Basic dimension

A basic dimension defines the ideal location, angle or size of a feature. It is shown boxed and has no plus or minus value. The allowed deviation comes from the geometric tolerance that refers to it.

What it means for the calculation

The boxed dimension on its own says nothing about cost; the frame that goes with it does. Pricing a boxed dimension as an ordinary dimension under the general tolerance values a hole pattern far too loosely and misses the fixturing and the measurement the position tolerance requires. The reverse also holds: if no frame refers to it, the boxed dimension is incompletely specified and the requirement cannot be priced without asking.

Where it appears on the drawing

As a number inside a rectangular box, with no plus or minus value, usually on a hole pattern, an angle or a spacing.

Common mistakes

Treating the boxed dimension as a dimension covered by the general tolerance in the title block. Also adding up the spacings on a hole pattern as if deviations stacked, while each hole has to meet its own position zone separately.

Why is there no tolerance next to it?

Because the tolerance sits in the feature control frame that refers to that dimension.

Does the general tolerance from the title block still apply?

No, a boxed dimension falls outside the general tolerance.

Can an angle be a basic dimension?

Yes, a boxed angle usually pairs with an angularity tolerance in a frame.