Why GD&T exists
Before GD&T, drawings used 'plus-minus' tolerances on every dimension. This worked for simple parts but failed for complex 3D geometries. Two parts could both be in tolerance per the drawing yet not assemble — because the drawing didn't fully specify the geometric relationships.
GD&T solves this by specifying: - Form (how flat, straight, round) - Orientation (relative angles between features) - Location (where features are positioned relative to each other) - Runout (how round and concentric in rotation)
Result: drawings unambiguously specify whether parts will fit, even before inspection.
ISO 2768 vs ASME Y14.5
Two competing standards dominate global CNC drawings:
ISO 2768 is European/international, applies general tolerances based on a feature's size and the chosen tolerance class: - ISO 2768-f (fine) for precision parts - ISO 2768-m (medium) for general parts — most common - ISO 2768-c (coarse) for rough machining - ISO 2768-v (very coarse) for cast/forged parts
Class m applied to a 30 mm dimension = ±0.2 mm tolerance.
ASME Y14.5 (American) provides more detailed feature-specific tolerancing. Used heavily in US aerospace and automotive. Applies different tolerance categories to each feature type explicitly.
Both standards can be combined: ISO 2768 for general dimensions + GD&T (per ASME Y14.5 or ISO 1101) for critical features.
Form tolerances: flatness, straightness, circularity
Form tolerances specify how perfect a feature's shape must be.
- ▸Flatness ⏥: How flat a planar surface must be. Specified value = max deviation from the perfect plane. Typical: 0.05 mm for sealing surfaces, 0.005 mm for precision bearing seats.
- ▸Straightness —: How straight an edge or axis must be. Used on cylindrical features (shaft straightness).
- ▸Circularity ⊙: How round a circular feature must be. Critical for bearing races, seal surfaces, valve disks.
- ▸Cylindricity ⊘: Combined circularity + straightness for cylindrical features. Tighter than circularity alone — controls form along the cylinder's full length.
Orientation tolerances: parallelism, perpendicularity, angularity
Orientation tolerances specify how the angular relationship between features must be controlled. Each reference a datum (the 'reference plane' for measurement).
- ▸Parallelism //: Two surfaces parallel to within X. Important for parts that bolt together with paired surfaces.
- ▸Perpendicularity ⊥: One feature perpendicular to a datum. Critical for hole-to-surface relationships, shaft-to-flange interfaces.
- ▸Angularity ∠: Feature at a specific angle to a datum. Used for ramped surfaces, oblique cuts.
Location tolerances: position, concentricity, symmetry
Location tolerances specify where a feature is located. The most common and most important class for CNC parts.
- ▸Position ⌖: Most common tolerance. Specifies how close a feature (typically a hole) must be to its theoretically correct location. Always specified WITH a datum reference. Typical: Ø 0.05 mm for hole positions.
- ▸Concentricity ◎: Centerlines of cylindrical features must align within X. Increasingly replaced by simpler runout tolerances.
- ▸Symmetry ⌯: Feature symmetric about a datum plane. Used for slots, keyways, paired holes.
Runout tolerances: circular and total
Runout tolerances combine circularity, concentricity, and angular relationships. Critical for rotating components.
- ▸Circular runout ↗: Measured at a single cross-section. Common for shaft features where the shaft axis is rotated and a dial indicator measures variation.
- ▸Total runout ↗↗: Measured along the full feature length. Tighter — controls both circularity AND axial wobble.
Datum references explained
Datums are the 'reference planes' for measurement. A datum reference establishes how the part is held during measurement and where measurements originate.
Standard datum naming: - [A] Primary datum (typically the largest reference plane, e.g., back of a flange) - [B] Secondary datum (constrains rotation in one axis) - [C] Tertiary datum (constrains rotation in the remaining axis)
A position callout looks like: ⌖ Ø 0.05 [A][B][C] = 'hole position within 0.05 mm relative to datums A, B, C'.
MMC, LMC, and RFS modifiers
Material condition modifiers change how a tolerance is interpreted as the feature size varies.
- MMC (Maximum Material Condition) Ⓜ: Tolerance applies when feature is at maximum material (smallest hole, largest shaft). Bonus tolerance available as feature departs from MMC. Common for hole patterns. - LMC (Least Material Condition) Ⓛ: Tolerance applies when feature is at minimum material. Used to verify minimum wall thickness. - RFS (Regardless of Feature Size): Default in ISO. Tolerance applies regardless of actual feature size. Default if no modifier shown.
MMC is especially useful for assembly fits: allows feature to be slightly off-position when it's smaller than maximum, since the smaller feature still assembles.
How tolerance choices affect cost
Tighter tolerances cost more — exponentially more. Approximate cost multipliers:
| Tolerance | Cost multiplier vs ±0.1 mm |
|---|---|
| ±0.5 mm (very coarse) | 0.8× |
| ±0.1 mm (standard machining) | 1.0× |
| ±0.05 mm (precision machining) | 1.3–1.5× |
| ±0.02 mm (close tolerance) | 1.8–2.5× |
| ±0.005 mm (CNC + CMM verification) | 3–5× |
| ±0.001 mm (grinding/EDM finishing required) | 5–10× |
Practical advice
Only specify tight tolerances on features that actually need them — bearing fits, seal surfaces, mating fits. For general dimensions, ISO 2768-m (±0.1-0.2 mm) is usually sufficient and saves 30-50% on part cost.
Conclusion
GD&T is a precise language for describing geometric relationships in parts. Understanding it lets you communicate functional requirements accurately to your CNC supplier — neither over-specifying (paying for unneeded precision) nor under-specifying (getting parts that don't assemble). Most CNC drawings need GD&T on only 3–8 critical features; the rest can be ISO 2768 general tolerances.