This paper has investigated an approach to reduce and compensate thermal displacement for high accuracy NC lathes. An efficient design and optimization method is proposed for a headstock structure design of NC lathes to minimize the thermal displacement of the spindle center position. Compared to the existing empirical methods, this method saves development time and cost. The Taguchi method and FEA method are used to identify the optimal headstock structure. The proposed method is verified by evaluating the spindle center transition of the headstock according to the optimization results.67746

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1. Introduction

The demand for higher accuracy NC lathes has increased dramatically with respect to machining accuracy requirements. Thermal deformation has significant effects on the machining accuracy. Much research has been carried out on this topic. However, not many good results were gained in practice.

The major studies on thermal deformation are summarized as follows. Moriwaki and Shamoto proposed an estimation compen- sation method for thermal displacement by using temperature sensor [1]. Brecher and Hirsche extended this work based on control internal data (e.g., axis feed and spindle speed) [2]. Spur et al. used non-metallic materials (e.g., carbon fiber reinforced plastics) to suppress thermal displacement [3]. Mitsuishi et al. applied the Finite Element Method (FEM) analysis on bearing preload and casting shape optimization to minimize displacement [4]. Jedrzejewski conducted compensation through a fin/fan- coupled thermal actuator controlled by strain gage-based thermal distortion feedback [5]. Shimizu et al. developed an algorithm to estimate the total machine thermal deformation by fitting the deformation modes to data obtained from eddy current type displacement sensors [6].

Several machine tool manufacturers adopt the methods of using temperature information from sensors or internal NC controller to estimate thermal displacement and conduct compensation. For a NC lathe, the thermal displacement is usually affected by machine structure, ambient temperature, state of heat sources (servo motors or machining heat), airflow and coolant usage, etc. And, to estimate the displacement involves complex interplay of these parameters and needs a large number of combinatory experi- ments. While it is possible to conduct accurate compensation for linear  thermal  deformation  along  each  axis,  the compensation

accuracy drops dramatically as deformation is accompanied with twisting or warpage.

Development of a new NC lathe involves modifying the structure of an existing machine and running experiments concurrently, which is usually time consuming and costly. In this paper, a novel approach is proposed to design a headstock for NC lathe immune to thermal deformation caused by random temperature deviation. By combining Taguchi method [7] with CAE analysis, an NC lathe spindle structure is determined and a headstock is manufactured based on the design result. The thermal deformation is evaluated to prove the efficiency of the proposed method.

2. Headstock structure and thermal displacement measurement

Fig. 1 shows the internal structure of an NC lathe’s spindle associated with parameters of parts and ambient  variables.  The goal is to design a headstock with thermal displacement concentrated on the Y axis instead of X axis since the thermal error on Y axis is considerably smaller than that on X axis in terms of direct diametrical influence. The heat sources are front bearing

housing, rear bearing housing and the  motor.

To measure thermal displacement, a hollow cylindrical work- piece was mounted into the spindle chuck while four eddy current displacement sensors were mounted on a fixture of cast iron with low thermal expansion. As shown in Fig. 2, the sensor fixture was attached to the machine body to measure the  relative  displace- ment between the body and the spindle [8]. Eqs. (1) and (2) were given as follows to calculate the thermal displacement on X and Y axis respectively.