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Datum Features Must Be of Sufficient Size
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

Datum selection is the most significant aspect of applying GD&T. In section 4.3 of the Standard it says that a datum feature should be of sufficient size to permit its use. Well, the major use of datum features is to arrest or take away the degrees of freedom or movement of a part so that it can be reproducibly inspected.

On this drawing the hole in the middle of the part is serving as the primary datum feature. As a cylindrical feature, the drawing is asking this feature to take away 4 degrees of freedom. Ridiculous! This hole isn’t long enough to reproducibly arrest 4 degrees of freedom. If you do this on a drawing, please do not tell anyone you have been reading my Tips.

The second drawing has the large flat surface (which is probably the mounting surface) as datum feature A. This feature can arrest 3 degrees of freedom reproducibly. The center hole is the secondary datum feature. Now it only has to take away 2 degrees of freedom which it can reproducibly do.

The datum features you pick have to be useable. By selecting practical datum features, your designs will perform better and you can let everyone know that you are a faithful reader of my Tips.

Back to Tips Tip added Jan 2008