[12,5] Basic dimension
Basic dimension · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
Boxed dimensions are theoretically exact (no tolerance of their own): they set the nominal location to which the geometric tolerance of the associated frame applies.
What controls
A theoretically exact dimension, with no tolerance of its own, that defines the nominal location, orientation or profile of a feature; it is drawn inside a box. The permissible variation is not in the dimension but in the associated geometric feature control frame: the basic dimension says where the feature should be, and the frame says how far it may depart from there. That is why a basic dimension is never checked against a ± tolerance and is not covered by the drawing's general tolerance note.
How to verify
A basic dimension is not measured as such: it is used as the nominal value against which the CMM software compares the actual feature, or as a dimension for the fixture or gauge. In practice, the theoretically exact location is programmed within the drawing's datum reference frame, and what gets reported is the deviation from it, not an absolute reading. If an inspection report shows a basic dimension with a tolerance assigned, someone has mistakenly treated it as a regular dimension.
Datums and notes
Basic dimensions are always measured from the datum reference frame, not from one feature to the next. Datum target locations and the theoretical angles of angularity and profile tolerances are also dimensioned this way.
Workshop guideline values
- General
- Basic dimensions paired with 0.1–0.3 mm position tolerances on ordinary hole patterns.
- Accuracy
- With 0.03–0.08 mm position tolerances on dowel pins and close fits.
- High precision
- With 0.005–0.02 mm position tolerances on reference plates and tooling.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Applying the drawing's general tolerance (ISO 2768 or similar) to it: a boxed dimension falls outside that note; its control is in the geometric frame.
- Stacking up tolerances across several chained basic dimensions: they do not add up, because each feature is evaluated relative to the datum reference frame, not relative to the previous feature.
- Confusing it with a reference dimension in parentheses: the basic dimension is mandatory and defines the theoretical location; the reference dimension is for information only.
How to get it in the workshop / if it comes out of tolerance
Shown in a box (ISO) or without a linear tolerance (ASME). Do not measure a basic dimension against a ± tolerance: the control is in the GD&T frame.
If the hole pattern comes out shifted as a whole but with the correct distances between holes, the problem is in the work offset or the datum reference frame, not the basic dimensions. If the distances vary from one feature to another, look at the machine: axis backlash, spindle thermal compensation or drift over the shift. When programming, enter the basic dimension as is, without breaking it down into intermediate toleranced dimensions.
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Other symbols in Datums and references
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