7 methods to detect the positioning accuracy of CNC machine tools
CNC machine tools are widely employed in the manufacture of machinery, and the precision of a CNC machine tool is used to judge its quality. With the ongoing development of precision machining technology, the precise requirements for CNC machine tools are increasing, therefore the accuracy must be positioned to determine whether or not the CNC machine is qualified. The next brief series of worldwide automation network will introduce you to numerous techniques of detecting positioning accuracy.
1. Linear motion positioning accuracy detection
The precision of linear motion positioning is often tested under no-load circumstances of the machine tool and worktable. The identification of CNC machine tools should be based on laser measurement, according to national requirements and International Organization for Standardization (ISO standard). In the absence of a laser interferometer, a standard scale can be utilized for regular users to compare measurements using an optical reading microscope. However, the measuring instrument's precision must be one to two classes greater than the measurement's accuracy.
The ISO standard requires that each positioning point compute the average value of five measurement data and the positioning point dispersion band generated by the dispersion-3 dispersion band in order to reflect all mistakes in multiple positioning.
2. Linear motion repeat positioning accuracy detection
The testing instruments are the same as those used to determine positioning accuracy. The general detection method is to measure at any three positions close to the midpoint and both ends of each coordinate stroke, position each position with rapid movement, and repeat the positioning 7 times under the same conditions, the value of the stop position is measured, and the maximum difference between the readings is obtained. As the repeated positioning accuracy of the coordinate, which is the most fundamental index representing the stability of the axis motion accuracy, one-half of the biggest difference in the three places is linked with a positive and negative sign.
3. Origin return accuracy detection of linear motion
Because the origin return accuracy is simply the repeated positioning accuracy of a specific point on the coordinate axis, its detection technique is identical to the repeated positioning accuracy.
4. Reverse error detection of linear motion
The reverse error of linear motion, also known as momentum loss, comprises the reverse dead zone of the drive elements (such as servo motors, servo hydraulic motors, and stepper motors) on the coordinate axis feed transmission chain, as well as the mechanical motion transmission pair. Errors like as backlash and elastic deformation are fully reflected. The bigger the mistake, the poorer the positioning accuracy and repeatability.
The reverse error detection method is to move a distance in advance in the forward or reverse direction within the stroke of the measured coordinate axis and use this stop position as the reference, and then give a specific movement command value in the same direction to move it for a certain distance. Then, move the same distance in the opposite direction and calculate the difference between the stop and reference positions. Take numerous measurements (typically seven times) at three places near the middle and both ends of the stroke, average the results, and use the maximum value of the acquired average value as the reverse error value.
5. Positioning accuracy detection of rotary table
Conventional measurement equipment include a standard turntable, an angle polyhedron, a circular grating, and a collimator (collimator), among others, which can be chosen based on the conditions. The measuring procedure is to turn the worktable forward (or backward) at an angle and halt, lock, and position it, then swiftly turn the worktable in the same direction, lock, and place it every 30 degrees, and measure. The indexing error is the maximum value of the difference between the actual rotation angle of each positioning location and the theoretical value (command value) after one cycle of measurement.
Every 30 points on a CNC rotary table should be utilized as a goal location. Fast positioning is conducted seven times from the forward and reverse directions for each target location. The position deviation is the difference between the actual attained position and the intended position, and then press GB10931- 89. The positioning accuracy error of the CNC rotary table is calculated using the method specified in "Assessment Method of Position Accuracy of Numerically Controlled Machine Tools," which is the difference between the maximum value of all average position deviation and standard deviation and the sum of the minimum value of all average position deviation and standard deviation.
Given the real usage requirements of dry-type transformers, it is often important to concentrate on the measurement of numerous right-angle equal points such as 0, 90, 180, and 270, and the accuracy of these points must be one level greater than other angular locations.
6. Repeated indexing accuracy detection of rotary table
The measuring procedure is to repeat the placement three times at any three points on the rotary table within one week, and to detect under forward and backward rotations, respectively. The difference between all readings and the theoretical value of the corresponding point with the greatest indexing accuracy. If it's a CNC rotary table, set one measurement point every 30 as the target position, then do five quick positionings of each target position in the positive and negative directions, measuring the difference between the actual and target positions.
That is, compute the position deviation first, and then the standard deviation using the technique given in GB10931-89. The standard deviation of each measurement point is 6 times the highest value, which is the CNC rotary table's repeated indexing accuracy.
7. Origin return accuracy detection of rotary table
The measurement method is to perform a return-to-origin from 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 that the detection of the existing positioning accuracy is measured under the condition of fast and positioning. For some CNC machine tools with a poor feed system, different positioning accuracy values will be obtained when positioning with different feed speeds. 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 use a semi-closed loop system, and most of the position detection components are installed on the drive motor, resulting in an error of 0.01~0.02mm within a 1m stroke. Not surprising. This is an error caused by thermal elongation, and some machine tools use pre-tensioning (preloading) methods to reduce the impact.
The repeated positioning accuracy of each coordinate axis is the most basic accuracy index reflecting the axis, which reflects the stability of the axis motion accuracy. It is impossible to imagine that a machine tool with poor accuracy can be stably used for production. At present, due to the increasing number of functions of the numerical control system, the systematic errors of the motion accuracy of each injector, such as pitch accumulation error, backlash error, etc., can be systematically compensated. Only random errors cannot be compensated, while the repeated positioning accuracy It reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by the numerical control system. Therefore, if the machine tool is allowed to be selected, the machine tool with high repeat positioning accuracy should be selected.
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