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Straightness - Unit Basis
   When Using "Unit Basis", Include an Overall Limit!

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 0.2/50mm has been specified. Since the overall part is 200mm long, what is the maximum flatness there could be over the entire length of the part?

The maximum radius of curvature allowed by the flatness of 0.2/50 is:

R2 = 252 + (R - 0.2)2
R2 = 625 + R2 - 0.4R + 0.04
0.4R = 625.04
R = 1562.6

Solving the other triangle for B:
1562.62 - 1002 = B2
B = 1559.4
Therefore the maximum out of flatness could be:
R - B = 3.2

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

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