Datums and Datum Reference Frames
Find datum features on a print, build a datum reference frame in the right order, explain how 3-2-1 locks a part, and see why changing the datum order changes the measurement.
Table of contents
A measurement only means something if everyone starts from the same place. Datums are that starting place. They tell the machinist how to locate the part, tell the inspector what to measure from, and tell the assembler which surfaces do the real locating work. If two people pick different starting surfaces, they can get two different answers from the same good part.
What You Will Be Able to Do
- Tell a datum feature from a datum and from a datum simulator.
- Explain the six degrees of freedom and how 3-2-1 locks them.
- Read the datum order in a feature control frame and explain why it matters.
- Say what a cylindrical datum feature locks and what it leaves free.
- Avoid the common mistakes that give a wrong reading on a good part.
Datum Feature, Simulator, Datum
These three words sound alike and mean three different things.
| Term | What it is | Example |
|---|---|---|
| Datum feature | The real, imperfect surface on the part | The machined bottom face |
| Datum feature simulator | The real tool that contacts it | A surface plate, a chuck, a gauge pin |
| Datum | The perfect plane, axis or point that results | The theoretical plane on the plate's surface |
A real surface has high spots, so you cannot measure from it directly. You rest it on something much more accurate, and the simulator stands in for the perfect datum. That is why a bottom face touches a surface plate only at its high points, and why the datum comes from those high points, not from an average of the surface.
On a drawing, a datum feature is marked by a capital letter in a box, joined to a triangle that touches the feature or its extension line. When the triangle lines up with a size dimension, the datum is the axis or center plane of that feature, not its surface. The letters are only labels. "A" does not have to be the first datum in a frame, and the order comes from the frame.
Degrees of Freedom
A loose part can move in six ways, called degrees of freedom. It can slide along X, Y or Z, and it can rotate around X, Y or Z. A datum reference frame is three mutually perpendicular planes built from the datums, and its job is to take those six motions away.
How 3-2-1 Locks a Flat-Sided Part
Picture a rectangular block set on a table, pushed against two walls.
- Primary datum, 3 points. The block sits on its bottom face. Three high points are the fewest that define a plane. That stops 3 motions: sliding up and down, and tilting in two directions.
- Secondary datum, 2 points. Push a long side against a wall. Two points define a line. That stops 2 more: sliding toward the wall, and spinning on the table.
- Tertiary datum, 1 point. Slide the block until its end touches the last wall. One point stops the final motion.
That is 3 + 2 + 1 = 6 points for 6 degrees of freedom. The same logic works when a fixture is built, when a part is clamped for machining, and when a coordinate measuring machine builds its alignment.
Primary should be the surface that matters most to function, and it is usually large and stable. A secondary needs a usable length so its two points are spread apart. Points close together let small errors swing the whole frame.
Fewer Than Three Datums
A frame may list only one or two datums. That is often deliberate. A perpendicularity callout on a face, to datum A, needs only A. The drawing asks you to lock only what matters for that feature. Do not add a datum that is not in the frame, and do not treat a missing one as a mistake.
Order Changes the Result
The frame reads left to right: first box is primary, then secondary, then tertiary. The primary gets all its contact points. The secondary only gets what the primary leaves. So A, then B, then C is a different frame from B, then A, then C, unless the part is perfect, and no part is.
Here is a worked example. A plate rests on bottom face A. Its left side, B, leans 0.003 over its 3.000 height, which is 0.001 per inch. A hole sits 4.000 along from the corner and 1.000 up, and it is exactly on location in a frame of A, then B.
- Measure with A primary. The plate sits flat on A, and B touches at its high point. The hole reads on location.
- Swap the order so B is primary. Now the frame rotates 0.001 per inch to lie along B.
- The hole now reads about 0.004 high (4.000 x 0.001) and 0.001 sideways (1.000 x 0.001).
- Straight-line error is about 0.0041. Position is reported as a diameter, so it reads about Ø 0.0082.
A Ø 0.005 position zone passes in one frame and fails in the other, on the very same part. Nothing about the hole changed. Always set up in the exact order in the frame.
