What Is GD&T?
GD&T stands for Geometric Dimensioning and Tolerancing (GD&T). It is a system of symbols used on engineering drawings to define geometric tolerances. Unlike linear dimensions with simple plus/minus values, GD&T symbols communicate design intent by controlling the shape, orientation, and location of features relative to datum features.
GD&T follows ASME Y14.5. This standard defines 14 geometric characteristic symbols, and those symbols fall into four categories: form, orientation, location, and runout. Each symbol controls a different type of geometric error. This guide covers all 14.
GD&T gives a larger usable tolerance zone than plus/minus dimensional tolerances. A circular zone (true position) is 57% larger than a square zone (± X and ± Y). This means lower cost with the same fit, and it communicates design intent more clearly to the shop floor.
Form Controls
Form controls define the shape of a single feature independent of its feature size or relationship to other geometry. They do not need a datum reference.
| Symbol | Name | What It Controls | CNC Example |
|---|---|---|---|
| — | Straightness | How straight a line or axis is | A shaft that must slide in a bearing |
| ◻ | Flatness | How flat a surface is | A sealing face on a manifold |
| ○ | Circularity | How round a cross-section is | A piston bore at any single slice |
| ☌ | Cylindricity | How round and straight a cylinder is | A bearing journal on a shaft |
Flatness is the most common form control in CNC work. Use it on mating surfaces that must seal or sit flush. Cylindricity is equally important for bearing bores, where the cylindrical tolerance zone ensures roundness and straightness simultaneously.
Orientation Controls
Orientation controls — also called orientation tolerances — define how a feature sits relative to a datum. They always need at least one datum reference.
| Symbol | Name | What It Controls | CNC Example |
|---|---|---|---|
| ⊥ | Perpendicularity | 90° relationship to a datum | A pin hole square to a mounting face |
| ∥ | Parallelism | How parallel to a datum | Top and bottom faces of a plate |
| ∠ | Angularity | A specific angle to a datum | A chamfer at exactly 45° |
Location & Runout Controls
Location tolerances define where a feature is. Runout controls check rotation errors around a datum axis. Both need datum references.
| Symbol | Name | What It Controls | CNC Example |
|---|---|---|---|
| ⊕ | True Position | Location of a feature center | Bolt hole pattern on a flange |
| ◎ | Concentricity | Center points align on an axis | Inner and outer diameters of a tube |
| ☰ | Symmetry | Feature is centered on a datum | A keyway centered on a shaft |
| ↗ | Circular Runout | Surface wobble at one cross-section | A pulley face checked on a lathe |
| ↗↗ | Total Runout | Surface wobble over full length | An entire shaft surface against centers |
| ⌒ | Profile of a Line | 2D cross-section shape | An airfoil cross-section |
| ⌒⌒ | Profile of a Surface | 3D surface shape | A contoured mold cavity surface |
True position is the most used location tolerance. It defines where a hole or pin center must be. Always specify it with a diameter zone (∅) and reference datums. For cylindrical features, the tolerance zone becomes a cylindrical tolerance zone centered on the ideal axis.
Datums Explained
A datum is a reference feature on the part. Common datum features include flat surfaces, holes, and edges. The shop uses datums to set up and inspect the part. A datum axis, for example, is the theoretical center line of a cylindrical datum feature. Every GD&T callout that controls orientation or location needs datums.
How to Pick Datums
- Datum A is the primary contact surface. Pick the largest, most stable flat face.
- Datum B locks rotation in one direction. A long edge or a hole works.
- Datum C locks the remaining movement. A second hole or edge works.
Do not pick a small or thin feature as Datum A. The shop will clamp on that surface. If it is unstable, the part will move during machining. Pick a large, flat face.
Datum Order Matters
The order of datums in a feature control frame changes the setup. Datum A goes first. Datum B goes second. Datum C goes third. Switching the order changes how the shop holds the part.
Feature Control Frame
The feature control frame is the box that holds a GD&T callout. It reads left to right.
| Box | Contains | Example |
|---|---|---|
| 1st box | GD&T symbol | ⊕ (true position) |
| 2nd box | Tolerance value | ∅ 0.010 |
| 3rd box | Material modifier (if any) | M (MMC) |
| 4th+ boxes | Datum references | A | B | C |
A complete callout reads like this: "The center of this hole must be within a ∅0.010 diameter zone at MMC, relative to datums A, B, and C." When the feature control frame includes a maximum material condition modifier, the part may receive bonus tolerance as the actual feature size departs from MMC.
Think of the feature control frame as a sentence. The symbol is the verb. The tolerance is how much. The datums are the reference points.
When to Use GD&T
GD&T is not always needed. Use it when the part function depends on feature relationships. For parts where a simple dimensional tolerance on linear dimensions is sufficient, standard plus/minus callouts work fine.
- Mating parts. Holes that accept pins or bolts need true position.
- Sealing surfaces. Faces that seal against a gasket need flatness.
- Rotating parts. Shafts and bores need runout or cylindricity.
- Aligned features. Holes that must line up across parts need true position to shared datums.
- Complex surfaces. Contoured shapes need profile controls.
For simple parts with no mating features, plus/minus tolerancing works fine. Do not add GD&T just to look thorough. Every GD&T symbol adds quality control and inspection cost, so only call out what the design intent truly requires.
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Upload CAD for Instant QuoteFrequently Asked Questions
What is the difference between GD&T and traditional tolerancing?
Traditional tolerancing uses plus/minus dimensional tolerances on linear dimensions. Geometric dimensioning and tolerancing (GD&T) controls shape, orientation, and location relative to datums using GD&T symbols in a feature control frame. GD&T gives the machinist a larger usable tolerance zone. This reduces cost while keeping the part functional.
When should I use GD&T on my drawing?
Use GD&T when parts mate with other parts, when holes must align with pins, when surfaces must be flat for sealing, or when the part function depends on feature relationships. For simple parts with no mating features, plus/minus works fine.
What is a datum in GD&T?
A datum is a reference feature on your part. It is usually a flat surface, a hole, or an edge. The shop uses datums to set up the part in the machine. Datums define where measurements start.
What does MMC mean in GD&T?
MMC stands for Maximum Material Condition. For a hole, MMC is the smallest allowed diameter. For a pin, MMC is the largest allowed diameter. When the actual feature size departs from MMC, the part receives bonus tolerance — extra positional allowance that helps reduce scrap.
Do CNC shops understand GD&T?
Most professional CNC shops understand GD&T and can interpret geometric tolerances on GD&T drawings. However, not all shops have CMM equipment to inspect every callout. Ask your shop about their quality control and inspection capabilities before adding complex geometric characteristic callouts to your drawing.
