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Consider flatness at MMC on your parts made from flat stock - 2009 & 2018 (#26A)

(In accordance with the ASME Y14.5-2018 standard)
(In accordance with the ASME Y14.5-2009 standard)

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Many parts are made from stock which is left in the as furnished condition. Rule #1 does not apply to the thickness when “STOCK” is specified on the size dimension. Therefore, the form variation of the part is whatever the mill supplies. Many designers will control the form of such parts with flatness applied to one surface. Flatness is often a very time consuming inspection. When flatness at MMC is applied to the material thickness, a simple envelope gage at the virtual condition may be used to limit form variation. Flatness applied to the size dimension controls the derived median plane (ASME Y14.5-2009 [5.4.2.1], ASME Y14.5-2018 [8.4.2.1]). In this example, the envelope gage used to verify flatness 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.

NOTE: In the 1994 Standard the derived median plane of a width was controlled using straightness, not flatness.

This tip is in accordance with ASME Y14.5-2009 and ASME Y14.5-2018.
This tip was originally released in July 1999.

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Back to Tips Tip added Jul 2020