Radial error measuring device based on auto-collimation for miniature ultra-high-speed spindles

Abstract

This study has developed a radial error measuring device for miniature ultra-high-speed spindles because it is very difficult to measure the radial error motion of miniature ultra-high-speed spindles by the conventional measurement method using capacitive-type displacement sensors. The authors have proposed an optical measurement method based on auto-collimation, which evaluates the radial error motion according to the movements of a laser beam reflected from a target sphere attached to the spindle end. This optical measurement method is suitable for radial error measurements of miniature ultra-high-speed spindles because of its applicability to a small target sphere, high-speed response and minor susceptibility to electric noise. In this paper, the measurement principle, and basic characteristics of the optical measurement method in addition to an approximate analysis are shown. The radial error motion of a miniature ultra-high-speed spindle with a steel ball 1 mm in diameter are measured by an optical measuring device designed and manufactured to implement the proposed method. The measurement results show that the optical measuring device is able to measure the radial error motion of ultra-high-speed spindles with a maximum rotational speed of 200 krpm.

Introduction

In recent years, micro-machine tools and micro-factories have been studied for the realization of space-, energy- and resource-saving production systems [1], [2]. The miniature spindle is one of the main components in such micro-machine tools as the miniature lathe and the miniature milling machine. It is comparatively easy for miniature spindles to run at ultra-high speeds because the rotating part of the miniature spindle is thin and lightweight. The rotational speed of the miniature spindle for machining must be high in order to increase the cutting speed on a thin shaft. Therefore, a miniature spindle system with a maximum rotational speed of 300 kilo revolutions per minute (krpm) has already been demonstrated [3].

The spindle axis is displaced by the rotation, and the width of the spindle run-out is called the rotation accuracy. The spindle run-out is composed of radial error motion, angular error motion and axial error motion. In general, the radial error motion is the principal component of the three error motions. Therefore, this study focuses on the radial error motion of the spindle. In this paper, the rotation accuracy is the width of the radial error motion. The measurement of the spindle rotation accuracy is very important for verifying the machining accuracy. In the conventional measurement method for spindle rotation accuracy [4], [5], capacitive-type displacement sensors measure a target such as a high-precision cylinder or ball attached to the spindle axis in the radial direction, as shown in Fig. 1. The target used in the conventional method is relatively large and heavy because capacitive-type displacement sensors have limitations in measuring the small diameter of the target [6], [7], while the target attached to the miniature spindle must be small and lightweight to avoid an imbalance of the spindle axis. In general, the minimum applicable diameter of the target is about four times the electrode diameter on the sensor head in the case of a cylindrical target and about five times in the case of a spherical target. Moreover, though the capacitive-type displacement sensor can respond at a high speed in principle, the signal to noise ratio (SNR) of the output signal in a broad frequency band is degraded not only by noise from the spindle motor but also by the structure of the target, which is electrically insulated by ceramics bearings built into the spindle. Therefore, it is very difficult to precisely measure the rotation accuracy of the miniature ultra-high-speed spindle by the conventional measurement method or other advanced methods [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] using the capacitive-type displacement sensors.

Some optical measurement systems for spindle rotation accuracy [19], [20], [21], [22], [23], [24], [25], [26], [27] have already been proposed in order to solve the measurement problems caused by the capacitive-type displacement sensors. The general characteristics of the measurement method based on the optical principle are high resolution, high-speed response, minor susceptibility to electric noise, non-contact, long working distance, and so on. Although the purpose of these optical measurement systems is mainly their application to high-precision and high-speed spindles, there are no systems applicable to the radial error measurements of a miniature spindle running at hundreds of thousand of revolutions per minute. Therefore, the goal of this study is to develop an optical measuring device for the radial error motion of miniature ultra-high-speed spindles. In the optical measurement method proposed in this study, the radial error motion of the spindle axis is evaluated according to the movement of the light beam reflected from the target sphere attached to the spindle end [28]. This method has advantages in (a) the applicability to a small target, (b) the availability of small and high-precision targets and (c) the ease of alignment of a small target in comparison to other measurement methods. The reasons behind these advantages are (a) condensing of a laser beam by a lens, (b) availability of high-precision steel balls for miniature ball bearings and (c) centering is the only alignment required for the target sphere to the spindle axis.

In this paper, first, the principle of the optical measurement method for the rotation accuracy of miniature ultra-high-speed spindles is presented, and the basic characteristics of the measurement method are analyzed. Then, the configuration of an optical measuring device based on the principle of the optical measurement method is described, and the data of the optical measuring device in the response test, noise test and calibration test are shown. After that, the radial error measurement of a miniature ultra-high-speed spindle with a maximum rotational speed of 200 krpm is demonstrated by the optical measuring device and the measurement results are shown in Lissajous figures. Finally, the results obtained from this study are summarized.