(12,5) Reference dimension
Reference dimension · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “(73.5) reference distance between holes” states a reference dimension requirement.
What controls
A dimension written in parentheses that repeats information already defined elsewhere on the drawing: it is a reading aid, not a requirement. Its value is derived from other dimensions (a sum of segments, a distance between two already-located axes), which is why it carries no tolerance of its own and nothing is accepted or rejected on it. What it does NOT mean is that it is an approximate or unimportant dimension: what it says is correct, but what governs are the dimensions and frames that produce it. If a reference dimension and the dimensions it comes from do not agree, the drawing has an error and it must be queried, not resolved by picking whichever is more convenient.
How to verify
It is not inspected or reported as a characteristic: it has no limits against which to judge the part. At most it serves as a common-sense check when programming the CMM or setting up the fixture (if the actual value is far from the reference value, something has been misunderstood on the drawing or in the setup). If a customer requires it to be measured, before agreeing, ask for it to be converted into a toleranced dimension or a geometric frame: as long as it is in parentheses, there is no possible acceptance criterion.
Common mistakes when interpreting it
- Applying the drawing's general tolerance (e.g. ISO 2768) to it: a dimension in parentheses falls outside that note because it is not a requirement.
- Confusing the parentheses with the box of a basic dimension: the basic dimension is mandatory and defines the theoretical location; the reference dimension only informs.
- Adding it to the inspection sheet to have one more check: besides adding nothing, it forces you to reject parts for something the drawing does not require.
How to get it in the workshop / if it comes out of tolerance
Use it to make sense of the drawing and to set up quick sanity checks, never as a manufacturing dimension. When transferring the drawing to a routing sheet or CNC program, do not enter it as a dimension to hit: if you chase it, you will end up shifting the feature that actually is toleranced. No line with a dimension in parentheses should appear on the inspection sheet.
If the reference distance does not come out right, do not correct it: look for the cause in the dimensions that produce it, because that is where the error is. A reference dimension that does not add up is very useful as an early warning of a wrongly set work offset or a misunderstood dimension chain. And if the drawing contradicts it with its own dimensions, stop and ask for clarification in writing before machining the batch.
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