DIY 8m Diameter Conduit Geodesic Dome
Recently I built a couple of geodesic domes. A lot of people in the USA build domes out of cheaply available EMT conduit. This conduit is nowhere near as ubiquitous and cheap in the UK so I found a fairly similar standard galvanised pipe used in the steam industry and put out for quotes from a number of steel stockholders.
Domes are the strongest, lightest and most efficient means of enclosing space, they are also pretty fun looking.
The above model is interactive, click and drag to orbit around the geodesic dome.
In the most basic terms, the design of a geodesic dome begins with an icosahedron (20-side form) inscribed within a hypothetical sphere. Each triangular face of the icosahedron is subdivided into n2 similar triangular tiles, n being the chosen degree of subdivision. The small triangles’ vertices are then projected onto the sphere, carrying the arrangement of edges along with them.
Making the frame
The poles that make up the frame were all manufactured by hand in my garage, and to a lesser extent my living room. I used a plumbers pipe cutter (a heavy duty old cast iron one) to cut all 165 struts. This was hard work as the pipe has a relatively thick wall (3.1mm) but it leaves a nicer finish and doesn’t make too much noise or require me to buy a cutoff saw. For this dome I cut three different lengths of strut, I called these different lengths A, B and C for my reference.
Once I had cut all of the pipes I used a set of dies that I made from some offcuts of 1/2″ steel plate to press the ends of the pipes flat. The dies allowed me to create a smooth transition between the circular cross section and the flats on the pipe, reducing any possible stress concentration points. They were a pretty basic shape, here is a picture:
I used a 12 ton hydraulic press to flatten the ends. I needed a reasonably high press force because the transition between circular and flat happens over about 150mm of the pipes length. After making three domes I had to replace the hydraulic press, this could be because it was the cheapest one available, but I think I was also pushing the press to the limit somewhat. Here is a picture of the one I used.
After pressing both sides of the pipes I drilled holes in each end using a pillar drill and a jig that I made from a wooden board. I initially used some titanium coated drill bits that someone recomended but ended up switching to HSS and just frequently regrinding the bits. Drilling 330 12mm holes in 6mm steel takes a surprising amount of time.
Once the holes were drilled I put a slight bend on each of the flats to make the angle of the resultant vertex match the shape of the sphere that the dome eventually makes. I used the press to clamp the ends of 10 struts at a time and then bent them by hand using a spirit level app on my phone to check the resulting bend angle. The three different strut lengths need to be bent to different angles to achieve a perfect sphere.
Finally I used an angle grinder to smooth out the ends of the struts so that they would not be at risk of damaging the cover should there be any movement between the frame and the material.
Assembling the dome took two days. I think it could easily be done in a couple of hours, we had difficulties with the weather, it had rained pretty much constantly for the previous month and was raining when we put the dome up. The ladders would sink into the ground and become unstable. We also didn’t have enough tall step ladders. We have subsequently put up similar domes in half the time. Here are a few pictures of assembling the frame.
A few tips I would suggest. Don’t immediately tighten up all of the joints as you go, wait until you have a couple of layers up as this allows you to move the dome to make troublesome struts fit. Use nuts and bolts of differing materials and apply some grease before you do them up. The first time we put up the dome we used stainless bolts and nuts, as you tighten the joints you pull all of the struts together against considerable force; this can create a lot of heat in the threads. In some cases this caused galling where the threads welded together making the nuts impossible to undo. Using differing thread materials and grease would prevent this.
Making the cover
If you want your dome to look nice then you need a proper cover, here’s what we started with:
And here is what we ended up making!
A big improvement, however the bin bags were surprisingly good!