Using a 4th (Rotary) Axis

  • I've been investigating converting an old printer in to a small rotary engraver and wondered whether it's possible yet to control with RepRap Firmware.
    How would you define a rotary axis?
    We have M673 for aligning a rotary axis but I can't find how you would define its travel. I figure it would need a travel defined in degrees and any motion would also need a direction parameter. The axis would also need a steps/degree configuration option. From what I have found the rotary axis on these machines is normally defined as axis A not that it really matters.
    It seems the only way to do it at the moment is using polar kinematics, where you would treat the centre of the rotary axis like the centre of the 'bed' then all moves would have to be G2 or G3 command with the centre of the arc being the centre of the axis.
    I have little experience with conventional CNC so would appreciate anyone pointing me in the right direction.

  • i could be wrong, but its my understanding that the CAM software takes care of dealing with the rotational nature of the 4th axis and as far as the printer is concerned its all business as usual, the only difference being you set the step/mm to actually work out to steps/degree.

    its on my list of things to test with the duet 3, but this is at least how fusion 360 and linux cnc dealt with it.

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    Just define it as a normal additional axes, but swap mm for degrees in all the configuration commands e.g. M92, M201, M203, M208, M566.

  • @dc42 Thanks for the response. I'll have to be cautious with absolute and incremental movements in that case. Without a direction parameter there are always 2 ways to get to a position.

    With absolute positioning, if for example towards the end of carving a thread around a dowel with 10 turns, you would end up getting to 3600 degrees, returning to 0 from that position would mean unnecessarily 'unwinding' the axis. A modulus function in meta commands would easily allow you to find the nearest 0 degree point again I suppose. It's just going to be case of wrapping my head round something new.

    Having had a very short play with machining outputs from fusion I found that using the Fanuc post processor was outputting I and J parameters so I assumed they were similar to G2/3 but their was no G parameter at all and other post processors were completely different. Still havent gone very in depth with it yet. Lots to investigate and experiment with.

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    @littlehobbyshop said in Using a 4th (Rotary) Axis:

    I'll have to be cautious with absolute and incremental movements in that case. Without a direction parameter there are always 2 ways to get to a position.

    This is true. The firmware has the means to handle rotary axes differently (it's used on polar and some SCARA machines), but currently there is no command to declare that an axis is rotary.

  • @littlehobbyshop please pass along your experience going forward. I occasionally add a 4th axis (currently just for indexing) to my Duet controlled micro mill. My CAM outputs 4th axis though I have yet to dedicate the time to explore simultaneous work. My current focus is an ATC for the machine but plan to start working again on the the "A" axis soon after that is complete

  • @dc42 Hi David, I am trying to get in contact with you concerning SCARA.
    I have posted a link on the forum , can you please have a look at it, I need your opinion.
    Thank you

    here is the link

  • I know you want an answer from dave... but... as @Phaedrux said in the other tread, Duet WiFi will definitely drive the scara geometry you show.

    Here is how to configure it:

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