Terminology and Terms in GD&T - Modifiers - Series 1

Modifiers

Modifiers are symbolic means of communicating information on engineering drawings.

Maximum Material Condition

Maximum material condition (MMC) is the size of a feature of size in a part that contains the maximum amount of material.

• For an internal feature of size, the MMC size is the smallest allowable size.
• For an external feature of size, the MMC size is the largest allowable size.

When the MMC modifier is used, it indicates that the specified tolerance applies when the feature of size is at the MMC size. As the size of the feature of size departs from the MMC size, the tolerance is increased by the amount that the size departs from the MMC size.

Internal & External feature of size

In each of the above case the part will have the maximum amount of material.

GD&T symbols to which MMC applies.

• Straightness (Axis)
• Parallelism
• Perpendicularity
• Angularity
• Position - Very Common

Reason for use

To ensure the two parts never interfere. 

For this you should make the MMC of the shaft is always less than the MMC of the hole, then you can guarantee there will always be clearance between the parts. 

Maximum Material Condition Modifier

Least Material Condition

Least material condition (LMC) is the size of a feature of size in a part that contains the minimum amount of material.

• For an internal feature of size, the LMC size is the largest allowable size.
• For an external feature of size, the LMC size is the smallest allowable size.

When the LMC modifier is used, it indicates that the specified tolerance applies when the feature of size is at the LMC size. As the size of the feature of size departs from the LMC size, the tolerance is increased by the amount that the size departs from the LMC size.

Internal & External feature of size

In each of the above case the part will have the minimum amount of material.

Reason for use

To ensure the two parts is always have a contact or press fit & no clearance. 

Gap between a hole/slot and edge of a part

For this you should make the LMC of the shaft is always larger than the LMC of the hole, then you can guarantee there will always be a tight fit between the parts. 

Least Material Condition Modifier

Regardless of Feature Size

This is the implied condition. All tolerance applies at RFS by default, which means no symbol is needed. 

RFS Modifier

Let us consider a simple example and apply all the above three conditions.

MMC, LMC AND RFS APPLIED

MMC Condition is applied

The geometric tolerance is applied at MMC feature of size. If the hole departs from MMC, additional tolerance is gained. Thus the geometric tolerance is applied to smallest size (MMC) of the hole, If the hole gets larger it gains additional tolerance to the position of the hole.

MMC Modifier is applied

LMC Condition is applied

The geometric tolerance is applied at LMC feature of size. If the hole departs from LMC, additional tolerance is gained. Thus the geometric tolerance is applied to largest size (LMC) of the hole, If the hole gets smaller it gains additional tolerance to the position of the hole.

LMC Modifier is applied

RFS Condition is applied

The geometric tolerance is implied at RFS. The size of the hole can be any, the geometric tolerance remains the same.

RFS Modifier is applied