TED Theoretically exact dimension
Theoretically exact dimension · Anotaciones de plano · ISO 1101
Example in plan
How to interpret it
On the drawing, “TEDs on profile + ⌓ 0.3 |A|B|C|” states a theoretically exact dimension requirement with a tolerance of 0.3 mm, measured from datums A, B, C in that order of precedence.
What controls
A dimension that defines exact nominal geometry (location, angle, size of a profile) without carrying a tolerance of its own: it is drawn in a box and its value is taken as exact by definition. This does not mean any deviation is allowed, nor that the deviation must be zero: the permitted variation is set by the geometric tolerance frame that governs that feature. That is why a dimension of this kind never appears on its own: without an associated feature control frame there is nothing to inspect.
How to verify
It is never measured against the dimension directly, but by evaluating the feature against the tolerance zone of the feature control frame, built around the theoretical geometry these dimensions define and referenced to the datum reference frame. In practice this means coordinate measurement: the part is aligned to the datum reference frame, the actual geometry is compared with the theoretical geometry and the deviation is reported against the value in the frame.
Common mistakes when interpreting it
- Applying the drawing's general tolerance to them: they are expressly excluded from it.
- Interpreting them as informational reference dimensions, when they are the nominal geometry that defines the tolerance zone.
- Measuring them in isolation with a conventional instrument instead of evaluating the feature control frame that governs them.
How to get it in the workshop / if it comes out of tolerance
When programming, these are the dimensions you enter as-is: they are the theoretical geometry the machine must try to reproduce. The tolerance is not in the dimension but in the feature control frame, and it is almost always expressed as a zone around that theoretical geometry. If you see a boxed dimension on a drawing with no feature control frame nearby governing it, that is a drawing error and should be queried.
If a part is rejected on one of these dimensions measured with a caliper, the error is one of method: a tolerance that does not exist is being applied. If the feature control frame result fails, act on whatever corresponds to the type of control (origin and workholding for position, tool and strategy for profile). And always check that the measurement alignment reproduces the drawing's datum reference frame, because these dimensions are always evaluated from there.
Related symbols
Other symbols in Plan annotations
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