7 Ways to Detect the Positioning Accuracy of CNC Machine Tools


The positioning accuracy of CNC machine tools describes the positional accuracy that may be accomplished by the movement of every coordinate axis of the device tool under the control of the numerical control device. The positioning accuracy of CNC machine tools may be understood because the motion accuracy of the device tool. Ordinary machine tools are manually fed. The positioning accuracy is mainly determined by the reading error. The movement of the CNC machine tool is realized by digital program instructions, and so the positioning accuracy is set by the numerical control system and mechanical transmission error.Hit on CNC Machine Tool Accuracy Calculation  to explore more about our services and sites. Hope you ll like our more services.


CNC machine tool is the abbreviation of digital control machine tool, which can be an automatic machine tool with a program control system. The control system is capable of logically processing a program having control code and other symbolic instructions and decoding it, using coded digital representations, and Nanjing Fourth Machine Tool Co., Ltd. inputs the numerical control device through the data carrier. Through the arithmetic processing, various control signals are issued by the differential control device to manage the movement of the device tool, and the parts are automatically processed according to the shape and size required by the drawings.


The movement of every moving area of the machine tool is completed under the control of the numerical control device. The precision that each moving part can perform under the control of the program command directly reflects the precision that the machined part can make. Therefore, the positioning accuracy is an essential test. Content.


1. Linear motion positioning accuracy detection
Linear motion positioning accuracy is generally performed under no-load conditions on machine tools and benches. In line with the national standards and the provisions of the International Organization for Standardization (ISO standards), the detection of CNC machine tools must certanly be based on laser measurements. In the absence of a laser interferometer, it can also be easy for the average user to utilize a standard scale by having an optical reading microscope for comparative measurements. However, the accuracy of the measuring instrument must certanly be anyone to two levels higher compared to accuracy of the measurement.


To reflect all the errors in multiple positioning, the ISO standard stipulates that each positioning point calculates the average value and the dispersion difference based on five measurement data, and the dispersion difference band formed by the dispersion band.


2, linear motion repeat positioning accuracy detection
The instrument used for the test is the same as which used to detect thepositioning accuracy. The overall detection method is always to measure at any three positions near the midpoint and the 2 ends of every coordinate stroke. Each position is quickly moved, and the positioning is repeated 7 times under the same conditions. The stop position value is measured, and the most reading difference is obtained. Taking one-half of the very most significant difference on the list of three positions, the positive and negative signs are attached because the repeated positioning accuracy of the coordinates, which can be the absolute most basic index reflecting the stability of the axis motion accuracy.


3, linear motion origin return accuracy detection
The origin return precision is basically the repeat positioning accuracy of a particular point on the coordinate axis, so its detection method is totally the same as the repeat positioning accuracy.


4. Reverse error detection of linear motion
The alternative error of the linear motion also called losing amount, includes the reverse dead zone of the drive position (such as servo motor, servo motor, and stepping motor) on the coordinate axis feed chain, and each mechanical motion transmission pair A comprehensive reflection of errors such as backlash and elastic deformation. The more expensive the error, the lower the positioning accuracy and the repeat positioning accuracy.


The detection method of the reverse error is to move a range forward or backward in the stroke of the measured coordinate axis and use the stop position as a research, and then offer a specific movement command value in exactly the same direction to make it move a distance. Then run exactly the same distance in the opposite direction and assess the difference between the stop position and the reference position. The measurement has performed a plurality of times (generally seven times) at three points near the midpoint and both ends of the stroke, and the average value at each position is obtained, and the most value on the list of obtained average values is the reverse error value.


5. Positioning accuracy detection of rotary table
Measuring tools include standard turret, angle polyhedron, circular grating and collimator (collimator), etc., which may be selected according to the specific conditions. The measurement method is to help make the table forward (or reverse) to an angle and stop, lock, and position. Utilize this position as a research, then quickly turn the table in exactly the same direction and measure every 30 locks. Each of the forward rotation and the reverse rotation is measured for 1 week, and the most value of the difference between the specific rotation angle of every positioning position and the theoretical value (command value) is the division error.If it is just a CNC rotary table, it should be a target position every 30, for each target position to quickly locate 7 times from the positive and negative directions, the difference between the area and the prospective position is in fact reached, and then based on GB10931- 89 The method specified in the “Method for Evaluating the Position Accuracy of Digital Control Machines” calculates the average position deviation and standard deviation, the difference between the most value of all the average position deviations and the conventional deviation and the sum of all the average position deviations and the conventional deviation. It's the positioning accuracy error of the CNC rotary table.


Thinking about the dry-type transformer to the specific use requirements, it is generally important to measure several equal-angle points such as 0, 90, 180, 270, etc., and the accuracy of these points is required to be improved by one level compared with other angular positions.


6. Repeated indexing accuracy detection of rotarytable
The measurement method is repeated 3 times in three places in 1 week of the rotary table, and the detection is performed in the forward and reverses directions respectively. The maximum value of the difference between the values of most readings and the theoretical value of the corresponding position. If it's a CNC rotary table, take one measurement point every 30 as the prospective position, and perform five fast positionings for each target position from the positive and negative directions respectively, and assess the difference between the specific arrival position and the prospective position. That's, the positioning deviation, and then calculate the conventional deviation according to the method specified in GB10931-89, which can be six times of the most value of the conventional deviation of every measuring point, which can be the repeating indexing precision of the numerical control rotary table.


7. The origin return accuracy detection of the rotary table
The measurement method is to execute the origin return from 7 arbitrary positions, assess the stop position, and use the maximum difference read because the origin return accuracy.


It must be noticed that the detection of the present positioning accuracy is measured under the condition of fast and positioning. For many CNC machine tools whose feeding system is not very good, different positioning accuracy values will be obtained when positioning with varying speeds of feed. Also, the measurement result of the positioning accuracy relates to the ambient temperature and the working state of the coordinate axis. At present, all the numerical control machine tools adopt a semi-closed loop system, and the positioning detecting components are mostly installed on the driving motor, which generates a mistake of 0.01 to 0.02 mm in a stroke of 1 m. It's not strange. This is a mistake brought on by thermal elongation, and some machines use a pre-stretch (pre-tightening) method to reduce the impact.


The repeating positioning accuracy of every coordinate axis reflects the absolute most basic accuracy index of the shaft, which reflects the stability of the motion accuracy of the axle, and it cannot be assumed that the device tool with poor precision may be stably used for production. At present, due to the increasing quantity of functions of the numerical control system, system errors such as pitch accumulation error and backlash error may be compensated for the motion accuracy of every sitting injector. Only the random error cannot be paid, and the repeat positioning accuracy is repeated. It reflects the absolute random error of the feed drive mechanism. It can't be corrected by the CNC system compensation. When it is found to be out of tolerance, only the fine adjustment of the feed drive chain is performed. Therefore, if the device tool is allowed to be selected, it is much better to decide on a device with high repeatability.


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