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Controlling Symmetrical Relationships with Positional
Tolerancing
Often designers need to create
symmetrically shaped parts. Symmetrical parts are usually
easier to assemble, look better and help maintain balance in a
design. Features shown symmetrical must be controlled to avoid
incomplete drawing requirements (2.7.3 of ASME Y14.5M-1994).
Symmetry is an option in these situations but it is difficult
to measure since it requires deriving the features' median
points to determine if they are contained within the specified
tolerance zone which is centered on the datum axis or datum
center plane.
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Position may also
be used to assure a symmetrical relationship. The
advantages of using Position include the ability to
modify the tolerance and datum reference at RFS (implied
in 1994 Standard), MMC or LMC. In addition, verification
is usually easier for Position than that required for
Symmetry since it is the center plane of the Actual
Mating Envelope (simulated by the inspection equipment)
that must be within the tolerance zone. |
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