How to Improve Positioning Accuracy of Each CNC Axes?
Introduction
Monitoring of the machine tool working condition and frequent testing of machine tool accuracy is very necessary in order to find and solve problems in a timely manner to improve the accuracy of parts processing, then how to improve the positioning accuracy of each axis of the machine tool and repeat positioning accuracy?
The same machine tool, due to the use of different standards, the positional accuracy obtained is not the same, so in the selection of CNC machine tool accuracy indicators, but also pay attention to the standards it uses. The positional standard of a CNC machine tool usually refers to the reverse deviation and positioning accuracy of each CNC axis. The determination and compensation of these two is a necessary way to improve machining accuracy.
Reverse deviation
Reverse Deviation and Its Causes in CNC Machine Tools
CNC machine tools have errors in drive components like servo and stepping motors. These errors cause reverse dead zones and reverse clearance in the feed drive chain. As a result, reverse deviation forms when axes change from forward to reverse motion. This phenomenon is also called reverse clearance or loss of motion. For semi-closed-loop servo systems, reverse deviation impacts positioning accuracy and repeatability. Consequently, it affects the overall machining accuracy of the product.
Impact of Reverse Deviation on Machining Accuracy
During G01 cutting movement, reverse deviation affects interpolation accuracy. If deviation is large, parts may be “not round enough” or “not square enough.” In G00 rapid positioning, it reduces positioning accuracy. This affects hole machining accuracy in drilling and boring. Moreover, as equipment operates longer, reverse deviation increases due to wear and tear. Therefore, regular measurement and compensation of reverse deviation for each axis are necessary.
Measurement Method of Reverse Deviation for Linear Axes
To measure reverse deviation, first move the axis forward or backward a certain distance. Use the stop position as the base. Then, move further in the same direction, and afterward move the same distance in the opposite direction. Measure the difference between the stop position and the base position. Measure three positions near the midpoint and ends of the stroke multiple times (usually seven). Calculate the average for each position. The maximum average value is the reverse deviation. Always move a certain distance first; otherwise, the measurement will be inaccurate.
Use measuring tools like micrometers or dial indicators for linear axes. If possible, use a dual-frequency laser interferometer for higher accuracy. Avoid extending the meter base or rod too far, as this causes errors from cantilever forces. Programming methods can simplify and improve measurement accuracy.
Example CNC Program for Measuring X-Axis Reverse Deviation
To measure X-axis reverse deviation on a vertical coordinate machine, press the table against the spindle’s cylindrical surface. Then run this program:
N10 G91 G01 X50 F1000; table moves right
N20 X-50; table moves left to eliminate transmission clearance
N30 G04 X5; pause for observation
N40 Z50; Z-axis raises to clear the way
N50 X-50; table moves left
N60 X50; table moves right to reset
N70 Z-50; Z-axis resets
N80 G04 X5; pause for observation
N90 M99;
Note that the measured reverse deviation varies with operating speed. At low speeds, the value is generally higher due to less overshoot. At high speeds, overshoot reduces the measured deviation. The axis load and resistance also affect results.
Measurement of Reverse Deviation for Rotary Axes
The measurement method for rotary axes is similar to linear axes. However, the detection instruments differ. Use suitable devices for rotary axis measurement. Follow the same procedure of moving forward, reversing, and measuring position differences. This ensures accurate determination of rotary axis reverse deviation.
Compensation Methods for CNC Machine Tools Without Built-in Functions
Domestic CNC machine tools often have positioning accuracy worse than 0.02mm and lack compensation functions. In some cases, programming can achieve unidirectional positioning to clear reverse clearance. When mechanical parts are consistent, low-speed unidirectional positioning reaches the interpolation start point. Then interpolation processing begins. During reverse interpolation feed, move by the reverse gap value before formal interpolation. This improves interpolation machining accuracy and meets part tolerance requirements.
Backlash Compensation in Advanced CNC Systems
Other CNC machines store reverse clearance values in dedicated memory addresses. When the axis changes direction, the CNC reads and applies backlash compensation automatically. This reduces reverse deviation and improves positioning accuracy. Generally, CNC systems use a single backlash value based on rapid movement. However, this may not balance rapid positioning and interpolation accuracy well. FANUC0i and FANUC18i systems offer two backlash compensation values for rapid (G00) and cutting feed (G01) motions. The CNC system selects the appropriate value automatically for better precision.
Enter backlash value A from G01 cutting feed into parameter NO11851. Enter backlash value B from G00 into parameter NO11852. To enable separate compensation, set the fourth digit of parameter 1800 (RBK) to 1. If RBK is 0, separate compensation will not execute. Note that G02, G03, and JOG motions use the same compensation as G01.
Positioning Accuracy
Positioning accuracy of CNC machine tools refers to the measured positional accuracy of the moving parts of the machine tool under the control of the CNC system movement can be achieved, CNC machine tools are different from ordinary machine tools is an important precision, which, together with the geometric accuracy of the machine tool cutting accuracy to produce an important impact, especially on the hole gap machining in the hole distance error has a decisive impact. A CNC machine tool can be judged from the positioning accuracy it can achieve its machining accuracy, so the positioning accuracy of the CNC machine tool to detect and compensate for the necessary way to ensure the quality of machining.
Determination of positioning accuracy
At present, the use of dual-frequency laser interferometer machine testing and processing analysis, the use of laser interference measurement principle, the real-time wavelength of the laser as a measurement reference, so improve the test accuracy and enhance the scope of application. The testing method is as follows:
Install the dual-frequency laser interferometer;
Install the optical measuring device in the direction of the coordinate axis of the machine tool to be measured;
Adjust the laser head so that the measurement axis and the moving axis of the machine tool are co-linear or parallel, i.e. the optical path is pre-collimated;
Entering the measurement parameters after the laser has warmed up;
Move the machine for measurement according to the specified measurement program;
Data processing and result output.
Compensation of positioning accuracy
If the CNC machine’s positioning error exceeds the allowable range, compensation is necessary. Commonly, technicians manually input pitch error compensation tables into the CNC system. However, three- or four-axis compensation may involve hundreds of thousands of points. Therefore, manual input is time-consuming and error-prone.
Now, using the RS232 interface, computers connect to CNC controllers. VB auto-calibration software synchronizes the laser interferometer and CNC machine. This achieves automatic detection and pitch error compensation. The process includes backing up existing parameters, generating CNC programs for point-by-point measurement, and transmitting them to the CNC system. The system then measures errors automatically and generates new compensation parameters. These parameters are uploaded, completing pitch compensation. Finally, accuracy verification repeats.
By using pitch error automatic compensation and reverse gap compensation, technicians can optimize each axis’s compensation points. This ensures the CNC machine reaches its best precision and greatly improves detection efficiency.
Positioning accuracy is vital for CNC machines. Although users may select high-precision equipment initially, wear increases errors over time. This negatively affects part processing and production. Therefore, accurately measuring and compensating reverse deviation improves positioning accuracy. This keeps the machine in optimal condition and ensures machining quality.