Seven methods for detecting the 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 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 by digital program instructions, so the positioning accuracy is determined by the CNC system and mechanical transmission errors.
CNC machine tool is the abbreviation of digital control machine tool, which is an automated machine tool equipped with a program control system. This control system is capable of logically processing programs with control codes or other symbolic instructions, decoding them, and representing them with coded numbers. Nanjing Fourth Machine Tool Co., Ltd. inputs them into the CNC device through an information carrier. After calculation and processing, various control signals are sent out by the CNC device to control the action of the machine tool. The parts are automatically processed according to the shape and size requirements of the drawing.
The motion of each moving component of the machine tool is completed under the control of the CNC 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. The detection of linear motion positioning accuracy is generally carried out under no-load conditions of machine tools and workbenches. According to national standards and the provisions 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 measured accuracy. In order to reflect all errors in multiple positioning, the ISO standard stipulates that each positioning point should calculate the average and scatter based on five measurement data, forming a positioning point scatter band.
2. The instruments used for detecting the repeated positioning accuracy of linear motion are the same as those used for detecting the positioning accuracy. The general detection method is to measure at any three positions near the midpoint and ends of each coordinate stroke. For each position, rapid movement positioning is used, and the positioning is repeated 7 times under the same conditions. The stop position value is measured and the maximum difference in reading is calculated. One half of the maximum difference between the three positions, accompanied by positive and negative symbols, is used as the repeated positioning accuracy of the coordinate, which is the most basic indicator reflecting the stability of axis motion accuracy.
3. The origin return accuracy of linear motion is essentially the repeated positioning accuracy of a special point on the coordinate axis. Therefore, its detection method is completely the same as the repeated positioning accuracy.
4. The detection of reverse error in linear motion, also known as loss of momentum, includes the reverse dead zone of the driving parts on the coordinate axis feed transmission chain (such as servo motors, servo hydraulic motors, and stepper motors), as well as the comprehensive reflection of errors such as reverse clearance and elastic deformation of various mechanical motion transmission pairs.
The larger the error, the lower the positioning accuracy and repeated positioning accuracy. The detection method for reverse error is to move forward or backward by a certain distance within the stroke of the measured coordinate axis, and use this stop position as a reference. Then, give a certain movement command value in the same direction to move it by 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. Conduct multiple measurements (usually 7 times) at three positions near the midpoint and ends of the stroke, and calculate the average value at each position. The maximum value of the obtained average value is the reverse error value.
5. The positioning accuracy detection and measurement tools for rotary workbenches include standard turntables, angle polyhedra, circular gratings, and collimators (collimators), which can be selected according to specific situations. The measurement method is to rotate the workbench forward (or backward) by an angle and stop, lock, and position it, using this position as a reference. Then, quickly rotate the workbench in the same direction and lock the position every 30 seconds for measurement. Measure forward and reverse rotation once each, and the maximum difference between the actual rotation angle at each positioning position and the theoretical value (command value) is the indexing error.
If it is a CNC rotary table, the target position should be set every 30, and each target position should be quickly positioned 7 times from the positive and negative directions. The actual difference between the position and the target position is the position deviation, and then the average position deviation and standard deviation should be calculated according to the method specified in GB10931-89 "Evaluation Method for Position Accuracy of Digital Control Machine Tools". The difference between the maximum sum of all average position deviations and standard deviations and the minimum sum of all average position deviations and standard deviations is the positioning accuracy error of the CNC rotary table. Considering the practical usage requirements of dry-type transformers, it is generally important to focus on measuring several right angle equal points such as 0, 90, 180, 270, etc. The accuracy of these points is required to be improved by one level compared to other angle positions.
6. Repetitive indexing accuracy detection of rotary worktable
The measurement method is to select three positions within one week of the rotary worktable and repeat the positioning three times, respectively, and perform the detection in the forward and reverse directions of rotation. The maximum value division accuracy of the difference between the theoretical value at the corresponding position and all read values. 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 positive and negative directions. The difference between the actual position and the target position should be measured, which is the position deviation. Then, the standard deviation should be calculated according to the method specified in GB10931-89. Six times the maximum standard deviation of each measurement point is the repeated indexing accuracy of the CNC rotary worktable.
7. The measurement method for measuring the accuracy of the origin reset of the rotary worktable is to perform one origin reset from seven 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 under fast and positioning conditions. For some CNC machine tools with poor feed system wind speed, different positioning accuracy values will be obtained when using different feed rates 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 use a semi closed loop system, and 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 an 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 that reflects the stability of the motion accuracy of that axis. It cannot be imagined that machine tools with poor accuracy can be stably used for production.
At present, due to the increasing number of functions of CNC systems, system errors such as pitch accumulation error, reverse clearance error, etc. in the motion accuracy of each seat injector can be compensated systematically. Only random errors cannot be compensated, and repeated positioning accuracy reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by CNC system compensation. When it is found to be out of tolerance, only fine adjustment and correction of the feed drive chain can be carried out. Therefore, if it is allowed to choose a machine tool, it is better to choose a machine tool with high repeated positioning accuracy.
