Application of Laser Interferometer in Measurement of CNC Machine Tools

Global competition and quality standards require higher positioning accuracy, tighter 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 three-dimensional volume positioning accuracy of machine tools.

Twenty years ago, the largest positioning errors of machine tools were the pitch error of the screw and the thermal expansion error of the screw. But now most of the above errors have been greatly reduced, and the main errors of machine tools have become perpendicularity errors and straightness errors. In order to achieve high three-dimensional spatial positioning accuracy of machine tools, all three displacement errors, six straightness errors and three perpendicularity errors on the machine tools must be measured and compensated. Measuring straightness and perpendicularity errors with traditional laser interferometers is difficult, time-consuming and expensive. It usually requires downtime for several days and requires experienced experts to perform the measurements.

Optodyne, Inc. has developed a new, breakthrough laser vector measurement technology for measuring three-dimensional volume positioning errors of machine tools (U.S. Patent 6,519,043, 2/11/2003). This measurement method can be completed in just a few hours instead of several days with traditional laser interferometers. Therefore, 3D volume positioning error measurement and compensation has become practical and can achieve higher accuracy and smaller tolerances.

JOBS SPA, an Italian company, has been manufacturing three-axis and five-axis high-speed linear motor driven standard machine tools since the 1980s. Two years ago, JOBS replaced the traditional laser calibration equipment with the laser Doppler interferometer (LDDM) patented by OptoMotion. Combined with 3D volume positioning error measurement technology, or combined with the step-by-step diagonal measurement technology invented by OptoMotion, LDDM makes it easy for JOBS to make accurate measurements and detect problems before production operations become critical. If the parts are not qualified, it will directly affect the production of the assembly and electrical departments. If the parts processing cannot meet the specified tolerances, it will take more time to assemble to ensure that the machine tool can meet the technical indicators of processing accuracy. With

OptoMotion's 3D volume laser calibration equipment, JOBS spent less time and obtained more complete data in a few measurements. This allows JOBS to clearly understand the errors of the machine tools and calibrate them in a timely manner, thus delivering users with more competitive quality and price.

The step-by-step diagonal measurement method uses four 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 errors can be used to generate a 3D volume compensation table, which can be uploaded to the Siemens 840D controller to calibrate any positioning errors, thereby improving positioning accuracy.

JOBS reported that with only a few measurements using the optical interferometer and the step-by-step diagonal measurement method, enough data can be obtained to clearly show the status of the machine tool. JOBS easily solves some common problems such as assembly errors, errors caused by temperature changes, and structural problems without increasing assembly time. The quality of JOBS products is getting better and better. Moreover, the step-by-step diagonal measurement for 3D volume calibration requires a maximum of 7 measurements, from which the type and size of most errors can be understood. JOBS has determined that this method can replace traditional instruments as optical collimators, rulers, and granite platforms on the assembly line.

The LDDM technology from Opto-Dynamics used by JOBS uses the single-beam MCV-500 and dual-beam MCV-2002 to reflect a modulated laser beam from a movable target. The beam with position information is detected and processed to generate a lookup table, which enables the controller to compensate for errors. Since the return beam does not require offsets like traditional laser interferometers, setup is very fast. Only two components need to be adjusted: a single-aperture laser head that transmits and receives the laser beam, and a flat mirror that serves as the target.

JOBS reports that the single-beam MCV-500 uses step-by-step diagonal measurement to obtain three-dimensional volume positioning errors with minimal assembly interruption time, thereby greatly reducing costs.

The laser and flat mirror are placed on the spindle and table, and move alternately in steps along each axis, X, Y, and Z, and repeat until they reach the diagonal corner. The diagonal positioning error is collected after each step of all three axes. This technology collects three times the amount of data and allows the displacement error to be measured as each axis moves.

The trajectory of the target movement is not a straight line, and the lateral movement is large. The traditional interferometer does not allow such a large lateral movement, and no data can be measured. The LDDM laser interferometer uses a plane mirror as a target. The movement parallel to the mirror will not transfer the laser beam, nor will it change the distance from the light source. Therefore, the measurement will not be affected.

Up to four working position temperature sensors can be connected to the automatic temperature compensation unit. Automatic temperature compensation also provides compensation for environmental factors such as changes in air temperature, atmospheric pressure, and machine tool temperature.