I went ahead and measured the weight of my z gantry with a luggage scale, and I hear the nut click over when I get to about 142oz. For those not familiar in oz, that’s just shy of 9 pounds. This is with a somewhat tall J primo (150mm Z), with 0.125 wall DOM, a mister with clunky metal flex hoze, and a Bosch Colt w/ my sorta clunky mount/vacuum setup. That has a few things going for it vs say one with like 50mm Z travel built with conduit and not mister/vacuum. My birds eye guess is this accounts for ~40oz extra weight on my gantry vs a typical short z conduit build. Still, 140oz doesn’t feel like much to me… the rest of the framing seems to be holding solid at that point anyways (but I really need to invest in a dial guage to quantify such claims).
I measured the installed length spring force for a few stock Tr8x4 backlash nuts I have on hand. Ryan and everyone else who said these would be of little help were correct… ~40oz more is all you’ll get, which is nothing for a mill. I found a spring with a lot more k in my spares box, which just so happened to fit the nuts well. I cut it such that I got 160oz of load out of it (installed length). This would mean my gantry won’t ‘click up’ until lift loads surpass 18 pounds.
That seems adequate to ensure I’m getting max load out of the framing… but jury is out on how long my nuts will last. I recently went to a 1 start T8, and have 2 brass and 2 POM anti backlash nuts coming this week. Guessing greased brass will be best here, but cheap enough to try the POM. My modified core just finished printing and looks very good (y’all are gonna fall in love the color I used, hehe). I might be in a position to get photos and put it to the test this weekend. Will take some tests to see… but I’m hoping to get better plunge performance, as well as ability to use a downcut once in a while.
[edit: DOH! I think I cut my new lead screw prematurely. That anti-backlash nut I think is going to slip off the end when move Z to the top. So I’ll have to order another screw and cut it about 1.5" longer than stock to work with this.]