Your culprit is, was and always will be friction.
All thread is designed for FASTENING. The pressure angle of the threads is conducive to locking - this is partly why nuts don't work loose, they essentially "wedge" themsleves into each other.
Moreover and more importantly, the first thread of engagement in the direction of loading sees a predominance of the load. When/as/if he thread deflects, the remaining threads will see/share the load.
Hence, in spit of your efforts, all your load is concentrated to a very small spot on your nut. Thus, it is going to be difficult to first generated an adequate lube film and then to maintain it. Hence, concentrated friction will readily occur. THis friction is what was raising the temp of the grease to the smoke point and inducing your scuffing/scoring/siezure problem.
Honing/lapping the threads did several things. It probably smoothed out the cut/torn surface induced by threading. It also increased the thread to thread clearance - this forced the nut to share more of the load over more threads insted of concentrating it to the initial engagement point. It also added more clearance for grease/lube to penetrate and remain in the clearance space.
WHen you get a chance to do some research, look into the contact angles of rolled or cut BOLT thread as opposed to the same angle of acme thread - you'll note that there IS a difference. That's because acme's are/were desiged for power transmission as opposed to FASTENING.
Yes, you can craft up all thread to get it to act like acme but, for the long term success of your plasma cutter, you might want to use stuff a bit better suited to the intended use. After all, there is a reason why even the cheapest, low buck import lathes use acme's (and brass nuts) on the lead screws.
In the mean time, I'm glad to have been of help to your short term development efforts  |