3D printed rim Joiner

 

The Platonic Solids, image courtesy of https://commons.wikimedia.org/wiki/File:Platonic_solids.jpg

Setup of joiner for cube - see line 2 of diagram below

Cube (table without tabletop) with one side missing. This is sturdy enough but it rocks a bit. Any bigger and it wouldn't fit through the door!

I thought this would be ok but it was a bit flimsy

Tetrahedron with now very rusty steel joiners

New joiners with internal washer spacers set up for cubes. This was not the final design for the bolts. The final design is in the pic below.

Joiner schematic

Some time ago now, I started experimenting with joining circular waste objects together to make mathematical objects like Platonic solids. The principle is that, aided and abetted by 3d printing or other joiners, circles can be made to act like regular polygons (squares, equilateral triangles, regular pentagons) in models and other constructions. Initial work started with beer cans and cds where joiners form a free-to-move hinge between the circular faces. Eventually I managed to publish a maths education paper on the subject.

Bike wheels are circular objects too, and for several years I've been using them in sculptures, however when this article was written, the joiners I'd used were timber or steel.

All that changed a few weeks ago, after I had gathered the time and energy to complete a bike rim icosahedron.  Although the build was successful, the parts had taken a considerable amount of work (sourcing, cutting, drilling, deburring, painting) to complete, and the results still weren't safe for my grandkids to play with, so it was back to the drawing board, or  rather, back to the laptop PC and 3d printer.

 A 3d printed part that would do the same thing as the steel icosahedron joiner could have lots of advantages compared to the steel parts:

* Able to make one part that's versatile for different platonic solids and prisms without extra effort

* Rustproof.

* Could most commonly be made from strong PLA+ material

* No need for deburring as 3d printing is with a 0.6 nozzle and edges are not sharp.

* Able to be resized during printing so steel or plastic or other pipe fits inside. This leads to greater strength and the possibility a pipe connecting 2 joiners together as per the photo below.

Construction showing plastic pipe used in a tetrahedron

However on the other side, 

* May not be as strong as steel, so care needs to be taken with assembly, and it is recommended to use rims without spokes or hubs in the structures. This is because full bike wheels may be too heavy for the 3d printed joiners.

* Resistance to UV rays is really not known. 

* Resistance to heat, ie 45C ambient temperatures is really not known.

* Extra care needs to be taken with building the full icosahedron, as without careful extra support, the structure weight may overwhelm the 3d printed joiners. To get around this, try building a yurt! In this case, it  is an icosahedron with 5 sides lopped off as shown below. The 5 rims touching the ground can be pegged to the ground for extra stability.

Just add a cover and you have a yurt or cubbyhouse!

For builds with this part, I started on a cube, and then built the icosahedron cubby house. Along the way, I refined the fasteners used for the construction, so that the bolt ends are inside the joiners. This makes the construction safer. The idea of a printed pad for the washers in the assembly worked well right from the start. 

Here is the list of parts needed for each join (see photos below) . This assumes you are using a rim which is like the ones I usually use, a 27" steel rim which is not hollow and has a low profile. High profile, aero 700c rims might need longer bolts.

3d printed rim joiner                   1

3d printed washer spacer             2

6mm hex head screw, 30mm       2

6mm washer approx 17mm od    4

6mm nyloc nut                              2

6mm spring washer                     2

 

 

The next shape I plan to build is an octohedron in the front yard.

Regards Steve Nurse

In this photo, the bolts are positioned about 138 degrees apart. This angle is set in the way the joiner is made, and matches the icosahedron angle and bolt positions shown in the table above

 Update Jan 27, 2025

I have put both the joiner and washer pad up on thingiverse as downloadable files. Here is the link, https://www.thingiverse.com/thing:6927455 

Regards Steve Nurse

2024 Icosahedron

 

Icosahedron at its current site

On the road

Et voila

Next step

Work in progress

Building in front yard

Drilling jig, drilled joiner, and painted joiner the single thru bolt.

Deburring inside using lathe

Dodecahedron made with the same techniques

Bringing home the wheels

Dumpster at a local environment park's bikeshed

Late last year I attempted to make an icosahedron sculpture from old bike wheels and failed but was not put off! I had tried to make joiners from cheap, weak wood, and also did not bother despokifying the wheels I was using. All this combined to make the thing collapse under its own weight.

So this year, when I was ordering some cut steel tube for shelving, I also ordered some tubes for icosahedron building. As well as getting the steel cut, the parts had to be drilled, but were also deburred, degreased, dried, primed and painted. The drilling was at the dihedral angle for the icosahedron of 138.2 degrees, and I 3d printed a jig that let me mark out and then drill the holes. Quite early on I realised a shortcut was possible and used a single bolt (instead of 2) in each of the 30 joins between rims.

When all the parts were ready including wheels with spokes removed, I started in, and managed to build an icosahedron in my front yard before Christmas (complete with lights and dangly things and windmill) . Then I transferred it to M's house in time for Christmas lunch. For short distances it can be busted into 3 rings of 5 with 5 loose wheels, and the rings fit (rather conspicuously) on the top of my small car. Not great for longer distances though!

With this success under my belt, I changed the icosahedron into a smaller "glamping" hut which seems to be quite good and could be slept in.  My grandkids were amused by the structure, but the general consensus was that the joiners were still too sharp for kids to play with it as a cubby house. It was back to the drawing board again but this time it wouldn't take a year!  I've been gathering bike wheels again, and a new design is almost ready! More in next post.