I hate seeing this on a print. I know I'm gonna have to explain why the big variation every time. So imagine taking a gauge pin the size of your A datum and putting a ring gauge on that pin 0.25 down on to the pin, you will most likely be able to easily tilt that ring gauge from side to side. There is just not enough cylinder there to hold a ring gauge in place. Only the bottom surface would be keeping it from tilting, not the a datum cylinder. This is the same way a machine is going to react when measuring it. A 0.0001" out of roundness or size difference is going to affect it. If you feel you can't get through to engineering on the matter you could take a plane around the cylinder before measuring it. (kind of lying basically) It makes it more consistent.
Thanks for the reply, I see what you are saying regarding the cylinder size.
When you say take a plane around the cylinder before measuring, what do you mean by that?
I think I figured out what you mean, basically measure a plane that slices through the cylinder?
Thanks for the reply, I see what you are saying regarding the cylinder size.
When you say take a plane around the cylinder before measuring, what do you mean by that?
I think I figured out what you mean, basically measure a plane that slices through the cylinder?