Day 2 of the dome fabrication weekend started with the construction of a jig for drilling the 2nd hole in each strut.
The distance between the bolt-holes in the ends of the struts is the most critical part of the project. The vertices of a geodesic dome must be of relatively precise length for the dome to properly approximate a sphere. If the vertices aren’t drilled correctly, not only does the dome look all wobbly, it loses the magical strength that geodesic domes are known for.
Following the instructions on desertdomes.com, I cut a V-notch in a length of 2×4 and then inserted a 7/16″ lag bolt near one end. Actually, the hardware store didn’t have 7/16″ lag bolts, so we bought a section of 7/16″ all-thread and cut it to length. This turned out to be quite fortuitous. Even though the hole in the wood was drilled to be tight, the all-thread still had perhaps 1/8″ of wobble, which would have ruined the precision we were going for. I clamped two pieces of scrap wood tightly around the extra length sticking out the bottom and screwed them down.
This extra “sandwich” of wood held the all-thread dead-steady through the whole process. If we had used a lag bolt, as called for by desertdomes, we wouldn’t have had the extra length to “sandwich”.
Here’s the finished “sandwich” of wood, holding the all-thread in place.
And here’s the top side of the jig. The protruding end of the all-thread has been tapered slightly on a bench grinder so that the holes that were drilled in the struts yesterday fit snugly over it. This will hold the strut in place while the hole is drilled in the other end, keeping the distance between the holes precise and consistent.
The jig is clamped in place so that the bit will drill into the strut at exactly the right length. After each set of struts is drilled, the jig is moved to the correct length for the next set of struts.
Before drilling the first hole of each set, we made a tiny pilot-mark and re-checked the measurements. We have about $600 invested in this EMT conduit, so we don’t want to screw up by drilling carelessly.
The first strut off the drill was double-checked for correct distance between the holes, and then designated as the “master” for the run. A bolt was set into one hole, and subsequent struts were laid onto the bolt when they came off the drill.
The distance between the holes was then compared to the “master” by sighting along the edge of a 6×6 support column of the barn.
The struts still had lots of sharp little metal bits sticking off from where they had been cut originally. Not only is this dangerous to the people occupying and building the dome, it can cut into the very-expensive dome cover. The cut ends of the struts were ground smooth.
… and then filed to remove any final burrs. Personally, I would have passed on the filing, but Keith thought it was important and seemed eager to do it, so who was I to stop him!
The last step in the fabrication is to bend the flat ends of the struts to the appropriate angle. The struts have to form a half-sphere, after all. For a 4v frequency dome, the strut ends must be bent to between 7 and 9 degrees. We used an inclinometer to bend the first pole to the right angle, and then made a mark on the leg of the table as a reference for all future bends.
As of now, we still have about 2/3 of the struts to bend, but we’re damn near done with the fabrication of the dome parts. All that remains is to purchase the hardware, such as the lag bolts and eye-bolts we’ll use to connect the struts to each other. We purchased the cover today: a white 60×60 tarp at around $350 plus $100 shipping! I’ll say this for domes: they’re not cheap.
We’ve scheduled a trial build for June 26. Given what an adventure the fabrication has been, we’ll see how the actual construction goes!
#1 by Clayton Gaar at August 2nd, 2010
Hey, Thanks for the great writeup! I just built a 3v 24′ dome out of 1/2″ conduit would like to build another one with 1″ conduit so that I can hang hammocks in it. Do you have any pics of the final dome erected? Im interested to see the final product and here how it went. Cheers
#2 by Joshua Bardwell at August 3rd, 2010
For your dome, did you go 3/8 or 5/8?
According to the Internet, with a 4v dome, 3/4″ conduit is sufficient to hang from the vertices. We haven’t tried it yet, though. How long are the struts on a 3v 24′? You might not need to go all the way up to 1″. Another approach I read about was using 3/4″ conduit with 1/2″ rebar on the inside as reinforcing. You put the rebar in before you squish the ends and then it’s just in there permanently. I’m not sure how that trades off in strength, size, weight, cost, and complexity of fabrication, compared to 1″ EMT, but I wanted to throw it out as an option.
