When Part Rejections Go Too Far: Lessons from Over-Tolerancing
Years ago, I worked with a machined plastic part that cost about $200 each. The drawing called for tight tolerances across most features — and rightly so. But this part got rejected for something minor: a tiny burr on an outer edge.
The drawing specified a 0.005-inch radius for the edge. In reality, this could be done with a simple hand deburring tool, but trying to measure that precisely was nearly impossible. The deburring wasn’t about function — it was mostly cosmetic.
The part was made from hard plastic. The burr removal had no effect on form, fit, or function. The so-called out-of-tolerance area was barely visible. Yet the part was scrapped.
Critical Dimensions vs. Non-Critical Details
Critical dimensions matter. If they’re wrong, the part won’t work. But calling a tiny deburring radius “critical” makes no sense.
The real requirement was simple: remove burrs and sharp edges so no one cuts themselves. That’s a safety issue, not a precision machining issue.
I’ve had much better success with a general note like:
“Remove all burrs and sharp edges.”
That keeps inspectors focused on what matters, without wasting effort on measuring unnecessary microns.
When Managers and Customers Over-Reject
The inspectors weren’t at fault — they were following the drawing. The real problem was over-tolerancing in the design.
Years later, a co-worker told me about a meeting where a manager wanted to reject parts over an almost invisible “flaw.” She showed the part, but no one could even see what she was talking about. The meeting ended without action — but valuable time had already been wasted.
I’ve also seen customers use supplier quality issues as leverage. A Japanese customer once tried to reject parts because the shipping box had uneven coloring. There was no specification for packaging color, and the box only protected the product. Yet they pushed the issue until someone finally clarified the spec.
The Real Cost of Rejecting Usable Parts
Rejecting parts for non-critical issues doesn’t improve quality. Instead, it creates waste:
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Higher costs from scrapping usable parts
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Irritated vendors and strained supplier relationships
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Unnecessary landfill waste when perfectly functional parts are discarded
Over-tolerancing and nitpicking details add friction instead of value.
The Bottom Line
In manufacturing, the goal is to balance function, safety, and cost. Over-tolerancing and rejecting usable parts undermine all three.
The lesson is simple:
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Define critical dimensions clearly.
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Keep non-critical features flexible.
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Focus inspections on what truly affects form, fit, and function.
That way, we avoid wasted resources, build stronger supplier relationships, and keep usable parts out of the scrap pile.
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org