Sound Waves Measure Part Quality

Manufacturing Engineering, Mar 2005

For the past year, Capstan Atlantic (Wrentham, MA) has been using sound waves for nondestructive testing of its powder metal (P/M) automotive components.

The company uses resonant acoustic method (RAM) NDT for automated 100% inspection of a variety of automotive structural parts, including precision gears for powertrain applications. Customers include domestic US automakers and Japanese transplant facilities.

Capstan Atlantic's 100,000 ft^sup 2^ (9300 m^sup 2^) production facility employs 230 people and houses a complex process that begins with an iron-based P/M alloy. The powder blends are pressed to varying densities depending on application requirements, then newly formed gears are sintered in 65' (20-m) long continuous-belt furnaces at temperatures above 2000°F (1100°C). The process is robust, but customers still require 100% inspection of parts for structural integrity.

Before investing in RAM NDT technology supplied by The Modal Shop Inc. (Cincinnati), Capstan Atlantic used a nondestructive torque test. But the method was subject to operator interpretation, it was slow and expensive, and it didn't guarantee 100% conformance. "Our previous method tested around 40-50 parts an hour," recalls VP of Engineering Rich Slattery. "Now, the RAM NDT unit tests 600-700 parts per hour, and when I go home at the end of the day I know every single gear produced on our line has been thoroughly and completely tested.

"It's hard to put a price tag on that, but it's easy to figure the consequences involved in a field failure. That's every manufacturer's worst nightmare."

After an initial learning curve, Capstan Atlantic personnel found the RAM NDT system relatively easy to use. The technology works on a principle analogous to that of a bell or tuning fork. When you strike either instrument, it vibrates, emitting a sound. An instrument that rings true produces a consistent sound, and this consistency reveals the structural integrity of the instrument.

In RAM NDT, the test piece is struck by a small anvil and emits a natural frequency as part of its structural response. This unique and measurable signature is then compared to signatures from both good and bad product. If a gear is cracked, is not fully densified, or misses other characteristics of a structurally sound product, the flaw will be exposed when its signature deviates from that of a good product.

The unit tests the part for external and internal flaws and provides an objective, quantitative analysis that eliminates operator errors. A dynamic sensor captures sound, and a high-speed analog/digital converter translates the sound into measurable data which is then compared to predefined data. In effect, RAM NDT listens to the structural response of a part and evaluates it against the statistical variation from a control set of good parts in order to screen defects.

"We've incorporated RAM NDT technology right on our assembly line," says Slattery. "components pass through it right before packaging. Any product that doesn't pass inspection is removed from the line automatically. And gears that pass inspection are immediately packaged and shipped."

According to Slattery, the system saves the company $0.33 on each gear produced over the previous method. Because Capstan Atlantic ships approximately 5000 gears a day, that amounts to savings of $1660 per day. And, by eliminating the need for a specially trained operator, it also reduces personnel costs.

Finally, the captured data provided feedback that contributed to other quality improvements. "The unit has enabled us to recognize things about our process and make improvements upstream that have increased our product yield by reducing process variation," Slattery concludes. Circle 309