4 axis harmonic rotary setup

Hi guys and girls. I currently have a 4 axis setup with a worm gear type gearbox. Accuracy isn't really great so I'm looking to upgrade to a more direct drive/harminic drive type, prefreably without breaking the bank.
I have been looking at something like this: https://www.aliexpress.com/item/1005...d=GqxhjiPyKy78

I allready have a Nema 34 closed loop stepper I can use and a 100mm chuck. What I'm struggeling with is understanding the ratios. I see 1:10, 1:50, 1:100 reducing ratios, what are the pros and cons of the different ratios? i'll be using my 4 axis for wood, plastics and phenolic so I don't need an enormous amount of torque.
If some of you have experience with the actual unit in the link or something similar, that would be great. If you have link to a better unit, prefreable in Europe and at a reasonable price, that's even better.

Thnaks.
Kim

Do you really have accuracy problems with a worm drive? My rotary axis is based on a standard dividing head with the worm driven by a stepper and is plenty accurate enough for most purposes in metal. Just what accuracy are you looking for in the materials you are using?

The only thing a harmonic drive gives you over a worm is zero backlash. If your accuracy problems are due to backlash then adopting the right cutting strategy can deal with them - basically make all your rotary cutting moves in the same direction so the backlash is "taken out". For example I have written a knurling wizard in which the whole knurl is made in one long cut with the rotation in the same direction while the work moves axially backwards and forwards relative to the cutter.

As for the ratios there is the usual tradeoff between resolution and speed. Assuming a 200 step motor with say 8x microstepping and a 10:1 ratio you will need 16000 steps per rev which is probably enough resolution for anyone! 0.0225 degrees per step.

Thanks for the quick reply John, I'm using the rotary axis mostly for doing ring billets for pool cues. I'll include a video (not mine) that illustrates what I'm looking for. I'm having issues with the cutter not comming back to the same slots, which gives me a sloppy fit. Maybe there's something in the settings I should do?
i use Acorn CNC and the worm drive is driven by a NEMA 34 closed loop stepper from stepperonline.

Thanks

Kim

Well that could be explained by backlash between the worm and wheel, so if you turn in one direction and take a cut, then say went a bit further round, but then reversed to take another cut, you wouldn't be at the same angular position. My rotary axis is based on a Myford dividing head, which has adjustable friction so the axis isn't deflected by cutting forces; and I use cutting strategies that make sure that all angular moves that count towards actual cuts are made in the same direction. You can turn in the other direction, say to get back to a starting position, but if so need to overshoot slightly then go in the "right" direction to actual zero. Your controller probably also has backlash compensation but I'm not sure how effective that would be on a rotary axis.

The product you linked to there isn't a harmonic drive - it has a planetary / epicyclic reduction stage. It's possible there is less backlash than you currently have but there's no way to be certain unless you take a punt..

I made my 4th axis using a true harmonic drive sourced from a Korean machine breaker. When combined with a used Yaskawa servo drive I have pretty much zero backlash and 60rpm (3000rpm and 50:1). Perhaps a bit OTT for your application but it can be done without breaking the bank.

https://www.youtube.com/watch?v=CprMqxnXcEE

https://www.youtube.com/watch?v=UFqE9NB2eWc&t=86s