When specifying the depth of a cut, should the dimension start from the datum surface or from the surface where the cut will be made? In the example below, a cut of 5mm is made from the surface opposing datum surface A. Is the appropriate dimension 5 or 10?


I'm told to do it from the datum surface but wouldn't it be more convenient for the machinist if it were from the surface from which the cut will be made? The former requires additional maths whereas the latter is more obvious in terms of "what has to be done on the part"

Ok, the maths is not hard, but imagine having to do it 10 times and scribbling all over the drawing.

  • $\begingroup$ See engineering.stackexchange.com/q/36771/10902 $\endgroup$
    – Solar Mike
    Jul 23 '20 at 14:51
  • $\begingroup$ I could change datum A but there are FOS on both surfaces so the question is a bit more general than changing datum surfaces. Do you mean to specify both surfaces as different datums? Is that advisable for two parallel surfaces, i.e. the thickness of the plate? $\endgroup$
    – Bawb
    Jul 23 '20 at 15:03
  • $\begingroup$ I don't know what the machine is, but as an example, using a milling machine with a DRO, you would move the tool to the datum height, and zero it. Then, to make the cut, if you were watching the readout, you would be approaching from above the part, i.e. -16, and would then stop at -10, which is the number on the drawing. There's no maths involved, no guesswork at all. $\endgroup$ Jul 23 '20 at 19:26

If the request is coming from whoever will be machining or inspecting the part, I would side with them. They know what they need to see to make sure the part ultimately meets the print.

Also, since your first datum is on the opposing face this implies it is getting machined first, hence why it is a reference datum. The geometry of the face you're making the cut in question (in terms of flatness, parallelism, etc) is not defined (at least not in the image you have shared), so measuring off that surface is less precise than measuring off Datum A.

For inspection purposes it is also much easier to have the distance from 'A' explicitly stated as this is the reference surface for the feature parallelism. I imagine surface 'A' would be on an inspection plate, a CMM or indicator would be set to measure the feature surface depth nominally 10 mm from the reference face and measure parallel. No extra math for them required, less room for error.

Generally if someone in manufacturing with more experience asks for something that doesn't immediately make sense to you and you don't have a strong argument against it, there's probably a good reason for it. Like wearing a mask in public.


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