4.2 RMB, MMB, and LMB on Datum Features

Material Boundary Modifiers on Datum Features

ASME Y14.5-2009 allows the drafter to specify the material boundary at which a datum feature is simulated. The three modifiers — RMB (regardless of material boundary), MMB (maximum material boundary), and LMB (least material boundary) — apply to the datum feature in the datum compartment of the feature control frame. They are written after the datum letter and are distinct from the RFS / MMC / LMC modifiers that apply to the toleranced feature in the tolerance compartment. Confusing the two is a common Senior-level error.

In Y14.5-2009, RMB is the default for datum features — no symbol is drawn. MMB uses the circled-M and LMB uses the circled-L in the datum compartment. The 2018 revision changed the default terminology to RFS for the toleranced feature and kept the datum-side language as "regardless of material boundary"; in 2009 the abbreviations RMB/MMB/LMB are the standard's own terms for datum modifiers.

What Each Modifier Does to the Simulator

RMB — Regardless of Material Boundary

At RMB the datum feature simulator is variable in size. It expands (for an internal feature) or contracts (for an external feature) until it makes maximum contact with the datum feature's actual mating surface. The simulator's size is therefore the actual mating size of the feature, which changes from part to part. The derived datum (axis or center plane) is the axis or center of that simulator.

• For an internal cylinder at RMB: an expanding mandrel grows until it contacts the bore at actual mating size.
• For an external cylinder at RMB: a contracting ring or collet closes until it contacts the pin at actual mating size.
• For a width (two parallel faces) at RMB: the simulator is two parallel plates that close until both contact.

RMB self-centers the part: the datum is the center of the actual feature. Repeatability is high because the simulator always matches the part.

MMB — Maximum Material Boundary

At MMB the simulator is fixed at the maximum material condition of the datum feature (with its size and any applicable geometric tolerance). The simulator is a constant-size gage — a fixed pin for an internal feature, a fixed ring for an external feature. Because the simulator is constant, a part whose datum feature is smaller than MMB (internal) or larger than MMB (external) can shift within the simulator. That movement is datum shift, covered in section 4.3.

• For an internal cylinder at MMB: a fixed pin at MMC size (plus any bonus from the feature's own tolerance if applicable).
• For an external cylinder at MMB: a fixed ring at MMC size.
• For a width at MMB: two fixed parallel plates at MMC separation.

MMB mirrors the functional assembly: a bolt goes into a hole at its smallest (MMC) condition, and a part with a larger hole has play. The gage is a functional go-gage.

LMB — Least Material Boundary

At LMB the simulator is fixed at the least material condition of the datum feature. LMB is used when the thin-wall / weakest state is the functional concern — typically for wall thickness, minimum material, or to ensure a feature does not break into an adjacent cavity. The LMB simulator is a constant gage at LMC size.

• For an internal cylinder at LMB: a fixed pin at LMC size (the largest the bore can be).
• For an external cylinder at LMB: a fixed ring at LMC size (the smallest the pin can be).

LMB is far less common than MMB but appears in castings, thin-wall parts, and calculations of minimum wall thickness.

Comparison Table

Repeatability and Choice

RMB gives the most repeatable datum because the simulator always matches the part — every part sets its own datum at its own center. MMB and LMB sacrifice some repeatability for functional realism: the gage is constant and represents the worst-case mating condition. The Senior-level decision is:

• Use RMB when the datum must self-center (bores that mate on their axis, widths that center on their center plane).
• Use MMB when the part assembles to a fixed external gage (a pin in a hole, a shaft in a bore) and the bonus or shift behavior matches function.
• Use LMB when minimum material or wall thickness is the design driver.

Worked Example: Bore Datum RMB vs MMB

A part has a bore datum A with size Ø25.00 ± 0.05, so MMC = Ø24.95 (smallest hole) and LMC = Ø25.05 (largest hole). The bore is referenced as |POS|Ø0.4|A| for a four-hole pattern.

• A at RMB: the mandrel expands to the actual bore size on each part. The datum axis is the bore's actual center. No datum shift. A part with a Ø25.02 bore and one with a Ø24.98 bore both self-center on their own axis.
• A at MMB: the gage pin is fixed at Ø24.95 (MMB). A part with a Ø25.02 bore has 0.07 of diametral clearance, allowing the part to shift on the gage by that amount. The inspector can use that shift to bring the four-hole pattern into tolerance.

Irregular Features of Size as Datum Features

The RMB/MMB/LMB rules are not limited to cylinders and widths. Y14.5 recognizes two kinds of irregular feature of size:

• Type (a): a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a sphere, cylinder, or pair of parallel planes — for example a set of four bosses whose common envelope is a cylinder.
• Type (b): a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope other than a sphere, cylinder, or parallel planes — a profile-defined pocket, a cam lobe, an extruded contour.

Both types can be datum features of size, and both therefore accept RMB, MMB, and LMB in the datum compartment:

The Senior-level subtlety is where the boundary comes from. On a regular feature of size the MMB falls out of the size limits plus any geometric tolerance. On a type (b) irregular feature of size there are no size limits — the boundary is derived from the true profile plus the profile tolerance applied to the datum feature. So a pocket defined by basic dimensions with a profile tolerance of 0.4 applied all around has an MMB offset 0.2 from the true profile on the material side, and a gage built to that boundary is what the part shifts within.

A common exam framing shows an irregular contour used as a secondary datum feature at MMB and asks where the shift comes from. The answer is the profile tolerance on the datum feature, not a size tolerance the feature does not have.

What the Revisions Actually Changed

Be precise here, because the exam is. RFS has been the default since Y14.5M-1994 — a geometric tolerance or datum reference with no modifying symbol applies RFS, and 2009 and 2018 both keep that default. What changed at 2009 was the vocabulary on the datum side: 1994 applied MMC and LMC to datum references, while 2009 introduced material boundary terminology — MMB, LMB, and RMB — to distinguish a boundary applied to a datum feature simulator from a material condition applied to a toleranced feature. Y14.5-2018 retains MMB/LMB/RMB unchanged. The circled-M and circled-L symbols still appear in the datum compartment in both revisions; only the words behind them were sharpened.