Probe Error Compensation Method for CNC Machine Probe

1. Drilling diagnosis of CNC machine tool probe

During the working process of CNC machine tools, vibration will inevitably occur, which will have a negative impact on the life of the tool. So, this text has developed a kind of modular USB system automatic diagnosis system based on NI CompactDAQ (wireless data acquisition system) type.

This article mainly considers the free vibration of the drill bit, using the Siemens SINUMERIC 840D CNC system (rated spindle speed 1500pm, maximum speed 8000rpm) to drill small diameter holes on a 500V/5 machining center to diagnose vibration.

The experimental conditions are as follows: auger drill diameters 2.85 and 4.7 mm (P18 steel), drilling depth 8 mm; 298x110x10 mm prismatic workpiece (14X17H2 steel). In drilling, the spindle speed is n=250500 and 600rpm, and the axial feed is 0.06mm/rev.

To reduce test time, no lubricant was supplied to the cutting area. Each experiment was performed at least three times; the number of replicates depends on the results proving to be similar. In this measurement system, the analysis of the vibration spectrum within a given frequency range not only determines the displacement of the signals along the X, Y and Z axes, but also displays the amplitude-frequency characteristics of these signals, giving a better indication of changes in drill bit operation during its wear and failures.

The results show that the wear of the small-diameter drill bit of the CNC machine tool probe is accompanied by the appearance and obvious increase of the vibration acceleration signal. The spindle speed increased from 250 rpm to 600 rpm, and the amplitude increased from 39.0 to 63.2 m/s. After the signal reaches a maximum value (characteristic frequency), the bit breaks.

When drilling small diameter (2.85 and 4.7 mm) holes in stainless steel workpieces, the drill bit wears at a characteristic frequency of 500 Hz, and adjacent harmonics in the range of 450-550 Hz. The bit life to complete wear and breakage is not very dependent on bit speed: in the range of 250-600pm, it is 187.1-229.3s.

2. Compensation method of CNC machine tool probe

Using a three-point motion probe usually causes a radius error greater than 10 μm, which is the main measurement error of CNC machine tools. The systematic error of a motion-triggered probe for a CNC machine tool can exceed the error of the machine tool itself. Therefore, the accuracy of the CNC machine tool probe largely determines the accuracy of the machine tool. Because the main part of detection error is systematic error, numerical error correction method is feasible.

In order to improve the accuracy, people usually use a numerical correction method for the probe system error, but this correction method requires a large calculation in the controller of the CNC machine tool. In order to reduce the amount of calculation, this study proposes a new method: by setting the measuring probe at a specific speed, the pre-travel of the probe in this direction is modified by modifying the measured value in a specific direction.

Because before measuring a new machine, all calculations can be done offline and used to determine the speed value in the G code of the CNC machine tool. The CNC machine tool performs error correction by changing a single measured speed value rather than by separately calculating the correction values in the three directions.

Applying the new method can make the measurement accuracy of the three-point motion probe comparable to that achieved by using more complex and expensive strain gauge probes. The same effect can be achieved by using numerical error correction on CNC machine tools, but the method proposed in this paper has a smaller calculation amount and stronger operability.

The method proposed in this paper has been tested on a motion probe type CNC machine tool, and the results show that the error can be reduced by at least 10 times using this method.