Eight testing methods for 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 tools under the control of CNC devices. The positioning accuracy of CNC machine tools can also be understood as the kinematic accuracy of machine tools. The common machine tool is fed manually, and the positioning accuracy is mainly determined by the reading error, while the movement of the CNC machine tool is realized by the digital program command, so the positioning accuracy is determined by the CNC 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. The accuracy of each moving part under the control of the program command directly reflects the accuracy of the processing part. Therefore, the positioning accuracy is a very important detection content.

1. Linear motion positioning accuracy detection
The positioning accuracy of linear motion is generally carried out under the no-load condition of the machine tool and the 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 based on laser measurement. In the absence of a laser interferometer, ordinary users can also use a standard scale and an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument must be 1~2 grades higher than the measured accuracy.
In order to reflect all the errors in multiple positioning, the ISO standard stipulates that each positioning point is a positioning point dispersion zone composed of the average value and dispersion - 3 dispersion zone calculated according to five times of measured data.

2. Repetitive positioning accuracy detection of linear motion
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 the travel of each coordinate. Each position is positioned by rapid movement, and the positioning is repeated 7 times under the same conditions. The stop position value is measured and the maximum difference between readings is calculated. One half of the maximum difference value of the three positions is attached with positive and negative symbols as the repeated positioning accuracy of the coordinate, which is the most basic index to reflect the stability of axis motion accuracy.

3. Detection of the Return Accuracy of the Origin 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 loss of momentum, includes the comprehensive reflection of the reverse dead zone of the drive parts (such as servo motor, servo hydraulic motor and stepping motor) on the feed transmission chain of the coordinate axis, the reverse clearance and elastic deformation of the mechanical motion transmission pairs. The larger the error is, the lower the positioning accuracy and repeated positioning accuracy are.
The reverse error detection method is to move a distance forward or backward in advance within the stroke of the measured coordinate axis and take the stop position as the reference, then give a certain movement command value in the same direction to move it for a distance, and then move the same distance in the opposite direction to measure the difference between the stop position and the reference position. Carry out multiple measurements (generally 7 times) at three positions near the middle point and two ends of the stroke, and calculate the average value at each position. The maximum of the average value is the reverse error value.

5. Positioning accuracy detection of rotary table
Measuring tools include standard turntable, angle polyhedron, circular grating and collimator (collimator), which can be selected according to specific conditions. The measurement method is to turn the workbench forward (or backward) for an angle and stop, lock, and locate it. This position is used as the reference, and then quickly turn the workbench in the same direction, lock and locate it every 30 for measurement. The forward rotation and reverse rotation shall be measured for one cycle respectively, and the maximum difference between the actual angle of each positioning position and the theoretical value (command value) is the indexing error. If it is a CNC rotary table, it should take every 30 as a target position. For each target position, it should be quickly positioned 7 times from the positive and negative directions. The difference between the actual position and the target position is the position deviation. Then, the average position deviation and standard deviation should be calculated according to the method specified in GB10931-89 Evaluation Method for the Position Accuracy of Digitally Controlled Machine Tools, The difference between the maximum value of all average position deviations and the standard deviation and the sum of the minimum value of all average position deviations and the standard deviation is the positioning accuracy error of the NC rotary table.
Considering the actual use requirements of dry-type transformers, generally focus on the measurement of 0, 90, 180, 270 and other right angle equally divided points. The accuracy of these points is required to be one level higher than that of other angle positions.

6. Repetitive indexing accuracy detection of rotary table
The measurement method is to select three positions at random within one week of the rotary table to repeat the positioning for three times, and detect them under the rotation in the positive and negative directions respectively. The maximum indexing accuracy of the difference between all readings and the theoretical value at the corresponding position. If it is a CNC rotary table, take one measuring point every 30 as the target position, and quickly locate each target position for five times from the positive and negative directions, measure the difference between the actual position reached and the target position, that is, the position deviation, and then calculate the standard deviation according to the method specified in GB10931-89. Six times of the maximum standard deviation of each measuring point is the repeated indexing accuracy of the CNC rotary table.

7. Zero point reset precision detection of rotary table
The measurement method is to carry out the original point reset once from 7 arbitrary positions, measure the stop position, and take the maximum difference read as the original point reset accuracy.
It should be pointed out that the existing positioning accuracy is measured under the condition of fast and positioning. For some CNC machines with poor feed system style, different positioning accuracy values will be obtained when positioning at different feed speeds. 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. It is not surprising that the error of 0.01~0.02mm occurs within 1m travel. This is the error caused by thermal elongation. Some machine tools adopt pre stretching (pre tightening) 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 assumed that the machine tool with poor accuracy can be stably used in production. At present, due to the increasing functions of the NC system, systematic compensation can be made for the systematic errors of the movement accuracy of each ejector mark, such as the pitch accumulation error and the reverse clearance error. Only the random error can not be compensated. The repetitive positioning accuracy reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by the CNC system compensation. When it is found that it is out of tolerance, only the fine adjustment and correction of the feed drive chain can be made. Therefore, if the machine tool is allowed to be selected, it is better to select the machine tool with high repeated positioning accuracy.