GlossaryKind of term: Manufacturing termBasic dimension
What is a basic dimension?
A basic dimension sits in a box with no tolerance of its own and gives the ideal location or form; the allowed deviation is set by the feature control frame.
A basic dimension, called a theoretically exact dimension or TED in ISO, is a dimension in a rectangular box with no plus-minus. It fixes the ideal location, orientation or form of a feature relative to datums or other features: the nominal position around which a position, angularity or profile tolerance lies. On a flange with four holes on a pitch circle, the ⌀80 pitch circle sits in a box, and so does the 90° between the holes unless it follows implicitly from equal spacing; the feature control frame says how far each hole may stray from that ideal location.
What does the standard say about a basic dimension?
ISO 1101 and ASME Y14.5 use the box in the same way. A general size tolerance, such as ISO 2768-1 called up in the title block, does not apply to a basic dimension. The dimension only gains a requirement through a geometric tolerance that refers to it; without one, the specification is incomplete.
How does a basic dimension work in practice?
For programming, the basic dimension is the nominal target: there is no size tolerance whose mid-point you could aim for, because the zone of the feature control frame sits around this dimension. The real requirement is in that frame, and with it in the setup and the measurement. What is inspected is the feature's deviation from the ideal location, measured from the datums, usually on a coordinate measuring machine or, with a Ⓜ in the frame, also with a functional gauge. A calliper between two holes does not prove that requirement.
What is a basic dimension often confused with?
A basic dimension is not a reference dimension: a reference dimension sits in brackets, is for information only and is not inspected. Nor is it a dimension with zero tolerance: deviation is allowed, it is just stated in the feature control frame instead of next to the number.
Frequently asked questions
What is the difference between a boxed dimension and one in brackets?
A boxed dimension is basic and belongs to a feature control frame that governs the deviation. A dimension in brackets is a reference dimension: for information only, with no requirement and no inspection.
Do I have to measure a basic dimension?
You measure the feature's deviation from the ideal location, as the feature control frame prescribes, from the datums. The dimension itself is the nominal target and is not inspected on its own.
What if no feature control frame goes with a boxed dimension?
Then the requirement is incomplete. Ask which tolerance is meant before you quote. The general tolerance does not apply to such a dimension, so assuming a value yourself can lead to a dispute at final inspection.