The positioning accuracy of a CNC machine tool refers to the positional accuracy that each coordinate axis of the machine tool can achieve under the control of the CNC device. The positioning accuracy of a CNC machine tool can also be understood as the motion accuracy of the machine tool. Ordinary machine tools rely on manual feed, and their positioning accuracy is mainly determined by reading errors. However, the movement of a CNC machine tool is achieved through digital program instructions; therefore, its positioning accuracy depends on the errors of the CNC system and the mechanical transmission.
CNC machine tools, short for numerical control machine tools, are automated machine tools equipped with a program control system. This control system logically processes programs with control codes or other symbolic instructions, decodes them, and represents them with coded numbers. Nanjing Fourth Machine Tool Co., Ltd. inputs these codes into the CNC device via an information carrier. After processing, the CNC device issues various control signals to control the machine tool's movements, automatically machining parts according to the shape and dimensions required by the drawings.
The movement of each moving part of a 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 machined parts can achieve. Therefore, positioning accuracy is a very important inspection item.
1. Linear motion positioning accuracy testing
Linear motion positioning accuracy is generally measured under no-load conditions on the machine tool and worktable. According to national standards and the International Organization for Standardization (ISO standards), laser measurement should be the standard for CNC machine tool inspection. In the absence of a laser interferometer, a standard ruler with an optical reading microscope can be used for comparative measurement by general users. However, the accuracy of the measuring instrument must be 1-2 grades higher than the accuracy of the object being measured.
To reflect all errors in multiple positioning operations, the ISO standard specifies that for each positioning point, the average value and variance are calculated based on five measurements, forming the positioning point variance band.
2. Linear motion repeatability accuracy detection
The instruments used for testing are the same as those used for testing positioning accuracy. The general testing method involves taking measurements at any three positions near the midpoint and both ends of each coordinate's travel. Each position is positioned using rapid traverse, and this process is repeated seven times under identical conditions. The values at the stopping positions are measured, and the maximum difference in readings is calculated. Half of the largest difference among the three positions, with a positive or negative sign, is taken as the repeatability accuracy of that coordinate. This is the most fundamental indicator reflecting the stability of the axis's motion accuracy.
3. Accuracy detection of origin return for linear motion
Origin return accuracy is essentially the repeatability accuracy of a specific point on the coordinate axis, so its detection method is exactly the same as that of repeatability accuracy.
4. Reverse error detection for linear motion
The reverse error of linear motion, also called loss of momentum, is a comprehensive reflection of errors including the reverse dead zone of the driving parts (such as servo motors, hydraulic servo motors, and stepper motors) in the feed transmission chain of that coordinate axis, the reverse backlash of each mechanical motion transmission pair, and elastic deformation. The larger the error, the lower the positioning accuracy and repeatability.
The method for detecting reverse error involves moving the coordinate axis a certain distance in the forward or reverse direction within its travel range, using this stopping position as a reference. Then, a certain movement command is given in the same direction, causing it to move a certain distance. This is repeated by moving the axis the same distance in the opposite direction, and the difference between the stopping position and the reference position is measured. Multiple measurements (usually 7 times) are performed at three positions near the midpoint and both ends of the travel range, and the average value at each position is calculated. The maximum value among these average values is taken as the reverse error value.
5. Positioning accuracy testing of the rotary table
Measuring tools include standard rotary tables, angle polyhedra, circular gratings, and collimators (collimators), which can be selected according to specific circumstances. The measurement method involves rotating the worktable forward (or backward) by an angle, stopping, locking, and positioning it. This position is used as a reference. Then, the worktable is rapidly rotated in the same direction, locking and positioning every 30 degrees, and measurements are taken. One full rotation is measured in both the forward and reverse directions. The maximum difference between the actual rotation angle at each positioning position and the theoretical value (command value) is the indexing error. For CNC rotary worktables, every 30 degrees should be considered as a target position. For each target position, rapid positioning is performed 7 times in both forward and reverse directions. The difference between the actual position and the target position is the position deviation. The average position deviation and standard deviation are then calculated according to the method specified in GB10931-89 "Evaluation Method for Position Accuracy of Numerical Control Machine Tools". The difference between the maximum sum of all average position deviations and standard deviations and the minimum sum of all average position deviations and standard deviations is the positioning accuracy error of the CNC rotary worktable.
Considering the actual usage requirements of dry-type transformers, it is generally necessary to focus on measuring several right-angled division points such as 0°, 90°, 180°, and 270°, requiring the accuracy of these points to be one level higher than that of other angle positions.
6. Repeatability testing of rotary table
The measurement method involves randomly selecting three positions within one revolution of the rotary table and repeating the positioning three times, performing the test in both forward and reverse directions. The maximum difference between all readings and the theoretical value for the corresponding position is the indexing accuracy. For CNC rotary tables, a measurement point is selected every 30 degrees as the target position. Each target position is rapidly positioned five times in both forward and reverse directions, and the difference between the actual position and the target position is measured, i.e., the position deviation. The standard deviation is then calculated according to the method specified in GB10931-89. Six times the maximum standard deviation among all measurement points is the repeatability indexing accuracy of the CNC rotary table.
7. Rotary table origin return accuracy test
The measurement method involves performing a return to the origin from each of the seven arbitrary positions, determining the stopping position, and using the maximum difference as the accuracy of the return to the origin.
It should be noted that current positioning accuracy tests are performed under high-speed, positioning conditions. For some CNC machine tools with less efficient feed systems, different positioning accuracy values will be obtained when positioning at different feed speeds. Furthermore, the measured positioning accuracy is related to the ambient temperature and the working state of the coordinate axis. Currently, most CNC machine tools use semi-closed-loop systems, and the position detection elements are mostly mounted on the drive motor. It is not surprising that an error of 0.01~0.02mm occurs within a 1m stroke. This error is caused by thermal expansion, and some machine tools use pre-tensioning (pre-tightening) methods to reduce its impact.
The repeatability of each coordinate axis is the most basic accuracy indicator reflecting that axis. It reflects the stability of the axis's motion accuracy; it's unrealistic to expect a machine tool with poor accuracy to be used stably in production. Currently, with the increasing functionality of CNC systems, systematic errors in the motion accuracy of each coordinate axis, such as pitch accumulation error and backlash error, can be systematically compensated. However, random errors cannot be compensated. Repeatability reflects the comprehensive random error of the feed drive mechanism, which cannot be corrected by CNC system compensation. When it is found to be out of tolerance, fine-tuning of the feed transmission chain is necessary. Therefore, if machine tool selection is possible, a machine tool with high repeatability accuracy should be chosen.
