Thursday, March 13, 2008

Tolerancing software

In keeping with my last post on process capability, here is some software that helps automate tolerancing

What I design is produced in low quantities. Consequently tolerancing does not get close attention. The items that are produced from my designs typically have 100% inspection. When I design I base my tolerances on published handbook values for the production process being used. My sources include Machinery's Handbook, Tool and Manufacturing Engineer's Handbook or Manufacturing Engineering and Technology.

In those references one can find charts and graphs like the one below.

Source: Manufaturing Engineering and Technology, Serope Kalpakjian, 1989.

The problem with these charts and graphs is that they don't give statistical properties. There is no information on the standard deviation of the process.
For me, in designing low production run items, not having the statistical properties doesn't matter. Generally, using the tolerance bands indicated in the charts and graphs produces parts that fit together.

Occasionally, a part might not fit. The cost of an occasional goof, however, is not worth the cost of the analysis it would take to remove that risk. In large production runs it becomes important to know the capability of the process.

The software, Tolerance Capability Expert, I linked to at the top of this article addresses the problem of not knowing the statistical distribution. It queries the user for the process, the size of the part, and the tolerance. It returns a Cp, the capability index for that specific feature.

I am not going to offer a formal review of the package. I tried a demo version of it a few years back but have never had access to the full package. The demo impressed me. For someone designing for large production runs I think it could be truly useful. For me, I didn't think I could persuade my boss or my customers to spring for it. The occasional extra bit of hand fit-up is seen as cheaper than the cost of the software.

Finally, a good reference on capability indices is Measuring Process Capability: Techniques and Calculations for Quality and Manufacturing Engineers by Davis Bothe.


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