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Primary datum for hole pattern

Question asked by Mike Pogue on Aug 25, 2014
Latest reply on Aug 25, 2014 by Timothy Bratton

Have been out of the SolidWorks world for awhile. The new forum is going to take some getting used to.

 

I frequently see the primary datum for a hole-pattern placed on the actual surface the hole is on. In fact, this is how I was taught to do it. It seems to me that this is impossible to check without a functional gage, and that the primary datum should be on the surface opposite the one the hole is on. I spent some time looking at ASME Y14, and at first glance, the datums (data?) look to be on the hole surfaces. But after doing some detective work, I'm about 75% sure those figures are first-angle projections, so I didn't trust that conclusion. I'd like to hear what the experts think.

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