Eight testing methods for positioning accuracy of NC machine tools

The positioning accuracy of NC machine tool refers to the position accuracy that can be achieved by the movement of each coordinate axis of the machine tool under the control of NC device. The positioning accuracy of NC machine tool can be understood as the motion accuracy of machine tool. Ordinary machine tools are fed manually, and the positioning accuracy mainly depends on the reading error, while the movement of NC machine tools is realized by digital program instructions, so the positioning accuracy depends on the NC system and mechanical transmission error. The movement of the moving parts of the machine tool is completed under the control of the numerical control device. The accuracy of the moving parts under the control of the program instruction directly reflects the accuracy of the machined parts. 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 machine tool and worktable. According to the provisions of national standards and international organization for Standardization (ISO standard), the detection of CNC machine tools shall be subject to laser measurement. In the absence of laser interferometer, for ordinary users, standard scale and optical reading microscope can also be used for comparative measurement. 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 stipulates that each positioning point calculates the average value and the dispersion-3 dispersion band of the positioning point according to the five measurement data.

2. Linear motion repeated positioning accuracy detection

The instrument used for detection is the same as that used for detecting 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 by rapid movement. Repeat the positioning for 7 times under the same conditions, measure the stop position value and calculate the maximum difference of readings. One half of the largest difference among the three positions is attached with positive and negative symbols as the repeated positioning accuracy of the coordinate, which is the most basic index reflecting the stability of 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 momentum loss, includes the reverse dead zone of the driving parts on the feed transmission chain of the coordinate axis (such as servo motor, servo hydraulic motor and stepping motor), and the comprehensive reflection of the errors such as 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 reverse error is to move a distance in the forward or reverse direction in advance within the stroke of the measured coordinate axis, take the stop position as the benchmark, then give a certain movement command value in the same direction to move it for a certain 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 the three positions near the midpoint and both ends of the stroke, calculate the average value at each position, and take the maximum of the average value as the reverse error value.

5. Positioning accuracy detection of rotary table

Measuring tools include standard turntable, angle polyhedron, circular grating and collimator, which can be selected according to specific conditions. The measurement method is to make the workbench rotate forward (or reverse) by an angle, stop, lock and position, take this position as the reference, and then quickly rotate the workbench in the same direction, lock and position every 30 for measurement. The forward rotation and reverse rotation are measured for one cycle respectively, and the maximum value of the difference between the actual rotation angle and the theoretical value (command value) of each positioning position is the indexing error. If it is a CNC rotary table, take every 30 as a target position, and quickly locate each target position for 7 times from the positive and negative directions. The difference between the actual position and the target position is the position deviation, and then calculate the average position deviation and standard deviation according to the method specified in gb10931-89 evaluation method of position accuracy of digital control machine tools, The difference between the maximum value of all average position deviation and standard deviation and the minimum value of all average position deviation and standard deviation is the positioning accuracy error of NC rotary table.

Considering the actual use requirements of dry-type transformer, several right angle bisection points such as 0, 90, 180 and 270 are generally measured, and the accuracy of these points is required to be one level higher than that of other angle positions.

6. Repeated indexing accuracy detection of rotary table

The measurement method is to select three positions within one week of the rotary table and repeat the positioning for three times, and detect them respectively under the rotation in the positive and negative directions. The maximum indexing accuracy of the difference between all read values and the theoretical value of the corresponding position. If it is a CNC rotary table, take a measuring point every 30 as the target position, quickly locate each target position from the positive and negative directions for 5 times, measure the difference between the actual arrived position and the target position, that is, the position deviation, and then calculate the standard deviation according to the method specified in gb10931-89. 6 times of the maximum standard deviation of each measuring point is the repeated indexing accuracy of the CNC rotary table.

7. Accuracy detection of zero point reset of rotary table

The measurement method is to reset the origin from 7 arbitrary positions, measure its stop position, and take the maximum difference read out as the origin reset accuracy.

It should be pointed out that the existing detection of positioning accuracy is measured under the condition of fast and positioning. For some NC machine tools with poor feed system, different positioning accuracy values will be obtained when positioning at different feed speeds. In addition, the measurement results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. At present, most CNC machine tools adopt semi closed-loop system, and most of the position detection elements are installed on the driving motor, which is not surprising to produce an error of 0.01 ~ 0.02mm within 1m stroke. This is the error caused by thermal elongation. Some machine tools use pre stretching (pre tightening) to reduce the influence.

The repeated positioning accuracy of each coordinate axis is the most basic accuracy index reflecting the axis. It reflects the stability of the motion accuracy of the axis. It is impossible to imagine that the machine tool with poor accuracy can be stably used in production. At present, due to the increasing functions of the numerical control system, the systematic errors of the standard motion accuracy of each seated ejector, such as pitch accumulation error and reverse clearance error, can be systematically compensated. Only the random error can not be compensated, and the repeated positioning accuracy reflects the comprehensive random error of the feed drive mechanism. It can not be corrected by the numerical control system compensation. When it is found that it is out of tolerance, only the feed transmission chain can be fine adjusted and corrected. Therefore, if the machine tool is allowed to be selected, the machine tool with high repeated positioning accuracy should be selected.