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CNCCookbook Beginner's Guide to GD&T: MMC, LMC, RFS, and Bonus Tolerances

Feature-Of-Size (FOS)

Surfaces and sets of parallel surfaces associated with a size dimension are called features-of-size (FOS). Typical examples of features of size include:

- Hole diameters (which are cylindrical surfaces)

- Plate thicknesses (two opposed parallel surfaces)

- Pin and Boss diameters (also cylindrical surfaces)

- Ball Bearing diameter (a spherical surface)

Maximum Material Condition (MMC)

Maximum Material Condition (MMC) refers to a feature-of-size that contains the greatest amount of material, yet remains within its tolerance zone. Some examples of MMC include:

- Smallest hole diameter

- Largest pin diameter

It is the smallest hole diameter because a larger hole removes material, hence the smallest diameter provides for the greatest amount of material. Likewise, it is the largest pin diameter because a smaller diameter would remove material.

On drawings, MMC is written simply as an M inside a circle:

MMC is a circled M...

Maximum Material Conditional is one of the dimensional limits on a part. The other side of the tolerance range would be the Least Material Condition.

The only GD&T symbols where you can apply Maximum Material Condition are:

- Straightness

- Parallelism

- Perpendicularity

- Angularity

- True Position (the most common use for MMC)

Why Use Maximum Material Condition?

Let's say you want to ensure two parts never interfere, or you'd like to limit the amount of interference between parts when they are at their worst tolerances. These are good uses of MMC.

For example, consider a shaft that must go through a hole with clearance between the two.

The MMC of the shaft would be the Maximum Diameter.

The MMC of the hole would be the Minimum Diameter.

If the MMC of the shaft is always smaller than the MMC of the hole, you have guaranteed there will always be clearance between the parts. MMC and LMC are defined the way they are--as maximizing or minimizing the amount of material--to make it easier to see and understand these relationships between tolerances.

Gaging Maximum Material Condition

Let's continue with our hole and shaft example. Suppose you wanted to make a functional gage for the part. We could use a pin gage that mimics the lower limit of the hole. In other words, the gage controls the Max Material Condition of the part for that hole since MMC for a hole is the minimum diameter. We call such a gage the "Go Gage" because the part must always go into it.

In practice, we'd need to make the pin that is our go-gage just a tiny bit smaller so it slides in and out easily. By making the pin smaller, we can also account for errors in straightness.

Bonus Tolerance

If you make the pin used for gaging even smaller than MMC, you're creating Bonus Tolerance. In GD&T, Bonus Tolerance = Difference between MMC and actual condition.

Least Material Condition

Least Material Condition (LMC) refers to a feature-of-size that contains the least amount of material, yet remains within its tolerance zone. Some examples of LMC include:

- Largest hole diameter

- Smallest pin diameter

It is the largest hole diameter because a smaller hole adds material, hence the largest diameter provides for the greatest amount of material. Likewise, it is the smallest pin diameter because a larger diameter would add material.

On drawings, LMC is written simply as an L inside a circle:

LMC is a circled L...

Least Material Conditional is one of the dimensional limits on a part. The other side of the tolerance range would be the Maximum Material Condition.

Why Use Least Material Condition?

Let's say you want to ensure two parts are always in contact or have a press fit. These are good uses of LMC.

Least Material Condition is used fairly rarely in GD&T. There are only a few reasons why an LMC would be called. Perhaps the most reason is when you have holes or other internal features that are close to the edge of the part.

Let's take the hole close to the edge of the part. If it is smaller than it's LMC, you can apply a bonus tolerance to the part because now the true center of the hole can be closer to the edge without minimizing the thickness of the material.

Gaging Least Material Condition

A gage intended to control the Least Material Condition is called a "No-Go Gage," A No-Go Gage is made as close to fitting as possible but without a fit being possible. For example, to make sure a pin always fits tightly into a hole, we could design a No-Go gage with a hole whose diameter was equal to the LMC of the Pin. If the pin won't fit the hole (a No-Go), then we know it is large enough to be a tight press fit.

The Problem with LMC

LMC has a weakness relative to MMC. With MMC you are defining the point the size cannot go past as the max material size + the geometric callout. For example, we can check hole diameter and perpendicularity with the same "Go" gage. It works fine because you have two positive tolerances.

With LMC you cannot create a functional gage that controls both. Take the diameter + perpendicularity example. Because LMC gages are "No-Go" gages, we can't check the perpendicularity with the same gage used to check diameter--that gage doesn't fit the hole and can't tell us anything about perpendicularity.

Because of this, LMC is seldom used to control for geometry and size. In fact, it is most commony combined with true position on thin walled parts.

Regardless of Feature Size

If there is no call out to MMC or LMC, the part is measured regardless of feature size (RFS). In fact, since RFS is the default, there isn't even a symbol for it--RFS is what you get in the absence of an MMC or LMC symbol.

Regardless of Feature Size simply means that whatever GD&T callout you make, it is controlled idependently of the size dimension of the part. RFS eliminates any potential Bonus Tolerance, allowing GD&T tolerances to be more tightly controlled.


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