I wanted to build a greenhouse at home, and this site was a great inspiration for me to want to build a geodesic dome type. But I struggled with the concept of the hubs in a hub and strut building technique. I wanted to make my dome out of wood, and hubs seemed the best way to connect the wood. But no one seemed to make or sell anything generic that could be used and obviously the angles needed to be absolutely right. So I had a think about how to make my own and decided to make a test dome just to practise on. I decided to make my hubs out of 68mm drain pipe, and the struts were 32mm square softwood. I then decided to make two drilling jigs, one hexagon and one pentagon, which would fit around the drain pipe and present holes for me to drill at the correct spots around the drain pipe hubs. Of course this would also be a good practise.
But how to make the jig the right size to fit correctly? Well for that I''m glad I paid attention to my trigonometry at school.
So here is a little picture. Of course the point is to make a hexagonal frame that will perfectly fit around the pipe. Then if we dill directly through the middle of each section of the frame we will drill holes around the hub at perfect angles.
So referring to the image, d is the diameter of the pipe. This is the outside diameter - we need to take into account the thickness of the pipe because our frame fits around the outside. So in this case d is 68mm, so r (the radius) is 34mm. Next we have the angle theta in the triangle. Because this is a regular hexagon we know this angle is 30 degrees. From this we can work out h1 which is the inside measurement for the jig.
Tan 30 = h1 / r or to rearrange it h1 = tan(30) * 34. Therefore we get h1 = 19.63, so the inside length of the strut is 39.26mm.
This is actually enough information to make it, but sometimes it''s better to measure the longer length. This length can be determined by Tan 30 = h2 / ( r + w ) or h2 = tan(30) * ( 34 + 32 ). Thus h2 = 38.11mm, and the outside length is 76.21mm.
The cut angles at either end are of course 60 degrees. So we can mark out 6 pieces of wood, measure the outside or inside lengths (or both) to get the length and cut the ends appropriately. The jogs don''t need to be fantastically strong so PVA wood glue is sufficient to glue them. I coats the ends in PVA and placed them around a small piece of the pipe and then strapped the whole thing with one of those straps you can use for camping - so I could pull it tight together and it would hold it firmly.
When the glue has dried, you can just take the strap off and the pipe should then just slide in and out of the jig. If you cut and measured accurately you should be able to slide the hub in and out easily, but it should also be firm enough to stay in place during the drilling.
Of course the hexagon is similar but the angles are different. Of course in this case theta would be 36 degrees and the cut angles 54 degrees and you only need 5 frame pieces.
My advise would be to drill the frame pieces before assembling the frames, so the drill hole is right in the middle of the piece and is vertical. Other than that they are very simple to make. Here is one of my finished and drilled hubs
And the completed dome (prior to staining and covering.)
Now I can''t wait to make the full sized one! :)
[ Comments 8 ]
This is a great job!! i only have one question.. what is the angle at the end of each struts??
If you go to the dome calculation tools pages the angles are at the bottom of the page.
the pic of the finished dome ! withthe cat ! so niecly expresses the spirit of the contributars of this site .. love this page and I will certainly start building a greenhouse as soon as possible ! cheers Mel
It's a nice idea but the struts don't radiate at 60 degrees from the hex hubs, it's 58, 58, 63, 58, 58, 63. Not a big difference, just something to be aware of.
Another way to have hubs is to buy
1) SIMPSON GT6Z gazebo connector plates
(go to their website and lok at the "projects" section) Use these for the 6 way connections
2) For the 5 way connections use a STARPLATE kit (more than enough in one kit)
Both products are available thru building supply stores
Guys - this is a great source of help - thank you. Just started my first 3v dome - looking good.