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Seven methods for positioning accuracy detection of NC machine tools

  • 2018-01-16 09:41:22

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The positioning accuracy of CNC machine tools, refers to the position accuracy of each axis in the NC machine tool device under the control of movement can be achieved. The positioning accuracy of CNC machine tools and can be understood as the movement precision of the machine tool. The common machine tool by manual feeding, positioning accuracy mainly depends on the error of reading, mobile and CNC machine tool is implemented by digital program instructions therefore, the positioning accuracy depends on the numerical control system and mechanical transmission error.


CNC machine tool numerical control machine tools referred to, is equipped with an automated machine tool 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's four bed Co. Ltd. through the carrier of information input by machine. Processing by the numerical control device controls the signal sent to control the machine movements, by drawing the shape and size requirements, will be automatically processed by the parts.


The positioning accuracy of CNC machine tools, refers to the position accuracy of each axis in the NC machine tool device under the control of movement can be achieved. The positioning accuracy of CNC machine tools and can be understood as the movement precision of the machine tool. The common machine tool by manual feeding, positioning accuracy mainly depends on the error of reading, mobile and CNC machine tool is implemented by digital program instructions therefore, the positioning accuracy depends on the CNC system and mechanical machine error. The moving parts movement is completed in under the control of the NC device, the 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 detection of the content.


1, location precision detection of linear motion

Linear motion positioning accuracy is generally carried out in the machine table and no-load conditions. According to the national standard and the international organization for Standardization (ISO), detection of CNC machine tools, should be based on laser measurement prevail. In the absence of laser interferometer, for the average user to also can use the standard scale. Compared with, measured by optical reading microscope. However, the accuracy of measuring instruments must be measured with high accuracy than 1~2 grade.

In order to reflect all the errors in multiple positioning, the ISO standard stipulates that every location point is calculated by the five measurement data to calculate the mean value and the scatter difference band of the dispersion -3 dispersion band.


2, precision detection of repetitive positioning in linear motion

Used for detecting instrument and detecting precision. The same general detection method is measured in arbitrary coordinate travel and near the midpoint at both 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 in three position. In one of the biggest difference between the 1/2, a sign, as repeated positioning precision of the coordinate, it is the most basic index to reflect the motion precision stability.


3, the precision detection of the origin of the linear motion

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


4, reverse error detection of linear motion

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 error is bigger, the location the accuracy and repeatability of positioning accuracy is low.

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. Stop in the position of the three point and close to the ends of the stroke were measured several times (usually 7 times), calculated the average value of each position, the maximum value to the mean value of reverse error value.


5, positioning accuracy detection of rotary table

A standard turntable angle measuring tool, polyhedron, circular grating and collimator (collimator), can be selected according to the specific conditions. 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, are measuring every 30 locking position, forward and reverse turn. Turn the measurement for a week, the actual angle and position the theory of value (instruction value) maximum difference for indexing error. If the NC rotary table, should be every 30 to a target position for each target position from the positive. The two direction of rapid positioning 7 times, reaching the actual position and the target position is difference of the position deviation method according to GB10931-89< digital control machine tool position accuracy evaluation method to calculate the average position > regulations and standard deviations, the average position of all partial The difference between the maximum value of the difference and the standard deviation and the minimum value of the deviation from all the average positions and the standard deviation is the positioning accuracy error of the numerical control rotary table.


Considering the actual requirements of dry-type transformers, several right angle bisection points such as 0,90180270 are generally measured. The accuracy of these points is increased by one level compared with other angles.


6, repeatability accuracy detection of rotary table

Three position measuring method is optional in the rotary table within a week of repeated positioning 3 times, respectively, were detected rotate in the opposite direction. The maximum value of all readings in the indexing accuracy from the corresponding theoretical value of the difference. If the NC rotary table, every 30 to take a measurement as the goal position, each target position from the positive, 5 anti rapid positioning in two directions, measure the difference between the actual arrival of the position and the target position, the 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 repeated in the indexing precision CNC rotary worktable.


7, the accuracy detection of the original reversion of the rotary table

The measurement method is to reversion the original point from 7 arbitrary locations and determine its stop position. The maximum difference is used as the accuracy of the original point.

It should be pointed out that the existing detection positioning accuracy is in rapid positioning, under the condition of measurement, 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 environmental temperature and the working state of the coordinate axis the 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 strange. This is the error of thermal elongation of the machine used some pre tension (preload) method to reduce the impact.


Repeat positioning accuracy of each axis is to reflect the most basic accuracy index of the shaft, it reflects the stability of the motion accuracy of precision machine tool, can not imagine can be stably used for production. At present, the CNC system function more and more, sit for each injector movement system standard error such as pitch error accumulation so, the backlash error can not only random error compensation system, compensation method, and the repetitive positioning accuracy is the reflection of the random error integrated feed drive mechanism, it cannot be modified by the compensation of CNC system, when it found that when the ultra poor, only the fine adjustment correction of feed transmission chain. Therefore, if allowed the selection of machine tools, machine tool should choose high repeat positioning accuracy as well.

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