The positioning accuracy of the CNC machining center 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 the CNC machining center 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, and the movement of the CNC machine tool is realized by digital program instructions, so the positioning accuracy is determined by the error of the CNC system and the mechanical transmission. The movement of the moving parts of the machine tool is completed under the control of the numerical control device. The accuracy of the moving parts under the control of the program instructions directly reflects the accuracy of the processed parts. Therefore, the positioning accuracy is a very important detection content.
1. Linear motion positioning accuracy detection
Linear motion positioning accuracy is generally carried out under no-load conditions on the machine tool and worktable. According to the national standard and the International Standardization Organization (ISO standard), the inspection of CNC machine tools should be subject to laser measurement. In the absence of a laser interferometer, for the average user can also use a standard scale with an optical reading microscope for comparative measurements. However, the accuracy of the measuring instrument must be 1 to 2 levels higher than the measured accuracy.
In order to reflect all errors in multiple positionings, the ISO standard requires each positioning point to calculate the average value and the dispersion difference-3 dispersion difference bands of the positioning point based on five measurement data.
2, Linear motion repeat positioning accuracy detection
The instruments used for testing are the same as those 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. The stop position value is measured and the maximum difference in reading is obtained . Take one-half of the maximum difference in the three positions, and attach positive and negative signs as the repeated positioning accuracy of the coordinate. It is the most basic index that reflects the stability of the accuracy of the axis motion.
3, linear motion origin return accuracy detection
The origin return accuracy is essentially the repeat positioning accuracy of a special point on the coordinate axis, so its detection method is exactly the same as the repeat positioning accuracy.
4. Reverse error detection of linear motion
The reverse error of linear motion, also called the amount of loss, which includes the reverse dead zone of the driving part (such as servo motor, servo hydraulic motor and stepper motor, etc.) on the coordinate axis feed transmission chain, and each mechanical motion transmission pair The comprehensive reflection of errors such as backlash and elastic deformation. The greater the error, the lower the positioning accuracy and repeated positioning accuracy.
The detection method of the reverse error is to move a distance in the forward or reverse direction in advance in the measured coordinate axis and use this stop position as a reference, and then give a certain movement command value in the same direction to move it a distance, Then move the same distance in the opposite direction and measure the difference between the stop position and the reference position. Measure multiple times (generally 7 times) at the three points near the midpoint and both ends of the stroke, and find the average value at each position. The maximum value of the average value is the reverse error value.
5. Positioning accuracy detection of rotary table
Measuring tools include standard rotary table, angle polyhedron, circular grating and collimator (collimator), etc., which can be selected according to specific conditions. The measurement method is to rotate the worktable forward (or reverse) by an angle and stop, lock, and position, using this position as a reference, and then quickly rotate the worktable in the same direction, locking and positioning every 30th, and measuring. The forward rotation and the reverse rotation are 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 indexing error. If it is a CNC rotary table, each 30 should be a target position, and each target position should be quickly positioned 7 times from the forward and reverse directions. The difference between the actual position and the target position is the position deviation, and then press GB10931- 89 “Evaluation Method for Positional Accuracy of Digitally Controlled Machine Tools” calculates the average position deviation and standard deviation, the difference between the sum of all average position deviations and the maximum value of the standard deviation and the sum of all average position deviations and the minimum value of the standard deviation Is the positioning accuracy error of the CNC rotary table.
Considering the requirements of dry-type transformers to actual use, it is generally important to measure the right-angle bisectors such as 0, 90, 180, 270, etc., and the accuracy of these points is required to be improved by one level compared to other angular positions.
6. Repetitive indexing accuracy detection of rotary table
The measurement method is to repeat the positioning at any three positions within a week of the rotary table 3 times, and detect under the rotation of 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 a measurement point every 30 as the target position, and perform five quick positionings on each target position from the forward and reverse directions, and 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 method specified in GB10931-89. The standard deviation of each measurement point is 6 times the maximum value, which is the repeating index accuracy of the CNC rotary table.
7. Return-to-origin precision detection of rotary table
The measuring method is to perform a return-to-origin from each of 7 arbitrary positions, measure the stop position, and use the maximum difference read as the return-to-origin accuracy.
It should be pointed out that the detection of the existing positioning accuracy is measured under the condition of rapid and positioning. For some CNC machine tools with poor feed system style, when using different feed speeds for positioning, different positioning accuracy values will be obtained. 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 detection elements are mostly installed on the drive motor, resulting in an error of 0.01 ~ 0.02mm within a 1m stroke. It is not strange. This is an error caused by thermal elongation. Some machine tools use pre-stretching (pre-tensioning) to reduce the impact.
The repeated positioning accuracy of each coordinate axis is the most basic accuracy index reflecting the axis. It reflects the stability of the motion accuracy of the axis. It cannot be assumed that machine tools with poor accuracy can be used stably in production. At present, due to more and more functions of the CNC system, system errors such as pitch accumulation error and backlash error can be compensated systematically for each sitting injector standard motion accuracy. Only random errors cannot be compensated, while repeated positioning accuracy It reflects the comprehensive random error of the feed drive mechanism. It cannot be corrected by CNC system compensation. When it is found to be out of tolerance, only the feed drive chain is fine-tuned and corrected. Therefore, if it is allowed to choose the machine tool, it is better to choose a machine tool with high repeat positioning accuracy.