Application of laser interferometer in CNC machine tool measurement

Global competition and quality standards require higher positioning accuracy, smaller tolerances and higher feed rates for machine tools. In order to meet these requirements and produce high quality and high precision parts, it is necessary to measure the 3D volumetric positioning accuracy of the machine.

Twenty years ago, the maximum positioning error of the machine tool was the pitch error of the screw and the thermal expansion error of the screw. However, most of the above errors have been greatly reduced, and the main error of the machine tool has turned into a vertical error and a straightness error. In order to achieve high machine tool three-dimensional positioning accuracy, all three displacement errors, six straightness errors and three perpendicularity errors on the machine must be measured and compensated. Measuring straightness and perpendicularity errors with conventional laser interferometers is difficult and time consuming. It usually takes a few days to shut down and requires experienced experts to take measurements.

Optodyne, Inc. has developed a new and breakthrough laser vector measurement technology for machine tool 3D volumetric error measurement (US Patent 6,519,043, 2/11/2003). This measurement method can be completed in a matter of hours instead of a few days with a conventional laser interferometer. Therefore, three-dimensional volume positioning error measurement and compensation becomes practical, and higher precision and smaller tolerances can be achieved.

JOBS SPA, an Italian company, has been manufacturing standard machine tools driven by three-axis and five-axis high-speed linear motors since the 1980s. Two years ago, JOBS replaced the traditional laser calibration equipment with a laser-approved laser Doppler interferometer (LDDM). Combined with the three-dimensional volumetric positioning error measurement technology, or combined with the step-by-step diagonal measurement technology invented by the company, LDDM makes it easy for JOBS to accurately measure and detect problems before the production operation crisis. If the part fails, it will directly affect the production of the assembly and electrical departments. If the part machining does not meet the specified tolerances, it takes more time to assemble the technical specifications to ensure that the machine can achieve the machining accuracy.

With the company's three-dimensional volume laser calibration equipment, JOBS took very little time and got more complete data with several measurements. This allows JOBS to clearly understand the machine's errors and calibrate these errors in a timely manner, thus delivering the user with more competitive quality and price.

The step-by-step diagonal measurement method uses 12 identical diagonal settings and collects 12 sets of data. Based on the measured data, all three displacement errors, six straightness errors, and three perpendicularity errors can be determined. The measured positioning error can be used to generate a three-dimensional volume compensation table that can be uploaded to the Siemens 840D controller to calibrate any positioning error, thereby improving positioning accuracy.

JOBS reported that the use of optical interferometers and step-by-step diagonal measurement methods yielded enough data with very few measurements to show the state of the machine very clearly. JOBS solves some common problems very easily, such as assembly errors, errors caused by temperature changes, and structural problems, without increasing assembly time. The quality of the products produced by JOBS is getting better and better. Moreover, step-by-step diagonal measurements for three-dimensional volume calibration require up to seven measurements, from which the type and size of most errors can be known. JOBS has determined that this method can be used as an optical collimator, ruler and granite platform on the assembly line instead of traditional instruments.

The LDDM technology used by JOBS uses a single-beam MCV-500 and a dual-beam MCV-2002 to reflect back the modulated laser beam from a movable target. The beam with position information is detected and Processed to generate a lookup table, thus enabling the controller to compensate for errors. Since the return beam does not have the same offset as a conventional laser interferometer, the setup is very fast. Only two components need to be adjusted: a single-hole laser beam that emits and receives a laser beam, and a flat mirror that serves as a target.

JOBS reported that the single-beam MCV-500 uses step-by-step diagonal measurements to achieve three-dimensional volumetric positioning errors with minimal assembly time, thus significantly reducing costs.

The laser and the plane mirror are placed on the main shaft and the table, and the X-axis, the Y-axis, and the Z-axis are alternately moved step by step along each axis, so that the repetition goes all the way to the diagonal of the diagonal. The positioning error of the diagonal after each step movement of all three axes is collected. This technology collects three times the amount of data and allows measurement of the displacement error for each axis movement.

The trajectory of the target movement is not a straight line, and the lateral movement is large. Traditional interferometers do not allow such large lateral movements and no data can be detected. The LDDM laser interferometer uses a flat mirror as the target, and the movement parallel to the mirror does not transfer the laser beam and does not change the distance from the light source. Therefore, the measurement will not be affected.

A temperature sensor with up to four working positions can be connected to the automatic temperature compensation unit. Automatic temperature compensation also compensates for changes in environmental factors such as air temperature, atmospheric pressure, and machine temperature.

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