Error precision detection method of CNC machine tool

The positioning accuracy of a CNC machine tool refers to the position accuracy that can be achieved by the movement of each coordinate axis of the machine tool under the control of the CNC device. The positioning accuracy of a CNC machine tool can be understood as the motion accuracy of the machine tool. Ordinary machine tools are fed manually, and the positioning accuracy is mainly determined by the reading error, while the movement of the CNC machine tool is realized by digital program instructions, so the positioning accuracy is determined by the CNC 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 accuracy of each moving part under the control of the program instruction directly reflects the accuracy that can be achieved by the machined part. Therefore, the positioning accuracy is a very important inspection. content.

1. Detection of linear motion positioning accuracy Linear motion positioning accuracy is generally carried out under no-load conditions of the machine tool and worktable. According to national standards and the provisions of the International Organization for Standardization (ISO standards), the inspection of CNC machine tools should be subject to laser measurement. In the absence of a laser interferometer, it is also possible for general users to use a standard scale with an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument must be 1 to 2 levels higher than the accuracy of the measurement. In order to reflect all the errors in multiple positioning, the ISO standard stipulates that each positioning point is calculated based on the five measurement data and the positioning point dispersion band composed of the average value and the dispersion difference -3 dispersion band. 2. Detection of repeat positioning accuracy of linear motion The instrument used for detection is the same as that used for detecting positioning accuracy. The general detection method is to measure at any three positions close to the midpoint and both ends of each coordinate stroke, each position is positioned with rapid movement, and the positioning is repeated 7 times under the same conditions, and the stop position value is measured and the reading is larger. value. Take one half of the difference between the larger of the three positions and attach the positive and negative signs as the repeated positioning accuracy of the coordinate, which is a basic indicator reflecting the stability of the axis motion accuracy. 3. Detection of the return accuracy of the origin of linear motion The return accuracy of the origin is essentially the repeated positioning accuracy of a special point on the coordinate axis, so its detection method is completely the same as the repeated positioning accuracy. 4. Reverse error of linear motion detection The reverse error of linear motion, also called loss of momentum, includes the reverse of the driving parts (such as servo motor, servo hydraulic motor, stepper motor, etc.) on the coordinate axis feed transmission chain. To the dead zone, the comprehensive reflection of errors such as backlash and elastic deformation of each mechanical motion transmission pair. The greater the error, the lower the positioning accuracy and repeat positioning accuracy. The detection method of the reverse error is to move a distance in the forward or reverse direction in advance within the stroke of the measured coordinate axis and use the stop position as the reference, and then give a certain movement command value in the same direction to make it move a certain distance. Then move the same distance in the opposite direction, and measure the difference between the stop position and the reference position. Perform multiple measurements (generally 7 times) at the three positions near the midpoint and both ends of the stroke, find the average value of each position, and take the larger value of the average value obtained as the reverse error value. 5. Detection of the positioning accuracy of the rotary table. Measuring tools include standard turntables, angle polyhedrons, circular gratings and parallel light tubes (collimators), etc., which can be selected according to specific conditions. The measuring method is to make the worktable rotate an angle in the forward (or reverse) direction, stop, lock, and position, use this position as the reference, then quickly rotate the worktable in the same direction, lock the position every 30, and measure. The forward rotation and the reverse rotation are measured one round each, and the larger of the 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, it should take every 30 as a target position. For each target position, fast positioning is performed 7 times from the forward and reverse directions. The difference between the actual reached position and the target position is the position deviation, and then press GB10931- The method specified in 89 "Digital Control Machine Tool Position Accuracy Evaluation Method" calculates the average position deviation and standard deviation, the sum of the larger value of all the average position deviations and standard deviations and the sum of the smaller values of all the average position deviations and standard deviations The difference is the positioning accuracy error of the CNC rotary table. Considering the requirements of dry-type transformers to actual use, it is generally necessary to perform key measurements on several right-angle equal points such as 0, 90, 180, and 270, and the accuracy of these points is required to be improved by one level compared with other angular positions. 6. Repeated indexing accuracy detection of the rotary table. The measurement method is to repeat positioning at any three positions within a circle of the rotary table for 3 times, and perform detection under the rotation of the forward and reverse directions respectively. The indexing accuracy of the larger value of the difference between all the readings 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 perform 5 rapid positioning of each target position from the forward and reverse directions respectively, and measure the difference between the actual reached position and the target position. That is, the position deviation, and then calculate the standard deviation according to the method specified in GB10931-89. The 6 times the larger value of the standard deviation of each measuring point is the repeat indexing accuracy of the CNC rotary table. 7. Detection of the return-to-origin accuracy of the rotary table. The measurement method is to perform a return-to-origin from 7 arbitrary positions, determine the stop position, and use the larger difference as the accuracy of the return-to-origin. It should be pointed out that the existing detection of positioning accuracy is measured under the condition of fast and positioning. For some CNC machine tools with poor feed system demeanor, when positioning with different feed speeds, different positioning accuracy values will be obtained. In addition, the measurement results of the positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. At present, most CNC machine tools adopt a semi-closed loop system, and most of the position detection components are installed on the drive motor, which produces an error of 0.01~0.02mm within a 1m stroke. It is not surprising. This is an error caused by thermal elongation. Some machine tools use pre-tensioning (pre-tensioning) to reduce the impact. The repeated positioning accuracy of each coordinate axis reflects the basic accuracy index of the axis. It reflects the stability of the axis motion accuracy. It is impossible to imagine that the machine tool with poor accuracy can be used in production stably. At present, due to more and more functions of the CNC system, the system error of the movement accuracy of each ejector mark, such as the accumulated pitch error and the backlash error, can be systematically compensated. Only the random error cannot be compensated, and the repeated positioning accuracy It just reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by numerical control system compensation. When it is found to be out of tolerance, only the feed drive chain can be fine-tuned and corrected. Therefore, if the machine tool is allowed to be selected, it is better to choose a machine tool with high repeat positioning accuracy. 1. Detection of linear motion positioning accuracy

  Linear motion positioning accuracy is generally carried out under no-load conditions of the machine tool and worktable. According to national standards and the provisions of the International Organization for Standardization (ISO standards), the inspection of CNC machine tools should be subject to laser measurement. In the absence of a laser interferometer, it is also possible for general users to use a standard scale with an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument must be 1 to 2 levels higher than the accuracy of the measurement.

  In order to reflect all the errors in multiple positioning, the ISO standard stipulates that each positioning point is calculated based on the five measurement data and the positioning point dispersion band composed of the average value and the dispersion difference -3 dispersion band.