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?

  • $\begingroup$ We were always told to have sufficient to make / define the piece, any more and it can be confusing or lead to errors. Also what face is first machined or is the first machining task the dowel hole? $\endgroup$ – Solar Mike Jul 20 '20 at 6:28
  • $\begingroup$ It’s ok to have a datum for the machining and then to also run inspection dimensions critical to fit from the dowel. $\endgroup$ – Jonathan R Swift Jul 20 '20 at 7:00
  • $\begingroup$ @SolarMike - The external faces are machined first just to determine the shape and external dimensions, but these aren't critical, i.e. they can vary by 5mm and I still wouldn't care. What matters are the positions of other FOS relative to the dowel. Yes, the dowel is dimensioned from the edges, but its absolute position is not important, only the relative positions of other FOS are $\endgroup$ – Bawb Jul 23 '20 at 14:46

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.

  • $\begingroup$ When producing a drawing and without further communication with the machinist, can I have 4 datums? I've been told this should be avoided, but it just doesn't make sense to reference other datums since these dimensions will need unnecessarily tight tolerances. Not sure why 4 datums is avoided - is this some kind of unspoken rule? $\endgroup$ – Bawb Jul 21 '20 at 8:50
  • $\begingroup$ You should only have as many datums (or other information for that matter) as you need on a drawing. Since your dowel hole is locating other features that should be a datum. Only using 3 datums may be a rule of thumb at the shop you're working with, but it's not uncommon for drawings to have many datums. $\endgroup$ – jko Jul 21 '20 at 13:00

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