What is mean by datum iin gd&t?
How its classifieds?
3 Answers
A Datum is a plane, axis, or point location that GD&T dimensional tolerances are referenced to. Typically, multiple features will be referenced by each datum, so they're a very important part of the whole thing. Datums tell us where to measure from.
In the drwaing below, A and B are the Datums used to establish the center of the hole using the Position Symbol and the Feature Control Block for the hole center..
Datums basically are surfaces, axis or points with zero tolerances.
Everything are measured with respect to a datum in engnn drawings.
Your question requires an in depth answer. Please see section 3 in the attached document for a detailed explanation of datums. A brief synopsis would be: Parts are checked (measured) while being held in place in a checking fixture. The part should be stable in the fixture to get accurate, repeatable measurements. The datums set up on the part drawing or CAD model should be the control aspects (clamping surfaces, points, gage pins, etc.) of the checking fixture. From there, all other part measurements can be made.
