A Practical Introduction to GD&T

Dimensional tolerances say how big a feature is. GD&T says what shape it is and where it sits. A working introduction to datums, form and position controls — without the extra formulas.

by Hassan Mehrabi· ·3 min read

A dimensional tolerance is a simple limit: this dimension must fall between these two numbers. But dimensions alone do not fully define a part.

The example that makes it obvious

You want a rod: 20 diameter, 100 long.

The maker delivers a rod measuring exactly 20 everywhere and 100 long. But the rod is bent.

Can you reject it? Not really. No dimension is out of tolerance. You controlled sizes, not shape.

This is where GD&T comes in: Geometric Dimensioning and Tolerancing.

First: datums

GD&T is meaningless without datums. A datum is the surface everything else is measured from.

Datums are marked with capital letters in a box: A, B, C.

How to choose one? Three rules:

  1. The face the part sits on in the assembly
  2. The face the part is clamped on during machining
  3. The largest, most stable surface

If all three coincide, ideal. If not, the first wins — how the part functions matters more than how convenient it is to make.

Usually three datums are defined, constraining the part completely like three walls of a room: A the primary face, B perpendicular to it, C perpendicular to both.

Form controls — no datum required

These control the shape of a surface in itself:

Flatness — how much a surface may wave. Critical for sealing and reference faces.

Circularity — how oval a circular section may be. Vital for a shaft turning in a bushing.

Cylindricity — circularity plus straightness along the length. This is what catches the bent rod.

Straightness — how much the axis may bow.

Orientation and location — datums required

Position is the most used and most useful GD&T symbol.

Say you have four bolt holes. With coordinate tolerancing you write 50 ±0.1 and 50 ±0.1. That puts the hole centre anywhere in a square 0.2 across.

But the hole is round, not square. With position you write ⌖ Ø0.28 A B C, putting the centre anywhere in a circle 0.28 across.

That circle has more area than the 0.2 square — 57 percent more tolerance with no loss of function. Fewer parts get scrapped and the price comes down.

Perpendicularity, parallelism and angularity control a surface's relationship to a datum.

Concentricity — one diameter's axis relative to the datum axis. Important for stepped shafts.

Runout — how much a surface rises and falls as the part rotates about its datum. For pulleys and gears this is the most practical symbol, because it measures exactly what you feel in service.

Reading a feature control frame

| ⌖ | Ø0.2 Ⓜ | A | B | C |

Left to right:

  1. Type of control — here, position
  2. Amount — a tolerance zone 0.2 in diameter
  3. Ⓜ — maximum material condition (optional)
  4. A, B, C — the datums it is measured against

Where to start

You do not need all fourteen symbols. Start with the three that cover ninety percent of real needs:

  1. Datums (A, B, C) — without these the rest means nothing
  2. Position (⌖) — for every hole pattern
  3. Runout (↗) — for every rotating part

Add the others when a part actually demands them.

The common mistake

The biggest beginner error is choosing the wrong datum. If your datum is a face that plays no part in the assembly, all your tolerances will be measured correctly and none of them will help.

Before adding any GD&T symbol, answer this: which face does this part sit on in the machine? That face is your datum A.

GD&T is a powerful tool but not for every part. For a simple bracket, ordinary dimensional tolerancing is both sufficient and correct. Bring GD&T in where form and position genuinely matter.

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