Add Grip Fins to joiners #1783
Add Grip Fins to joiners #1783
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This is the version I've used to make grip fins for holding PTFE tubes for my 3d printer. They aren't the same shape as the ones in the Slant 3D video (for small holes, I think these might be better) and aren't "attachable" yet.
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The second version is preferable to the first, but the difference is trivial. In the first video, he makes fins protruding from the side of the hole. In the second version, he makes fins by cutting slots into the side of a smaller hole. The second version is far, far better because the resulting fins taper, distributing the flexing loads more evenly across the span of the fin. If the fins are constant thickness like in the first video, the flexing load gets concentrated at the base of each fin, which increases the likelihood of fatigue and breaking. Or in the case of PETG, the plastic doesn't break but deforms permanently, eliminating the friction-fit feature that was intended in the first place. Standard design guidance for flexing snap-connectors also advise that the base of the flexing part should be double the thickness of the tip, to reduce fatigue and the chance of failure. Therefore, I would abandon the first idea and focus on the second. |
Nice! I'll use that as the default. I assume optimal is around 2x and not "at least 2x"? With the size I was using, it's actually that the tip is twice the size it needs to be because a single extrusion did not hold well, so it needed to be at least 2x the line width (or, with arachne, 1.5x? I wrote 1.75x but I think it goes down to 75% line width so that also needs confirming). |
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The guidance I've seen is to taper the cross-section 2X from base to tip. It doesn't have to be exact. A higher ratio moves the stress toward the tip, and a lower ratio moves it toward the base. Often I've had to fudge it due to space constraints, going less than 2X. See https://fab.cba.mit.edu/classes/S62.12/people/vernelle.noel/Plastic_Snap_fit_design.pdf pages 8-9 -- that isn't the design guide I originally found, but it says basically the same thing. Some of my own designs where I've used this concept:
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What's the status of this? Is it going to move forward? |
Yes, just not my biggest priority right now, unless it's causing anybody problems. |
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I was just trying to clean up the PRs. Ideally there would be zero, but if you're planning to return to it then we can leave it for now. |
3 participants
Grip fins allow a cylindrical object (or even ovoid-cross-section) to be held tightly, but also removable and reusable.
My searching didn't find anybody else having made these in SCAD.
In this old Slant 3D video, he shows straight fins used. This is the one implemented so far.
In this newer Slant 3D video he uses a different shape of fin that I haven't implemented yet -- at one point I tried, but they don't work at small scale. Judging from the video it's theoretically as easy as taking the one I made and diffing it out.
This is the version I've used to make grip fins for holding PTFE tubes for my 3d printer. They aren't attachable yet and the docs aren't finished and the naming is dumb and the tests aren't in the right place, which is why it's draft.