In my opinion, one of the most underutilized tools in the GD&T toolbox is the tangent plane modifier. It was introduced in the 1994 ASME standard, yet some people still think of it as a new concept. Shown in the example below, the tangent plane modifier (circled T) can save money by only controlling the high points of a surface, rather than every point:
To understand the drawing above, first realize what parallelism controls if no T modifier is given. Regular parallelism requires that every point on the top of the part be within a tolerance zone of 0.2 mm. This means that regular parallelism inherently controls flatness to the same specification.
But there might be times when a designer does not need to control flatness. Perhaps another mating part will contact the top of our part, and we only care about the angle at which the mating part sits. In that case, we don’t need the surface to be flat within 0.2, since our mating part will only feel the high points anyhow.
In that case, the tangent plane modifier makes sense. It does not give us a “bonus tolerance” as the MMC modifier does with features of size, but it does have the advantage of being more forgiving of the surface’s form error. The following is an illustration of the possible error that would be allowed by the tangent plane modifier:
Notice that portions of the surface can actually go below the tolerance boundary; this is because the tolerance is imposed only on the imaginary tangent plane — this can easily be inspected by placing a gage block or a flat plate on the top and then measuring the gage block’s parallelism.
Of course, there are times when this is not desired, such as when there are concerns about fluid leaking between this part and a mating part. And it should also be noted that the tangent plane modifier can be used not just on parallelism, but also on perpendicularity, angularity, and even profile of a surface.