6. Repeatable indexing accuracy test of rotary table
The measurement method is to repeat positioning 3 times at any three positions within one week of the rotary table, and test in the forward and reverse directions respectively. The maximum indexing accuracy of the difference between all readings and the theoretical value of the corresponding position. If it is a CNC rotary table, take a measuring point every 30 as the target position, and perform 5 rapid positioning of each target position from the forward and reverse directions respectively, measure the difference between the actual position and the target position, that is, the position deviation, and then calculate the standard deviation according to the method specified in GB10931-89. 6 times the maximum value of the standard deviation of each measuring point is the repeatable indexing accuracy of the CNC rotary table.
7. Origin return accuracy test of rotary table
The measurement method is to perform origin return from 7 arbitrary positions respectively, measure its stop position, and use the maximum difference read as the origin return accuracy.
It should be pointed out that the existing positioning accuracy detection is measured under fast and positioning conditions. For some CNC machine tools with poor feed system style, different positioning accuracy values will be obtained when different feed speeds are used for positioning. In addition, the positioning accuracy measurement results are related to the ambient temperature and the working state of the coordinate axis. At present, most CNC machine tools use a semi-closed loop system, and the position detection elements are mostly installed on the drive motor. It is not surprising that an error of 0.01~0.02mm is generated within a 1m stroke. This is an error caused by thermal elongation. Some machine tools use pre-stretching (pre-tightening) methods to reduce the impact.
The repeatability of each coordinate axis is the most basic accuracy indicator of the axis. It reflects the stability of the axis's motion accuracy. It cannot be assumed that a machine tool with poor accuracy can be used stably in production. At present, due to the increasing functions of CNC systems, the system errors of the motion accuracy of each ejector, such as the accumulated pitch error and the reverse clearance error, can be compensated by the system. Only the random error cannot be compensated, and the repeatability accuracy reflects the comprehensive random error of the feed drive mechanism. It cannot be corrected by CNC system compensation. When it is found to be out of tolerance, only the feed transmission chain can be fine-tuned. Therefore, if it is allowed to choose a machine tool, it is better to choose a machine tool with high repeatability accuracy.