1、A new method and instrument for measuring circular motionerror of NC machine toolsH.L. Liu*, H.M. Shi, B. Lia, X. LiNational CNC Control System Research Center, Mechanical Department, Huazhong University of Science and Technology, Wuhan 430074, ChinaZhanjiang Ocean University, Zhanjiang, Guangdong 5
2、24025, ChinaAbstract A new method and instrument for measuring circular motion error of numerical control (NC) machine tools are described in this paper. The instrument consists of a linear displacement transducer bar with two balls at each end and a high accuracy rotary encoder. The radius variatio
3、ns are detected by the transducer and the rotation angle of the bar is measured by the rotary encoder while the machine is moving in a circular path. The measuring area is circular except for a small area around the center of the disc. The bar can be expanded and contracted along its axis for differ
4、ent application. The instrument has a compact structure and can be installed on a machine tool simply and quickly. It is shown by the experimental results that the instrument has good repeatability and high precision of measuring circular motion trajectories. The instrument can be widely used especi
5、ally in the error-compensation and error-source project in the industrial application.q 2005 Elsevier Ltd. All rights reserved.Keywords: NC machine tool; Motion error; Measurement instrument; Circular measurement1. Introduction In recent years, the precision machining process has attracted much atte
6、ntion from numerous investigators. One of its important tasks for error-compensation and errorsource diagnosis is to map the volumetric errors of a machine tool 1. Current techniques have the ability to measure parametric error function for each of the machines axes, for instance, the positioning an
7、d linear motion accuracy. However, it is still difficult to measure precisely the circular motion error let alone a more general motiontrajectory. Several devices and methods are usually used to measure the trajectory accuracy of circular motions, and described as follows:(1) A test bar and a one-di
8、mensional probe 2.(2) A disk (or a ring gage) and a two-dimensional probe 3.(3) The double ball bar system (DBB) consists of a flexuralbar at each end and two magnetic balls constrained by special sockets with a spherical surface. The deviation of the relative distance between balls is measured usin
9、g the flexural bar while the machine moves in circular motion 4,5.(4) The double-bar linkage and two rotary encoders are set at the root end of the link separately to detect the rotation angles of the links 6,7.(5) The KGM circular test system 8. The above-mentioned instruments and methods have real
10、ly used to assess the trajectory accuracy of circular motion and some usefully results have been gained. There exist some problems to some extent in their practical applications as those pointed out by several researchers 1,6. In (1) and (2), there is limitation by its own stranded disk accuracy. Hi
11、gh accuracy can be obtained by (3) but only in the radius direction. Although circular motion errors along x axis and y axis in XY plane (two-dimensional error) can be got in (4), the instrument is very complicated and will be made hardly. KGM circular test system, also called cross-grid encoder, ha
12、s an excellent performance, but it is very costly and the biggest measuring zone is merely a circular of about radius 120 mm. A new method and instrument for measuring the circular motion error of NC machine tools is presented in this paper. When the machine is stopped at points along the circular p
13、ath where radius and angle information can be gained and commanded and actual points can be compared, errors of x and y axis can be measured which is a two-dimensional error. Moreover, this method is simple and convenient in practical application, while the instrument is of a compact structure with
14、low cost.2. Outline of the measurement instrument A schematic diagram of the presented prototype measurement instrument is shown in Fig. 1. This instrument consists of a linear transducer bar with two balls at each end of the bar similar to DBB and a rotary encoder, which is set at root end of ball
15、1 to detect rotation angles of the bar. Ball 2 is connected to machine tool spindle by ball holder. The base of the instrument is fixed on table of an NC machine tool to be measured, for example, xy stage of the machine center. The rotation plane of the linear transducer bar is parallel to xy plane
16、of the machine table and perpendicular to the rotation axis of the spindle. The measuring coordinate frame can be set as that the rotation axis of the encoder on the root of ball 1 is defined as the zaxis, which is parallel to the rotation axis of the spindle denoted as Z. The x and y axes are set to be parallel respectively to the machine tool and denoted as X, Y, so the xy plane is located on the rotatio
