Corner Welding Jig for Pentagonal Dodecahedra (aka Dodecahedrons)

If you look at a dodecahedron, it's very easy to describe: it has 12 surfaces of which each has 5 vertices.

If it comes to making it, it's very hard due to the threedimensionality. If you just start with a pentagon, you don't know where the up going edges exactly meet.

To get all the angles correct, I create a jig that helps to weld one corner at a time. From there on, it's easy to create the whole thing. The idea is to start with a single vertice of a dodecahedron. All the other angles are a piece of cake.

For this instructable I worked on the concrete floor in my workshop. This because my welding table is a big dodecahedron jig already (so principally it doesn't make sense for me to create another jig). Also you can see that it's easy to make even on the floor. If you have a proper working and/or welding bench, you will get a more precise result than mine.

Yes, I know, I'm not the best welder there is. I'm an artist. Please be polite ;-)

Supplies

6 x flat iron bar - any dimension, the bigger the better, mine are 5 mm x 40 mm x 150 mm (1/5" x 1 2/3" x 6")

[3 x scrap metal - if you want it to last forever]

30 x iron rod (for the dodecahedron) - any dimension, mine are 8 mm x 20 cm (1/3" x 8")

Step 1: Create One Vertice of a Pentagon (2D)

The only thing we need to know for this step is the inner angle of the pentagon which is 108° (180-360/5). 2500 years ago, people where drowned for knowing this, so please be careful.

We weld two bars together at this angle, that's it.

If youre unsure, you can print out any pentagon and lay the bars along two sides.

Step 2: Optional: Harden the Jig to Last Forever

Since the bars are welded at one point only, your jig could break apart if used a lot. To make it stronger, we can simply apply some scrap metal to connect the two bars.

Step 3: Assemble Three Pentagon Vertices to the Dodecahedron Vertice (3D)

To shape the vertice of the dodecahedron, you have to assemble three of the pentagon vertices. I could tell you the angle (it's 116,...°), but usually that doesn't help much because you don't have a possibility to measure it while setting it up.

Since we have three pieces and we know that they have to line up, we can simply underlay some wood until the edges meet and then we set some welding points. After it's correctly aligned, we weld them on the outside.

If your flat irons are the same size (like mine), you can also align the pieces upside down to form a little pyramid. Since we don't want to weld inside our jig anyway, this is the method of choice. I used taped to help me while aligning. With the support of two heavy stoppers, it was an easy thing to find the right angle.

Et voilà, your jig is ready.

Step 4: Creating the Dodecahederon - Make 4 Vertices

Align 3 rods to meet in the middle of the jig and weld them together. Repeat until you have 4 vertices.

If you do it for the first time: don't be afraid if the rods don't align too exact - we're not building a rocket ;-)

If you are doing more dodecahedra, you will learn what is important to get an even better result.

Step 5: Assemble 2 Antenna Rings

By taking two of our vertices and one rod, we can build a pentagonal ring. The angles are all given by the three pieces.

We weld them together to form a simple ring with two antennas.

Step 6: Expand Them to Hedgehog Rings

Our Hedgehog has 5 spines going upwards. To find the direction they have to point, we use our jig.

Step 7: Close the Spikes on One Ring to Form the Crown Ring

To add another pentagon to the hedgehog ring, I just align two more rods. The angles are all given again.

Since we are welding only little and the material is thin, not everything is in perfect angle, we can adjust it once it's all welded together.

I have used the jig here to have a heavy weight on my crown ring. You could also use it to weld the two rods together first. But since they are bending inwards toward the welding point anyway, I do it right away.

Close all the upfacing pentagons. Now we have a crown or bowl.

Step 8: Put the Rings Together

Now we can stack the two rings on top of each other. If they don't fit perfectly, don't worry, wen can still bend them.

I used the jig to attach one of the vertices, the others I did free hand. After the 20th spot, the dodecahedron is finished for the first time.

Now you can work on it to get all angles right - I usually don't do it because I put mine somewhere in nature and they fit perfectly without being perfectly angled out.

Step 9: Close the Gaps

Up th now we welded on the inside of the vertices. To make the form more stable and finish the welding, we close the outlying gaps by welding them up.

After that you can go at it with an angle grinder or whatever you like.