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Use the Correct Size and Center
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

The Y14.5 standard has always talked about controlling centers (center points, axes and center planes). It also talks about calculating additional tolerance (bonus tolerance) and datum shift depending on the produced “size” of the feature when MMC or LMC is applied. The problem was that it was never clear how to determine the correct size or center to use. When inspecting a part, there are lots of different ways to find a center or size. The 2009 revision of the Standard clears this all up. It defines several “envelopes” which are used to determine centers and size for calculating bonus and datum shift. At first these might seem confusing. So, to clear it up, we have created a reference card and models to explain the various envelopes. Here is a sample of the Reference Card.

Items 1 and 2 are unrelated actual mating envelopes (UAME) and their centers. These are used to establish primary datum features, apply position and orientation tolerances and calculate bonus tolerance. The items numbered 3 and 4 are related actual mating envelopes (RAME) and their centers. They are used with secondary and tertiary datum features to establish datums and calculate datum shift where these datum features apply at MMB (used to be MMC). There are also envelopes where LMC and LMB apply. If you are not using these envelopes, you may be introducing too much uncertainty into your inspection results. Tec-Ease’s Envelope Reference Card and models should clear this all up for you. Consider joining our Premium site today.

Back to Tips Tip added Sep 2010