Join me in perhaps the greatest position debate of this entire minute.
When you draw a feature control frame for position of a hole, the primary datum should be a plane perpendicular to the axis of the hole. True or false?
I'm reading Geometric Dimensioning and Tolerancing by David Madsen. I believe this book was published in 1995, so it's a little dated. But it says in the positioning chapter "The true position centerline is perpendicular to the primary datum."
R. Dean Odell also says as much in his position video. Time stamp 21:10
https://youtu.be/XbTbYxdrRYg?t=1270
I've got a part like this. And I feel like that hole on the left should have its own datum along its respective plane to serve as the primary datum in the FCF. Is that dumb?
This got me wondering how necessary perpendicularity of the primary datum is. So I set up the Zone3 training part on a CMM and on purpose I measured the left side first. This was not the face that the hole is born into. I'm kind of following the second example in my first image. So I measured 3 planes, and I measured the hole as a cylinder. I think it's also worth noting that I'm not using datum simulators, I'm touching off the part itself and I'm using reference geometry.
And it didn't seem to matter what order I fed the datums into the evaluation, I got the same calculation. If the axis of the hole is not true to the primary datum, I would expect an extreme failure. So what gives? Does the fact that I'm using reference geometry somehow override the logic of the datum precedence?
Thanks.