I have one problem. In my drawing, I have two datum planes. One is parallel datum A and one is perpendicular datum B.

Can I show them in any vertical or horizontal line as a co-datum A-B?



Hi Yogesh,

It is not clear from the drawing exactly which feature(s) of the part is datum A and which feature(s) is datum B, but from what I can see, you cannot reference these two as co-datum features.

In your example, it looks like datum A is a mounting face that orients the part, and datum B is a feature or features that locate the part. In this case, datum A should be referenced as the primary datum feature and datum B as the secondary datum feature, just like it’s shown in the parallelism callout.

Per ASME Y14.5, two features are referenced as co-datum features when both are required to properly orient and locate the part in its assembly. In other words, one feature by itself is unable to properly orient or locate the part.

A good example of co-datum features is bearing diameters at each end of a shaft. Both are required to orient and locate the shaft in its assembly. Another example is the mounting surfaces of an intake manifold on a V8 engine that are at a 45 degree angles. Both are required to properly orient and locate the manifold on the engine block.

Thanks for your question,

Roy Cross

ASME GDTP Senior Level
Dimensional Engineering Mentor