Crusarius - for what it's worth I started out 4 years ago with b7 threaded rod and a stack of nuts for the up/down with 2-1/2", 3/16" wall square tubing over 2", 3/16" wall square tubing for the sliding action. It's worked fine for many trips up and down. I had chain and sprocket gearing that allowed 4 turns to the inch and the threaded rod was pitched at 16 TPI so it was easy to count the teeth on the sprockets (as gears) to make that happen. I used #40 roller chain for linking them.
I've since upgraded to the up/down system Linn Lumber uses on his mills - the acme rods and those nice brass nuts/blocks. There is no comparison. It's weird now because this new system has a pitch of 5 TPI. I simply left everything else alone and reinstalled as it was, just replaced the rods and nuts. Of course the two rods are still chained together at the top to lock them together to turn at the same time and I used this place to plumb in the wheelchair motor to drive the up/down. The cranking force by hand is a little tougher than before (going up only) but it moves roughly 3 times as fast! It flies down. I haven't tried the motor out on it yet, but with the gigantic people I see these little motors pushing around I think it'll be ok. I think I'll be able to exit a cut, fly the head up and over the log and fly backwards on the carriage just like the big boys do. I'll be making the electric system for the motors 24V because it doubles the speed range available and doubles the power (torque) available as well. For the cost of an extra blemish battery from Interstate I think it's worth it in the long run for sure.
I keep the slides for up/down lubed with ATF/kerosene 50/50 mix occasionally while sawmilling, with a quick spray. It scraped the paint off of course but then stopped there and I don't see any more wear really. Theoretically it'll rub right through one day but I think it'll take millions of up/down cycles to get there. Not in my lifetime! The threads for up/down get a liberal lubing. I use toilet brush ends, the ones with the wire in the middle and are formed in a loop with a plastic handle, to wrap around the top and bottom of the nuts for the up/down and this keeps sawdust from packing in the threads. Just cut the handle off and you have an easily formed brush for multiple purposes. And they're pretty cheap too.
I've only had to replace one bolt that held on the roller guide on the discharge side because it broke in half. I assume it work hardened from vibrations and cracked and then broke. It's the only thing I've had to do after sawing for 4 years and who knows how many bf. Maybe 25,000 bf? It takes massive time to cut this much doing it all manually is all I know. I haven't had to do any more adjustments since the initial setup. I was very picky about everything being as perfect as possible while building and adjusting and it's paid off. It's a good design that holds up well. I followed the plans to a T as far as construction was concerned.
Whew. Time for a break.