GD&T: Material Modifiers, what are they? How do they work?

If you have been around part drawings and manufacturing long enough, there's a pretty good chance you've seen a feature control fame with a circled letter after the tolerance. The examples/images shown below were created in Autodesk Inventor.

Probably the most common circled letter seen is "M". This means that the tolerance is to be applied at the Maximum Material Condition.

 

Let's take a deeper look into the different types of material conditions that can be applied to tolerances.

As the image above shows there are four different material conditions that can be applied and each one behaves differently than the others.

1. Regardless of Feature Size (RFS): This is the implied condition. All tolerances apply at RFS by default, which means no symbol is needed.
2. Maximum Material Condition Modifier (MMC): The circled M. This provides a feature with additional geometric tolerances as the feature's size departs from its maximum material conition.
3. Zero at MMC: Behaves just like the MMC. The differences will be discussed later on.
4. Least Material Condition Modifier (LMC): The circled L. This provides a feature with additional geometric tolerances as the feature's size departs from its least material conition.

There were a couple of important terms introduced in that list: Maximum Material Condition and Least Material Condition.

• Maximum Material Condition: This is the condition where a feature has the maximum amount material possible. Examples being the smallest hole size, or the largest pin diameter.
• Least Material Condition: This is the exact opposite of MMC, a feature has the least amount of material possible. Examples being the largest hole size, or smallest pin diameter.

Now let's see how these different conditions impact the position of a simple hole on a machined part.

Here we have a Ø2.500 +.010/-.000 hole and its position tolerance is Ø.010. Each of these conditions described above will have a different effect on the allowed position of this hole.

RFS: When a tolerance is implied at RFS, the specified tolerance is independent of the feature's actual size. This means that no matter what the actual hole diameter ends up being, the applied tolerance is always the stated value. In this case .010. Here's a table to show what that means:

Feature Size	Position Tolerance Allowed
2.500	.010
2.501	.010
2.502	.010
2.503	.010
2.504	.010
2.505	.010
2.506	.010
2.507	.010
2.508	.010
2.509	.010
2.510	.010

 

MMC: The stated tolerance applies at the MMC size of the feature, and additional tolerance is gained as the feature departs from the MMC size. This means that at the MMC (smallest diameter), the stated tolerance is applied. As the holes gets larger the position gains additional tolerance. Here's a table showing that:
 

Feature Size	Position Tolerance Allowed
2.500	.010
2.501	.011
2.502	.012
2.503	.013
2.504	.014
2.505	.015
2.506	.016
2.507	.017
2.508	.018
2.509	.019
2.510	.020

Zero @ MMC: This is very similar to MMC, except that the stated tolerance is zero. At the MMC feature size, the allowed positional tolerance is zero. As the feature departs from the MMC, additional tolerance is gained. Here's a table showing the effect:

Feature Size	Position Tolerance Allowed
2.500	.000
2.501	.001
2.502	.002
2.503	.003
2.504	.004
2.505	.005
2.506	.006
2.507	.007
2.508	.008
2.509	.009
2.510	.010

LMC: This is the exact opposite of MMC. The stated tolerance applies at the feature's LMC (largest diameter) and as the feature departs from the LMC more tolerance is allowed. Here's a table showing that effect:

Feature Size	Position Tolerance Allowed
2.510	.010
2.509	.011
2.508	.012
2.507	.013
2.506	.014
2.505	.015
2.504	.016
2.503	.017
2.502	.018
2.501	.019
2.500	.020

 

Now that the effect of each modifier has been shown, how do you decide which one (if any) to use? 

• RFS: Used when locating a press fit pin or shaft.
• MMC: When the hole is being used in a clearance application.
• Zero @ MMC: Can be used in place of MMC.
• LMC: Used in a locational application.

Important note: LMC is not commonly used. RFS is usually the best option when using a feature to locate another.

 

I hope this post clears up any confusion with material modifiers and helps you to more accurately design parts in the future!

 

Thanks!

-Brady

              

Comments

[Lenin Mathialagan]

Hi,
I notice that MMC or LMC is always used with mostly position control. Can it used with, say, a run out? Some literature say no. I just can't understand why? Can you explain?
Thank you.

[Brady Scoffield]

Lenin,

In my experience and the resources I've got say that MMC/LMC cannot be used with runout tolerances. Both circular runout and total runout are surface controls and feature modifiers (MMC/LMC) are not applied to surface controls.