Straightness and flatness per unit length are required when it is necessary to avoid sharp changes over a feature’s length or surface unit. Figure 1 illustrates an example of the straightness requirement for a unit of length.

Figure 1

The functional constraint for the straightness requirement per unit length illustrated in Figure 1 arises in this case from an interface with an opposite cylinder at a length of 100 that may be at any place within a range of 400.

In such cases it is also important to check whether there is a need to limit the overall straightness of the feature.

The straightness requirement is continuous (across one hundred units of length, no matter where) and not in four 100-length segments.

Figure 2 shows the basis for calculating the total straightness deviation.

Figure 2

First, calculate the maximum curvature radius from the small triangle:

The total straightness deviation (B-R) is calculated from the large triangle (dotted-line):

B – R = 1.6

If the design intends that the total straightness deviation should not exceed 0.4, then a limit should be added to the total deviation as shown in Figure 3.

Figure 3

The top line limits the STRAIGHTNESS deviation all along the feature; the bottom line limits the maximum deviation per each 100 units of length.

 In summary:

STRAIGHTNESS per unit length and FLATNESS per unit surface are designed to ensure functionality; the length / surface unit size is derived from a functional constraint. In order to limit the total deviation, composite tolerancing can be used (Figure 3).

Bottom line is,

even if we did something “a thousand times” and “everyone” understands what it means,

it may be common and accepted,

but it does not make it conform to a standard!