Terry,
Believe me, I am well aware of the geometry. The arm needs to rotate 140 degrees, which is a LOT. Basically, both ends of the travel end up being near a critical point, where it is at the very limit of usable range. If I move the cylinder further out, then the ATC can no longer retract far enough to clear the table. Right now, it's going right up to the edge at the quill end of the travel, but it DOES work reliably. It takes a fair amount of tension in the cylinder to pull it off the quill, hence the "jump", but once it's moved the first 3/4" or so, it's fine. One simple fix is to just add a very small (<1/2") air cylinder that is fed by the same air line that feeds the big cylinder to retract the ATC. This small cylinder will pop out, and "push" the arm away from the head to get it started, which will completely resolve this small problem.
I could, of course, also go with a more complex linkage, but I really don't want to go there unless I have no choice. This simple linkage is close enough, I have no doubt I can make it work perfectly with just a few minor tweaks.
Regards,
Ray L.