Analyzing tolerance limits

Analyzing tolerance limits

by Bill Drake

Three dimensional analysis of tolerances proves they can be eased on a fixed disk drive

Using three dimensional tolerance analysis, specification limits were relaxed on a critical subassembly for a 2.5-inch fixed disk drive from Seagate Technology in Scotts Valley, Calif., saving several million dollars in manufacturing costs.

This disk drive incorporates an overmold assembly containing components of a voice coil motor (VCM), which is used to move the drive's recording-head assembly across its disk surfaces. In the overmold process, the components are placed in a mold and plastic is injected around them. The process had not previously been used in high-volume production, and there were concerns about maintaining stringent variation requirements.

Among the critical issues was the variation in air gaps between the coil and the magnets. The gaps must be controlled to very tight specifications to ensure optimal motor performance. Based on preproduction quality data from the supplier, an unacceptable proportion of the overmold assemblies would fail to meet quality acceptance criteria.

Seagate engineers used VSA 3D simulation modeling and analysis software from Variation Systems Analysis Inc. in St. Clair Shores, Mich., which predicts variation levels in assemblies due to specified design tolerances and measured manufacturing variation. A 3-D design-intent simulation model showed which product tolerances were most critical.

Rather than use normal distributions, actual quality data were applied to the model. Probability distributions for the critical tolerances mostly approximated normal distributions, but one important tolerance approximated a U shaped distribution. This resulted in an unnecessary penalty on the production parts, because the normal distribution assumed extended "tails" that could not exist. Accordingly, larger tolerances were possible.

The software was used to simulate 10,000 assembly builds, and it accurately predicted how much variation occurred for each quality parameter as well as which tolerances and locating schemes contributed most significantly to the variation.

Engineers tried approximately 25 potential solutions involving tolerance reallocation before one was found meeting all requirements. All assembly dimensions were well within specifications, and production start-up was trouble free.

The above was adapted from an article by Bill Drake, senior staff engineer for advanced concepts development at Seagate Technology in Scotts Valley, Calif.. The full text may be found in the June 1997 issue of Mechanical Engineering magazine. © 1997 ASME International.© To obtain a copy of this issue, click here.