GD&T
Also called: Geometric Dimensioning and Tolerancing · ASME Y14.5
GD&T is the symbol language of technical drawings that defines form, orientation and location of surfaces, beyond plain sizes.
GD&T (Geometric Dimensioning and Tolerancing) is a symbol system that states not only the size of a feature but also how flat, perpendicular, concentric or well located it must be. The most used standards are ASME Y14.5 (US) and the ISO GPS family (ISO 1101 and related).
How it works
Each geometric tolerance is written in a feature control frame: the symbol (flatness, perpendicularity, position, runout…), the tolerance zone value and, when it makes sense, the datums it refers to. Instead of ± dimensions stacking error, a hole’s position is given relative to a datum reference frame that mirrors how the part is assembled. That frees tolerance where it does not matter and tightens it only where function demands.
Practical example
A cover with 4 mounting holes. With chained ±0.1 dimensions, the holes can stack 0.4 of error and the cover will not fit. With position ⌀0.2 relative to faces A, B and C, each hole has a clear cylindrical zone, inspection measures against the same reference and the assembly works.
Common mistakes
- Putting geometric tolerances on everything, raising part cost without functional gain.
- Choosing datums that do not match how the part is fixtured and assembled.
- Mixing ASME and ISO on the same drawing without stating the standard.
FAQ
Is GD&T mandatory?
No, but on parts with precise assemblies it prevents arguments between design, manufacturing and inspection. On simple parts, well-planned linear dimensions may be enough.
ASME or ISO?
Use the customer's standard or the part's market. What matters is stating it in the title block and keeping it across the whole assembly.
Updated
Related service
Manufacturing documentation
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