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Maximum and Least Material Condition Explained

Posted: Tuesday, February 24, 2015

People are often confused by the terms Maximum Material Condition and Least Material Condition in GD&T and, therefore, get some anxiety when they see all those little M’s and L’s in circles on a print. People learn that Maximum Material Conditional occurs when a feature weighs the most or has the most mass and Least Material Condition has the least mass. This sounds simple when you think of something like the diameter of a pin. The largest allowable size for the diameter of the pin would cause the pin to weigh the most and, therefore, be the Maximum Material Condition. It starts to get confusing when you look at holes and realize that it’s the smallest allowable hole that would have the most mass. So, sometimes the largest allowable dimension is the Maximum Material Condition and sometimes the smallest one is. What’s up with that? We don’t care how much the part weighs, just if the parts meet the print.

So why all of this confusing stuff about Maximum and Least Material Conditions? To understand why this is really important to GD&T and designers, we have to remember that GD&T is a tool to ensure that parts function. A very important function of a part is that it fits together with other parts in an assembly. Think of a very simple example: a pin fitting in a hole. Why does a designer need to worry the most about how these parts fit? We definitely need to worry about the largest allowable size of the pin. The biggest pin is going to be the biggest problem when it comes to fitting in a hole. When we think of the hole, we worry about the smallest allowable size for the hole. Every designer’s worst nightmare is that biggest pin trying to get into that smallest hole. Hey, that sounds familiar: biggest pin and smallest hole? Maximum Material Condition has to do with something more than weight. It also leads to the tightest allowable fit between mating features. This is something important to designers and manufacturing.

The next thing we usually concern ourselves with when designing the fit between a hole and a pin is how loose it can get. Loose can sometimes mean a sloppy fit that may not function anymore. Loose happens when holes are big and pins are small. In addition to fits being loose, features are pushed to an extreme that may impact strength. A thin pin may shear and a large hole may cause a stress riser in the part. Like Maximum Material Condition, Least Material Condition is important for a whole lot of reasons beyond just the feature having the least mass. We can now see why these features, Least and Maximum Material Conditions, are important for designers to consider when making sure that parts function. Having an easy way to refer to these conditions is important to GD&T because it allows tolerances to be expanded in special cases without impacting function. We will look at that next…

By: Ron Litke
Engineering Manager
Orchid Design