外文文献原文.pdf

Investigation on the spindle thermal displacement and its compensation of precision cutter grinders Yung-Cheng Wang a, Ming-che Kaoa, Chung-Ping Changb aInstitute of Mechanical Engineering, National Yunlin University of Science and Technology, No. 123, Uni. Road, Section 3, 640 Douliu, Yunlin, Taiwan bDepartment of Mechanical Engineering, National Central University, Taiwan a r t i c l ei n f o Article history: Received 24 July 2010 Received in revised form 7 March 2011 Accepted 16 March 2011 Available online 30 March 2011 Keywords: Cutter grinder Spindle thermal displacement Laser displacement sensor Compensation mechanism a b s t r a c t The axial performance of cutter grinders will have direct effects on the precision of ground cutters. Therefore, error elimination will be benefi cial for the precision improvement of cutter grinders. One of the main axial error sources is the spindle thermal displacement. General cutter grinders are not equipped with the cooling systems, because of their rel- ative lower rotating frequencies (8000 rpm), small working range and slower thermal gra- dient. On condition without cooling, there will be obvious thermal errors at the beginning hours of operations or the restarting of machines. That will cause materials waste and lower working effi ciency. In this investigation, an effi cient measurement system and the compensation mechanism have been established for the minimization of thermal displacements. About 80% of the thermal displacement can be reduced and the thermal displacement error becomes less than 5lm. With this solution, the precision of cutter grinders can be improved. ? 2011 Elsevier Ltd. All rights reserved. 1. Introduction Precision cutter grinders (Fig. 1) can be widely applied in either traditional mechanical industries or high-tech industries. The so-called maternal genetic effect available in products would mean that the precision of products de- pends mainly on the machine tools, of which the errors can be classifi ed as: geometrical errors, temperature rise errors and dynamic errors, etc. According to relevant studies, 4070% of errors of machine tools result from the thermal displacement 1,2, where the main source of heat lies in the friction between spindle and transmission belt, and that between spindle and ball bearings. So far, some companies have developed the sensors laid inside the spindles by which to compensate the errors of thermal displacement 3. Yet machines equipped with such spindles would increase working cost for small and middle sized cutter grinder manufacturers, while there will be certain limits on the varieties and adoption of spin- dles, and on the promotion of performances of used machines. In this study, we have integrated an optical displace- ment sensor system, a temperature sensor system, and a signal acquisition and analysis system to construct an independent module measurement system of spindle, by means of which it can be easy to fi nd out the mathematical correlation between temperature and thermal displace- ment for the spindle of cutter grinders. That can be used to test the quality of spindles and also provides the basis for compensation of thermal displacement during practical manufacturing process. 2. Construction of measurement system Temperature sensors and a displacement sensor were installed in the measurement system (Figs. 2 and 3). From the spindle test system, temperature and displacement sig- nals have been gained. After processes of signal converters 0263-2241/$ - see front matter ? 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.03.018 Corresponding author. Tel.: +886 5 5342601x4122; fax: +886 5 5312062. E-mail address: .tw (Y.-C. Wang). Measurement 44 (2011) 11831187 Contents lists available at ScienceDirect Measurement journal homepage: Fig. 1. Five-axis cutter grinder. Fig. 2. Module measurement system of spindle. Fig. 3. Installation of sensors. 1184Y.-C. Wang et al./Measurement 44 (2011) 11831187 and signal acquisitions, they would be recorded and ana- lyzed in the measurement program, by which the correla- tion between temperature drift and thermal displacement can be established (Fig. 4). 2.1. Temperature measurement system Temperature sensors were fi xed on the experimental machine, in which the spindle was installed. To acquire the internal temperature relatively proximate to that of the spindle and reduce the infl uences of external environ- ment, the temperature sensor was installed inside the wall of spindle holder to contact the surface of spindle nearly. The PT-100 temperature sensor 4 was selected to mea- sure the temperature on different positions of spindle. It is generally appropriate to restrict the current of PT- 100 within the range of 0.5 mA2 mA. Its resolution can be set at 0.1 ?C. After testing it can be shown that temper- ature drifts of all temperature sensors are similar (Fig. 5). To simplify the measurement process and obtain a better Fig. 4. Signal processing. Fig. 5. Temperature changes of different positions. Fig. 6. Different starting temperatures. Y.