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Size

Another Size Tip

When Size is Really Important, Throw Away Your No-go Pin Gages.

According to Rule #1 of the National Standard, the least material size (largest internal feature or smallest external feature) of a feature of size should be verified at each cross section. No-go pin gages do not detect form errors. If a hole, for instance, is out of round (which they usually are) a no-go pin will not detect that some cross sections of the hole are actually larger than the size will allow.

In order to detect that the size is being controlled, two-point measurements must be made. The hole illustrated would pass a no-go pin but would fail a two-point check. In other words, this hole fails a proper inspection of size.

This problem can be avoided by remembering the two-line phrase for measuring size:

Full form checks at MMC
Two point checks at LMC

This part, for instance, is not acceptable if size is checked in accordance with the Y14.5 Standard

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