will this 4th axis work

[Robin Hewitt]

I thought these bolt to the bed rotating chuck things usually came with a worm drive and an index wheel/pin.
Sticking a stepper in place of the index handle is a CNC conversion.
Replacing the worm wheel with a toothed pulley sounds like a Chinese CNC conversion. More for selling than using.
Maybe you want to engrave toothpicks and this is exactly what you need.
Alternatively, maybe if you bought it as an incomplete conversion, played with it, decided what resolution your usage required, replaced their timing pulley with a nice anti-backlash worm from HPC gears, tried it with the stepper then replaced the stepper with one of those singleton servo drives doing the rounds on eBay.
That could work.

[magicniner]

I have one of the type in the first link in the OP, I chose it for the decent through capacity which you don't get with harmonic drive versions, it worked well enough as standard but I added a link bar and bearings outboard at the back behind the belt to take bending load off the stepper shaft and was then able to add a belt tensioning pulley bearing.
A few hours work and a few quid in bearings yielded something which is up to 4-axis machining in Ally and Steel using tooling up to 12mm and 7000rpm.

I had a good indexing head but I believe that a converted/ home brew worm/wheel job is more of a bodge than pulleys, belts & bearings because long term under CNC use gears and plain bearings will require more maintenance, more expensive maintenance and more complex maintenance.

- Nick

[johnsattuk]

I chose it for the decent through capacity which you don't get with harmonic drive versions, - Nick

Picked up this harmonic drive some time ago for a 4th axis project, not yet got round tuit. 50 -1 with 25mm through and pre-loaded cross roller bearing on the output ring

[Robin Hewitt]

I just found these things existed but I already feel a deep need to own one, was this an e-bay find?

[johnsattuk]

I just found these things existed but I already feel a deep need to own one, was this an e-bay find?

Yes, came across it after I had bought a 5phase stepper with 50-1 harmonic drive ( 25000 steps /rev ) which I was going to belt drive a head 2-1 so that I could incorporate a through hole. I also have some very low backlash 5 phase 10-1 (5000 steps/rev ) epicyclic drives that I thought I might use with perhaps ~4-1 belt drive.

[Robin Hewitt]

I have now bought a 50:1 harmonic to play with, eBay item number: 291926642351
I thought maybe connect it to a shaft using my favorite Myford change wheels. Not easy to cut the middle out 100% concentric so I would have to spring two together, that seems to do the trick if you want low backlash with Myford wheels. I think I could enjoy this. Somewhere in my junk boxes I have a new pair of 40mm self-aligning, flange bearings, featuring grub screws on the extended inner race. They could easily become a part of this.

[johnsattuk]

They come in various sizes, 11 is quite a small one, do you know shaft sizes

[magicniner]

I thought maybe connect it to a shaft using my favorite Myford change wheels..

Surely you'll be using Anti-Backlash gears from HPC Gears as it would be a strange choice to add backlash in the primary drive (regardless of how small an error this would introduce) of a system designed specifically to avoid backlash?

[magicniner]

Picked up this harmonic drive some time ago for a 4th axis project, not yet got round tuit. 50 -1 with 25mm through and pre-loaded cross roller bearing on the output ring

Yeah,
Harmonics with through capacity exist but are not fitted to the Chinese eBay 4th axis models under discussion here, nice thing to have in the parts drawer, please post some photos when you've completed a project with that,
Regards,
Nick.

[magicniner]

Double Post, Sorry guys, gremlins on the interface!