The advantage of CNC machine tools is their high accuracy and efficiency. It's very important to calibrate and correct their positioning accuracy periodically. The original accuracy of CNC machine tools is usually calibrated before sales with two-frequency laser interferometer or step gauge. Most people think the calibration with two-frequency laser interferometer is more dodern and higher accuracy. In fact, it is not. In most cases, the calibration with step gauge is higher accuracy and more practical. Why ?
The standard JJF1064-2004« Standard for calibrating coordinate measuring machines », drafted by National Institute of Metrology (NIM), agreed by metrologists and issued by the Chinese government, definitely recommends that step gauge, the mechnical reference artefact of length, shall be used for calibrating coordinate measuring machines. Only when the maximum length of mechanical reference artifact can not reach 66% of diagonal of distance to be measured is the two-frequency laser interferometer advised to be used. This standard strongly recommends that the reference artefact used for calibration and the object to be calibrated must have similar expansion coefficient. The expansion coefficient of step gauge is about 10.5 X10-6 and the expansion coefficient of the driving ball screw to be calibrated is about 11.5 X10-6, both of their expansion coefficients are quite similar. But the expansion coefficient of laser of two-frequency laser interferometer is almost zero.In fact, the step gauge's measurement uncertainty caused by ambient temperature change (even the tiny change of 0.2C° per hour) is significantly smaller than that of two-frequency laser interferometer. Measurement uncertainty is the main basis for judging whether a certain specific measuring method is appropriate or not. The positioning accuracy of CNC machine tools is quite close to that of coordinate measuring machines. The calibration methods for both of them are also similar. Therefore, it is higher accuracy to calibrate under normal circumstances with step gauge. Under special circumstances, namely under the strict condition of national laboratory (strict with temperature, humidity, air pressure, aggressive air and insulation of vibration, etc.) or the expansion coefficient of the object to be calibrated is near zero, such as fused quartz, iridio-platinum,etc., two-frequency laser interferometer is advised to be used. As a matter of fact, it's rare to meet the strict condition outside laboratory.
In most cases, the difference between the expansion coefficients of two-frequency laser interferometer and the object to be calibrated is big, because the expansion coefficient of laser of two-frequency laser interferometer is almost zero.Therefore, under normal circumstances, the uncertainty of two-frequency laser interferometer is bigger than that of step gauge.Step gauge can meet different needs, because it has less demanding on environment and the advantage of low price. That's why step gauge is more practical. The National Physical Laboratory (NPL) of U.K recommends step gauge in its web site, saying "Mechanical reference artefacts, such as a step gauge, provide a cheap and effective way of assessing the performance of a Co-ordinate Measuring Machine (CMM) for a length measuring task. Step gauges are particularly well suited for performing formal performance verification procedures described in national or international standard specifications, such as EN BS ISO 10360 Part 2."
Mr. Wang Wei Nong , the head of Thread / coordinate measurement technology research laboratory under Division of Metrology in Length and Precision Engineering of NIM, published an article The component of uncertainty caused in the repeatability of calibration of CMMs in 12 periodicals 2005 of china Metrology. He analysed the measurement uncertainty caused during the calibration of CMMs based on his many years of experience of calibration work and made a conclusion: "When the component of uncertainty caused in the repeatability is the main source of uncertainty, higher accuracy of gauge blocks can't improve the uncertainty of calibration. Specific limits should be calculated according actual situation."
We now can make a summary that two-frequency laser interferometer can't achieve higher accuracy than step gauge in calibrating CNC machine tools.In most cases under non-strict conditions, calibrating by step gauge is more accurate and reliablbe than that by two-frequency laser interferometer.
The end users need to calibrate the positioning accuracy of CNC machine tools when checking before acceptance or maintaining them. The simple reason is: the original positioning accuracy often changes after transportation (vibration is inevitable), installation, running test and actual operation of a few months, because the conditions on the worksite and the stress of the main parts of the machine tools often vary from those of their manufacturers. Therefore, it's necessory to calibrate them with step gauge in stead of two-frequency laser interferometer. For example, a certain manufacturer of measuring tools in Guilin bought two machining centers and found the accuracy of one was not as good as the other. They calibrated its positioning accuracy with step gauge and made some correction. After this, its accuracy became better than the other. A manufacturer of wire cut electrical discharge machine uses step gauge for calibration and correction of its machines before and after sales, its positioning accuracy ranks the best in its field.
The conclusion is that on worksite calibrating CNC machine tools by step gauge is is more accurate and practical than two-frequency laser interferometer.
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