grinding chatter - origin and suppression. Annals of the CIRP, 50, pp [2] Hesselbach, J., Adaptronik fuer Werkzeugmaschinen: Forschung in Deutschland. Shaker Verlag, Aachen. [3] Pahl, G., and Beitz, W., Konstruktionslehre. Springer Verlag, Berlin, Heidelberg. [4] Simnofske, M., Raatz, A., and Hesselbach, J., Design 7 Copyright c 2012 by ASME

8 process for adaptronic machine tools. Production Engineering, 3, pp [5] Boldering, A., Simnofske, M., Raatz, A., and Hesselbach, J., eds., Active Vibration Reduction to Optimize the Grinding Process, ASME International Design Engineering Technical Conferences & Computers and Information in Engineering Conference (IDETC/CIE), ASME. [6] Boldering, A., Simnofske, M., Raatz, A., and Hesselbach, J., Schleifmaschine mit adaptronisch erhoehter statischer und dynamischer steifigkeit. In Adaptronik fuer Werkzeugmaschinen, J. Hesselbach, ed., Forschung in Deutschland. Shaker Verlag, Aachen, pp [7] Hesselbach, J., Hoffmeister, H.-W., Schuller, B., and Simnofske, M., eds., Adaptronik fuer Werkzeugmaschinen, Adaptronic Congress. [8] DIN ISO 603-4, Schleifkoerper aus gebundenem Schleifmittel. Teil 4: Schleifmittel fuer Flachschleifen/Umfangsschleifen, mai ed. Beuth Verlag, Berlin. [9] Simnofske, M., DE : Werkzeugaufnahme. German Patent and Trademark Office, Munich. 8 Copyright c 2012 by ASME