Tolerances get all the attention on a drawing. Datums decide whether those tolerances can be held at all.
What a datum actually does
A datum tells the machinist and the inspector which surface everything else is measured from. It is the reference the part is built and judged against.
If a drawing has no datums, or has them scattered without a clear scheme, the supplier picks their own. That choice affects how the part is fixtured, which affects where the errors accumulate. Two suppliers making the same drawing to the same tolerances can produce parts that both pass inspection and still do not interchange in your assembly.
Choose datums from the assembly, not from the model
The most reliable rule we know: the datum should be the surface that locates the part in its real assembly.
If a bracket bolts to a plate on its flat back face and is located by two dowel holes, then that face and those two holes are the datum scheme. Dimensioning from a convenient model origin instead is where interchangeability problems begin.
This sounds obvious. In practice, a large share of the drawings we receive are dimensioned from whatever the CAD origin happened to be.
The cost consequences
Fixturing. A sensible datum scheme lets the part be held once, on the surfaces that matter. A poor one forces extra operations to reach features referenced to different faces.
Ispezione. Features referenced to a clear datum can be measured in one CMM setup. Features referenced inconsistently need repositioning, and each reposition adds measurement uncertainty and time — which shows up in your unit price on any part with a full inspection requirement.
Argument at first article. When a dimension is out and the drawing is ambiguous about what it was measured from, nobody can settle it. This is the single most common cause of first-article disputes we see.
The three-plane scheme
For prismatic parts the standard approach is three mutually perpendicular datums: primary (usually the largest flat, controls two rotations and one translation), secondary (controls one rotation and one translation), tertiary (controls the last translation).
For round parts, an axis and a face. For a part that locates on dowels, the face plus the two holes as a pattern.
You do not need to be a GD&T specialist to get this right. You need to answer one question: what does this part touch when it is installed, and in what order does it settle into position?
What to check before you send a drawing
- Is every critical feature referenced to a datum, not to another feature?
- Are the datums the surfaces the part actually locates on?
- Can the datums be physically touched by an inspection probe? A datum on a surface that is later covered or removed cannot be verified.
- Is the datum sequence stated, so primary/secondary/tertiary is unambiguous?
La versione breve
Datums cost nothing to specify and are expensive to get wrong. Pick them from the assembly, state the sequence, and make sure they can be measured. It is the highest-leverage five minutes you can spend on a machining drawing.
