How to Enhance Precision in CNC Machine Tools?

With the ongoing expansion of the manufacturing sector, CNC machine tools have become indispensable in the realm of precision machining. However, as the demand for higher precision in machined parts continues to grow, CNC machine tools are under greater scrutiny. Many factors can affect the accuracy of these machines, including installation, usage, and wear and tear. Hence, regular monitoring and precision testing are essential for maintaining and improving the accuracy of CNC machine tools.

Accuracy in CNC Machine Tools

When procuring CNC machine tools, the primary concern for users is the positional accuracy, particularly the accuracy of each axis and repeat positioning accuracy. However, real-world statistics reveal that many machines do not meet their technical specifications during installation. Furthermore, the majority of CNC machines are used in less-than-optimal conditions. As a result, improving machining precision demands consistent monitoring and precision testing of these machine tools.

Positional Accuracy Testing

Presently, CNC machine tools undergo positional accuracy testing based on international standards like ISO230-2 or national standards such as GB10931-89. These standards determine the positional accuracy of the machine tool; however, the choice of standards can influence the results. Positional accuracy usually encompasses both backlash and axis positioning accuracy, and to achieve enhanced machining precision, compensation for these factors is critical.

Backlash

Backlash, also known as the reverse bias, is a pivotal aspect of CNC machine tools. It arises from a combination of factors, including mechanical motion transmission errors, reverse dead zones within drive chain components (such as servo motors and stepper motors), and backlash in mechanical motion transmission elements. This phenomenon results in a loss of accuracy when axes transition from forward to reverse movement. Backlash can detrimentally affect various aspects of machining, including interpolation movements, drill hole accuracy, and more. Over time, wear and tear can increase backlash. Hence, it is essential to periodically measure and compensate for axial reverse deviations in CNC machine tools.

Measuring Backlash

The measurement of backlash involves conducting measurements at different points along the axis of travel. You begin by moving the axis in one direction by a specific distance and then returning it to the reference point. By measuring the difference between the stopping position and the reference position at various points along the axis, you can determine the average value for each position. The maximum average value obtained is the measured value of the reverse deviation. It's crucial to ensure that there is a certain distance of motion during the measurement process; otherwise, you will not obtain an accurate value for the reverse bias.

For linear axes of motion, measurement instruments, such as percentage gauges, percent gauges, or dual-frequency laser interferometers, are employed. When using dial indicators, the extension of the table base and rod should be neither too long nor too short, as excessive cantilever lengths can lead to inaccurate readings. Additionally, it's possible to measure the reverse deviation through programming, which simplifies the measurement process.

Backlash Compensation

Most CNC machines have backlash compensation to ensure accurate positioning. These machines store backlash values for each axis, and when a direction change command is given, the CNC system will automatically read the backlash value for that axis and compensate for any coordinate displacement. The proper setting of these values helps ensure that the machine accurately reaches the commanded position.

For some CNC systems, like FANUC0i and FANUC18i, there are two types of backlash compensation available, one for rapid motion (G00) and the other for low-speed cutting feed motion (G01). The CNC system will automatically select the compensation value based on the feed rate, enhancing machining precision for different modes.

Positioning Accuracy

Positioning accuracy in CNC machine tools represents the capability to achieve specific movement positions in the CNC system. It is crucial for various machining applications and is especially critical for hole machining, as it can significantly impact pitch accuracy.

Measuring Positioning Accuracy

Positioning accuracy is determined using a dual-frequency laser interferometer that employs laser interferometry principles to measure wavelength deviations. This approach enhances measurement accuracy and expands the scope of application.

To measure positioning accuracy, you need to follow these steps:

Install a dual-frequency laser interferometer.

Install an optical measuring device in the direction of the machine's coordinate axis.

Align the measuring axis parallel or co-linear to the machine's movement axis.

Warm up the laser, then enter measurement parameters.

Execute machine movement based on the specified measurement program.

Process the collected data and generate results.

Compensating Positioning Accuracy

If the measured positioning error exceeds the permissible error range for the CNC machine tool, compensation is necessary. The standard approach involves calculating a pitch error compensation table and entering it into the CNC system to ensure enhanced machining precision.

In conclusion, to maintain and enhance the accuracy of CNC machine tools, consistent monitoring and precision testing are essential. Monitoring backlash and compensating for it, as well as addressing any inaccuracies in positioning, can significantly improve the quality and precision of machined parts.