Seven Ways to Detect the Positioning Accuracy of CNC Machine Tools

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.

CNC machine tool is the abbreviation of digital control machine tool, which is an automatic machine tool equipped with a program control system. The control system can logically process programs with control codes or other symbolic instructions, decode them, and express them with coded numbers. Nanjing Fourth Machine Tool Co., Ltd. inputs them to the numerical control device through the information carrier. After arithmetic processing, the numerical control device sends out various control signals to control the action of the machine tool, and automatically process the parts according to the shape and size required by the drawing.
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 on 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 calculates the average value and dispersion based on the five measurement data, and the dispersion zone constitutes the positioning point dispersion band.

2. Detection of repeat positioning accuracy of linear motion
The instrument used for testing is the same as that used for testing 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 maximum difference in the reading is obtained. . Take one-half of the largest difference among the three positions and attach the positive and negative signs as the repeated positioning accuracy of the coordinate, which is the most basic indicator reflecting the stability of the axis motion accuracy.
3. Detection of the return accuracy of the origin 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 completely the same as the repeated positioning accuracy.

4. Reverse error detection of linear motion
The reverse error of linear motion, also called loss of momentum, includes the reverse dead zone of the driving part (such as servo motor, servo hydraulic motor, stepper motor, etc.) on the coordinate axis feed transmission chain, and each mechanical motion transmission pair A comprehensive reflection of errors such as backlash and elastic deformation. 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 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), 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 for one circle each, and the maximum value of the difference between the actual rotation angle and the theoretical value (command value) of each positioning position 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 difference between the maximum sum of all average position deviations and standard deviations and the sum of the minimum sum of all average position deviations and standard deviations , 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 rotary table
The measuring method is to repeat positioning at any three positions within a circle of the rotary table for 3 times, and perform detection under rotation 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 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 maximum value of the standard deviation of each measuring point is 6 times, which 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 maximum difference read out as the precision 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 result of positioning accuracy is 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 is the most basic accuracy index reflecting the axis, and it reflects the stability of the axis’s movement accuracy. It is impossible to imagine that the machine tool with poor accuracy can be used in production stably. At present, as the CNC system has more and more functions, 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.

Post time: Jun-01-2021