Geometric Dimensioning and Tolerancing, or GD&T, is a symbolic language used on engineering drawings to describe the allowable variation in part geometry. It helps designers, machinists, and inspectors agree on what features matter for function, assembly, and interchangeability. Instead of only giving plus or minus dimensions, GD&T controls shape, orientation, position, and movement of features in a precise way.
This makes manufacturing more efficient because parts can be checked against functional requirements, not just nominal sizes.
Key Facts
- A feature control frame gives the geometric characteristic, tolerance value, modifiers, and datum references for a controlled feature.
- Datums are theoretically exact reference planes, axes, or points used to locate and orient a part during inspection.
- Position tolerance controls the allowable location error of a feature, often using a cylindrical tolerance zone for holes and pins.
- Maximum material condition means the feature contains the most material, such as the smallest hole or largest shaft.
- Total tolerance for a bilateral size dimension is upper limit minus lower limit, so T = Lmax - Lmin.
- Runout tolerance controls how much a surface or feature varies as the part is rotated about a datum axis.
Vocabulary
- Datum
- A datum is an exact reference used to establish the coordinate system for measuring and controlling part features.
- Feature control frame
- A feature control frame is the boxed GD&T note that states the geometric control, tolerance, and datum references.
- Tolerance zone
- A tolerance zone is the allowable region within which a feature or surface must lie to be acceptable.
- Position tolerance
- Position tolerance controls how far a feature may deviate from its true theoretical location.
- Runout
- Runout is the variation of a surface during rotation around a datum axis, combining effects of shape and alignment.
Common Mistakes to Avoid
- Treating GD&T as decorative drawing symbols, which is wrong because each symbol defines a specific inspection requirement and functional limit.
- Ignoring datum order in a feature control frame, which is wrong because primary, secondary, and tertiary datums constrain the part in a specific sequence.
- Using position tolerance like a plus or minus coordinate tolerance, which is wrong because position usually creates a circular or cylindrical zone rather than a square zone.
- Confusing flatness with parallelism, which is wrong because flatness controls surface form by itself while parallelism controls orientation relative to a datum.
Practice Questions
- 1 A shaft diameter is specified as 20.00 mm +0.02 mm and -0.01 mm. What are the maximum diameter, minimum diameter, and total size tolerance?
- 2 A hole has a position tolerance of diameter 0.30 mm at maximum material condition. If the hole is at its maximum material condition size and its measured axis is 0.12 mm from true position, does it pass the position requirement?
- 3 A bracket face must be flat and also perpendicular to a mounting base. Explain why a drawing might need both a flatness tolerance and a perpendicularity tolerance instead of only one of them.