will this 4th axis 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

[Edward]

I use lots of harmonic drives for my motion control rigs. I buy them second hand from ebay, but I have also bought a couple of modern brand new ones from the Harmonic Drive rep in the UK. They are very expensive new. HQ is in Germany but usually made in Japan, they own the monopoly, so the cheapo Chinese contraptions may not live up to expectations re. "zero" backlash, quality of bearings and so on. A brand new size 25 HD will usually cost a couple of thousand. I paid £1500 for each size 11 for a camera rig. They are beautifully made.

The one above from Johnattuk is a proper one with very good angular bearings. These usually come from small robotic machines that are decommissioned, a lot from S Korea, and a bit old, but perfectly usable. Some work perfectly, some have seen better days and will work, although they may be a bit bumpy at certain speeds, which you wouldn't probably notice on cnc rigs, but you do notice in motion control where perfect smoothness throughout the range is paramount.

The one you bought, Robin is too small, being size 11, with small bearings. You need at least a size 25, with size 14 being a bit marginal. Typical price for a good decent second hand size 25 with hollow shaft is around £450.

Here is a render of one of mine showing both ends

Edward



And this will give you a perspective on sizes. Size 11, size 14 and size 25

[Robin Hewitt]

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

I think it is 6mm in, 12mm out.

[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?

[Robin Hewitt]

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?

Wow, it almost designs itself.