CNC machine tool is the abbreviation of digital control machine tool. It is an automatic machine tool equipped with a program control system. The control system can logically process the program with the control code or other symbolic instructions, decode it, and represent it with a coded number. Nanjing No. 4 Machine Tool Co., Ltd. enters the numerical control device through the information carrier. Through the arithmetic processing, various control signals are sent from the numerical control device to control the movement of the machine tool, and the parts are automatically processed according to the shape and size required by the drawings.
The positioning accuracy of CNC machine tools refers to the positional accuracy that can be achieved by each coordinate axis of the machine tool under the control of the numerical control device. The positioning accuracy of CNC machine tools can be understood as the precision of the machine tool. Ordinary machine tool by manual feed, positioning accuracy is mainly determined by the reading error, and CNC machine tool movement is achieved by digital program instructions, so 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 that each moving part can achieve under the control of the program instruction directly reflects the precision that can be achieved by the processing part. Therefore, the positioning accuracy is a very important test. content.
1. Linear Motion Positioning Accuracy Detection Linear motion positioning accuracy is generally performed under no-load conditions on the machine tool and table. According to the national standards and the provisions of the International Organization for Standardization (ISO standards), the detection of CNC machine tools shall be subject to laser measurement. In the absence of a laser interferometer, it is also possible for the average user to use a standard scale with an optical reading microscope for comparative measurements. However, the measuring instrument accuracy must be 1 to 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 dispersion of the positioning point consisting of the mean value and the dispersion difference of -3 dispersion bands with five measurements.
2. The instrument used for the detection and detection of the repeatability of linear motion repeats the same accuracy as that used for detecting the positioning accuracy. The general detection method is to measure at any three positions close to the midpoint of each coordinate stroke and at both ends. Each position is positioned by rapid movement, repeated seven times under the same conditions, the stop position value is measured and the maximum difference between the readings is obtained. . One-half of the maximum difference between the three positions, plus the positive and negative signs, as the positioning accuracy of this coordinate, it is the most basic indicator to reflect the stability of the axis movement accuracy.
3. The return accuracy of the origin of linear motion detection The return accuracy of the origin is essentially the repeat positioning accuracy of a particular point on the coordinate axis, so its detection method is completely the same as the repeat positioning accuracy.
4. Inverse Error of Linear Motion Detection The reverse motion error of linear motion, also called the loss of motion, includes the inverse of the driving position (such as servo motor, servo hydraulic motor and stepping motor, etc.) on the feed chain of the coordinate axis. Into the dead zone, the backlash of mechanical transmission pairs and the elastic deformation and other errors are comprehensively reflected. The greater the error, the lower the positioning accuracy and repeatability.
The detection method of reverse error is to move a distance forward or backward in the travel of the measured coordinate axis and use this stop position as a reference, and then give a certain movement command value in the same direction to make it move a certain distance. Then move the same distance in the opposite direction and measure the difference between the stop position and the reference position. The measurement is performed multiple times (usually seven times) at three points near the midpoint of the stroke and both ends, and the average value at each position is determined, and the maximum value among the obtained average values is the reverse error value.
5, rotary table positioning accuracy measurement and measurement tools are standard turntable, angle polyhedron, circular grating and collimator (collimator), etc., can be selected according to the specific circumstances. The measurement method is to make the workbench forward (or reverse) to an angle and stop, lock, and position. Take this position as a reference, then rotate the workbench in the same direction quickly, and lock and position every 30 times to measure. For one revolution of the forward rotation and one reverse rotation, the maximum difference between the actual rotation angle and the theoretical value (command value) of each positioning position is the graduation error. If it is a numerically controlled rotary table, it should take every 30 as a target position. For each target position, it is quickly positioned seven times from the positive and negative directions. The actual difference between the position and the target position is the position deviation, and then press GB10931- 89 The method specified in "Assessment of Positional Accuracy of Digitally Controlled Machine Tool" calculates the average position deviation and standard deviation, the difference between the maximum value of all average position deviations and standard deviations and the sum of all the average position deviations and the standard deviation. , is the positioning precision error of CNC rotary table.
Considering the requirement of dry-type transformers for practical use, generally several key points such as 0, 90, 180, and 270 are measured in key points, and the precision of these points is required to be one grade higher than that of other angles.
6. The measurement and measurement method of the repeated index precision of the rotary table is repeated three times at any position within one week of the rotary table, and the detection is performed in the positive and negative directions respectively. The accuracy of the maximum value of the difference between the readings and the theoretical value of the corresponding position. If it is a numerically controlled rotary table, take one measurement point every 30 as the target position, and perform five rapid positionings for each target position from both positive and negative directions, and measure the difference between the actual position and the target position. That is, the position deviation, and then calculate the standard deviation according to the method specified in GB 10931-89, the maximum value of the standard deviation of each measurement point is 6 times, which is the repeat indexing accuracy of the CNC rotary table.
7. The return-to-origin precision detection and measurement method of the rotary table is to perform an origin return from seven arbitrary positions and measure the stop position. The readout maximum difference is used as the origin return accuracy.
It should be pointed out that the detection of the existing positioning accuracy is measured in the case of rapid and positioning. For CNC machine tools whose flight system is not very good in certain feed systems, different positioning accuracy values will be obtained when positioning with different feed speeds. In addition, the measurement results of the positioning accuracy are related to the ambient temperature and the working status of the coordinate axis. At present, most CNC machine tools use a semi-closed-loop system, and most of the position detection elements are mounted on the drive motor, which produces an error of 0.01 to 0.02 mm in a stroke of 1 m. It is not strange. This is an error due to thermal elongation, and some machines use pre-tensioning (pretensioning) to reduce the effect.
The repetitive positioning accuracy of each coordinate axis reflects the most basic accuracy index of this axis, it reflects the stability of the axis movement accuracy, and it cannot be assumed that the machine tool with poor accuracy can be used stably for production. At present, due to more and more functions of the numerical control system, system errors such as pitch accumulation error and backlash error can be compensated for each seat injector's target motion accuracy. Only random errors cannot be compensated, and repeat positioning accuracy can be compensated. It is a comprehensive random error that reflects the feed drive mechanism. It cannot be corrected with the CNC system compensation. When it is found to be out of tolerance, only the feed drive chain is fine-tuned. Therefore, if the machine tool is allowed to be selected, it is better to select a machine with high repeatability.
