How to convert Profile Tolerance(GD&T) into equal bilateral tolerance

In this post, we will see how to convert profile tolerances to an equivalent bilateral tolerances.

Why should we convert Profile Tolerance into Equal Bilateral Tolerance?

A GD&T callout cannot be used for performing Tolerance Analysis and Calculation. For performing tolerance analysis, all the involved tolerances should be in an equal bilateral format.

Profile Tolerances

Of all the GD&T callouts, profile tolerance is the easiest to convert into plus/minus format.

So, what is a profile tolerance?

Part with profile tolerance

Profile tolerance specifies the width of a tolerance zone within which the actual surface profile exists. The profile tolerance that is positioned around the nominal surface can be unilateral, equal-bilateral, or unequal bilateral.

Equal bilateral profile tolerancing(Tolerance zone not to scale)

In the above picture, the profile tolerance is equally, bilaterally distributed around the nominal surface. That is, the tolerance zone boundaries are offset at equal distances from the true profile(nominal surface).

As shown in the figure, half(2.5) of the entire tolerance zone width(5) is outside the component and the other half is inside the component.

Now, a doubt may arise in your mind?

The same kind of tolerancing effect can be applied by just adding a ±2.5 tolerance to the height dimension.

50 ± 2.5

Then why are we preferring a profile tolerance over the traditional tolerancing in this situation?

The answer
Profile tolerances are superior to traditional tolerances due to the below advantages.

1. They are related to a datum reference frame. This provides a precise tolerance zone location concerning other toleranced features.
2. Profile tolerance can be applied to any surface without bothering about its shape, location, and orientation. It doesn’t matter whether it is a curved surface, conical surface, or inclined surface. You cannot specify the ± tolerance to a curved or inclined surface. Right?

Converting Profile tolerance into Equal, Bilateral tolerance

By now, we are clear with Profile tolerance. Now, let’s see how to convert a Profile tolerance into Equal, Bilateral tolerance. We will take the same example for this calculation.

Equal bilateral profile tolerancing(Tolerance zone not to scale)

Step 1:

From the figure, we can deduce that the total tolerance range is the profile tolerance.

Total tolerance range = Profile tolerance = 5

Step 2:

Now, divide the Profile tolerance(total tolerance range) by 2. This will give you equal bilateral tolerance.

Equal bilateral tolerance value = 5/2 = 2.5

Step 3:

Now, take the dimension which relates to the surface and the datum that’s specified in the profile tolerance feature control frame. Add the above calculated Equal bilateral tolerance to it.

50±2.5

The final result

This is how we convert profile tolerance into equal bilateral tolerance. The above method holds good even if the surface under consideration is curved in nature.