Posts made by Apsu

@fcwilt Joint slop is quite easy to take out with one of these types of arrangements, as preloading radial bearings is exactly the method used for doing so in machinery. There are many articles and data sheets discussing taking slop out of radial bearings by preloading, as well as different preload approaches (and when you might want to use angular contact or tapered rollers, etc) out there in the wild. Like https://www.nationalprecision.com/info-library/technical-data/bearing-preload/ or https://www.skf.com/binaries/pub12/Images/0901d1968065f1f4-Bearing-preload_tcm_12-299896.pdf for example.

Back to back and face to face are both very common setups for wheels and shafts, from bicycles to engines to skateboards. Getting the joints stiff is pretty easy. Designing the housings and connections and arms to handle the large lever and twisting forces is the real challenge.

@fcwilt said in Triperon Motion:

What is the raison d'etre for such machines?

There are very few machine designs -- for printers or otherwise -- that are linked in parallel, in 3 or more axes, and also linear kinematic mechanisms. Of those, the multipteron family are the simplest.

The general benefit of parallel machines is high dynamic stiffness without needing to move one actuator with another (serial linkage), which can compound the error of one axis with the other due to the coupling. The side effect is you can build a 3D printer with a fixed bed and all fixed motors.

@droftarts https://i.imgur.com/o2IsTWH.png is the original joint I settled on in my 2016 build. Double-shear, dual-back-to-back preloaded radial bearings (608), with steel washers used for compressing plastic between metal, and separating inner from outer bearing races.

Tapered rollers or angular contact sets have been suggested before, and I think they're a fine idea, but I think larger bearings (like the 6806s in Seward's latest videos) and using square tube (printed or otherwise) is a much bigger improvement over my old 2020 arm idea. Extrusion does not resist torsion force very well due to the small core diameter, and there's a large number of elbow angles where the force on one or more of the arm segments is largely torsional.

My most recent Tripteron prototype (https://www.youtube.com/watch?v=7LAmA4Q6-o4) build is using 2040 segments with single-shear joints, radial bearings on each end 40mm apart, and a thrust bearing inbetween. It's decently stiff, but I still think single-shear needs much larger bearings to improve the surface area over which the joint halves interact (stiffening the lever arm of the interface).

Seward's current joints in the video mockups are pairs of 6806s in single-shear with bolted in caps to secure them together, and it's a very reasonable approach. I also believe you could use a 6806 and 608 in combination with a similar setup and it'd be fine. The biggest question overall here is honestly joint 'wrist' geometry/cross-section, and the material/cross-section of the arm segments given their torsional and bending forces. Everyone focuses in on the joint itself but these other two points are much bigger in my experience. And I built dozens of these things to test

Anyhow, that's my $0.02

@o_lampe said in Triperon Motion:

@fma Right, designing these joints and then print them are two different shoes...

Sure, but these machines do work. It's not like they haven't been built and we're just guessing. Questions about specific rigidity/resonance targets are a separate thing, but one of the benefits of parallel kinematic systems is that they are redundantly constrained (not overconstrained). Meaning the rigidity and dynamic behavior isn't just a question of individual joints but the system as a whole.

Or to think of it another way, the whole is stiffer than the sum of its parts

@droftarts said in Triperon Motion:

I don’t think it’s a delta movement. If the angle of the Tripteron effector is 45 degrees, a horizontal movement is translated into a 1:1 movement in the corresponding axis. However, I don’t know for sure; @apsu or @oliof would be the ones to ask!

You are indeed correct that it is not delta movement. It is completely linear, and described by a linear matrix/system of linear equations. All Tripterons are (including my 2016 Delteron/Colinear design), and as such are uniquely different from SCARAs or Deltas.

And lastly, just to say it, the orthogonal Tripteron layout is also Cartesian.

@hackinistrator Correct. I didn't finish getting a more polished CAD mockup into diagram form, just was setting up the repo to support discussions I was having in realtime on Discord.

Here's a bit more complete rendering of the idea, with more of the gaps filled in and specific ideas about how to approach the interconnections and belt routing. Hope that's a little clearer.

@droftarts I think it's not necessarily the case that rigidity issues with the cross-carriages result in unwanted Z motion, but rather that it results in unwanted Z force. I.e., some of the connected pieces help constrain each other, and 'activate' the differential modes of each actuator. However, if you connect the outside actuator units together in the corners, so you have effectively one square Z unit on a bunch of Z guides, the constraint is very high, and it becomes much more difficult for those stray forces to result in unwanted differential motion.

But yes, the cross-connection is a great place to focus on getting rigid as it's in a nicely balanced position to help constrain and cancel the torques.

@droftarts I'm the creator of the Mir design, and have been delayed in finishing up the parts for my own prototype build, so I haven't shared print files yet, but I do intend to.

I'm happy to have a discussion about the design theory and build considerations here if desired, but most conversation does happen on Discord.

Is there anything about the theory of operation that isn't clear I could help clarify?