TECHNICAL REPORT Technical Approach for Preventing Thermal Distortion in Machine Tools Y. KUBO Thermal distortion in machine tools greatly affects the dimensional tolerances of workpieces and causes various defects in production process. This report presents a technical approach for preventing thermal distortion that would contribute to substantial improvement in quality, manufacturing efficiency and energy saving. Key Words: thermal distortion, centerless grinding machine, optical linear scale, temperature compensation 1. Introduction Thermal distortion in machine tools greatly affects the changes in dimension of workpieces and causes various defects as listed below: q Decrease in productivity and loss of resources due to defectives. w Energy loss due to preliminary running. e Personnel cost due to additional works The purpose of this study is to establish a technology to minimize such losses in order to contribute to the quality improvement and cost reduction for customers and to assure the fundamentals of the company in the market. This paper presents a technical approach for thermal distortion in order to omit preliminary running based on the compact, ultra-precision centerless grinding machine C315F (Fig. 1) that was developed in July 21. Grinding wheel wheel Fig. 2 Schematic drawing of centerless grinding machine Rotational driving force is generated on a workpiece when it is ground by the grinding wheel. However, as the driving force is braked by the frictional force from the regulating wheel, the workpiece rotates slowly with the surface speed of the regulating wheel independent of that of the grinding wheel. Forward, backward Guide plate Stopper Fig. 1 Compact ultra-precision centerless grinding machine, C315F 2. Centerless Grinding Machine A centerless grinding machine consists of a grinding wheel, regulating wheel and blade. A workpiece is placed between the grinding wheel and the regulating wheel and supported on the V-shaped surfaces composed of the outside periphery of the regulating wheel and the top surface of the blade (Fig. 2). Koyo Engineering Journal English Edition No.164E (24) <Through-feed grinding> wheel Forward, backward <In-feed grinding> Fig. 3 Grinding method in centerless grinding machine Cylindrical grinding machines are also used for grinding the outside periphery of a shaft as well as centerless grinding machines. However, centerless grinding machines are used more extensively for mass production of low cost and high precision parts because of the advantages over the cylindrical 57
grinding machines as shown below. q High precision and high efficiency grinding is possible due to high support rigidity of the workpiece as the whole surface of the workpiece is supported. w No special preceding process like center hole machining for supporting the workpiece is needed. e Rounding effect that improves the roundness of the workpiece is large. However, in spite of such advantages of the centerless grinding machine, there are disadvantages, too, attributable to its structure. Figure 4 shows its structure. The two main components, the grinding wheel and the regulating wheel are supported on the grinding and the regulating respectively free to rotate. The grinding and the regulating are connected through the bed below. 3. Measures for Thermal Distortion in Centerless Grinding Machine 3. 1 Characteristics of Thermal Distortion Figure 5 shows the construction of the compact, ultraprecision centerless grinding machine C315F used in this study. and grinding wheel spindle cover dresser wheel dresser table The upper part of opens and closes wheel Optical linear scale table wheel table Fig. 4 Structure of centerless grinding machine This type of construction is called C-frame structure. As the grinding position is on a higher point than the top face of the bed, the workpiece tends to be affected easily from static and dynamic rigidity, and the thermal distortion of the machine. This can cause negative effects on the grinding accuracy as demonstrated in Table 1. Fig. 5 Construction of C315F The most important feature of this grinding machine is the optical scale installed between the grinding and the regulating, and that the full closed loop control is always watching the distance between the grinding wheel and the regulating wheel (Fig. 6). In fact, this distance has most critical impact on grinding accuracy of the workpiece. As a first step, the effects of this construction on the thermal distortion were verified. Monitoring the distance between grinding wheel and regulating wheel Connecting bar Table 1 Effects of C-frame structure on grinding accuracy " Large effect # Some effect Grinding accuracy table wheel Dimensional accuracy Roundness Cylindricity Roughness Static rigidity # # # Dynamic rigidity " # Thermal distortion " # It is comparatively easy to grasp the characteristics of static and dynamic rigidity, and these can be resolved by utilizing test data etc. However, thermal distortion is extremely complicated and difficult to be solved. Scale head (attached to regulating through connecting bar) Scale base (attached to grinding wheel ) Fig. 6 Arrangement of optical linear scale The purpose of using the optical scale is to achieve micro positioning of.1lm step. In this test, however, the limited position through-feed grinding was done after making the full closed loop control ineffective and the declination between the scale base and the scale head (distortion between the grinding wheel and the regulating wheel ) was recorded as time series data. This distortion is the thermal distortion measured underneath the optical scale mount. Figure 7 is a comparative 58 Koyo Engineering Journal English Edition No.164E (24)
