# Do drawing dimensions have to start from datum?

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.

• Jul 23 '20 at 14:51
• 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?
– Bawb
Jul 23 '20 at 15:03
• 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. Jul 23 '20 at 19:26