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Dual Dimensions

Question asked by Barna Madau on Feb 24, 2009
Latest reply on Sep 27, 2010 by John Ehrhardt
I thought I'd start a new thread regarding the proper way to use dual dimensions on drawing. First off, I strongly dislike dual dimensions, but the company I'm contracting for insists my drawings conform to their company standard. My hands are tied, though I will question their decision for using them...

So, I understand now, though I had not previously given it any thought, how dual dimensions create contradictory specifications. Simple example- You have ".039/.035 [1/0.9]" on a drawing- the part comes in at .0393", do you reject it? it's outside of the .039", but still inside of 1mm (which is approximately .03937"). Is there any precedence for calling out the alternate units as reference? That would indicate that the inspector should inspect the parts to the primary units, and ignore the tolerance on the alternate unit.

I can see why some companies use them. Most US CNC machines all operate in inches. Most of the tooling available in the US is made in inches (end mills, drills, reams, etc..). So, if you give a machinist a drawing in mm, you are forcing them to convert units which costs time (which = $$).