Scanning for Ideas: High-Speed CMM Compensates for its Own Spee

Nov. 7, 2008
The new Renscan5 coordinate-measuring machine (CMM) from Renishaw Inc., Hoffman Estates, Ill. (, uses a touch probe or stylus to make measurements at up to 500 mm/sec (or 4,000 data points/sec), which is up to 30 times faster than conventional CMM systems. But at those speeds, the stylus bends, which would destroy any accuracy in a normal CMM.

Edited by Stephen J. Mraz

Instead of fighting the bending, which would require a thicker, shorter, and stiffer stylus and limited access to complex parts, the company used a laser to compensate for the bending. Here’s how it works:

The probe consists of a tip mounted on a hollow, 500-mm low-mass tube. A laser beam travels up the tube to a reflector that sends it back down the tube. The tube is designed to bend about 50 μm at the tip under acceleration, so the laser takes longer to travel up and down the slightly longer or shorter tube. A position-sensing device monitors this difference. The difference is then translated into a measurement output. A software algorithm combines the data with the probe geometry, letting it derive the tip’s exact (or corrected) position on the fly during scanning.

With the compensation in place, accuracy is 1 μm at 250 mm from the axis of rotation at 500 mm/sec on the five-axis machine. Early adopters reported that the new CMM lets them measure jet-engine blisks in one-ninth the time of their previous systems.

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