Measuring method for positioning accuracy of CNC metal lathe


1 Linear motion positioning accuracy detection

The positioning accuracy of linear motion is generally carried out under the no-load condition of machine tool and workbench. According to the national standards and the provisions of the International Organization for Standardization (ISO standards), the detection of CNC metal lathe shall be subject to laser measurement. In the absence of a laser interferometer, a standard scale and an optical reading microscope can also be used for comparison and measurement by ordinary users. However, the accuracy of the measuring instrument must be 1-2 grades higher than the accuracy of the measured instrument. In order to reflect all errors in multiple positioning, the ISO standard stipulates that the average value and dispersion of each positioning point are calculated according to the five measurement data, and the dispersion zone constitutes the dispersion zone of the positioning point.


2 Repetitive positioning accuracy detection of linear motion

The instrument used for detection is the same as that used for detection of positioning accuracy. The general detection method is to measure at any three positions near the midpoint of each coordinate stroke and at both ends. Each position is positioned by rapid movement. Repeat the positioning for seven times under the same conditions, measure the stop position value and calculate the maximum difference of readings. One half of the maximum difference of the three positions is attached with positive and negative symbols as the repeated positioning accuracy of the coordinate, which is the most basic index reflecting the stability of the axis motion accuracy.


3 Origin return accuracy detection of linear motion

The origin return accuracy is essentially the repeated positioning accuracy of a special point on the coordinate axis, so its detection method is the same as the repeated positioning accuracy.


4 Reverse error detection of linear motion

The greater the reverse error of linear motion, the lower the positioning accuracy and repeated positioning accuracy. The detection method of reverse error is to move forward or backward for a distance within the stroke of the measured coordinate axis and take the stop position as the reference, then give a certain movement command value in the same direction to move it for a distance, and then move the same distance in the opposite direction to measure the difference between the stop position and the reference position. Carry out multiple measurements (generally 7 times) at three positions near the midpoint and both ends of the stroke, and calculate the average value at each position, and take the maximum value of the average value as the reverse error value.


5 Detection of positioning accuracy of rotary table

Measuring tools include standard turntable, angle polyhedron, circular grating and collimator (collimator), which can be selected according to specific conditions. The measurement method is to turn the workbench forward (or backward) for an angle and stop, lock, and position it. Use this position as a reference, and then quickly turn the workbench in the same direction, lock and position it every 30 seconds, and measure. Measure forward rotation and reverse rotation for one cycle respectively, and the maximum difference between the actual rotation angle of each positioning position and the theoretical value (command value) is the indexing error. If it is a CNC rotary table, take every 30 as a target position, and quickly locate each target position seven times from the positive and negative directions.

The difference between the actual position and the target position is the position deviation. Then calculate the average position deviation and standard deviation according to the specified method, the difference between the maximum value of all average position deviation and standard deviation and the minimum value of all average position deviation and standard deviation, It is the positioning accuracy error of the NC rotary table. Considering the actual use requirements of dry-type transformer, several right-angle bisection points such as 0, 90, 180 and 270 are generally measured with emphasis, and the accuracy of these points is required to be improved by one level compared with other angle positions.


6 Repeated indexing accuracy detection of rotary table

The measurement method is to select three positions within one week of the rotary table and repeat the positioning for three times, and then conduct the detection under the positive and negative rotation respectively. The maximum graduation accuracy of the difference between all reading values and the theoretical value of the corresponding position. If it is a CNC rotary table, take one measuring point every 30 as the target position, and quickly locate each target position five times from the positive and negative directions, 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 specified method. Six times of the maximum standard deviation of each measuring point is the repeated indexing accuracy of the CNC rotary table.


7 Detection of zero point reset accuracy of rotary table

The measurement method is to reset the original point from seven arbitrary positions, determine its stop position, and take the maximum difference read as the accuracy of the original point reset. It should be pointed out that the existing positioning accuracy detection is measured under the condition of fast and positioning. For some CNC metal lathe with poor feed system, different positioning accuracy values will be obtained when positioning at different feed speeds. In addition, the measurement results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis.

At present, most CNC metal lathe adopt semi-closed loop system, and the position detection elements are mostly installed on the drive motor. It is not surprising that 0.01-0.02mm error is generated within 1m travel. This is the error caused by thermal elongation. Some machine tools use the method of pre-stretching (pre-tightening) to reduce the impact. The repeated positioning accuracy of each coordinate axis is the most basic accuracy index of the axis, which reflects the stability of the movement accuracy of the axis. It cannot be assumed that the machine tool with poor accuracy can be stably used for production. At present, due to the more and more functions of the numerical control system, the system error of the standard motion accuracy of each injector, such as the pitch accumulation error and the reverse clearance error, can be compensated systematically.

Only the random error can not be compensated, and the repeated positioning accuracy precisely reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by the compensation of the numerical control system. When it is found that it is out of tolerance, only the fine adjustment and correction of the feed drive chain can be carried out. Therefore, if it is allowed to select the cnc metal lathe, it is better to select the machine tool with high repeated positioning accuracy.