Author Topic: 4th axis harmonic drive build  (Read 744 times)

Offline JayMcClellan

  • Jr. Member
  • **
  • Posts: 30
  • Country: us
    • BrainRight
4th axis harmonic drive build
« on: March 23, 2017, 11:15:41 AM »
Here's the new 4th axis assembly I am building using a harmonic drive gearbox:

<a href="http://www.youtube.com/watch?v=mmOnktzifeg" target="_blank">http://www.youtube.com/watch?v=mmOnktzifeg</a>

I just finished making the drivetrain, which I show in this video, and next I need to build the enclosure and finally machine the faceplate to fit the chuck.
I posted more details on my website at http://brainright.com/Projects/FourthAxis/.

Offline Joules

  • Hero Member
  • *****
  • Posts: 849
  • Country: gb
Re: 4th axis harmonic drive build
« Reply #1 on: March 23, 2017, 12:21:28 PM »
Jay, what 3D printer are you using.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline JayMcClellan

  • Jr. Member
  • **
  • Posts: 30
  • Country: us
    • BrainRight
Re: 4th axis harmonic drive build
« Reply #2 on: March 23, 2017, 01:49:21 PM »
I have a Lulzbot Taz 4, https://www.lulzbot.com/, which has since been superseded by the Taz 5 and then Taz 6. I chose the Taz partly because of its large print volume and because it's all open-source. I upgraded the print bed to PEI (polyetherimide) like the latest Taz printers have and I've been very happy with that upgrade. Among other things I use it to make stuff for the shop like brackets, featherboards and so on. If you look closely in the video you can see the black dust hood that connects my vacuum hose to the spindle on the CNC router, and that was 3D printed too.

Offline JayMcClellan

  • Jr. Member
  • **
  • Posts: 30
  • Country: us
    • BrainRight
Re: 4th axis harmonic drive build
« Reply #3 on: April 08, 2017, 08:23:19 PM »
I just published the Part 2 video of this 4th axis build:

<a href="http://www.youtube.com/watch?v=xaEhkF9S0mY" target="_blank">http://www.youtube.com/watch?v=xaEhkF9S0mY</a>

Offline PK

  • Sr. Member
  • ****
  • Posts: 285
  • Country: au
Re: 4th axis harmonic drive build
« Reply #4 on: April 08, 2017, 08:44:06 PM »
Great job! And a big job too on a mill that size. 
Do you have a feel for how much torque the harmonic drive is good for?

Offline JayMcClellan

  • Jr. Member
  • **
  • Posts: 30
  • Country: us
    • BrainRight
Re: 4th axis harmonic drive build
« Reply #5 on: April 10, 2017, 12:51:51 PM »
Thanks PK. The continuous output torque rating of this gearbox is 60 N-m = 8496 oz-in.  Momentary start/stop torque rating is about twice that, and peak torque (shock load) is about 3x. Theoretically at 3A drive current this stepper can deliver 1.9 N-m = 269 oz-in of input torque, so with a gear ratio of 45x  that means 85.5 N-m = 11,400 oz-in of output torque, which is 40% over the gearbox rating. I'm only driving it at 2.8A but it could still over-stress the gearbox by about 30% if the output were stalled at full power. However the gearbox has a 3x safety margin for peak torque so it should not cause any damage if it's only done occasionally (crash!) and not in regular use. In normal operation I should be able to get the full 60 N-m of torque and if it has a typical torque curve the stepper should deliver the required 1.3 N-m of input torque at 2.8A up to roughly 500 RPM which gives an output speed of about 10 RPM. On a 2-inch diameter part that's a cutting feed rate of about 60 inches = 1500 mm per minute, a very usable range for most materials. And that should be WAY more torque than I need to resist cutting forces with the small cutters I'm likely to use so I should be able to push the feed rate to perhaps twice that and still get maybe 30 N-m = 4250 oz-in of output torque. That's 265 lb of force at a 1" radius which is enough to snap most of the cutters I have. The stepper's output drops rapidly above 1000 RPM so I think useful max output speed will top out around 20 RPM. Sorry if that's too much information in answer to your simple question.

Offline PK

  • Sr. Member
  • ****
  • Posts: 285
  • Country: au
Re: 4th axis harmonic drive build
« Reply #6 on: April 10, 2017, 05:48:32 PM »
It looks nice and stiff too. Done any heavy cuts?

Offline JayMcClellan

  • Jr. Member
  • **
  • Posts: 30
  • Country: us
    • BrainRight
Re: 4th axis harmonic drive build
« Reply #7 on: April 10, 2017, 08:10:42 PM »
No, so far the heaviest cuts were on the side of its faceplate, and that wasn't too heavy at about 10 thou. I think I might run into limitations of the 3-jaw chuck before limitations of the gearbox. My plan is to make a secondary faceplate that mates to this one in place of the chuck and has T-slots, which would permit some heavier milling. I might mill the T-slots with it mounted on the 4th axis so they align with the center, haven't decided about that yet. Actually most of the projects I have in mind for it in the near future would be in wood on the CNC router, where it would be unlikely to push the limits of stiffness. I just built it heavily so I can use it on the mill too, but it won't be much more than an indexing head until I get the CNC conversion done on the mill.