Gaging Our Success

When Al Caravella went into business in 1956, I am sure he dreamed of the successful manufacturing facility that Sterling Machine Company has become. His two sons, Greg and Tom, bought the Hartford area job shop in 1994 and over time the business has grown into an 18,000 square foot precision machining facility employing 40 full-time workers.

Columns From: 1/3/1999 Modern Machine Shop,

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When Al Caravella went into business in 1956, I am sure he dreamed of the successful manufacturing facility that Sterling Machine Company has become. His two sons, Greg and Tom, bought the Hartford area job shop in 1994 and over time the business has grown into an 18,000 square foot precision machining facility employing 40 full-time workers.

A significant amount of Sterling's business is manufacturing different sizes of round parts. They had purchased a variety of gages to measure these parts, but measuring repeatability and accuracy problems constantly plagued them.

Enter Steven T. Bidwell—foreman, tool designer and chief engineer. Toward the end of 1994, Steve was approached by one of Sterling's lathe operators. "The operator was machining a part approximately 10 inches in diameter with a very thin wall," Steve remembered. "The part had a very close tolerance and the operator mentioned that turning and boring the part was not a problem. What concerned him was trying to find a way to accurately measure the size of the completed part."

The operator explained that rolling the part on a surface plate under a bar-type diameter gage would distort the part due to the spring pressure of the indicator and, thus, give a false measurement. The dial bore gage was also giving distorted readings from gage pressure, not to mention that it was scratching the finish of the part.

Other types of measuring tools such as micrometers, inside mikes or pie tapes were not accurate or reliable enough for their aerospace customer. To make matters worse, five people measuring the same circumference would sometimes result in five different readings. So the operator ended up running back and forth to a CMM to take a series of measurements around the circumference of the workpiece and then calculate an average.

After giving the problem a great deal of thought, Steve had an idea he hoped would resolve the internal measuring problem. He built a prototype of his idea specifically for the problem part. (See diagram). Rather than using a theoretical centerline as most cylindrical part measurement systems do, his gage set provided an automatic physical centerline. The key to the design was its centerline hub assembly. This component, together with a non-rocking swing gage, provided repeatable, accurate, full circle measurements quickly and economically. It could be set up in a matter of minutes, with accurate and repeatable readings generated with a simple turn of the gage through a 360-degree arc. Moreover, it could inexpensively make these measurements that otherwise would require much more costly instruments, such as coordinate measuring machines. The gage was simpler to set up and operate, and it was completely portable.

Mr. Bidwell has since moved on to develop the idea into a standard product—the EZ-ID-OD Gage Set. Two basic parts comprise the gage set. The centerline hub assembly consists of a base, a centerline post and expandable jaws. It automatically creates an accurate physical centerline, while the weight reducer assembly, or optional datum plate, removes virtually all the weight of the swing gage from the part being measured.

Also, with the free-style swing gage—which consists of a precision indicator, contact probe, a front and rear swing body and a pair of side bars—contact probes touch the measured part with a force that is limited to a few grams. The unit is particularly suited for parts with uneven top edges, such as castings with burrs and flashings, or flexible, thin-walled parts. It also permits height adjustments to check diameters for taper.

Early in 1995, Bidwell Corporation was formed and patents were applied for, both domestically and internationally. The product has been used by manufacturers in New England and soon will be available throughout North America and selected international markets.

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