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Checking System and
SPC Improve Gage Studies

 Fairfield Manufacturing, October 1996

To attain the high precision part quality needed by gear manufacturers, ISO 9001 certification is not always enough. In the case of Fairfield Manufacturing Company, Inc., QS 9000 plus a range of industry specifications, in addition to the special needs set by individual customer companies, are more the quality norm for this largest independent North American producer of a broad range of loose gears and complete power transmission assemblies.

Currently, the company produces more than 10,000 different part numbers totaling about two million gears ranging in size from 1 to 60" in diameter.

Gage Repeatability and Reproducibility

For Fairfield, which has been able "to reduce costly, post-production inspection, the major challenge has been to increase our on-line measuring capabilities so that we can have optimum control of our manufacturing process in order to meet the multi-level goals of our customers," says Dan Phebus. Director of Quality. - In turn, we make our customer assembly operations more reliable because the gears conform precisely to their own design drawings. We believe that GR&R is the key to reducing both our own and other users' variations."

By increasing its measuring capabilities, Fairfield has made major gains in preventing variations through better process control, significantly reducing rejects by correcting the manufacturing process beforehand. Highly accurate instrumentation became necessary to succeed. Through its Production Part Approval Process (PPAP), Fairfield bolstered its GR&R program. "We had to verify the process statistically. That would allow us to meet the rigorous tolerances we not only set for ourselves but which were critical for our customers," Phebus says.

The key instrument first acquired in 1992 was a Fowler/Trimos Mini-Horizontal Measuring System which enables Fairfield to measure both internal and external gear parts accurately in one pass. It replaced "mikes and pins," which caused gage errors of 25% or higher "under ideal conditions," he adds, More recently, five more Mini-Horizontals have been acquired.

In House Studies to Process Control
Phebus says the company had conducted an intensive in-house study on gaging errors. For example, it found that measuring a part with a total tolerance of 0.0018. made with 3"- 4" micrometers and calipers. primarily hand-held tools, showed a gage error of 51.7 %, with column-mounted "mikes and pins" a 33.7% error.
However, when using its new Mini-Horizontal Measuring
System only a 7.4% error was evident.
This is why, Phebus says "today's measurement capability has to exceed production process capability. We have transformed our operation from post-production inspection to on-line process control, reaping the twin benefits of better quality improvements and cost- reductions."
How this was accomplished was due in large part to an idea of Ron Gellenbeck, the company's chief tool designer, who suggested putting the Fowler/Trimos instruments on movable carts adjacent to the production line. This was a superior and quicker way to get results than by trucking over the finished gears to an inspection laboratory. "We did it all on-line and thus could immediately detect variations. Then we could alter the process to gain the uniformity that was our goal."

High Volume Measuring...
Fairfield's basic business is gears and power transmission systems for OEM customers manufacturing off-highway and construction equipment, mining, railroad, agricultural, industrial and material handling equipment of all kinds. This makes big demands on its products' accuracy and conformance to design specifications. "We have a high volume of gears to be measured and sorted according to gear size," says Gellenbeck. "They are boxed and shipped by classifications, and some of our customers require reverse classification. This means that the smaller gear pitch diameter needs a larger code number. In contrast to all other instruments on the market, the Fowler/Trimos Mini-Horizontal System can handle the correct size code in reverse order.

In addition, he explained, the stable base of the Mini-Horizontal units complemented the movable cart system Fairfield had devised to per- form the on-line measurements prior to heat treating the gears and the grinding operations.

Six primary benefits of the new system have resulted:
  1. Obtaining a true process control technique.
  2. Saving substantial amounts of time over post-production inspection.
  3. Reducing customer assembly time.
  4. Significant dollar savings for the process by reducing rejects and producing faster verification of part sizes.
  5. Better human relations in the plant, because measurement operators like the ease of using the system, encountering less fatigue than before.
  6. The new ability to measure both externally and internally. The latter advantage was achieved by Fairfield technicians who expanded the utility of the Trimos instrument and its Sylvac probe by devising new anvils to measure gear sizes up to 12"in diameter.

Ruggedness, Reliability and Accuracy!
Gellenbeck adds that the original selection of the instrument depended heavily on its 0.00006"accuracy and the fact that the anvils were removable and thus could be replaced with ball-type or cylindrical pins to measure pitch diameters, The system has a Sylvac 1" travel probe and a digital readout display box. A direct two-way RS-232C data output allows connection to a printer.

"With this gage," concludes Phebus, "we detect variations quickly and simply and are sure we're not getting false signals about our process." The company is continuing to refine its SPC program and improve the accuracy of its instrumentation, since they are mutually dependent for ultimate product precision.