Cylindrical Datum Features
Not every datum is a flat surface. A hole or a shaft used as a datum feature gives you an axis, and it behaves differently from a plane.
| Datum feature | Gives you | Locks | Leaves free |
|---|---|---|---|
| Plane, primary | A plane | 3: one slide, two tilts | Two slides, one spin |
| Cylinder, primary | An axis | 4: two slides, two tilts | Sliding along the axis, spinning around it |
| Cylinder, secondary to a plane | An axis square to the plane | 2: the two slides | Spinning around it |
| Slot or pin, tertiary | A clocking reference | 1: the spin | Nothing left |
A common frame is a flat face as A, a center hole as B, and a slot or second hole as C. Face A sets the part down, hole B centers it, and C stops it from spinning. That last step is called clocking. Without something to stop the spin, any feature off to the side has no fixed location, because the part can rotate about B.
If a long shaft is primary, its axis locks the tilt, and a shoulder face as secondary then locks the slide along the axis. In that frame the shoulder may rock against the simulator a little, because the shaft already set the tilt. Swap them, and the shoulder sets the tilt while the shaft only centers the part. As with flat datums, the order changes the frame.
The simulator for a cylinder must center on it. If the frame shows no modifier, the simulator has to expand or close until it fits the feature tightly, like an expanding mandrel, a collet or a chuck. A loose pin in a hole is not a proper simulator, because the part can wander on it. If an (M) follows the datum letter, a fixed-size simulator is correct, and the looseness is allowed on purpose. That allowance is called datum shift.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Measuring from a convenient edge, not the datum | Numbers that are repeatable and wrong |
| Swapping the datum order | Good parts rejected, or bad ones passed |
| Averaging a rocking surface | The datum shifts between setups, so readings drift |
| Dirt, burrs or chips on a datum feature | The part sits on the burr, and everything is measured tilted |
| Clamping hard enough to bend the part | You measure the bent shape, not the free one |
| Using more than the needed contact points | The fixture forces the part flat, so it reads better or worse than it is |
| Adding datums that are not in the frame | You restrict a feature the drawing left free |
A rocking part deserves a closer look. If a bowed bottom face rocks on the plate, it has more than one possible seat. Do not just push on one corner and take a reading. Use the method your shop documents, and note it on the report, so the next person can get the same answer.
Make it a habit to find the datum features on a print before you read anything else. Before you make a chip or take a reading, deburr and clean them. Machining and inspecting from the same datums also saves a lot of argument. If a shop locates from one surface and inspects from another, any error between those two surfaces shows up as a reject.
Check Your Understanding
1. A casting's top face has a large boxed "A" on it, but the frame for a bore reads B first, then A. Which is primary for that bore?
B. The order in the frame decides, not the letter. The letter is only a label.
2. A fixture holds a thin panel on eight hard stops on its bottom face. The panel reads flatter in the fixture than on the bench. Why?
Three points set a plane, and the five extra stops can push the panel flatter than it really is. You are measuring the forced shape, not the free one.
3. A disc has a flat face A, a center bore B and a small keyway. A hole near the rim is dimensioned to A and B only. An inspector asks how far around the rim it should be. What is missing?
Nothing locks the spin about the axis of B. If the hole's angle around the rim matters, the frame needs a third datum, such as the keyway. If the drawing leaves it out on purpose, then only the hole's distance from center is controlled.
4. Your reading changes each time you reseat a part on the plate. What should you look at first?
Check the datum feature for burrs, chips and rocking. Then check that you are seating it on the right surface in the right order. A changing reading usually means the datum changed, not the feature.
Key Takeaways
- A datum feature is the real surface, the simulator is the tool that touches it, and the datum is the perfect result.
- A frame takes away six degrees of freedom. On flat surfaces that is 3 points, then 2, then 1.
- The frame's order sets the priority, and changing it changes the measurement.
- A cylinder gives an axis. As a primary it locks four motions and leaves slide and spin.
- A stable seat, clean datums and the correct order come before any reading.