Jason discusses form controls on a per unit basis using the example of controlling flatness on a large surface in this Question Line Video.

We received a question from a student, wanting clarification on a feature control frame calling out flatness. This feature control frame included additional information after the initial tolerance zone value, and our student wanted to know what those values referred to.

In the example below, we have a part with two feature control frames calling out flatness for the surface of the part. The top feature control frame includes only the flatness symbol and the tolerance. This is saying that the entire surface of the part must be held to a flatness of .125”. This means that the entire surface must remain within two parallel planes spaced .125” apart. This is a pretty large tolerance, considering the size of the part.

Figure 1: Entire Surface Flatness Control


The bottom feature control frame is also calling out flatness, but it is refining the form to a tighter tolerance over a smaller area. This feature control frame is indicating that a flatness tolerance of .030” must be held over a 6”x6” square. This tighter tolerance over a small area is limiting the rate of form change, ensuring that though we still have a .125 overall flatness tolerance, we won’t have a drastic change within a small area.

Figure 2: Surface Flatness Refinement


In the example above, you can see that the surface of the part in the 6”x6” square is within the .125” flatness tolerance zone specified in the top feature control frame, but it is clearly outside of the .030” tolerance zone required by the bottom feature control frame. Therefore, this part would fail the flatness per unit basis requirement.

Flatness form controls on a per unit basis are often used to make sure there are no large steps in material due to manufacturing processes. In our example, this refinement was based on a square area. This can also be done over a diametric area, in which case the diameter symbol would be shown in the feature control frame, rather than the square symbol.

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