Giant geodesic sphere from scrap

Principle 7: Design from patterns to details

The completed sphere made from scrap polypipe and recovered bolts
My friend Dylan spoke to me of making of making a sphere from polypipe, as a project for kids at the upcoming fete at his local primary school. He discovered the video below that explained how to do it, so we spent the day (my birthday) working on it. My best birthday yet!

We soon realised that the project was a bit beyond the abilities of a primary school student.

The key bits of information that we gleaned from this video was: using the soccer ball as a guide, the calculations for the lengths that we needed and the quantities of lengths.

A soccer ball (football) is made up of 20 hexagons and 12 polygons, all of the stitching is the same length. We called this length 'Normal' (N). The diameter of the sphere is about 5 x N. We wanted to make a 2m high sphere so N = 400mm. We needed 90 lengths of polypipe at 450mm, 25mm extra at each end to give us room to drill a hole. We wondered if we would need to include the star shapes within the polygons (P) and hexagons (H) as was done on the video clip for a large dome, so we thought that we would leave this for later. After all, the soccer ball didn't use them. We recorded the lengths of the 'stars' anyway, just in case.
N (400mm) x 90 lengths 20 hexagons and 12 polygons in a sphere on a soccer ball
6 triangles in a Hexagon, 5 in a Polygon.
H = 20 x 6 = 120. P = 12 x 5 = 60
P (400mm x 0.8696 = 348mm) x 60 lengths
H (400mm x 1.0224 = 409mm) x 120 lengths
Dylan cuts short lengths of discarded 25mm polypipe where it is kinked
A jig is set up to cut the polypipe to the correct length (N + 50mm = 400mm)
A jig is set up to drill a hole that is centred  and 25mm from the end of the pipe
Our jig for drilling the second hole of the polypipe at 400mm (N), with a coach bolt with it's top cut off to hold the pipe at one end.
Polygons and hexagons made up using the the soccer ball (football) as the guide
The construction got too complicated to work on the ground so we hung it up in a tree
The completed 'sphere', like a flat balloon without the triangle shapes for support
If we had thought about it more we would have realised that the sphere wouldn't hold it's shape without the triangle shape for support. A triangle is the most stable form, it holds its shape. So we needed to make up the stars to fill in the gaps.

We started with the hexagons, and with each one we added the sphere became more stable. It required some pressure to add 6 pieces of polypipe to a single bolt, which was fine when we were assembling the stars on the workbench. It was a much greater challenge when fixing them to the sphere. The joins needed to be on the ground so that we could stand on them, the job would be much easier with some sort of compression tool.

When calculating the length for the polygon stars I neglected to add the 50mm to our 348mm (P), so they were all too short. We continued with the H stars and found that the sphere held it's shape without the P stars, and also gave us access to the inside of the sphere. I'm sure that the sphere would be more stable with the extra support, but it's fine without it.

Six pieces of polypipe fixed together with a single bolt to make a six pointed 'star' -
Dylan fixing the first of 20 'stars' within the hexagons, each making 6 triangles that gives the sphere strength

The project took the entire day, and was quite exhausting. Great fun. We'd like to make another, using a home made tool to help compress the joins. Thinking about it we could probably use 3 pieces at twice the length (H) with a hole in the middle for the stars in the hexagons, which would make the job a bit easier. Amazing what you can make from bit's of other peoples waste, just for the fun of it.


dweeze (Johnno) said…
One of my neighbours did a half dome out of polypipe. Covered in bird net, it makes a brilliant vegie patch.

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