Here’s a link to the constructed dome.
http://www.facebook.com/photo.php?pid=4273734&l=d850c4e106&id=526167355
#3 by JDB at March 22nd, 2011
Great blog posts. We are looking to build one similar. What do you use for the skin? Also can this be assembled without a ladder?
#4 by Joshua Bardwell at March 22nd, 2011
For the cover, we used white tarps. Getting a single tarp in a workable size was impossible (for example, sites listed 60×60 tarps for sale, but nobody actually had them in stock), so what we ended up doing was buying three 10×30 tarps to wrap around the base and one 30×30 to go over the top, like a “hat.” It worked pretty well. We had a hard time getting the 30×30 straight. What ended up working was having a bunch of people grab the edges and shake to make waves while others pulled on ropes attached to the corner. It was a windy day, and a few times the wind caught it like a sail and actually helped a lot, since it “flew” up off the dome and didn’t have much friction.
The only way to assemble a dome this size without a ladder would be to build it top-down. This is most commonly done with a crane, for very large (e.g. 50-foot, 100-foot) domes. That way, you hang the very top from the crane, lift it up five or six feet, put together the next layer, lift it up, etc… It’s wonderful, except that you need a crane. You can do top-down builds without a crane if you have a bunch of people who can lift the dome and hold it while other people assemble the layer. The weight is manageable if you have enough people–e.g. our dome weighs about 600-700 lbs. With ten or fifteen people, that wouldn’t be too much weight. But here’s the problem: if anybody drops their end, or if the dome becomes too out-of-level, you’re asking for a bent pole or, worst-case, for the dome to collapse. Also, all those people have to be holding up the dome for a long time while everybody else builds. Also, you have to find ten or fifteen people who are willing to help. All in all, I don’t think top-down makes any sense for a dome this size unless you have a crane. I did consider rigging up some kind of “jack” that could raise the dome up from the inside, but in the end, it just wasn’t worth it.
In other words, you need a ladder. Several ladders are better. With this dome, the vertex is 16 feet off the ground. We did it once with a ten foot ladder, making up the remaining distance by putting the ladder up on a big table. (No, OSHA did not approve.) The second time, we bought a 14-foot step ladder used off of Craigslist. It makes things much safer.
My suggestion would be to have at least two, and maybe three step-ladders. This allows multiple people to work at the same time. One person can be on the tallest ladder, assembling vertices, and other people can be coming along behind on lower ladders tightening down the vertices. You don’t want to tighten down the vertices on a layer until the layer is complete, but once you get the first three or four layers done, it actually really helps a lot for someone to come along behind and cinch down the first three layers. This forces the base into a proper circle, and means that you have a lot less forcing to do on the upper layers. Nothing like being 14 feet off the ground trying to shove a bunch of struts onto the end of a bolt. After that, you can keep working your way up, and the “cinchers” can follow along a layer or two behind you.
One ladder should be a 14-foot ladder for the top. If you have infinite money, buy a second 14-footer. The upper layers are reachable from a shorter ladder like a ten-footer, but it’s way more comfortable on a bigger ladder. If you don’t have infinite money, get an eight or ten-footer and a six-footer. The smaller ladders will be useful for the lower levels of the dome and will be easier to move around than the enormous 14-footer. It’s actually pretty hard for one person to move the 14-footer, so that makes it a two-person job even though only one person is actually assembling the dome. If you only have one 14-footer, there will be parts of the dome that only one person can get to, and so they will go twice as slow, as that person has to go around once to assemble the layer and again to tighten it down.
This all, of course, assumes you’re building a dome as big as ours. If yours was just a smidge smaller, say, a 12-footer or 14-footer, you could probably do the whole thing with more reasonable ladders.
You (or other readers) might be wondering why you can’t just climb on the dome as you assemble it. The reason is that the dome is actually not very strong until it is complete. Also, the struts we used are NOT strong enough to take a person’s weight. If you were very careful to step only on the vertices, maybe, but the problem is that the struts are long enough that you can’t really get from one vertex to another without stepping on a strut.