chart of the distortion and the workpiece dimensional changes. As to the test method, the machining was started immediately after the machine start up without any preliminary running. The grinding fluid temperature was kept constant using a fluid temperature controller attached to the grinding fluid tank. dimensions become minus due to temperature rise of grinding table Above the Distortion, lm 8 4 2.5h dimensional change Distortion between grinding wheel and regulating wheel Ambient temperature After 2.5 h Running parallel. 1. 2. 3. 4. 5. 6. 7. 8. 9. 1. 11. 12. Hours after machine start up, h 32. 28. 24. 2. 16. 12. Fig. 7 Thermal distortion generated underneath and workpiece dimensional changes As a result of this test, the followings were learned: q The distortion between the grinding wheel and the regulating wheel depends on the ambient temperature and moves to the plus side of workpiece dimensional changes. w After 2.5 hours of machine start up, the workpiece dimensional changes and the distortion between the grinding wheel and the regulating wheel ran parallel. Thus, it is possible to compensate the thermal distortion beneath the scale mount (beneath each ) by running full closed loop control while keeping the optical scale effective. On the other hand, within 2.5 hours of machine start up, the workpiece dimensional changes moved drastically to the minus direction. It was already verified that most of the causes was in the thermal distortion of the grinding. From the above, the followings could be learned on the characteristics of the thermal distortion of centerless grinding machines. The part beneath and above the generates thermal distortion separately from each other due to different factors and the two types of thermal distortion combined affect the dimensional changes of the workpiece. Therefore, when studying the whole thermal distortion of the machine, it is necessary to look into the parts beneath and above the separately and different solution must be given individually for each factor (Table 2). Temperature, : Grinding wheel dimensions become plus due to outside temperature rise wheel Fig. 8 Thermal distortion characteristics in centerless grinding machine 3. 2 Thermal Distortion of Grinding Wheel Table As stated above, the thermal distortion in C315F beneath the can be ignored, when full closed loop control is adopted. Consequently only the thermal distortion of the grinding was focused in this study of the thermal distortion in machine tools. It is apparent that the thermal distortion in the grinding comes from self-heating of the machine. There are two factors: 1. 2. spindle There are 3 modes of heat transfer, i.e. conduction transfer, convection transfer and radiation transfer. The conduction transfer, in other word the heat transfer from the contact surface, is dominant in case of the thermal distortion in the grinding. Figure 9 shows the structure around the grinding wheel spindle head. Beneath the Table 2 Factors and causes of thermal distortion Above the Beneath the Thermal distortion factors affecting workpiece dimensions table Feeding screw shaft Factor of thermal distortion spindle Ambient temperature Grinding fluid temperature Koyo Engineering Journal English Edition No.164E (24) 59
First of all the conduction transfer from the grinding wheel motor was examined. Shielding of heat transfer was attempted from the beginning by inserting the heat insulating material between the contact surface of the motor and the grinding. However, the heat generating from the motor was beyond the capacity of the heat insulating material and the decrease in workpiece dimensions during the continuous grinding operation was accelerated as shown in Fig. 1. A water-cooled spacer was inserted between the motor and the grinding for countermeasures and the heat from the motor could be completely shielded as shown in Fig. 1..2..2.4.6.8 1. 1.2 1.4 1.6 1.8 Rolling bearing section of grinding wheel spindle cover After countermeasures on heat transfer from grinding wheel Heat insulating material 1 2 3 4 5 6 In the next place, the heat transfer from the grinding wheel spindle comes from the heat generation in the grinding wheel spindle bearing section. There is no way to shield the heat transfer from this section owing to the constructional restriction. The other conceivable method than shielding the heat transfer is to minimize the heat generation in the bearing section as near to zero as possible by, for example, "circulating coolant fluid in the spindle shaft," or "controlling the pressure fluid temperature by adopting hydrostatic bearings." However, either of the methods is not desirable from energy saving point of view as both methods involve investment of incidental equipment and consume a lot of energy. Consequently, attention was directed to the mounting structure of the grinding. As a result of modification of the mounting structure, while shielding the heat transfer from the, the thermal distortion in the grinding was successfully reduced to 6.5lm or 1/2 of the previous level from the original value of 13.2lm (Fig. 11). 