Since everything seemed to be mechanically sorted out after the last post, I was gearing up for a little test to figure out settings for a good surface finish last weekend, and had a full Z-Axis crash during a text v-carve pass. Something broke, and the whole assembly fell down like a rock. Luckily I was watching it, e-stop in hand, and caught it immediately.
Image: Crash Result
Turns out what had happened was the threads had completely broken out of a (brass??) reducer which the lead screw goes through, which was screwed into an aluminum block that bolts with a plastic spacer directly onto the X-Axis bearing plate assembly or whatever you call it. Turns out this has happened before, as there were a bunch of those reducers included with the various surplus hardware/junk that came with the machine, and at least 4-5 of them where the threads were stripped out in exactly the same way....
Image: Trashed Threads
After taking the whole thing apart it seems like a pretty sketchy setup. The motor connects directly with the screw by means of an aluminum coupling with 2 pairs of 2 set screws, 1 pair for the motor shaft, and one pair to tighten the coupling onto the screw, which is somewhat messily ground down to make flat-ish spots for the screws to dig into. At the time of disassembly there was only 1 set screw present on the screw side, which put the screw off center, since it's a 3/8" screw in a 1/2" coupling going to a 1/2" motor shaft.
Image: Motor-Screw Coupling
Is there a better type of setup I could put together relatively simply with the same overall configuration of the machine? Coherent mentioned oldham couplings for the motor to screw connection, and that seems like a good idea. I'm guessing I could get a new lead screw, since this one is worn and slightly bent anyway, and use the clamping type hubs as shown here https://www.mcmaster.com/oldham-couplings without having to grind the screw down?
What about the reducer/block thing. Is there a better way of doing that? Or is using a static threaded brass block to keep a screw holding a 40lb Z assembly and router in place a common practice? It just makes me really nervous about leaving the machine unattended, especially for long 3D passes, since I know this failure has happened multiple times in the past.