Faster Inspection On Existing CMMs

Software that adjusts for dynamic error lets a CMM inspect at higher rates without losing precision.

Article From: 8/16/2004 Modern Machine Shop, ,

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Differences between the two measurements

Here, the same circle is measured once at a slow speed and once at a fast speed. The differences between the two measurements show the CMM controller where the CMM's dynamic error lies.

The chief obstacle to faster inspection on coordinate measuring machines is dynamic error. At different speeds and accelerations, the machine's inertia gives the CMM different levels of error in different regions of the work zone. When the machine is scanning, this dynamic error affects the measurement results. The error is so complex and unpredictable that the typical strategy is simply to run the CMM at such a slow speed that inertial effects don't matter.

But it may be possible to scan faster than this, even on an existing machine. The dynamic error, though unpredictable at first, remains consistent from one workpiece to the next. Renishaw (maker of CMM probes and CMM controllers) has taken advantage of this fact to develop an algorithm for improved measurement efficiency.

The company's "Renscan DC" adjusts for dynamic error by inspecting the same workpiece twice—once at standard speed and once at a faster speed. The differences between the two measurement results show the CMM controller precisely where the dynamic error lies. As a result, the next piece—and all pieces thereafter—can be inspected accurately using only the faster speed.

There are caveats. The algorithm only applies to applications in which the same part number is inspected multiple times. The map of dynamic error for one part does not apply to another part, nor does it apply if the same part is set up in a different location on the CMM.

But in those cases where multiple pieces of the same part number are all inspected the same way, significant time savings become possible. In a test case involving a V8 engine block, the faster inspection routine using dynamic error compensation took just 17 percent of the time that the standard inspection cycle had required.

The algorithm is available on Renishaw's "UCC" line of universal CMM controllers. The company points to Renscan DC as a principal reason why it developed its own CMM control in the first place. Adapting an algorithm such as this for the various proprietary CMM controllers would have made the functionality more expensive.

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