The positioning accuracy of the CNC lathe refers to the position accuracy that can be achieved by the coordinate axes of the lathe under the control of the CNC device. The positioning accuracy of the CNC lathe can be understood as the movement accuracy of the lathe. Ordinary lathes are fed manually, and the positioning accuracy is mainly determined by the reading error, while the movement of the CNC lathe is realized by digital program instructions, so the positioning accuracy is determined by the CNC system and mechanical transmission errors. The movement of the moving parts of the lathe 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, so the positioning accuracy is a very important detection content.
The following 7 items explain the main content of the CNC lathe positioning accuracy detection in detail, I hope to help you:
1. Linear motion repeat positioning accuracy detection
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 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. . 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.
2. Linear motion positioning accuracy detection
Linear motion positioning accuracy is generally carried out under no-load conditions on the lathe and the worktable. According to national standards and the International Standardization Organization (ISO), the inspection of CNC lathes shall 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 1-2 levels higher than the measured accuracy.
In order to reflect all the errors in multiple positioning, the ISO standard requires each positioning point to calculate the average value and dispersion ± 3 dispersion bands of positioning points based on five measurement data.
3. Linear motion origin return accuracy detection
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. Positioning accuracy detection of rotary table
Measuring tools include standard turntable, angle polyhedron, circular grating and collimator (collimator), etc., which can be selected according to the specific situation. 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 taken as a target position, and each target position should be quickly positioned 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 according to GB regulations The method calculates the average position deviation and standard deviation, and the difference between the maximum value of all average position deviations and the standard deviation and the sum of all the 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 several right-angled equal points 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.
5. Reverse error detection of linear motion
Reverse error of linear motion, also called loss of momentum, which includes the reverse dead zone of the driving part on the coordinate axis feed drive chain, the comprehensive reflection of the reverse clearance and elastic deformation of each mechanical motion transmission pair. 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 obtained average value is the reverse error value.
6. Repeated indexing accuracy detection of rotary table
The measuring method is to repeat the positioning three times at any three positions within one week of the rotary table, and detect them by rotating 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 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 by GB. 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 accuracy 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 out as the return-to-origin accuracy. It should be pointed out here that the detection of the existing positioning accuracy is measured under the condition of fast and positioning. For some CNC lathes with poor feeding system, when different feeding speeds are used 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 lathes 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 lathes use pre-stretching (pre-tensioning) to reduce the impact.
The repeat positioning accuracy of each coordinate axis is the most basic accuracy index reflecting the axis, which reflects the stability of the motion accuracy of the axis. It cannot be assumed that a lathe 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 the lathe is allowed to be selected, a lathe with high repeat positioning accuracy should be selected.
