Friday, August 7, 2009

Tolerances, from Aviation Week.

The F-35 fighter (the latest, even later than F22) is having a few problems with costs. Some parts have having a 50% scrap rate, i.e. half the parts produced are out of tolerance and are tossed into the scrap bin.
Then we enconter this odd statement.
"The issue is not just producing parts that are within tolerance, Brig. Gen Heinz says, but insuring the variability within the tolerance band in minimized so that when parts are assembled the tolerance "stackup" is also within limits. "It's too early to tell" whether GE/Rolls will encounter problems."
Uh Oh. General Heinz doesn't understand manufacturing. Each part has a drawing, which calls out the dimensions, and the allowable tolerance on those dimensions. The shop folk work from the drawing. If the drawing says dimension "x" has a tolerance of + or - 0.005 inches, then the parts will vary by 0.005 inches. If the tolerance is tighter, the parts will be held to tighter tolerances. The shop builds to the drawing.
Tolerance "stackup" is a problem when the overall design isn't right. If all the parts in the assembly are at the limit of the tolerance, say they are all slightly oversize, they may not fit together, or require excessive force (a big hammer) to jam them together. The solution is to tighten the tolerances on the parts until the worst case will fit.
This in the principle of interchangeable parts. In the bad old days, parts were filed to fit the assembly. This required a lot of expensive skilled handwork, slowed production, and made it impossible to fix assemblies in the field. If the parts don't interchange, you can't cannibalize parts from a wrecked unit to fix another one. In this country Eli Whitney pioneered interchangeable parts way back in the end of the 18th century, and it is the basis of mass production.
If the F-35 requires "minimizing the variability within the tolerance band" before things fit, it's a good bet that parts don't interchange, they have to be custom fitted. Which is a bad thing.

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