The use of CNC machine tools has become increasingly widespread in the manufacturing industry. Ensuring the accuracy of these machines is a critical part of their acceptance and ongoing operation. The precision of CNC machine tools—covering aspects such as repeatability, positioning accuracy, and axis inversion—is fundamental to their overall performance. On January 1, 1990, the State Bureau of Technical Supervision officially issued the standard for evaluating the positional accuracy of numerically controlled machine tools—GB10931-89, titled "Evaluation Method of Positional Accuracy of Digitally Controlled Machine Tools." This document clearly defines terminology, formulas, and measurement methods. However, it does not specify strict limitations on the instruments used for measurement. Currently, common instruments for measuring positional accuracy include laser interferometers, precision scales with reading microscopes, reference step gauges, and lever dial gauges (or electronic micrometers). Among these, the method using a dial gauge ranks second only to laser interferometry. Yet, this approach is costly, and its step size may not fully align with the requirements outlined in GB10931-89. As a result, there is often a mismatch between cost and performance. When a company receives imported CNC equipment, we designed a custom gauge similar to a step gauge, which we call a "standard step gauge." Its structure is illustrated in the figure. The pitch of the gauge is set according to actual needs but must comply with the standards specified in GB10931-89. The top holes at both ends are not only for processing but also for securing the CNC lathe between the headstock and tailstock. Plane N ensures that the gauge can be easily placed on the machine tool table. During the manufacturing process, multiple aging treatments are performed, and cold treatment is recommended to minimize residual stress and improve dimensional stability. After production, the distance between adjacent sides (measured to 0.001mm) is recorded using high-resolution instruments like a tool microscope, and the values are engraved onto the corresponding positions of the gauge. If available, a coordinate measuring machine is ideal for measuring the spacing between adjacent faces. To test the CNC lathe using the quasi-step gauge, the fixture is mounted between the headstock and tailstock, and the lever dial gauge is fixed on the tool holder, positioned close to the gauge. The distance between the end faces is input into the numerical control system, and the testing program is prepared. When testing coordinate position accuracy, the quasi-step gauge is placed on the tool post with its plane parallel to the feed direction. The lever dial gauge can then be mounted on the bed, and the test can begin. For X and Y-axis measurements, the gauge is placed on the workbench, and the lever dial gauge is attached to the spindle. When measuring coordinates, the gauge’s plane is fixed to the workbench using a square ruler, and the rest of the procedure follows the same steps as with the CNC lathe.

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Self-made quasi-step distance structure chart

The quasi-step gauge features a simple design and is easy to manufacture, making it suitable for in-house production within a factory. It can be used for testing various CNC equipment, including lathes, machining centers, and milling machines. The distance between adjacent end faces can be customized based on the specific needs of the enterprise. Moreover, the measured distances do not have to be integers. According to paragraph 5.4 of the "Evaluation Method," the target position Pj = (J - 1)t + r, where J is the target position number, t is the pitch (an integer not equal to the drive screw lead), and r is any decimal value. Therefore, the spacing of the measuring surfaces on the gauge meets the requirements of the "Evaluation Method."

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