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?