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06
October
2016
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Summary of testing methods for positioning accuracy of CNC machine tools
CNC machine tool is the abbreviation of digital control machine tool, it is a kind of automatic machine tool with program control system. The control system is capable of processing logic control rules encoding or other symbolic instruction program, and decoding the digital code, said, Nanjing fourth machine tool limited company through input information into the machine. The operation of the numerical control device to send out a variety of control signals, control the movement of the machine, according to the requirements of the shape and size of the drawings, automatic parts processing out.
The positioning accuracy of CNC machine tools is the position accuracy of the motion of 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 kinematic accuracy of the machine tool. The ordinary machine tool is fed by hand, the positioning accuracy is mainly determined by the reading error, and the movement of the numerical control machine tool is realized by the digital program, so the positioning precision is decided by the numerical control system and the mechanical transmission error. The moving parts of the machine tool movement is completed under the control of numerical control devices, each moving parts can be achieved in the program instructions under the control of the precision machining parts directly reflect the precision of the positioning accuracy, so it is a very important content detection.
1, linear motion positioning accuracy detection
Linear motion positioning accuracy is generally carried out under the condition of the machine tool and the working platform with no load. According to the provisions of the national standard and the International Standardization Organization (ISO standard), the detection of CNC machine tools should be based on laser measurement. In the absence of a laser interferometer, the standard scale can be used for the average user, and the optical reading microscope can be used to compare the measurement. However, the measurement accuracy of the instrument must be higher than the measured accuracy of 1~2 grades.
In order to reflect all the errors in the positioning, the ISO standard provides that every positioning point is calculated by the average value of the data of the five times and the positioning point scattered by the +-3.
2, linear motion repeat positioning accuracy detection
The instrument used for testing is the same as that used for detecting and locating accuracy. The general method is measured in arbitrary coordinate travel near the midpoint and the ends of the three locations, each location with fast mobile positioning, under the same conditions were repeated 7 times, measured the stop position and calculate the maximum difference numerical readings. 1/2, the largest one of the three positions, with positive and negative signs, as the coordinates of the repeated positioning accuracy, it is reflected in the stability of the accuracy of the axis movement of the most basic indicators.
3, linear motion of the origin return precision detection

The precision of return of the origin is essentially a repeat positioning accuracy of a special point on the axis, so its detection method is the same as the repeated positioning accuracy.

4, linear motion of the reverse error detection
Reverse error of linear motion, also called the loss of momentum, it includes the axis feed drive chain drive parts (such as servo motor, servo motor and stepper motor in the reverse dead zone, etc.) in order to reflect the mechanical motion of gear backlash and elastic deformation error. The greater the error, the lower the accuracy of the positioning accuracy and repeatability.
Detection method of reverse error is measured in the axis of travel in advance to move forward or reverse a distance to the stop position as the benchmark, then give a mobile command value in the same direction, to move a distance, then go the opposite direction the same distance, position and the reference position the difference of measurement. In the middle of the trip and the three ends of the two positions were repeatedly measured (usually 7 times), the average value of each position, in the average value of the maximum value for the reverse error.
5, rotary table positioning accuracy detection
A standard measurement tool, turntable angle polyhedron, circular grating and collimator (collimator), according to the specific circumstances of the. The measuring method is to make the working table (forward or backward) to a point of view and stop, locking and positioning, this position as a reference, and then to the same direction fast rotating table, every 30 locking and positioning, measurement. The maximum value of the difference between the actual rotation angle and the theoretical value (instruction value) of each position is the difference between the forward and reverse rotation measurements. If the NC rotary table, should be every 30 to a target position for each target position from the positive and negative direction of the two position fast 7 times, reaching the actual position and the target position is difference of the position deviation method, and then press GB10931-89 "digital position control of machine accuracy assessment methods" provisions of the calculation the average position and standard deviation, average deviation and standard deviation of all the maximum and average of all position deviation and the standard deviation of the minimum value and the difference is the positioning accuracy error of CNC rotary worktable.
Taking into account the requirements of the actual use of dry transformers, 0, 90, 180, 270, and several other right angle division point for the measurement, the accuracy of these points is more than the other point of view to improve a grade.
6, rotary table of repeated indexing accuracy detection
The measuring method is to repeat the position 3 times within one week of the rotary table, and the measuring method is carried out under the rotation of the positive and the reverse direction. The maximum dividing accuracy of the difference of the theoretical value of all readings and the corresponding position. If the NC rotary table, every 30 to take a measurement point as the target position, each target position from positive and negative two directions 5 times fast positioning, measure the difference between the actual arrival of the position and the target position, namely position deviation, according to the GB10931-89 standard method to calculate the standard deviation. 6 times the maximum value of the standard deviation of each measurement point is the repeated indexing precision CNC rotary worktable.
Origin return 7, precision rotary table
The measuring method is an origin return from 7 arbitrary position respectively, determine the stop position, the maximum difference as the origin of the return of the accuracy of readout.
It should be pointed out that the existing detection positioning accuracy is measured in fast, positioning case, CNC machine tool feed system in some manner is not very good, with different feed speed positioning, positioning accuracy will get different values. In addition, the determination results of the positioning accuracy and the ambient temperature and the axis of the working state, most of the current CNC machine adopts semi closed loop system, position detection components are installed on the drive motor, the error from 0.01~0.02mm 1m in the itinerary is not surprising. This is the error caused by the thermal elongation, some machine tools will be used to pre stretch (preload) method to reduce the impact of.
The repeated positioning accuracy of each coordinate axis is the most basic precision index, which reflects the stability of the accuracy of the axis movement. At present, the CNC system function more and more, for each jet is standard system error motion accuracy such as pitch error accumulation, etc. can be opposite clearance error compensation system, not only the random error compensation method, and the repetitive positioning accuracy is the reflection of the random error of the integrated feed drive mechanism, it cannot be modified by numerical control the system of compensation, when it was found when the ultra poor, only the fine adjustment correction of feed transmission chain. Therefore, if allowed to choose the machine tool, you should choose to repeat the high precision of the machine tool is good.

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