Consider Straightness on Your Parts Made from Flat
Many parts are made from stock
which is left in the as furnished condition. Rule #1 does not
apply to the thickness of these parts. Therefore, the form
variation of the part is whatever the mill supplies. Many
designers will control the form of such parts with flatness.
Flatness is often a very time consuming inspection. When
straightness at MMC is applied to the material thickness, a
simple envelope gage at the virtual condition may be used to
limit form variation. In this example, the envelope gage used
to verify straightness would measure 1.025. The actual local
size of this part must be checked with a micrometer type
measurement to assure that, at any cross section, the
thickness is between .995 and 1.005.