Datums and datum reference frames

A feature control frame that reads ⌖ ⌀0.2 Ⓜ A B C spends three of its five compartments on datums. Beginners read those as labels. They are not labels — they are the instruction for how to hold the part, and everything measured afterwards depends on them.

A cast aluminium housing clamped in a machining fixture on a milling machine table, seated on three flat locating pads with two side rest buttons against its flank, held down by toe clamps, with coolant residue on the table and the mill spindle above. — The pads and rest buttons under this casting are the datum reference frame made physical. Where they touch is where the part is measured from — which is why the scheme has to be settled long before anyone designs the fixture.

Six degrees of freedom, and taking them away

A part floating in space can slide along three axes and rotate about three. That is six ways it can move, and until all six are removed, asking where a hole is has no answer — the hole moves when the part moves.

A diagram of the 3-2-1 location principle. A rectangular part is shown seated on three points on its primary datum plane A, pushed against two points on its secondary datum plane B, and against one point on its tertiary datum plane C. Arrows and a table show which degrees of freedom each stage removes, building from six down to zero. A second panel shows the same part seated in the wrong precedence, with B taken first, and the resulting shift of a located hole relative to the frame. — Six contact points, six degrees of freedom. Take them in the stated order and the part sits one way only. Take them in a different order and the same part measures differently.

3-2-1 location removes them in order:

Stage	Contact	Removes
Primary (A)	Three points	One translation, two rotations
Secondary (B)	Two points	One translation, one rotation
Tertiary (C)	One point	The last translation

Six points. Six degrees of freedom. The part is now fully constrained and can be measured repeatably by anyone, anywhere.

Precedence is not alphabetical order

A B C does not mean A is the most important surface. It means the part is seated on A first and completely, then brought against B, then against C.

Reverse that order and the part rocks onto a different seat. Every located feature moves with it. A part that passed now fails, and nobody has touched the part.

Datums that are not flat faces

Holes. The axis of a hole, simulated by a pin or an expanding mandrel, removes two translations. A second hole or a slot removes the remaining rotation. This hole-and-slot scheme is on almost every sheet metal and casting fixture you will ever see, and it is why stamped parts carry two locating holes that look unnecessarily precise.

Datum targets. On castings, forgings and large stampings there is no surface flat enough to trust as a whole. Datum targets nominate the exact points, lines or small areas where the part is to be touched — so the foundry, the machine shop and the inspection room all seat it identically instead of each finding its own high spots.

Modifiers on the datum reference

A locating pin sitting in a clearance hole really does let the part shift a little. A material condition modifier on the datum reference releases that shift as extra tolerance, because it is genuinely available.

Where the locator is a press fit or a precision spigot there is no shift, so the reference stays at regardless of feature size. Same rule of thumb as on the toleranced feature itself — see GD&T fundamentals for the clearance argument in full.

What to take away

A part has six degrees of freedom, and a datum reference frame exists to remove all six.
3-2-1 — three points, then two, then one — does exactly that.
Precedence sets the seating order, and changing it changes the measurements.
The datum feature is the real surface; the datum is the perfect plane derived from it.
Choose the datum the part is located on in service, not the convenient one.
Holes and slots make perfectly good datums, and are the norm on sheet metal.
Datum targets pin down where a rough part is touched, so everyone seats it the same way.
A fixture that does not reproduce the drawing’s frame produces unrepeatable, arguable numbers.