Multiple controllers - 4th Axis

First post here! (searched, but found nothing matching, so please send me to another thread if you know of one)

I’m ordering an SO3 XL. At some point I’d like to add a small rotary axis, but the grbl controller isn’t really suitable to adding a 4th axis without doing some things I’d rather not do. I think a Tiny G would be a better direction.

Has anyone actually set up a second controller on their SO 3, say a Tiny G (and other necessary electronics), and simply switched controllers via cable swaps (using cable connectors and such) and software profiles? This leaves the original controller in place to run the machine in “stock” mode. Interested in anything you all have seen.


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I haven’t seen one but the Wiki would be worth looking at. I was going to experiment with a rotary controller but I’d planned to steal the Y axis.

There are a couple of links on the wiki:

I’ve got a design for an indexed front axis which translates machine motion back and forth into rotary motion — it’s apparently been done a couple of times, see: Off-the-hook on DIY 4th axis writeup

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Not a computer controlled 4th axis, but somewhere on here is Apollo’s jig for four-sided machining on the Nomad with the vise. Let’s see…

Ah ha! Here it is:

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Not 4th axis, but two controllers has definitely been done. Check out user @jimidi , he’s done a really slick board swap mod to run two different controllers/firmware.


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I might just steal the y axis as well. I make mutes for trumpets from the 17th/18th century and wanted something as functional as possible… but if I can get the grbL to do I good job on a rotary axis, then maybe that’ll be the way. Thanks!

@tito Thanks so much for this! Definitely a possibility. I guess I’d need some sort of indexing system (pins maybe?) on the spoil board in order to place the vice correctly and repeatably? Thanks!

@DanoInTx Thanks so much for the info… that’s going to be quite helpful!

No problem at all, I’d love to see a rotary axis implementation, but I’m too far behind on 20 other projects to even give it a shot. Jim from the link above is super knowledgeable and his work shows it. I have no clue what his schedule looks like, but if you have questions you might try pinging him.