-C. Wang et al./Measurement 44 (2011) 118311871185 resolution, only temperatures of the top position are mea- sured for analysis, because of its more temperature changes. 2.2. Displacement measurement system For thermal displacement measurement of the rotating spindle, high resolution in submicrometer range and non- contact measurement method are required. Thus the tradi- tional mechanical measurement devices are unsuitable for it. What used in this study was a laser displacement mea- surement device with the resolution of 0.2lm 5. The sta- bilitytestwascarriedoutundergeneralworking condition, while the drift of displacement signal was less than 2lm. That would meet the requirement of thermal displacement measurement. 3. Experiments and results Both the structure of machine tool and its surrounding conditions can become the factors affecting the processing precision. Obviously, the system errors of machine tool cannot be maintained at a fi xed level and can vary as time goes by. Besides, the thermal deformation created due to the operation of machine tool can lead to the thermal dis- placement, whose development can be very intricate 6. Though theoretical simulations as differential equations and fi nite element method can provide a great deal of information for the analysis of thermal displacement, the most effective way of analysis of such development would be the direct measurement of thermal displacement by means of experiments. Therefore, this investigation was based on measurements of temperature and thermal dis- placement of spindles. With these data, their correlation can be formulated and then the formulation is applied for compensation of thermal displacement, while only the temperature is measured during practical manufactur- ing process. It is impossible for ordinary processing factory to place the machine tool in a state of constant starting tempera- ture. Besides, the starting temperatures of operation can differ in each of the four seasons. To inspect the effect of different starting temperature, measurements were made on different starting temperatures of spindle on the cutter grinder. For instance, in Fig. 6, this test shows the resulted thermal displacement curves are fast no differences when the starting temperature was 19 ?C and 27 ?C separately. Thus for a spindle, the thermal displacement is only depen- dent on temperature changes, the starting temperature of operation should have no infl uence on it. Because the rotary speed of spindle is different at the practical applications, it is necessary to investigate, if the rotary speed of spindle will affect the thermal displace- ment amount due to a fast or slow speed. According to thedisplacementdifferencebetween3000 rpmand 6000 rpm as measured (Fig. 7), a fast or slow rotary speed Fig. 7. Thermal displacements in different rotary speeds. Fig. 8. Results of compensation. 1186Y.-C. Wang et al./Measurement 44 (2011) 11831187 would only result in a difference of the temperature changes, but have no effects on the thermal displacement amount. And there would be a very high repeatability. Thus, this feature can even more benefi t the compensation ofthermaldisplacement.Thecorrectionequationis L3000= 3.44T + 5.68 at 3000 rpm, and L6000= 3.95T + 4.95 at 6000 rpm. (where L is the displacement, and T is the temperature change) Above experiments show that temperature and thermal displacement changes of one spindle can be used to estab- lish a correlation, when both changes are increasing. To verify this correlation also suitable for falling temperature, experiments for rising and falling temperature have been performed. The results show that the curve of thermal dis- placement in rising temperature can match with that in falling temperature. Hence, the physical properties of the spindle would not change due to either rising or falling temperature. According to the thermal displacement curve from experiments, results of compensation for thermal displace- ment of spindle are shown in Fig. 8. By the correlation ac- quired in previous experiments, the spindle is operated at a speed of 6000 rpm and the thermal displacement will be 34lm after 2 h. By the compensation of each 2 ?C temper- ature interval, the thermal displacement can be reduced to approximately 4lm. That corresponds to a relative reduction of about 80%. By the compensation process the precision of cutter grinder will be enhanced distinctly. 4. Conclusion By the application of temperature and displacement sensors, the correlation equation between thermal dis- placement and temperature has been formulated. And with this equation, thermal displacement of spindle can be compensated during the manufacturing process. The experimental results have shown that effective compensation could make it possible to use the theoretical estimate for compensation. By compensation processes for each of above experiments, the thermal displacement can be reduced to 4lm in short time. The reduction ratio is about 80%. It was obviously proved that the processing error could be decreased to the range of micrometer so that the enhancement of the precision and working effi ciency could be indeed achiev