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.