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. 
 