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Six-Arm Simpson

Posted by Annirak 
Six-Arm Simpson
August 05, 2013 03:31AM
I've been thinking about 6-dof bots and the Sextupteron. The toolhead rigidity in Sextupteron concerns me, so I've been thinking about other geometries.

What about a six-arm Simpson with a hexagonal toolhead? Actually, a hexagonal toolhead isn't ideal, it needs to have three long sides and three short sides to prevent twisting in the toolhead. This would still give a very wide work range, but it has the advantage that all six arms work together to bolster the rigidity, rather than having each axis's rigidity determined only by a pair of arms. Some might be concerned that Simpson's torsion springs will be a limiting factor on milling force (if used as a mill instead of a printer) , but remember that with six arms, three are pulling (driven) while three are pushing (un-driven).

Simpson's apex even makes a natural toolhead switching area.
Re: Six-Arm Simpson
August 05, 2013 04:55AM
Here's how a commercial CNC mill with 6-arms works:

Six-Arm Simpson would just be this inverted; the Annirak Drive instead of overhead arms.
Re: Six-Arm Simpson
August 07, 2013 07:06PM
I can't figure this out in my head but say we have a standard Maggie with 6 arms instead of 3. We change the hub to a hexagon with 6 ball joints (or other 3DOF joint) at each of the vertices. Will the hub be fully constrained?
Re: Six-Arm Simpson
August 08, 2013 09:09AM
It's not fully constrained if you have an equilateral hexagon. It is fully constrained if you have a non-equilateral hexagon. In that respect, it has a lot in common with the Stewart Platform. In fact, you could use the Stewart Platform as a model for a Six-arm Simpson

There are two ways to constrain it: either make the base non-equilateral, or make the hub non-equilateral, or, even better, both. The closer you get to triangular on either the base or the hub, the more rigid the hub's position becomes. You can see that in the video I linked, they made both the hub and the base non-equilateral. That is, the arms are in pairs on the base, but they are in opposite pairs on the hub.
Re: Six-Arm Simpson
August 08, 2013 12:55PM
Okay so I have 6 arms that connect in the alternating triangle pattern you mentions. What do I need for hub and shoulder joints? Do the bottom ones need to be 2DOF and the top ones need to be 3DOF? The Stewart platforms I have looked at have a u-joint on the bottom and u-joints on a swivel up top.

Separate Note: A Stewart platform usually has a very limited range of motion. I guess I can get around some of that by having actuators that have a larger range of motion than what they usually use. Does that track?
Re: Six-Arm Simpson
August 15, 2013 07:19PM
I believe spider / six arm machines are promising:

Also, this is inspiring:

Edited 1 time(s). Last edit at 08/17/2013 03:46PM by Buytaert.
Re: Six-Arm Simpson
August 15, 2013 07:26PM
That 5DOF machine in the last one is amazing. It is a cross between Simpson and Wally. The only thing I don't like is the one linear component. Thanks so much for the link!
Re: Six-Arm Simpson
August 15, 2013 07:36PM
I think their bulky design is limited by weight and super precision need. So there're rooms that you can improved.
Thank you!
Re: Six-Arm Simpson
August 16, 2013 08:58AM
It's a bit off topic, but how would you create toolpaths for such a device? Commercial 5-axis CAM software for CNC milling looks to run thousands of US$. I don't even know if there's a concept of 5-axis FDM to find software for.

I'm comfortable enough with the concept of 3-axis fdm that I could probably muddle through writing a slicer. I'm not sure where I'd start with 5DOF.
Re: Six-Arm Simpson
August 16, 2013 09:22AM
Also, and maybe I'm missing something here, but I think 5DOF is the most you can use in a device like this: 3DOF for position and 2DOF for orientation. Controlling tool axial rotation seems rather silly. Could you get away with 5 arms?
Re: Six-Arm Simpson
August 16, 2013 09:23AM
I have experience with the algorithms and the necessary math. However, at first I plan to make one-off gcode programs to demonstrate one advantage at a time which will hopefully drive others crazy until they write a general solution. :-)

Edit: 6DOF is for symmetry. Besides you can add a light duty gripper for assembly.

Edited 1 time(s). Last edit at 08/16/2013 09:25AM by nicholas.seward.
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