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. Instead of measuring every part with a digital caliper, manufacturers use "Go/No-Go" gauges: Must fit into or over the part. No-Go Gauge:

DIN 7161 acts as a bridge between the designer’s dream and the machinist’s reality. While a designer might imagine a perfect 50mm shaft, the machinist knows perfection is impossible. DIN 7161 provides the "Tolerance Tables"—a set of rules that define exactly how much "imperfection" is allowed for the machine to still function perfectly. The Mechanics of the Story

This ensures that the tolerance is proportional to the difficulty of manufacturing a specific size.

Official Purchase: You can find historical versions or the current replacement at DIN Media.