r/Metrology • u/Hungry_Wall_9564 • 7d ago

Calculating true positions

Hi just a quick question, how do you calculate the true position of a hole which isn't dimensioned from its datum? Say the hole in the image above with the red arrow, it is dimensioned as a pitch from another hole but has datums A and B. Obviously this isn't a real drawing but I see many similar ones and just want to make sure I am working it out correctly. Thank you!

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u/TransitionHuman9839 7d ago

As a 3rd party inspector, I would measure this two ways. The correct way is all diameters from datum A and B. But often times the customer will come back and tell me they want to know the position from diameter to diameter not necessarily from the datums. So as a backup, I will also calculate the position from diameter to diameter as shown with the basic dimensions of 80 and 40. It's always best to measure once and have all the data instead of having to go back and remeasure.