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Save money by understanding variance and tolerancing.

Article Excerpt
Manufacturing processes are inherently variable, which results in component and asembly variance. Unless process capability, variance and tolerancing are fully understood, incorrect design tolerances may be applied, which will lead to more expensive tooling, inflated production costs, high reject rates, product recalls and excessive warranty costs. A methodology is described for correctly allocating tolerances and performing appropriate analyses.

Effects of incorrect tolerances

Tolerances on component and assembly dimensions are crucial to the success of a medical device. Incorrectly specified tolerances can lead to more expensive tooling, high reject rates, product recalls and substantial warranty costs. However, specifying tolerances correctly requires an understanding of process variance and process capability and this is not always available to the engineer or draftsman working on the product or process.

Process variation, typically the result of small changes in the tooling, process parameters and materials, means there will always be variation in the measured dimensions of components. Similarly, assemblies exhibit variance, partly because of the variance they inherit from the constituent components and partly because of variance in assembly processes.

A medical device's performance can be strongly influenced by component and assembly variance, thus every functionally critical component and assembly dimension is toleranced. The intention is that every device will assemble and function as intended, providing the tolerances are not exceeded because of the process variance.

However, an incomplete understanding of tolerancing and the related subject of process variance can lead to inappropriate tolerances being applied. Even if undertaken with the best of intentions, a subsequent tolerance analysis will give...

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