How Stable/repeatable?

Joshua - thanks for the comments...

A lathe tailstock is a very reasonable analog for what I'm trying to build. There are some differences however, I only need about 1" of travel, "precision" in the Z-axis is not necessary, repeatability in the Z-axis is not necessary, and I would like to keep the overall length (in Z) reasonably small (say 2.5 to 3"). In my latest (mental) model the threads are simply a way of controlling the motion in the Z-axis. Once the proper position is achieved I need some way to lock the ram. A split-collar on the smooth portion of the ram would require the ram to be longer, that is why I was thinking of simply splitting the female threaded portion and having that serve double duty as both the positioner and the lock...

I suggested using Al for the nut because it seemed like that would give more and make for an easier split-collar. I'll give that more thought...
 
@kizmit99
Ahhh, OK :)
I was under the impression you need precision and repeatability.

For what you are looking for then,
I would drill transverse to the thread and fit a brass pin with a light press fit. Something like a 1/4" pin would be good for a 1/2" to 5/8" ram. Scale up as needed. Leave the pin maybe 3/8" long.
Loctite the pin with the weaker Loctite.
Bore your big hole, and single point a nice tight/smooth thread for your ram.
Heat and pull the pin with plyers, cut the pin down to disappears into the hole.
counter bore and thread the pin hole for a grub screw.
carefully insert the pin so the threads line up (you did remember to mark the pin and body?)

Now you have a nice threaded ram, that you can adjust the lash, and lock it securely down. This should get you the maximum performance from the minimum space
 
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