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When Using Unit Basis, Include an Overall Limit
(In accordance with Y14.5M-1994 standard)

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Flatness and straightness on a unit basis are good controls to use when you don’t want an abrupt change in a surface. If you don’t limit the overall variation, however, a larger than expected out of flatness or straightness may result. For the part shown below, flatness on a unit basis of 0.2/50 mm has been specified. Since the overall part is 200 mm long, what is the maximum flatness there could be over the entire length of the part?

To avoid this it is usually best to include an overall flatness control when applying flatness or straightness on a unit basis.

Back to Tips Tip added Jun 2000