GD&T measurement 101
Reading an engineering drawing, the five kinds of geometric tolerance, and how each one is measured.
An engineering drawing turns a design into something that can be made. It carries everything needed to make the part, it works as the contract the manufacturer has to meet, and it sets the quality the part must reach, tolerances included. Once a drawing uses Geometric Dimensioning and Tolerancing (GD&T), inspection has to go beyond length and width to check a feature’s form, orientation, and location. This guide covers how to read a drawing, the basics of GD&T, and how each kind of geometric tolerance is measured.
Reading an engineering drawing
Most drawings have three parts besides the views themselves:
- Title block: the part name, drawing number, scale, and material.
- Revision block: the changes made to the drawing over time. Always check you’re measuring to the current revision.
- Notes: instructions that don’t fit on the dimensions themselves, such as general tolerances, finishes, and treatments.
The part is shown in standard views, often several on one sheet:
| View | What it’s for |
|---|---|
| Isometric | A 3D-style picture that gives a quick sense of the part’s shape |
| Orthographic | Flat views (front, top, side) drawn to scale, where most dimensions go |
| Section | A cut through the part to show internal features |
| Detail | An enlarged view of one small area |
| Assembly | How several parts fit together |
| Exploded | An assembly pulled apart along its axes to show how the parts relate and the order they go together |
GD&T basics
Datums are the reference points, lines, or surfaces, usually labeled A, B, C, that other features are measured from. They give everyone the same frame of reference, so the part measures the same no matter who inspects it or how it’s fixtured.
A feature control frame is the boxed callout that sets a geometric tolerance. Read left to right, it gives the characteristic being controlled (as a symbol), the size and shape of the tolerance zone, and the datums it’s measured from, in order of priority.
A GD&T drawing carries two kinds of tolerance:
- Size tolerance: how much a dimension, such as a diameter or width, can vary. This is the familiar plus/minus tolerance.
- Geometric tolerance: how much a feature’s shape, orientation, or location can vary, such as flatness, perpendicularity, or position.
Together they control not just a part’s size but its shape and where its features sit. That’s why GD&T inspection takes more than a caliper.
The five kinds of geometric tolerance
| Category | What it controls | How it’s usually measured |
|---|---|---|
| Form | Flatness, straightness, circularity (roundness), and cylindricity: a feature’s shape on its own, with no datum | Roundness measuring machine, CMM scanning, or a surface plate and dial indicator |
| Profile | Profile of a line and profile of a surface: how closely an outline or surface follows its true, theoretical shape | Contour measuring machine or CMM, compared against the CAD model |
| Orientation | Parallelism, perpendicularity, and angularity relative to a datum | CMM, or a sine bar and dial indicator for angles |
| Location | Position, plus concentricity and symmetry under ISO standards (ASME Y14.5-2018 dropped those two) | CMM. For a slot or tab, measure from both sides and take half the difference to find the center’s offset |
| Runout | How much a surface wobbles as the part turns on a datum axis: circular runout at one cross-section, total runout over the whole surface | Dial indicator on the rotating part, or a CMM or roundness machine |
Why position tolerances give you more room. A plus/minus tolerance on a hole’s X and Y location makes a square tolerance zone. A position tolerance makes a round zone. A circle drawn through the square’s corners covers about 57% more area, so parts that sit near those corners, which a plus/minus drawing would reject, still pass, while the hole stays just as far from where it should be at worst.
Knowing which category a tolerance falls into tells you the inspection method and the equipment you need.
Machines for GD&T inspection
- CNC coordinate measuring machine (CMM): the most versatile. Programmed probing checks form, orientation, location, profile, and runout on complex parts in one setup.
- Vision measuring machine: non-contact measurement, well suited to 2D features and to small or delicate parts a probe could move or mark.
- Roundness measuring machine: built for circularity, cylindricity, and runout on turned parts.
- Contour measuring machine: traces a profile and compares it with the CAD shape, for profile tolerances on grooves, radii, and threads.
For how these compare on accuracy, investment, and production volume, see Choosing the right measuring machine. For standards, errors, and calibration, see Measurement basics.
Adapted from the Metrology Application Group’s seminar on interpreting engineering drawings. We teach a hands-on Basic GD&T course in Pasig or at your plant.
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