Several detection methods for positioning accuracy of CNC machine tools

The positioning accuracy of CNC machine tools refers to the positional 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 understood as the motion accuracy of the machine tool. Ordinary machine tools are manually fed, and the positioning accuracy is mainly determined by reading errors, while the movement of CNC machine tools is achieved through digital program instructions, so the positioning accuracy is determined by the CNC system and mechanical transmission errors. The movement of each moving component of the machine tool is completed under the control of the numerical control device, and the accuracy that each moving component can achieve under the control of program instructions directly reflects the accuracy that the machined parts can achieve. Therefore, positioning accuracy is an important detection content.

1. Linear motion positioning accuracy detection

The positioning accuracy of linear motion is generally carried out under the condition of no load on the machine tool and worktable. According to national standards and the regulations of the International Organization for Standardization (ISO standards), the testing of CNC machine tools should be based on laser measurement. In the absence of a laser interferometer, for ordinary users, a standard scale can also be used with an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument must be 1-2 levels higher than the accuracy being measured.

In order to reflect all errors in multiple positioning, the ISO standard stipulates that each positioning point shall calculate the average value of five measurement data and a positioning point dispersion band consisting of a dispersion band of -3 dispersion bands.

2. Linear motion repetitive positioning accuracy detection

The instruments used for detection are the same as those used for detecting positioning accuracy. The general detection method is to measure at any three positions near the midpoint and ends of each coordinate stroke, and use rapid movement positioning for each position. Repeat the positioning 7 times under the same conditions, measure the stopping position value, and calculate the maximum difference in readings. Take half of the difference between the largest of the three positions, with positive and negative symbols attached, as the repeated positioning accuracy of that coordinate. It is the most basic indicator reflecting the stability of axis motion accuracy.

 3. Accuracy detection of origin return in linear motion

The accuracy of returning the origin 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 reverse dead zone of the driving parts on the feed transmission chain of the coordinate axis (such as servo motors, servo hydraulic motors, stepper motors, etc.), the comprehensive reflection of the reverse clearance and elastic deformation errors of various mechanical motion transmission pairs. The larger the error, the lower the positioning accuracy and repeat positioning accuracy.

The detection method of reverse error is to move a distance in the forward or reverse direction within the stroke of the measured coordinate axis and use this stop position as a reference. Then, a certain movement command value is given in the same direction to move a certain distance, and then the same distance is moved in the opposite direction to measure the difference between the stop position and the reference position. Perform multiple measurements (usually 7 times) at three positions near the midpoint and ends of the journey, and calculate the average value at each position. The maximum value obtained from the average value is taken as the reverse error value.

 5. Positioning accuracy detection of rotary worktable

Measurement tools include standard turntables, angle polyhedra, circular gratings, and parallel light tubes (collimators), which can be selected according to specific situations. The measurement method is to rotate the worktable forward (or backward) by one angle, stop, lock, and position it, use this position as a reference, and then quickly rotate the worktable in the same direction, locking and positioning it every 30 minutes for measurement. Measure one revolution each for forward and reverse rotation, and the maximum difference between the actual rotation angle at each positioning position and the theoretical value (instruction value) is the division error. If it is a CNC rotary worktable, the target position should be set at every 30, and quick positioning should be performed 7 times from the forward and reverse directions for each target position. The actual difference between the position and the target position is the position deviation. Then, the average position deviation and standard deviation are calculated according to the method specified in GB10931-89 "Evaluation Method for Position Accuracy of Digital Control Machine Tools". The difference between the maximum value of all average position deviations and standard deviations and the sum of the minimum value of all average position deviations and standard deviations is the positioning accuracy error of the CNC rotary worktable.

Considering the practical requirements of dry-type transformers, it is generally necessary to focus on measuring several right angle equal points such as 0, 90, 180, 270, etc., and to improve the accuracy of these points by one level compared to other angle positions.

 6. Repetitive indexing accuracy detection of rotary worktable

The measurement method is to select three positions and repeat the positioning three times within one week of the rotary worktable, and perform detection under forward and reverse rotation. The maximum division accuracy of the difference between all read values and the theoretical value at the corresponding position. If it is a CNC rotary worktable, a measurement point should be taken every 30 as the target position, and each target position should be quickly positioned 5 times from the forward and reverse directions. The difference between the actual arrived position and the target position, that is, the position deviation, should be measured. Then, the standard deviation should be calculated according to the method specified in GB10931-89. The maximum value of the standard deviation of each measurement point, which is 6 times, is the repeatability accuracy of the CNC rotary worktable.

 7. Accuracy detection of origin reset for rotary worktable

The measurement method is to perform an origin reset from 7 arbitrary positions, determine their stopping positions, and use the maximum difference read as the accuracy of the origin reset.

It should be pointed out that the current detection of positioning accuracy is measured quickly and accurately. For some CNC machine tools with poor feed system performance, different positioning accuracy values will be obtained when using different feed speeds for positioning. In addition, the measurement results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. Currently, most CNC machine tools adopt a semi closed loop system, and the position detection components are mostly installed on the drive motor. It is not surprising that an error of 0.01~0.02mm occurs within a 1m stroke. This is the error caused by thermal elongation, and some machine tools use pre stretching (pre tightening) methods to reduce the impact.

The repeated positioning accuracy of each coordinate axis is the most basic accuracy indicator reflecting the stability of the axis's motion accuracy. It cannot be imagined that machine tools with poor accuracy can be stably used in production. At present, due to the increasing functionality of numerical control systems, system errors such as pitch accumulation error and reverse clearance error in the motion accuracy of each seat injector can be compensated systematically. Only random errors cannot be compensated, and the repeated positioning accuracy reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by numerical control system compensation. When it is found that it exceeds the tolerance, only fine adjustment and correction of the feed transmission chain can be carried out. Therefore, if machine tool selection is allowed, it is better to choose a machine tool with high repeatability positioning accuracy.