GD&T Toolkit — ForgeCalc

① True Position

Measured axis deviation from the basic (true) location → actual position diameter, bonus tolerance, and pass/fail against the feature control frame.

Δx from true position

Δy from true position

Stated position tolerance — zone ⌀ (FCF value)

Material modifier

Feature type

MMC size

LMC size

Actual size

Coordinate ↔ position converter. Equal-bilateral ± coordinate tolerance ⇄ equivalent cylindrical position zone (⌀ = 2√2 · t).

± Coordinate tol (each axis)

Position zone ⌀

② Material Condition — Bonus & Virtual / Resultant Condition

MMC/LMC bonus tolerance from actual size, and the fixed worst-case boundaries used for assembly clearance and functional gauging.

Feature type

Modifier

MMC size

LMC size

Actual size

Geometric tolerance at stated condition (t_geo)

Definitions & formulas

Bonus tolerance t_total = t_stated + |actual − MMC| (MMC modifier). Capped at the full size tolerance |MMC − LMC|.

Virtual condition — the constant worst-case boundary mating parts must clear: external VC = MMC + t_geo; internal VC = MMC − t_geo. A fixed-size functional GO gauge is made to the VC.

Resultant condition — the opposite worst-case boundary: external RC = LMC − (t_geo + bonus_max); internal RC = LMC + (t_geo + bonus_max).

RFS grants no bonus and cannot be functionally gauged; reserve MMC for clearance/assembly features and use RFS on alignment-critical features.

③ Feature Control Frame Builder

Assemble a control, zone, modifiers, and datums → live FCF render and a plain-English reading.

Characteristic

Zone shape

Tolerance value

Material modifier

Projected zone Ⓟ (height, blank = none)

Primary datum

Secondary

Tertiary

Datum A modifier

Datum B modifier

Datum C modifier

Feature control frame

④ Tolerance-Zone Reference & Stack Contribution

The 14 characteristics — zone shape and how each contributes to a tolerance stack. Click a row to visualize its zone.

Sym	Control	Datum	Zone shape	Stack contribution

Orientation tilt amplification

A flatness/orientation zone of width t over feature length L is a tilt that grows into a lateral offset at distance d.

Zone width t

Feature length L

Distance d

The two conversion patterns

Direct lateral (location-type) — position, runout, concentricity, profile: the axis/surface can be off by half the zone, δ = ±t/2.

Tilt that propagates (orientation/form-type) — flatness, parallelism, perpendicularity, angularity: θ ≈ t/L, growing to δ(d) = d·t/L at distance d. Orientation errors amplify with distance — they dominate tall optical stacks.

Bonus (Ⓜ on the feature) and datum shift (Ⓜ on a datum of size) are always additive worst-case terms.

Built from the vault GD&T set (ASME Y14.5). Analytical first-order models — verify against the standard and your inspection method for release drawings.