Gears, Coordinate Measuring Machines and SCEPTRE™

by Rod Kleiss

Editor's Note: We reported on Kleiss Gears surprising results in gear inspection last newsletter using one of the first production models of our CUBIT Coordinate Gaging Module. For this issue we asked Rod Kleiss to discuss CMMs, gears and his experience with SCEPTRE System.

Gears present a unique difficulty in measurement. Apart from its features, the geometry of a gear is defined primarily by its method of generation. The quality is not just dependent upon the accuracy of the gear teeth, but also on relationship of the teeth around a 360 degree cylinder. The descriptive geometry can become a nightmare of jargon and misinformation to the inspector.

Number of teeth, Diametral pitch, and pressure angle are just the beginning. Pitch circle, base circle, root circle, addendum, dedendum, whole depth, crown, lead, circular tooth, thickness, base tooth thickness, index variation, total index variation... The list goes on forever. In fact, terminology is one of the greatest barriers to precision inspection of gears. The gearing industry has created its own tower of babble (pun intended).

A complete industry has evolved to address the precision inspection of gears. Coordinate measuring machines of great girth and tremendous sophistication have been developed to measure gears along. Gearing software is built into the machines, and these devices become monoliths for all to revere. Gears are fixtured on precision rotary tables, and dynamic measurement is begun by properly sequencing the software procedures. After all is said and done, the results of the inspection are printed and the Great God of gear inspection speaks as to the accuracy of its offspring, but therein lies the problem.

As these machines were developed to inspect the profiles of gears, the software became precious to its developers. They decided on just what features to inspect and how to inspect them. They developed their own criteria for mathematically smoothing and sorting the data, as well as adjusting for forces and speeds. They even developed their own methods and "artifacts" for qualifying the accuracy of their equipment.

I'm sure that all of these decisions are very convenient and lend themselves to a turnkey operation for the inspection of gears, but they do leave much to be desired by the conscientious quality engineer. For instance, how well do these artifacts, which consist of one tooth on a precision bore, correlate to real world gears? No one knows.

What data did the machine actually collect for its measurement? Most of the manufacturers don't share raw data with their customers. The philosophy seems to be: "trust us, we're the experts." That objection may actually be going away. The American Gear Manufacturer's Association (AGMA) has actually created a sub-committee of a committee for developing guidelines to electronically transfer gear measurement data. Will wonders never cease?

Another very big frustration with this equipment is its exclusivity to gears. Here is machinery valued at many hundreds of thousands of dollars, with reputed accuracy to microns, and you can't measure anything but gears on it -- what a waste.

Into this world of smoke and mirrors enters SCEPTRE System from EMD. They do the unthinkable.

They take data on a 3D geometry with traceability to NIST and let the customer decide what to do with the results. We can mimic all the measurement algorithms of the "gear CMM's" if we care to, or we can go beyond them with our own data measurement and interpretation. We have total control of probe force, speed, and point density. We don't have to rely on a committee to electronically share the data. It is sitting there waiting for us. We can use EMD's {SCEPTRE's??} own involute profile comparator or develop our own interpretation software.

At Kleiss Gears, we have done just that. We lead the industry in the profile inspection of plastic gears. We can now uniquely measure all of the shrinkage and gear characteristics of plastic gears and the mold cavities that create them. This is also true for power metal and forged gears. Even without a rotary table, we can measure many features on a helical or worm gear. Bevel gears are straightforward.

The simple reason for this capability is that the SCEPTRE System software and hardware allow the user to control the entire measurement process. That singular capability, along with the certifiable accuracy of the equipment, provides us with an unprecedented ability to measure what we make.

Rod Kleiss can be reached at www.kleissgears.com. (Use your Browser's BACK button to return to this page.)