Laser Calibration System Verifies Linear And Circular CNC Accuracy

How do you document that your CNC machine tool is within tolerance? That's becoming a very familiar question as more and more companies implement quality assurance programs, such as ISO 9000, TQM and predictive maintenance. These programs have a requirement for procedures for calibration and documentation of performance for traceability and/or trend analysis, which ensures machine tools maintain maximum performance and are within tolerance. The rationale for these programs is that if the process is correct, then the part will be correct. The result is higher quality parts, lower costs and shorter delivery times.

Centroid Corporation (Howard, Pennsylvania) manufactures PC-based CNC motion control systems for machine tools. The company's controls are utilized on knee mills and small to medium size bed mills, ATC (auto tool changer) bed mills, small to medium size lathes, and a variety of other machine tools. Because the controls are either integrated offsite by a machine tool builder or retrofitted by a local subcontractor, proving accuracy can be a critical issue.

"We had experienced situations where documenting the accuracy of the machine tool and our CNC control systems became critical," says Joe McCulloch, chief engineer and co-owner. "So, we purchased an MCV-500 laser calibration system, made by Optodyne Inc. (Compton, California), to provide data for feeding our compensation software and providing a hard copy of machine tool accuracy for both the machine tool builder and customers. We also utilize the laser calibration system as a diagnostic tool for after-sale maintenance."

The machine tool calibration system utilizes a stabilized laser with linear accuracy that is NIST (National Institute of Standards and Technology) traceable and features a resolution of up to one-microinch. However, Centroid controls are utilized primarily in the "tenths" (0.0001-inch) range, which is the lower limit of accuracy tolerances for machine tools in this market segment. Optodyne provides data collection software, supported by Windows.

"Before we purchased the laser calibration system, we and our dealers used dial indicators and a collection of precision bars and standards, which required a series of laborious and error-prone manual measurements. At times we could only spot check the ballscrew and overall machine accuracy," says Mr. McCulloch. "The laser calibration system quickly generates error correction data that we utilize to create an error map of the entire ball screw."

Based on Optodyne's Laser Doppler Displacement Meter (LDDM) technology, the system utilizes electro-optics, optical heterodyne techniques and phase-demodulators to obtain the position information. Since polarization and stray light are non-issues, precision and special optics are not required. Windows can be inserted in the beam path and simple mirrors can be used to reflect the laser beam to any angle. The system is usually mounted on the machine tool, which provides higher accuracy. Basically, the Laser Calibration System reflects a modulated laser beam off of a movable target (mirror). The beam is detected and processed for displacement information, which is used to determine position accuracy of the CNC servo loop.

"Our CNC software was written so that pitch error data generated by the laser calibration system can be uploaded by floppy disk to the CNC control," says Mr. McCulloch.

The Laser/Ballbar System allows both linear accuracy checks as well as contouring tests. By pointing the laser beam in the X-axis direction and mounting a flat mirror target on the spindle, the X-axis coordinate of the spindle motion can be measured even with a large Y-axis direction movement. Repeating measurement in the Y-axis direction, the Y-axis coordinate of the spindle motion can be measured. By combining the X-axis and Y-axis coordinate data, a polar plot of the measured circular path of the spindle can be generated.

There are only two components to align, the laser and a retro reflector. Both components are mounted to the machine tool, typically on the spindle and table, which eliminates the need for a tripod, as well as the need to remove covers or partially dismantle the machine. The system takes measurements at speeds up to 160 inches per second.

"A person can do all three axes in under three hours," says Mr. McCulloch. "The first-time user will take a day getting familiar with it. I've calibrated three axes in an hour, not with a lot of repeated runs, so it may not be a fair comparison. But to do three axes in two hours is very doable."

The operator moves the table into position with the machine's control. Since the system automatically senses table movement, data collection is automatically triggered after a pre-set, user-defined interval as the table begins to move and stop.

"We have two systems," says Mr. McCulloch. "One in Taiwan so our people can go to the factory and check out the mill or lathe, and one in the U.S. that, because of its compact size, is easy to send in the field."

"For two years the Optodyne system has helped us determine where accuracy problems were originating and correct them before they get out in the field," says Mr. McCulloch. "It's helped customers buying a machine with a Centroid control on it."