Plush Tetrapod

Tetrapods are pretty neat. Here is a (simplified) tetrapod, modelled using OpenSCAD. You can view/edit it in OpenSCAD Playground:

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$fs = 0.01;
$length = 10;
$radius_bottom = 4;
$radius_top = 2;
$angle = acos(-1/3);
union() {
    for(rx = [
        [0,0,0],
        [0,$angle,0],
        [0,$angle,120],
        [0, $angle, -120]
    ]) {
        rotate(rx)
            translate([0,0,$length/2])
                cylinder(h=$length, r1=$radius_bottom, r2=$radius_top, center=true);
    }
}

While at 38C3 I had the idea for a plush tetrapod So I prototyped one with felt, assembled with a mix of hand and machine sewing.

As as there is nothing new under the sun, I was able to find some prior art and a pattern from Almar Joling.

This comprises of two distinct shapes, which I’m calling the “tube” and the “end” shape, both of which form a “leg” of the tetrapod. 4 legs, one tetrapod.

I made two alterations to the pattern:

• I found (by a failed first attempt) that the size of the circular “end” wasn’t enough to allow hand stitching of it into the end of the “tube”. I found that, for the big variant of the pattern (page one of the pdf) a circle of diameter ~68mm worked fine.¹
• I added some extra length to allow for stitching on the straight edges of the “tube” piece.

The pattern did not come with any suggested order however, so here’s very haphazardly what I did:

1. I started by machine sewing the 4 “tube” parts together along their 3 “batwing” edges. This is probably the most tricky part to think about - sufficiently so that my tired brain is struggling to do so now without the physical bits in front of me. It’s easier to reason about physically, I promise. Importantly, leave one of these seams unsewn - you need an opening to invert the tetrapod later.
2. Machine sew the “tubes”’ straight edges together to form, well, tubes. You can easily still turn the partly assembled bits inside out to do this.
3. Hand sew the “end” parts into the ends of the tubes. You can still easily turn all the tubes inside out through the opening you left in step 1.
4. Hand stitch any loose or dodgy bits - I needed to do this mainly at the points where the 3 tube pieces meet.
5. Stuff it with something - I used generic synthetic wadding fluff. It needs more than you think!
6. Close up the opening you left in step 1.

I’d like to make this a better guide at some point - before then, if you want to make your own and have any questions feel free to reach out!