GD&T inspection starts with the drawing—not with a measuring tool. The correct method depends on the controlled feature, the tolerance zone, the datum references, production volume, and the information the team needs from the result.
Establish the datum setup first
For any control that references datums, first simulate the datum features in the correct order. A measurement taken from an arbitrary surface or an incorrect fixture may look precise but does not prove the GD&T requirement. The drawing’s datum reference frame defines the setup.
Common measurement tools
A surface plate and height gage can be useful for planar features. A dial indicator can evaluate runout when the part is rotated about the correct datum axis. A CMM can collect feature data and construct axes, center planes, and profiles. Functional gages can quickly check certain combined assembly requirements.
Match the method to the requirement
An indicator reading on a rotating diameter measures runout; it does not automatically prove concentricity. A few sampled points may not establish full-surface flatness. Choose a method that evaluates the actual feature, tolerance zone, and datum simulation defined by the drawing.
Inspection planning question
Before selecting a tool, ask: What feature is controlled? What is the tolerance zone? Which datums must be established? The individual symbol guides include practical measurement notes, such as total runout inspection and position inspection.
Frequently asked questions
How to Inspect GD&T: CMM, Height Gage, Dial Indicator, and Functional Gages FAQ
Can a CMM inspect every GD&T control?
A CMM can evaluate many controls, but the measurement strategy, datum simulation, sampling plan, and reporting method must match the drawing requirement.
Is a dial indicator enough for GD&T inspection?
It can be appropriate for some controls, including runout, but it does not automatically evaluate every geometric characteristic.
Why are datums important during inspection?
They create the repeatable reference system used to orient and locate the part before the controlled feature is evaluated.
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