1.6lm Time, min table Fig. 9 Structure around grinding wheel spindle head Fig. 1 Dimensional changes in continuous grinding operation.3lm After countermeasures 34 32 3 28 26 dimensional changes 24 Dimensional changes before countermeasure 22 2 18..5 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. 5.5 6. 6.5 7. 7.5 8. Hours after machine start up, h 2 2 1 6 14 dimensional changes before countermeasures dimensional changes after countermeasures on the thermal distortion of grinding Temperature changes of grinding The movement of 2 curves are symmetrical on this line Temperature changes of grinding 6.5lm 13.2lm Fig. 11 Effects by improving thermal distortion of grinding wheel spindle head However, the dimensional changes were still large and further improvement was needed. As a final measure, the method of temperature compensation was experimented. 3. 3 Implementation of Temperature Compensation The outline of temperature compensation is to detect the position of temperature rise in relation to the dimensional changes and to compensate the thermal distortion by moving the toward the temperature changes. The temperature compensation in this way was considered to be effective as the correlation between the workpiece dimensional changes and the temperature changes of the grinding was observed as shown in Fig. 11. Figure 12 shows the system diagram of temperature compensation. Temperature sensor (thermocouple) Optical linear scale table temperature Full closed loop control AD Temperature conversion Control device input module f (T) = a T Optical linear scale In Fig. 12, the temperature sensor is incorporated into the grinding and the temperature change data of the grinding was transmitted to the control device at real time to control the position against the temperature change value. Here, the position of temperature measurement was properly selected by testing. Temperature, : table Fig. 12 System diagram of temperature compensation 6 Koyo Engineering Journal English Edition No.164E (24)
And, a calculation formula was used to convert the temperature change value perceived in the control device to the compensation value. This formula is defined in a simple linear expression as shown below. This is the most important in this study. Slide compensation value The compensation factor "a" was determined from the relationship between the workpiece dimensional changes and the temperature changes of the grinding as shown in Fig. 11. By adopting all the countermeasures on thermal distortion as detailed above, the workpiece dimensional changes right after the machine start up could be dramatically reduced from the original value of 13.2lm to within 2lm. The result is shown in Fig. 13. Compensation factor (lm) (lm / ;) f (T) = a T 2 6 1 14 After all countermeasures on thermal distortion. 1. 2. 3. 4. 5. 6. 7. 8. Hours after start up of machine, h Temperature change value of grinding (;) Results after all countermeasures on thermal distortion Dimensional changes before countermeasures 1lm 13.2lm important not to try to solve all the cases with temperature compensation method as errors may occur because of complicated calculation formula and larger compensation factors. 4. Conclusion A technical approach to thermal distortion has been discussed in this report. It is important to grasp precisely the direction and the amount of thermal distortion in each section that influences the workpiece dimensional accuracy and at the same time to analyze fully its dependent factors in order to be able to take appropriate actions step by step. We have taken up the countermeasures on thermal distortion in the centerless grinding machine and adopted the methods described above. We could finally succeed to restrain thermal distortion by adopting temperature compensation. It is certain that this method is applicable to other machines than centerless grinding machines. On the basis of this technology, we are sure that Koyo can establish the fundamental technology that enables us to contribute to the world market with manufacturing quality and cost. Fig. 13 Results after all countermeasures on thermal distortion The effect of the thermal distortion countermeasures was described as 2lm in the above because the dispersion of repeated confirmation tests for reproducibility was taken into account. 3. 4 Summary of Countermeasures on Thermal Distortion in Centerless Grinding Machine The followings are the summaries of the countermeasures on thermal distortion in centerless grinding machines. Thermal distortion must be examined separately on beneath and above the section. Thermal distortion beneath the depends largely on the circumstances such as ambient temperature, coolant temperature. Consequently substantial effects can be expected if full closed loop control is to be implemented, placing the scale at the optimal position. Thermal distortion above the can be controlled effectively by restraining as much as attainable heat transmission from self-heating factors and by modifying its structure not to be affected easily by heat. A calculation formula of temperature compensation was defined in the simple linear formula. But this formula was obtained after eliminating thermal distortion factors as much as possible by several countermeasures. It is Y. KUBO * * Research & Development Department, Koyo Machine Industries Co., Ltd. Koyo Engineering Journal English Edition No.164E (24) 61