QM true positioning

Bonus tolerances for GD&T

A true balancing act

By Richard Clark

A bonus tolerance is arguably one of the most difficult concepts (in Geometric Dimensioning and Tolerancing) to explain to the powers that be. For almost all other characteristics called out on a drawing, here is our specification… this is our minimum, this is our maximum, and anything outside of these parameters is non-conforming product (scrap or rework).

When True Position (positional location) is called out on a drawing, the tolerance is applied at LMC (least material condition), MMC (maximum material condition), or RFS (regardless of feature size). If LMC or MMC is listed, we can calculate a wider tolerance, or bonus, depending on the actual size of the measured feature.

We should start with understanding RFS because it allows no bonus to our location tolerance zone. If a drawing calls for positional location and does not specify LMC or MMC, the center of the measured feature must lie within the tolerance zone specified, regardless of the feature size.

This on the drawing...

...means this on the part.

The concept of a bonus tolerance is simple. Diameter is a tolerance of size. However, the location also affects the fit and function of the feature to its mating part(s). Bonus tolerances allow the tolerance amount not used in the diameter to be applied to the location. This allows the maximum use of tolerances while keeping the fit and function intact.

Imagine we're trying to assemble a 5" long plug into a 4" deep bore. If the diameter of the bore was 2.0000" and the diameter of the plug was 1.9999", the plug would have to be straight within (or probably below) 0.0001" to ensure it would fit and function. On the contrary, if our plug diameter was 1.9995" it could have a straightness deviation of almost 0.0005" and still fit. This is a form example as opposed to location but the concept is very similar.

In our example, we'll use an inside feature. Our drawing states we have a diameter tolerance of 1.5000" +0.0005" (so our MMC would be 1.5000"). In addition, we have a positional location tolerance of 0.001" applied at MMC.

The intent of positional location when applied at LMC or MMC is to ensure an acceptable amount of clearance between the part you're making and its mating part. If the size of our inside feature is at MMC (smallest diameter) we need to be within 0.001" location for the mating part to fit (our part) properly. Let's look at a graphic illustration (not to scale) of our part at MMC (smallest diameter) and largest diameter.

The larger we make our inside diameter, the more “room” we have for the location to be outside of the 0.001" location tolerance zone, but the clearance remains acceptable. Whatever amount above MMC (above our minimum) our diameter becomes is applied as a bonus to the tolerance of the positional location. If the diameter of our feature was 1.5005", the 5 tenths we are above MMC would be applied to the location tolerance which then becomes 0.0015".

If we look at the graphic illustration with the mating part added, you can see why the bonus tolerance is applied to our location. If we are at MMC (smallest diameter), we have no “room” to spare with our location. If we are at the largest diameter, we have room to spare so a bonus tolerance is applied.

It's a textbook example of the Axiom of Equality (addition) found in a Jr. High level Algebra book. If we look at the diameter tolerance and the location tolerance as two sides of a balanced equation, whatever we add to one side we also add to the other side to keep the two sides in balance:

1.5000" (diameter at MMC) = 0.001" (location)

But if we were to add:
1.5000" + 0.0003" (diameter) = 0.001" + 0.0003" (location)

1.5003" (diameter) = 0.0013" (location)

And that's all there is to it. Trust me, if it were any more difficult than this, yours truly couldn't teach it. So let's always try and break it down to counting beads on a string… shall we?

Author's note: This article is a follow up to “Exposing the Myth of True Position” which ran in the October 2003 issue of Tooling and Production Magazine.

Richard Clark is a CMM Programmer and Trainer who is affiliated with CMM Technology Inc. (San Clemente, CA). To receive a copy of “Exposing the Myth of True Position” and/or a FREEWARE copy of his Bonus Tolerance calculator for Excel, e-mail feedback to rcmetrology@yahoo.com.