How to detect the positioning accuracy of CNC machine tools

The positioning accuracy of the controlled machine tool refers to the positional accuracy that can be achieved by the movement of each coordinate axis of the machine tool under the control of the numerical control device. The positioning accuracy of CNC machine tools can be understood as the motion accuracy of the machine tool. Ordinary machine tools are manually fed. The positioning accuracy is mainly determined by the reading error. The movement of the CNC machine tool is realized by digital program instructions. Therefore, the positioning accuracy is determined by the numerical control system and mechanical transmission error. The movement of each moving part of the machine tool is completed under the control of the numerical control device. The precision that each moving part can achieve under the control of the program command directly reflects the precision that the machined part can achieve. Therefore, the positioning accuracy is an important test. content.

1. Positioning accuracy detection of rotary table

Measuring tools include standard turret, angle polyhedron, circular grating and collimator (collimator), etc., which can be selected according to the specific conditions. The measurement method is to make the table forward (or reverse) to an angle and stop, lock, and position. Use this position as a reference, then quickly rotate the table in the same direction and measure every 30 locks. Each of the forward rotation and the reverse rotation is measured for one week, and the maximum value of the difference between the actual rotation angle of each positioning position and the theoretical value (command value) is the division error. If it is a CNC rotary table, it should be a target position every 30, for each target position to quickly locate 7 times from the positive and negative directions, the difference between the position and the target position is actually reached, and then according to GB10931- 89 The method specified in the “Method for Evaluating the Position Accuracy of Digital Control Machines” calculates the average position deviation and standard deviation, the difference between the maximum value of all the average position deviations and the standard deviation and the sum of all the average position deviations and the standard deviation. Is the positioning accuracy error of the CNC rotary table.

Considering the dry-type transformer to the actual use requirements, it is generally important to measure several equal-angle points such as 0, 90, 180, 270, etc., and the accuracy of these points is required to be improved by one level compared with other angular positions.

2. Repeated indexing accuracy detection of rotary table

The measurement method is repeated three times in three positions in one week of the rotary table, and the detection is performed in the forward and reverse directions respectively. The maximum value of the difference between the values of all readings and the theoretical value of the corresponding position. If it is a CNC rotary table, take one measurement point every 30 as the target position, and perform 5 fast positionings for each target position from the positive and negative directions respectively, and measure the difference between the actual arrival position and the target position. That is, the position deviation, and then calculate the standard deviation according to the method specified in GB10931-89, and the maximum value of the standard deviation of each measurement point is 6 times, which is the repeating indexing accuracy of the numerical control rotary table.

3. The origin return accuracy detection of the rotary table

The measurement method is to perform the origin return from 7 arbitrary positions, measure the stop position, and use the maximum difference read as the origin return accuracy.

It should be pointed out that the detection of the existing positioning accuracy is measured under the condition of fast and positioning. For some CNC machine tools whose feeding system is not very good, different positioning accuracy values will be obtained when positioning with different feed speeds. In addition, the measurement result of the positioning accuracy is 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 detecting components are mostly mounted on the driving motor, which produces an error of 0.01 to 0.02 mm in a stroke of 1 m. It is not strange. This is an error caused by thermal elongation, and some machines use a pre-stretch (pre-tightening) method to reduce the effect.

The repeated positioning accuracy of each coordinate axis reflects the most basic accuracy index of the axis, which reflects the stability of the motion accuracy of the axis, and it cannot be assumed that the machine tool with poor precision can be stably used for production. At present, due to the increasing number of functions of the numerical control system, system errors such as pitch accumulation error and backlash error can be compensated for the motion accuracy of each sitting injector. Only the random error can not be compensated, and the repeat positioning accuracy is repeated. It reflects the comprehensive random error of the feed drive mechanism. It can't be corrected by the CNC system compensation. When it is found to be out of tolerance, only the fine adjustment of the feed drive chain is performed. Therefore, if the machine tool is allowed to be selected, it is better to select a machine with a high repeatability.