The positioning accuracy of CNC machine tools refers to the positioning accuracy of the coordinate axis of the machine tool under the control of the CNC device. The positioning accuracy of the CNC machine tool can be understood as the motion accuracy of the machine tool. Ordinary machine tools are fed by manual, and the positioning accuracy is mainly determined by the reading error, and the movement of CNC machine tools is realized by the digital program instruction, so the positioning accuracy is determined by the numerical control system and mechanical transmission error. The movement of each moving part of the machine tool is completed under the control of the numerical control device, and the accuracy of each moving part can achieve under the control of the program instruction directly reflects the accuracy of the processing parts, so the positioning accuracy is a very important detection content.

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01 | Detection of linear motion alignment accuracy
The linear motion positioning accuracy is generally carried out under the no-load conditions of the machine tools and workbench. According to the national standards and the provisions of the International Organization for Standardization (ISO standards), the detection of CNC machine tools should be subject to the laser measurement. In the absence of a laser interferometer, the general user can also use a standard scale, equipped with an optical readout microscope. 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 locations, the ISO standard stipulates that each site calculates the average value and the divergence-3 dispersion band according to the five measurements.
02 | Repeated alignment accuracy detection for linear motion
The instrument used for the detection is the same as that used for detecting the positioning accuracy. The general detection method is to measure at any three positions near the midpoint of each coordinate stroke and both ends. For each position, the positioning is repeated 7 times under the same conditions, to measure the value of the stop position and find the maximum difference of the reading. With the largest one of the three positions, attach positive and negative symbols, as the repeated positioning accuracy of the coordinate, it is the most basic indicator of the stability of the accuracy of the axis of motion.
03 | Origin return precision detection for linear motion
The origin return accuracy, which is essentially the repeat positioning accuracy of a special point on this axis, so its detection method is exactly the same as the repeat positioning accuracy.
04 | Reverse-error detection of the straight-line motion
The reverse error of linear motion, also known as motion loss, includes the comprehensive reflection of the reverse dead area of the driving part (such as servo motor, servo hydraulic motor and stepping motor) on the transmission chain, and the reverse clearance and elastic deformation of the mechanical motion transmission pair. The larger the error, the lower the positioning accuracy and the repeated positioning accuracy.
The detection method of reverse error is to move a distance to the forward or reverse in advance in the stroke of the measured coordinate axis and take the stop position as the reference, and then give a certain movement instruction value in the same direction to move a distance, and then move the same distance in the opposite direction to measure the difference between the stop position and the reference position. At the midpoint near the stroke and the three positions at the two ends for multiple measurements (generally 7 times), find the average value at each position, and take the maximum value in the obtained average value as the reverse error value.
05 | Positioning accuracy detection of rotary bench
The measuring tools include standard turntable, Angle polyhedra, circular grating and parallel light tube (collimator), which can be selected according to the specific situation. The measurement method is to make the workbench forward (or reverse) to an Angle and stop, lock, positioning, this position as a reference, and then quickly turn the workbench in the same direction, every 30 lock positioning, measurement. Forward rotation and reverse rotation are measured for one week each, and the maximum value of the difference between the actual turn Angle and the theoretical value (instruction value) of each positioning position is the dividing error. If it is a CNC rotary table, should be every 30 as a target position, for each target position from the positive and reverse two directions for fast positioning 7 times, the difference between the position and the position deviation, according to the GB10931-89 average position deviation and standard deviation and the difference of the minimum value and all the average position deviation is the CNC rotary table positioning accuracy error.
Considering the use of dry type transformer to the actual use requirements, generally 0,90,180,270 and other right Angle points are measured, the accuracy of these points is required to improve a level of other Angle positions.
06 | Repeat grading accuracy detection of rotary bench
The measurement method is to repeat the position three times in one week of the rotary table, and detect the test in the positive and negative direction. Maximum split accuracy of the difference between the theoretical values and the corresponding positions in all read values. If it is a CNC rotary workbench, to take every 30 a measuring point as the target position, respectively for the target position from the positive and negative direction five rapid positioning, measuring the difference between the actual position and the target position, the position deviation, according to the method specified by GB10931-89 standard deviation, the standard deviation of the measurement point of 6 times, is the maximum repeated separation accuracy of CNC rotary workbench.
07 | Origin recovery accuracy detection of the rotation bench
The measurement method is to repeat the origin once from 7 arbitrary positions, and determine its stop position, with the maximum difference of the readout as the origin recovery 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 less good feeding system demeanor, different positioning accuracy values will be obtained. In addition, the determination results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. At present, most CNC machine tools use semi-closed-loop system, and most of the position detection elements are installed on the drive motor. It is not surprising that 0.01~0.02mm error is generated within 1m travel. This is the error of thermal elongation, some machine tools will use the pre-stretching (pre-tightening) method 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 motion accuracy of the axis. It cannot be imagined that the machine tools with poor accuracy can be used stably for production. At present, due to the numerical control system function more and more, for each sitting injector motion accuracy system errors such as pitch accumulation error, reverse gap error can be system compensation, only random error can't compensate, and repeated positioning accuracy reflects the comprehensive random error, it cannot use the numerical control system compensation to correct, when found it is poor, only for the transmission chain correction. Therefore, if the machine tool selection is allowed, it is better to choose the machine tool with high repeated positioning accuracy.
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