What is the difference between datum dimensioning and chain dimensioning?

Answer 1

Datum dimensioning is where a single reference point is used for all dimensions in a single direction. E.g. If Point A is declared the reference point, then Points B, C, D, etc, will all be measured relative to Point A, not to each other.

Chain dimensioning is where each point is measured to the next. E.g. Point B is measured from Point A, Point C is measured from Point B, Point C is measured from Point B, and so on.

The problem with chain dimensioning is that each measurement relies on the accuracy of the previous measurement. If there is a tolerance associated with each measurement (+/- X), then the tolerance for each subsequent dimension accumulates and becomes the number of measurements times the tolerance of each measurement (+/- n*X).

Since datum dimensioning uses only one reference point, the tolerances cannot accumulate as each measurement is isolated, rather than relying on the accuracy of the previous measurement.

Common practice is to only use datum dimensioning in design drawings to ensure greater accuracy over the entire design without requiring tighter tolerances.