2 Replies Latest reply on Aug 25, 2014 1:32 PM by Timothy Bratton

    Primary datum for hole pattern

    Mike Pogue

      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.

        • Re: Primary datum for hole pattern
          Jamil Snead

          I'm no expert, but I did a little bit of research into this a while ago. I read that it is indeed better to make the datum be the opposite surface because they will sit the part on that surface during inspection to it makes aligning to that datum easy. However for blind holes it might not make sense to have the opposite side be a datum because any perpendicularity control should be to the side the hole is on. So when I use positional tolerances I use the opposite surface as the primary datum for through holes and the hole side for blind holes.

          • Re: Primary datum for hole pattern
            Timothy Bratton

            Part function and what is critical is the factor, there is no blanket answer.