Seven Main Methods for Detecting Positioning Accuracy of CNC Machine Tools

The positioning accuracy of a CNC machine tool is the positional accuracy achievable by each coordinate axis under the CNC device’s control. 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 controlled by digital program instructions; therefore, its positioning accuracy depends on the CNC system and mechanical transmission errors.

A CNC machine tool is short for Digital Control Machine Tool, an automated machine tool equipped with a programmable control system. This control system can logically process programs with control codes or other symbolic instructions, decode them, and represent 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 drive the machine tool’s movements, automatically machining parts to the shapes and dimensions specified in the drawings.

The CNC device controls the movement of each part of the machine tool. The accuracy of each moving part under program control directly reflects the accuracy of the machined parts. Therefore, positioning accuracy is a very important testing criterion.

1. Linear Motion Positioning Accuracy Testing

Linear motion positioning accuracy is generally tested 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 testing CNC machine tools. 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 measuring instrument’s accuracy must be 1-2 grades higher than the accuracy being measured.

To account for all errors in multiple positioning operations, the ISO standard specifies that the average and dispersion of five measurements at each positioning point are calculated, forming the positioning point dispersion band.

2. Linear Motion Repeatability Positioning Accuracy Testing

The instruments used for testing are the same as those used for testing positioning accuracy. The general testing method involves measuring at any three positions near the midpoint and both ends of each coordinate’s travel. Each position is set using rapid traverse, and the process is repeated 7 times under the same conditions. The values ​​at the stopping positions are measured, and the maximum difference in readings is calculated. The largest difference among the three positions, with a positive or negative sign, is taken as the repeatability positioning accuracy of that coordinate. It is the most basic indicator reflecting the stability of axis motion accuracy.

3. Linear Motion Origin Return Accuracy Detection

Return accuracy is the repeatability accuracy of a specific point on the coordinate axis. Therefore, its detection method is identical to that for repeatability accuracy.

4. Linear Motion Reverse Error Detection

The reverse error of linear motion, also called loss of momentum, is a comprehensive reflection of errors, including the reverse dead zone of the drive components (such as servo motors, hydraulic servo motors, and stepper motors) in the feed transmission chain of the coordinate axis, the backlash of various mechanical motion transmission pairs, and elastic deformation. The larger the error, the lower the positioning accuracy and repeatability.

The method for detecting reverse error is to move the coordinate axis a certain distance in the forward or reverse direction within its stroke, using this stopping position as a reference. Then, a certain movement command value is given in the same direction, causing it to move a certain distance. Then, it is moved the same distance in the opposite direction. The difference between the stopping position and the reference position is measured. Multiple measurements are performed (usually 7 times) at three positions near the midpoint and both ends of the stroke. The average value at each position is calculated, and the maximum of these averages ​​is the reverse error value.

5. Positioning Accuracy Testing of Rotary Tables

Measuring tools include standard rotary tables, angle polyhedra, circular gratings, and collimators, which can be selected according to specific circumstances. The measurement method involves rotating the table forward (or backward) by a certain angle, stopping, locking, and positioning it. Using this position as a reference, the table is rapidly rotated in the same direction, locking and positioning at every 30 degrees, and measurements are performed. 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 tables, 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 and target positions 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 sum of all minimum sums of average position deviations and standard deviations is the positioning accuracy error of the CNC rotary table.

Considering the practical requirements of dry-type transformers, key measurements are typically taken at right-angle division points such as 0°, 90°, 180°, and 270°. The accuracy of these points should be one grade higher than other angular positions.

6. Repeatability Accuracy Test of Rotary Table

The measurement method involves randomly selecting three positions within one revolution of the rotary table, repeating the positioning three times, and 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 repeatability accuracy. For CNC rotary tables, a measurement point should be selected every 30° as the target position. For each target position, rapid positioning is performed five times in both forward and reverse directions. The difference between the actual reached position and the target position is measured, i.e., the positional 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 accuracy of the CNC rotary table.

7. Origin Return Accuracy Test of Rotary Table

The measurement method involves performing an origin-return test at seven arbitrary positions, determining the stopping position, and using the maximum difference as the origin-return accuracy.

It should be noted that current positioning accuracy tests are conducted under high-speed, positioning conditions. For some CNC machine tools with less efficient feed systems, different positioning accuracies will be achieved at different feed speeds. Furthermore, the positioning accuracy measurement results are related to ambient temperature and the working state of the coordinate axis. Currently, most CNC machine tools use semi-closed-loop systems, with position detection elements mostly mounted on the drive motor. An error of 0.01~0.02mm within a 1m stroke is not surprising. This is an error caused by thermal expansion; some machine tools use pre-tensioning (pre-tightening) methods to reduce its influence.

Repeatability accuracy for each coordinate axis is the most basic indicator of the axis’s motion accuracy and stability. It is unthinkable that a machine tool with poor accuracy can be used stably in production. Currently, with the increasing functionality of CNC systems, systematic errors in the motion accuracy of each component, such as pitch accumulation error and backlash error, can be compensated. However, random errors cannot be compensated. Repeatability accuracy reflects the overall random error of the feed drive mechanism and cannot be corrected by the 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.