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Seven ways to detect the positioning accuracy of CNC machine tools

The positioning accuracy of a CNC machine tool refers to the position accuracy that can be achieved by the movement of each axis of the machine tool under the control of a CNC device.The positioning accuracy of CNC machine tools can also be understood as the motion accuracy of machine tools.Ordinary machine tools are fed manually, and the positioning accuracy is mainly determined by the reading error, while the movement of CNC machine tools is achieved by digital program instructions, so the positioning accuracy is determined by the CNC system and the mechanical transmission error.
CNC machine tool is the abbreviation of digital control machine tool, which is an automated machine tool equipped with a program control system.The control system can logically process programs with control codes or other symbolic instructions, and decodes them, represented by coded numbers, and Nanjing No. 4 Machine Tool Co., Ltd. enters the CNC device through the information carrier.After calculation and processing, various control signals are sent by the CNC device to control the action of the machine tool, and the parts are automatically processed according to the shape and size required by the drawings.

The movement of each moving part of the machine tool is completed under the control of a CNC device. The accuracy that each moving part can achieve under the control of program instructions directly reflects the accuracy that the processed part can achieve. Therefore, positioning accuracy is a very important test content.

1. Linear motion positioning accuracy detection Linear motion positioning accuracy is generally carried out under no-load conditions of machine tools and workstations.According to national standards and the regulations of the International Organization for Standardization (ISO standards), laser measurement shall prevail for the testing of CNC machine tools.In the absence of a laser interferometer, for ordinary users, a standard scale can also be used with an optical reading microscope for comparative measurements.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 dispersion according to the five measurement data, and the dispersion band of the positioning point composed of the dispersion band.

2. The instrument used for detecting the repeated positioning accuracy of linear motion is the same as that used to detect the positioning accuracy.The general detection method is to measure at any three positions close to the midpoint and both ends of the stroke of each coordinate. Each position is positioned with rapid movement, and the positioning is repeated 7 times under the same conditions. The stop position value is measured and the maximum difference in reading is obtained.Take one-half of the difference between the largest of the three positions and attach a positive and negative symbol as the repeated positioning accuracy of the coordinate. It is the most basic index that reflects the accuracy and stability of the axis movement.

3. The return accuracy of the origin of linear motion detects the return accuracy of the origin, which 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. The reverse error of linear motion detects the reverse error of linear motion, also known as the loss of momentum. It includes the reverse dead zone of the drive parts (such as servo motors, servo hydraulic motors, and stepper motors) on the axis feed transmission chain, and the comprehensive reflection of the errors such as the reverse gap 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 forward or reverse in advance within the stroke of the measured axis and use this stop position as a reference, and 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.Multiple measurements (generally 7 times) were made at three positions near the midpoint of the stroke and at both ends, and the average value at each position was obtained, and the maximum value in the resulting average value was used as the reverse error value.

5. The positioning accuracy detection and measurement tools of the rotary table include standard turntable, angle polyhedron, circular grating and parallel light tube (collimator), etc., which can be selected according to specific circumstances.The measurement method is to turn the workbench forward (or reverse) at an angle and stop, lock, and position it. This position is used as a reference, and then quickly rotate the workbench in the same direction, locking and positioning every 30 seconds for measurement.Forward and reverse turns are measured for one week each, and the maximum value of the difference between the actual angle of each positioning position and the theoretical value (instruction value) is the indexing error.

If it is a CNC rotary table, every 30 should be used as a target position, and each target position 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, and then the average position deviation and standard deviation are calculated according to the method specified in GB10931-89 “Evaluation Method for the Position Accuracy of Digital Control Machine Tools”. The maximum value of all average position deviations and standard deviations and the sum of the minimum values of all average position deviations and standard deviations is the positioning accuracy error of the CNC rotary table.Considering the actual use requirements of dry transformers, key measurements are generally made at several right-angle points such as 0, 90, 180, and 270, and the accuracy of these points is required to be one level higher than that of other angular positions.

6. Detection of repeated indexing accuracy of rotary table
The measurement method is to repeat the positioning 3 times in any three positions within a week of the rotary table, and to detect the rotation in the positive and negative directions respectively.The maximum indexing accuracy of the difference between all reading values and the theoretical values of the corresponding position.If it is a CNC rotary table, take a measurement point every 30 as the target position, and quickly locate each target position 5 times from the positive and negative directions, and measure the difference between the actual reached position and the target position, that is, the position deviation, and then calculate the standard deviation according to the method specified in GB10931-89. The standard deviation of each measurement point is 6 times the maximum value, which is the repeatability of the CNC rotary table. Indexing accuracy.

7. The detection and measurement method of the origin return accuracy of the rotary table is to perform an origin return from any of the 7 positions, determine its stop position, and take the maximum difference read out as the origin return accuracy.

It should be pointed out that the detection of existing positioning accuracy is measured in the case of fast and positioning. For some CNC machine tools with poor feed system demeanor, 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 axis. At present,

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