Accurate compensation system for thermal deformation of machine tools
Harumitsu Senda(OKUMA Corporation) , Reiji Satou(OKUMA Corporation)

1. Summary

 Due to higher efficiency and higher accuracy for the machine tools, minimization of the thermal deformation, which causes to the machining dimensional deviation is required strongly. The thermal deformation is mainly caused by three factors. First, internal heat creation by machine operation, such as motor and main spindle running. Second, change of the environmental room temperature. Third, heat generation by machining process. In the past, compensation for this problem has been taken by human machining engineering technology. Therefore, we have developed a compensation system that mechanical construction itself can compensate thermal deformation automatically. This system utilizes temperature factors, which have a close relation between thermal deformation and key components. Now, this efficient system has been adapted for various kinds of machine tools.

2. Description of technology
 Figure 1 shows the conceptual diagram of a vertical machining center that is equipped with our thermal deformation compensation system. This system which is controlled by Okuma own developed CNC equipment estimates and computes a thermal deformation based on temperature data from exclusive thermo-sensors which are allocated on major machine construction, such as main spindle and column. And a large amount of compensation is transferred to overlap the applicable axis servo signal in real time.
 While the machine tool is in operation, the thermal deformation is seldom in a stable condition, but almost continuously in a transient state of thermal deformation. For example, during the work piece machining, main spindle rotation which changes frequently in a short time and the cutting fluid temperature transition usually cause the thermal deformation. In the long-time machining of workpieces such as dies and molds, room temperature transition also causes the thermal deformation of entire machine. Therefore, in order for the operator to use a machine with confident as if there were no thermal displacement, estimating the thermal deformation in a wide variety of operating conditions (transient states) accurately was a main aim of development.
1) Technology for estimating thermal deformation of main spindle
 The highest percentage of thermal deformation attributable to internal heating is produced by main spindle rotation. This characteristic of the thermal deformation depends on the spindle speed. And thus it was difficult to estimate the thermal deformation accurately. To solve this problem, we expressed these deformation characteristics through experimental formulas by using the parameters of the spindle speed and incorporated the formula for compensating in a continuously transient state into the computation for estimating thermal deformation. This has enabled to estimate the thermal deformation at all rotation speeds accurately. As a result, when the time constant of thermal deformation changes greatly due to the spindle rotating and stopping, or when the transient state of thermal deformation occurs successively, it is possible for this new system to control any change of thermal displacement within 5m.
2) Technology for estimating thermal deformation in the mechanical structure
 Machining heat (from cutting fluid temperature) and room temperature change have an effect on the stability of machining accuracy. In addition, as the coefficient of heat transfer varies depending on the presence or absence of cutting fluid during machining, we have developed technology to set up temperature sensors in the appropriate structural elements to analyze the thermal flow. This makes it possible to estimate the thermal deformation accurately in a wide variety of operating conditions. Under the condition changing room temperature by 8, we have achieved a stable machining condition within 10m dimensional change.
3) Securing the quality of the machining surface through our compensation system
 With the evolution of machining technologies in recent years, the machining surface roughness has been improved, and in some cases even an instruction for a compensation of 1m in thermal deformation affects the machining surface quality. For this reason, though the CNC device uses 1m-unit  in normal instructions, our compensation system uses 0.1m- unit. This makes our thermal deformation compensation system accurate highly and produces fine quality machining.

3. Conclusion
 We have developed a thermal deformation compensation system that compensates for machining tools under a wide variety of operating conditions accurately. A great number of models including multi-function machines, lathes, machining centers and grinders have been equipped with this thermal deformation compensation system and it has already contributed to the enhancement of our users' productivity. It has also helped to save energy by eliminating the warm-up time of machines to secure accuracy and by modifying the environmental conditions.
 Furthermore, this technology has proved to be valuable for developing ideas of new design concepts because it is enable to achieve further innovations for accuracy in tandem with a mechanical structure that takes the thermal deformation into close account.

Fig.2 Machine tools with thermal deformation compensation system


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