Unilateral Profile Tolerancing

Let’s go back to the question box —

“Why is profile allowed to be designated as one-sided? Can other GD&T symbols also be one-sided?” First, let’s address a couple of points: there are two profile symbols: profile of a line and profile of a surface. Yes, each of them can have the tolerance amount that is unilateral (one-sided) or bilateral (both sides of the intended profile). Also realize that the profile symbols can be “unequal bilateral” where there is tolerance on both sides of the perfect shape, but more tolerance exists on one side.  Some examples:

No other GD&T symbol can use these options; there is no such thing as unilateral flatness or unilateral parallelism. Why is that? Well, when we talk about flatness or parallelism, the feature in question has no curvature. So the surface “is where it is.” Profile tolerances, however, have a curve (usually) and also one or more basic dimensions that describe the radius or other values. The prescribed radius is important, and if the curve dips in or out, it is directly impacting the given radius. With this in mind, I should point out the tolerance zones for some other GD&T symbols can float to one side or the other. For instance, a parallelism tolerance of 0.5 allows a surface to tilt 0.5 mm in either direction, but that is not considered a unilateral tolerance, because it is a flat surface. (If it curves in or out, it doesn’t have to follow a particular radius.)

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  1. I have a unilaterial profile call out that has a 1 on each side of the circled U. Can you let me know what this means please


    Thank you,


  2. Hi Randy — what you have is the new way to show unequal tolerances in GD&T. Rather than using graphical means to communicate the idea (such as the phantom line in the picture shown above), the 2009 standard offers the circled U, which stands for “unequal.”

    The first number represents the total tolerance. Then, after the U, the second number represents how much of the total tolerance lies outside the part (in other words, adding material). So for your example, it is a tolerance of 1 which is entirely going out into air, away from the part.

    Using the U modifier, it’s possible to create an unequal bilateral tolerance. If your example had 1 followed by U followed by 0.2, that would mean 0.2 is on the outside and 0.8 toward the inside.