2.4 GD&T Symbols, Modifiers, and Communication

The 14 Geometric Characteristic Symbols

ASME Y14.5-2009 organizes 14 characteristics into five groups:

Note that Y14.5-2018 removes concentricity and symmetry from the standard; on the 2009 Senior exam they remain in scope. For each characteristic, the symbol is fixed — you cannot substitute orientation for location, or form for profile. A Senior-level scenario will show a functional requirement (e.g., "the face must remain perpendicular to the shaft axis within 0.1") and ask which symbol to apply; the answer is ⊥ | 0.1 | A, not flatness (which ignores the axis) and not profile (which would also control the surface's form relative to a true profile, a different intent).

Material Condition Modifiers — MMC, LMC, RFS

A material condition modifier is applied to the tolerance of a feature of size in an FCF. It defines the boundary at which the tolerance applies and whether bonus tolerance is available:

• Maximum Material Condition (MMC, Ⓜ) — the condition where the feature contains the most material: largest pin, smallest hole. The stated tolerance applies at MMC, and as the feature departs toward LMC, the difference in size becomes additional (bonus) tolerance. Use MMC when the functional requirement is assembly or clearance — a pin at MMC must fit into a hole at MMC.
• Least Material Condition (LMC, Ⓛ) — the condition where the feature contains the least material: smallest pin, largest hole. Use LMC when the functional requirement is minimum wall thickness or minimum material — e.g., a bore where the wall between the bore and an outside diameter must never be too thin.
• Regardless of Feature Size (RFS) — the tolerance applies at the actual mating envelope, with no bonus. In Y14.5-2009 RFS is the default when no modifier is shown; in Y14.5-2018 MMC becomes the default and RFS is shown with the Ⓢ symbol. This reversal is a key revision difference.

Bonus Tolerance — How It Behaves

Consider a hole Ø10 ± 0.1 with ⌖ | Ø0.2 Ⓜ | A|B|C. The stated tolerance zone is Ø0.2 at MMC. The MMC of the hole is Ø9.9 (smallest hole, most material). If the actual hole measures Ø10.05, the bonus is 10.05 − 9.9 = 0.15, so the effective position zone is Ø0.2 + Ø0.15 = Ø0.35. Bonus is always additive to the position zone and only available with MMC or LMC. With RFS there is no bonus — the zone stays Ø0.2 regardless of actual size.

Modifier Traps

• Bonus tolerance applies to the controlled feature, not to the datum feature, unless the datum reference also carries an MMC/LMC modifier. A datum referenced at MMC (e.g. ⌖ | Ø0.2 Ⓜ | AⓂ | B) allows datum shift — a different concept from bonus tolerance and a frequent Senior confusion.
• LMC bonus grows toward larger holes / smaller pins — the opposite direction from MMC bonus. A candidate who applies MMC logic to an LMC-modified control will compute the bonus backwards.
• RFS in 2009 is silent — no symbol. A 2018-trained candidate may incorrectly add Ⓢ to a 2009 drawing.

Supplementary Symbols and Modifiers

These modify how the control is applied:

The projected tolerance zone (Ⓟ) is especially important at Senior level: a threaded hole's position must often be controlled over the height of the engaged fastener, not just at the hole's own surface. Without Ⓟ, the position zone exists only at the part surface, and an angled threaded hole can pass position while misaligning the fastener above the surface. A question that mentions a mating fastener engaging 12 mm above the surface typically requires ⌖ | Ø0.2 Ⓟ 12 Ⓜ | A|B|C.

Selecting the Right Symbol and Modifier for Function

The Senior exam repeatedly tests the function → symbol → modifier mapping. Examples:

• A pin must always fit into a mating hole regardless of how the parts are assembled → position at MMC (bonus tolerance reflects clearance).
• A thin wall between a bore and an outside diameter must never be less than 1.2 mm → position at LMC (bonus protects the wall).
• A shaft's axis must run true to a journal regardless of size → runout or position at RFS (function is size-independent).
• The flatness of a gasket face, independent of any datum → flatness, no datum (form control).
• The orientation of a mounting face relative to a datum plane, ignoring surface irregularity → perpendicularity with tangent plane ⓣ.
• A threaded hole for a standoff engaging 15 mm above the surface → position with projected tolerance zone Ⓟ 15.

The Senior-level test is whether you recognize that the modifier is a design intent statement, not just a calculation toggle. MMC says "assembly matters," LMC says "material minimum matters," RFS says "function is independent of size." Reading the modifier tells you the designer's intent; choosing the modifier is how you communicate your own.