Choice of Datum Features and Number of Datum Features

Question

Consider a rectangular stock section with a dowel hole and other features of size (FOS) on it. The dowel hole serves to locate the entire part.
Should my choice of datum features be what is most convenient for the machinist or for the Q&A engineer? Given that it is rectangular, it is easiest to set up the datums as the 3 orthogonal faces/edges of the part.
But the external faces/edges serve no engineering purpose and is pointless to dimension any FOS from the edges. They should be dimensioned from the dowel hole instead since this determines the part's final position/orientation during assembly
In this scenario, can I have more than 3 datum features, i.e. including the dowel hole as a datum FOS to give 4 datums?

jko · Accepted Answer

Typically in my experience, your 3 orthogonal edges of the rectangular blank are your first 3 datums. Your dowel hole would be the first machined feature and since it locates the rest of the features, this would be your 4th datum. Your subsequent features would reference that 4th datum, as well as any of the original 3 to tolerance the features.

Correct answer by jko on August 31, 2021

Choice of Datum Features and Number of Datum Features

One Answer

Add your own answers!

Ask a Question