V-Wheels for Mini Kossel (OpenBeam 1515 Extrusions)

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Kamil Jiwa

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Apr 23, 2020, 5:28:59 PM4/23/20
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Hello again. This is related to my post about layer shifting issues on my Mini Kossel. Something I'd like to try is replacing the linear guide rails with vertical rollers. However, the designs I've seen are all for 2020 extrusions. For example, Haydn Huntley's Kossel Plus and Kumu 3D use this carriage: https://www.thingiverse.com/thing:2183355.

I've also seen Johann Rocholl's prototype for a recirculating carriage using small Delrin balls at https://github.com/jcrocholl/recirculating. However, I'm hesitant to use it because the design doesn't seem finalized and doesn't include details about what size the balls should be.

Has anybody used v-rollers with 1515 extrusions? Any suggestions for a good design I can try? Thanks.

Kamil

Hamp856

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Apr 23, 2020, 6:03:16 PM4/23/20
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Kamil,

Looks like you have 15 mm linear rails. I would not replace those with clever rollers. In my opinion the rails are much better. If they have axial rotation about the rail, I would augment them with roller on each side while keeping the linear rail. This would give you great lateral stability. Just some thoughts.

Edward Simpson

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Apr 24, 2020, 1:10:02 PM4/24/20
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Kamil, layer shifting on Deltas is usually the result of the CPU missing stepps or a bad calibration, not mechanical issues with the rails. I'd start by trying DC42's Delta Calibration Wizard while tuning the printer, then see if the layer shift is still present. Next step would be to check the belt tensions, if that's ok then try putting some lubricant on the rails, search Ryan Carl and lubricant to find an old thread in here discussing that. And last thing to consider is if you're using an 8-bit CPU, they sometimes have trouble with the Delta Kinematics equations when pushed too much.

As to the V-wheels on 1515 extrusion, I haven't ever seen that done since the shift to 2020 extrusion or bigger happened very early on in Kossel/Delta development, so there aren't any stock designs for V-wheels on that extrusion size. That said, Hyden does have an alternate design concept (used in the Kumu design I think?) that runs the v-wheels on the corner of the extrusions instead of in the channels, advantage is it allows for using T-slot extrusion for frames instead of V-slot/openbuilds extrusion, but you do need non-solid v-wheels to make it work. Here's the Giithub for Kumu-3D, it looks like the printed parts were done with open-scad, so you should be able to re-dimension them to work with 1515 extrusion, that or scale the STLs down a bit and it should work. I will note that making that jump would amount to total rebuild since the corners need to be replaced as well to get that trick to work.

Haydn Huntley

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Apr 24, 2020, 4:36:07 PM4/24/20
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I owe credit for the idea of running V-wheels on the diagonals of extrusions to Arnold Garcia of Blue Eagle Labs, who designed the Kossel Clear.
It works quite well. Also, using 4 wheels per carriage and using both inside and outside carriage pieces makes the carriages remarkably stable, yet is still inexpensive.

The disadvantage is that it requires designing the corner pieces to rotate the vertical extrusions by 45 degrees, so that a corner faces the center, rather than a face.
It also requires using square extrusions.

Ryan Carlyle

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Apr 27, 2020, 2:56:26 PM4/27/20
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1515 printers with plastic corners are FLIMSY. There was a time in this group when we'd get a post every other week from people asking about their frame wobbling, or unable to print the first layer because none of their frame angles were quite right. Having linear rails bolted to the extrusions will hugely stiffen up a 1515 frame, although printed plastic corners are still a weak point. 

Minimum size you should be using for wheels on extrusion is 2020. I would personally go straight to 2040 for columns on most delta designs, but you can make a good printer with 2020. 

Haydn Huntley

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May 3, 2020, 1:46:57 PM5/3/20
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I used two printer with 1515 extrusions for a long time, and my fix for their initial wobblyness was to add a little bit of diagonal bracing.
Rectangles are inherently weak -- triangles are strong.
There is more than one way to do it.  Initially I used X-bracing, but it sometimes interfered with getting my hands inside the printer in a hurry, so I switched to A-bracing.
Here are pictures of two 1515 extrusion based printers:

    




I still use A-bracing on my newer 2020 extrusion based printers, because it makes them a lot stiffer.  https://github.com/haydnhuntley/kumu-3d

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