# 4th axis harmonic drive build



## JayMcClellan (Mar 23, 2017)

I've started building a 4th axis assembly for my CNC router and milling machine using a harmonic drive gearbox, and I just published a video of the first part of the build:




I also posted some additional details about it on my web site at http://BrainRight.com/Projects/FourthAxis/. I hope to have part 2 finished in a week or so.


----------



## cmantunes (Mar 23, 2017)

Looks awesome! I'm looking forward to the finished project and I'd be curious to know what kind of backlash you end up with.


----------



## Jonathans (Mar 26, 2017)

Thanks! I really enjoyed watching that.  I'll be wanting to make one myself prrhaps albeit a little larger.


----------



## bpratl (Mar 26, 2017)

Jay, that's a great design and write-up/Video. A while back I converted a rotary table to a 4th axis but your concept is so much more compact and better. Thanks for sharing. Bob


----------



## cozmogeek (Apr 5, 2017)

Good stuff. Thanks for posting it!


----------



## JayMcClellan (Apr 8, 2017)

I just published the Part 2 video of this 4th axis build:


----------



## Zamfir (Apr 22, 2017)

Great 
Videos Jay!  I Enjoyed them tons!  

Looks like what I need too do but maybe just the next size up on the harmonic gear..Or is that gear sturdy enough to handle an 8" chuck?  I need to use a servo on mine and was thinking about belting the servo to the gear so it would not be so long..it would just be more thick..wrap around kinda thing.


----------



## JayMcClellan (Apr 23, 2017)

Zamfir said:


> Great
> Videos Jay!  I Enjoyed them tons!
> 
> Looks like what I need too do but maybe just the next size up on the harmonic gear..Or is that gear sturdy enough to handle an 8" chuck?  I need to use a servo on mine and was thinking about belting the servo to the gear so it would not be so long..it would just be more thick..wrap around kinda thing.



Glad you liked the videos. If you follow the link to my web page there's more info about the harmonic drive, including a link to their "catalog" that has specifications for different models of gearboxes. They give specs for various kinds of applied loads, so that may help you determine which size is sturdy enough to handle the loads you want to apply.

Using a belt drive is definitely a tradeoff, potentially making things more compact and giving you the option of changing the ratio with the pulleys. On the down side, it will sacrifice some rigidity and positional accuracy. It's hard to say how much would be sacrificed since it depends on the construction details, but anecdotally I see that timing belts are commonly used on 4th axes for woodworking applications but are much less commonly used for metalworking. Or perhaps more to the point, belt drives are commonly used for decorative applications where tolerances tend to be loose or even nonexistant. I'm not saying you can't get good results using a timing belt, but if you want to do something precise like gear cutting then a direct-coupled drive may be preferable.


----------



## jbolt (Apr 25, 2017)

The X & Y axis on my PM-932 CNC conversion are driven by timing belts. No rigidity or positioning issues.


----------



## Silverbullet (Apr 25, 2017)

May I had my thanks also. Enjoyed the build , something to strive for. At this point all I can do is watch and hope for the future.


----------



## JayMcClellan (Apr 29, 2017)

jbolt said:


> The X & Y axis on my PM-932 CNC conversion are driven by timing belts. No rigidity or positioning issues.


Good to know, as I'm planning to convert my PM-25MV someday and I have limited space around the machine for the Y axis stepper either front or back. I expect a belt would work okay for a 4th axis on the input side of a gearbox like this, as any errors would be reduced by 1/45 (or whatever gear ratio you have).


----------



## JayMcClellan (Jun 8, 2017)

This week I finished this 4th axis build, by powder coating the steel housing:


----------



## Jonathans (Jun 8, 2017)

Great job Jay!  I've really enjoyed watching your process on this project.  When can you start working on mine?


----------



## JayMcClellan (Jun 8, 2017)

Jonathans said:


> Great job Jay!  I've really enjoyed watching your process on this project.  When can you start working on mine?


Thanks. I'll add yours to my to-do list right away. Alas the list is already pretty long, plus my wife keeps adding things at the top, so you might better go ahead on your own.


----------

