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When Using “Unit Basis”, Include an Overall Limit - 1994, 2009, 2018 (#36)

(In accordance with the ASME Y14.5-2018 standard)
(In accordance with the ASME Y14.5-2009 standard)
(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 0.2/50mm has been specified. Since the overall part is 200mm long, what is the maximum flatness variation there could be over the entire length of the part?

We can use two right triangles to calculate the maximum flatness variation.   The maximum radius of curvature allowed by the flatness of 0.2/50 is:

R= 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.

Revised Drawing: 

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Back to Tips Tip added Jan 2021