1、中文 3940 字 Procedure for performing flatness measurement Flatness measurements are performed to check the flatness of CMM tables and surface plates. It determines whether any significant peaks or troughs exist and quantifies them. If these errors are significant then remedial work, such as lapping, m
2、ay be required. To understand the basic principles and techniques of flatness measurement refer to: The principles of flatness measurement. Standard methods of assessing flatness. Principle of flatness measurement The flatness measurement kit is used to measure flatness. The angular interferometer i
3、s attached to the turning mirror and the angular reflector is attached on top of the selected flatness base. The angular interferometer is placed in the path between the laser head and the angular reflector. Figure 1 - Principle of measurement The laser beam is split into two by the beam-splitter in
4、side the angular interferometer. One part of the beam (the measurement beam A1) passes straight through the interferometer and is reflected by one of the twin reflectors of the angular reflector back through the interferometer and into the laser head. The other beam (measurement beam A2) passes thro
5、ugh the periscope part of the angular interferometer to the second reflector from where it returns through the interferometer and into the laser head. An angular measurement is produced by comparing the path difference between the beams A1 and A2, (i.e. the measurement is independent of the distance
6、 between the laser and the interferometer). The flatness reading displayed by the software is the incremental height between the front and back feet of the flatness baseplate on which the angular reflector is fitted. This incremental height is calculated from the angular measurement and knowledge of
7、 the distance between the centres of the front and back feet of the flatness baseplate. This distance, termed the foot-spacing, must be entered into the calibration software before measurement starts. For each measurement line (see Standard methods for assessing flatness), the angular interferometer
8、 (mounted on the flatness turning mirror) stays stationary, while the reflector (mounted on a flatness base) moves along the line in incremental steps defined by the foot-spacing. A flatness measurement is carried out by taking a series of incremental height readings as the angular reflector is move
9、d along the measurement path. Figure 2 - Incremental measurements drawing changes In Figure 2 above: Position I is the initial position at which the interferometer reading is normally datumed. Position II is one foot-spacing beyond Position I. The interferometer reading will be the incremental dista
10、nce d1, which is the difference in height (with respect to the datum line) of the front and back feet of the flatness baseplate. Position III is one foot-spacing beyond Position II. The interferometer reading will be d2, i.e. the difference in height between the front and back feet of the baseplate
11、in its new position. Similarly, the reading at Position IV will be d3, and so on for all subsequent positions on the measurement line. The actual flatness of the measurement line will be the algebraic sum of the reading d1, d2 etc (plus the reading at the datum position (I) if it had not been zeroed
12、). Note: Environmental compensation is not required when taking flatness measurements, as the difference in path lengths between the two beams is so small that the error due to environmental effects is negligible. Standard methods for assessing flatness To measure the flatness of a surface, a number
13、 of measurement lines need to be taken over the surface. There are two standard methods of carrying out flatness measurements: Moody method Grid method The flatness of a surface can be defined as the separation of two planes which are parallel to the general trajectory of the surface and which just
14、enclose the measured surface as shown in Figure 1. Figure 1 - Flatness specification Flatness deviations for a surface plate of a known size can be compared with permitted deviations in national standards. Moody method With the Moody method, measurement is restricted to the eight prescribed lines as
15、 shown in Figure 2. Figure 2 - Moody map of surface plate The Moody method was first proposed by J.C. Moody in 1955 and has subsequently achieved wide acceptance. The method provides a relatively quick method of calibrating a surface plate, with the results being presented as a contour plot along the eight measurement lines tested, in a format acceptable for certification.
