1、A new method and instrument for measuring circular motionerror of NC machine toolsAbstractA 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 ball
2、s at each end and a high accuracy rotary encoder. The radius variations 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 d
3、isc. The bar can be expanded and contracted along its axis for different 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 c
4、ircular motion trajectories. The instrument can be widely used especially in the error-compensation and error-source project in the industrial application.1. IntroductionIn recent years, the precision machining process has attracted much attention from numerous investigators. One of its important ta
5、sks 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 and linear motion accuracy. However, it is still difficult t
6、o measure precisely the circular motion error let alone a more general motion trajectory.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-dimensional probe 2.(2) A disk (or a ring gage) and a two-di
7、mensional probe 3.(3) The double ball bar system (DBB) consists of a flexural bar 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 using the flexural bar while the machine moves in circular mo
8、tion 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 really used to assess the trajectory accuracy of circular moti
9、on and some usefully results have been gained. There exist some problems to some extent in their practicalapplications as those pointed out by several researchers 1,6. In (1) and (2), there is limitation by its own stranded disk accuracy. High accuracy can be obtained by (3) but only in the radius d
10、irection. 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, has an excellent performance, but it is very costly and the b
11、iggest 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 path where radius and angle information can be gained and comm
12、anded 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 low cost.2. Outline of the measurement instrumentA schematic
13、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 endof the bar similar to DBB and a rotary encoder, which is set at root end of ball 1 to detect rotation angles of the bar. Ball 2 is connected to
14、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 of the machine table andperpendicular to the rotation axis of t
15、he 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,
16、 Y, so thexy plane is located on the rotation plane of the linear transducer bar.Fig. 1. Schematic diagram of the prototype measurement.3. Principle of the measurement methodIn order to avoid the velocity lag of the axis servo, which always cause the actual point dropping behind the commanded point in machine moving, machine must beprogrammed to move in such a way that a point P(xi,yi) is moved along a circle and stopped at an
