In the past, the method of checking the flatness error of the machine tool work table has been the "two-point method", that is, using parallel rulers, micrometers, contour pads, frame level gauges, etc., to measure along certain straight lines of the work table. It is understood as a large straightness error on any section. However, it is inconsistent with the flatness definition rules in the national standard of the public service of shape and position. The current JB2670-82 "Metal Cutting Machine Inspection Regulations" points out that the inspection and measurement of flatness errors are based on the appearance of the containment practice and the interval is small. Space between parallel planes

From this definition, it can be seen that the orientation of the containment plane as the basis for identification also needs to be determined based on the principle of minor conditions. Therefore, it is necessary to fully grasp the error status of the tested appearance first, and then determine the appraisal standard according to the principle of certainty. In this way, it is decided that the measurement must be divided into two processes, that is, first measure the height of a number of points on the practical surface related to an arbitrary aspiration plane (measurement datum), and then through benchmark conversion and other methods, in order to find the error that fits the definition. value.