In case that description of the push/pull mechanism wasn't clear, here's some pics...
First, a center hole (1/2x13) was drilled and tapped right between the two plates. Not shown (but you'll see later) a 1/4" hole was drilled at the bottom of tapped hole. The area around the hole was milled flat to later accommodate a metal retaining plate for the adjustment screw.
Once the tapping was finished, the plate that's fixed was milled to remove the threads from that half of the hole. The semi-circle was bored to 9/16".
Now it might start making sense why the extra hole was drilled. I'll make a special screw that has a leading piece on the end that will support the end of the bolt; otherwise, it would just drop down into the 9/16" side of the semi-circle.
Here are the mated pieces. The special screw thats made will be held in the front by the fixed plate. I'll put some collars on the front of the screw and a metal plate will support the front part of the screw. I though it would be nice to use an ACME thread but, I don't have ACME taps on hand -and they're quite expensive. A 1/2-13 will have to do. When the adjustment screw is turned, the bottom/fixed plate will remain stationary and the top plate that supports the Morse holder disk will slide in the keyed grooves.
Taking a step backward in time, here's a shot of tapping the holes for the plate that will support the front of the screw. What I really wanted to point-out here is how the mill spindle is used to hold the tap straight. There's a pointy piece in the chuck that either fits in the divot in the end of the tap or, it fits well enough into the opening of the tap handle. I apply light pressure with the spindle for the first few turns. BTW: I made that tap handle when I was about 13 years old in my dad's garage shop. One of my favorite tools in the shop and everyone who uses it wants me to make them one -and I usually do. I can make them in my sleep.
Hopefully tomorrow, I'll have time to make the adjustment screw.
Until we meet again...
Ray