Seven ways to detect the positioning accuracy of CNC machine tools

Seven ways to detect the positioning accuracy of CNC machine tools

The positioning accuracy of CNC machine tools 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 CNC machine tools can also be known as the motion accuracy of the machine tool.

Ordinary machine tools are manually fed, and the reading error determines the positioning accuracy, so the movement of CNC machine tools is realized by digital program instructions, so the positioning accuracy is determined by the CNC system and mechanical transmission errors. The movement of each moving part of the machine tool is completed under the control of the numerical control device, and the accuracy that each moving part can achieve under the control of program instructions directly reflects the accuracy that the machined parts can achieve. Therefore, positioning accuracy is a very important test content.

  1. Linear motion positioning accuracy detection
    The positioning accuracy of linear motion is generally carried out under no-load conditions of the machine tool and the worktable. According to the national standards and the provisions of the International Organization for Standardization (ISO standard), the detection of CNC machine tools should be based on laser measurement. In the absence of a laser interferometer, a standard scale can also be used for comparison measurements with an optical reading microscope for ordinary users. However, the accuracy of the measuring instrument must be 1 to 2 grades higher than the accuracy of the measurement.
    To reflect all the errors in multiple positioning, the ISO standard stipulates that each positioning point calculates the average value of five measurement data and the positioning point dispersion band formed by the dispersion-3 dispersion band.
  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 value of the stop position is measured, and the maximum difference between the readings is obtained. . One-half of the largest difference in the three positions is attached with a positive and negative sign, as the repeated positioning accuracy of the coordinate, which is the most basic index reflecting the stability of the axis motion accuracy.
  3. Origin return accuracy detection 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 known as the loss of momentum, includes the reverse dead zone of the drive parts (such as servo motors, servo-hydraulic motors, and stepper motors) on the feed transmission chain of the coordinate axis, and each mechanical motion transmission pair. The comprehensive reflection of errors such as backlash and elastic deformation. The larger 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 within the stroke of the measured coordinate axis and use this stop position as the reference, and then give a certain movement command value in the same direction to move it for 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 (usually 7 times) at three positions near the midpoint and both ends of the stroke, obtain the average value of each position, and take the maximum value of the obtained average value as the reverse error value.
  5. Positioning accuracy detection of rotary table
    Measurement tools include standard turntable, angle polyhedron, circular grating, and collimator (collimator), etc., which can be selected according to specific conditions. The measurement method is to turn the worktable forward (or reversely) at an angle and stop, lock, and position it, take this position as the benchmark, and then quickly turn the worktable in the same direction, lock and position it every 30 degrees, and measure. The forward rotation and reverse rotation are each measured for one cycle, 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, every 30 points should be used as a target position. For each target position, fast positioning is performed 7 times from the forward and reverse directions. The difference between the achieved position and the target position is the position deviation, and then press GB10931- 89 The method specified in “Assessment Method of Position Accuracy of Numerically Controlled Machine Tools” calculates the average position deviation and standard deviation, 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. , is the positioning accuracy error of the CNC rotary table.
    Considering the actual use requirements of dry-type transformers, it is generally necessary to focus on the measurement of several right-angle equal points such as 0, 90, 180, and 270, and the accuracy of these points is required to be one level higher than other angular positions.
  6. Repeated indexing accuracy detection of rotary table
    The measurement method is to repeat the positioning three times at any three positions within one week of the rotary table, and perform detection under the forward and reverse rotations 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 measurement point every 30 as the target position, perform 5 rapid positioning of each target position from the positive and negative directions respectively, 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 repeated indexing accuracy of the CNC rotary table.
  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.
    We should point out that the detection of the existing positioning accuracy is measured under the condition of fast 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. Now, 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. Currently, due to the increasing number of functions of the numerical control system, the systematic errors in 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 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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