GD&T Least material condition

A brief conceptual description of LMC or Least material condition in GD&T. What it means and what are its implication on tolerancing?

What is Least material condition?

Let’s take a drawing of a part as shown which is a block and a center hole.

Dimensions and size tolerances added:

There are three features of size dimensioned in the drawing.

When we consider least material condition (when material is lowest) , the values for each of these features of size would be.

The largest hole (15.1) and smallest widths (49.7 and 34.7).

LMC is nothing but the condition in which the part is lightest with the “least material.”

Least Material condition modifier:

When a symbol L is used along with the geometric tolerance in feature control frame targtted at a feature of size ,whether position or orientation is being controlled. The meaning is that Geometric tolerance is dependent on the size of the feature of size

At LMC , the geometric tolerance is as mentioned in Feature control frame and as the size of the feature of size moves away from LMC , the total geometric tolerance increases. Meaning that as the FOS is manufactured away from its LMC size there is more acceptance in terms of variation of its position or orientation whatever is being controlled.

LMC modifiers are generally used on feature of size like holes , slots when the minimum wall thickness has to be maintained. Generally not used for feature of size which will undergo assembly/mating.

Learn more about how to Interpret GD&T callouts from this Free e-book on GD&T interpretations