Understanding Material Condition Modifiers

Material modifiers help the designer to better describe what is dimensionally acceptable by providing insight into additional tolerances caused by the presence or absence of material on a part or feature. For this blog, let’s learn and understand Material Condition Modifiers.

What are Material Condition Modifiers?

Material Condition Modifiers provide one of three callouts that describe whether a feature contains the maximum or minimum amount of material when fabricated, thus affecting the overall tolerance of the feature.

MMC – Maximum Material Condition Ⓜ

Maximum Material Condition is defined as the condition of a feature which contains the maximum amount of material, that is, the smallest hole or largest pin, within the stated limits of size.

— Effect of Maximum Material Condition

Where a geometric tolerance is applied on an MMC basis, the allowed tolerance is dependent on the actual mating size of the considered feature. The tolerance is limited to the specified value if the feature is produced at its MMC limit of size. Where the actual mating size of the feature has departed from MMC, an increase in the tolerance is allowed equal to the amount of such departure. The total permissible variation in the specific geometric characteristic is maximum when the feature is at LMC. Likewise, referencing a datum feature on an MMC basis means the datum is the axis or center plane of the feature at the MMC limit. Where the actual mating size envelope has departed from MMC, a deviation is allowed between its axis or center plane and the axis or center plane of the datum.

LMC – Least Material Condition Ⓛ

Least Material Condition is defined as the condition in which there is the least amount of material, the largest hole or smallest pin, within the stated limits of size.

— Effect of Least Material Condition

Where a positional tolerance is applied on an LMC basis, the allowed tolerance is dependent on the actual mating size of the considered feature. The tolerance is limited to the specified value if the feature is produced at its LMC limit of size. Where the actual mating size of the feature has departed from LMC, an increase in the tolerance is allowed equal to the amount of such departure. The total permissible variation in position is maximum when the feature is at MMC. Likewise, referencing a datum feature on an LMC basis means the datum is the axis or center plane of the feature at the LMC limit. Where the actual mating size of the datum feature has departed from LMC, a deviation is allowed between its axis or center plane and the axis or center plane of the datum.

RFS – Regardless of Feature Size (Default)

Regardless of Feature Size is the default condition of all geometric tolerances and requires no callout. Regardless of feature size simply means that whatever GD&T callout you make, is controlled independently of the size dimension of the part.

— Effect of Regardless of Feature Size

Where a geometric tolerance is applied on an RFS basis, the specified tolerance is independent of the actual size of the considered feature. The tolerance is limited to the specified value regardless of the actual size of the feature. Likewise, referencing a datum feature on an RFS basis means that a centering about its axis or center plane is necessary, regardless of the actual size of the feature.

Why Use Material Conditions Like MMC?

MMC defines the worst case condition of a part that will still guarantee, because it is still within the prescribed tolerances, the assembly between pin(s) and hole(s). When a hole is at its smallest (MMC) and a pin is at its largest condition (also MMC), we can be sure that we will still be able to assemble that part. Thus, MMC is widely used in cases where clearance fits are common.

What is Bonus Tolerance?

As the size of the pin departs from MMC toward LMC, a bonus tolerance is added equal to the amount of change. Bonus tolerance equals the difference between the actual feature size and the MMC of the feature.

Bonus Tolerance = MMC – LMC

2 Rules of Material Conditions

Rule # 1 Individual Feature of Size

Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form, as well as size, are allowed.

Boundary of Perfect Form at MMC – External Feature of Size

Boundary of Perfect Form at MMC – Internal Feature of Size

Rule #2 All Applicable Geometric Tolerances

RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified. MMC or LMC must be specified on the drawing where it is required.

Tolerances of Position – RFS is Implied

Applicability of Modifiers

All Applicable Geometric Tolerances (Rule #2). RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified. MMC or LMC must be specified on the drawing where it is required.

[All material referenced from DCS GD&T Training Manual]

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