Under Construction

The Materials listed above are for a 14 ½ foot Dome.

And are only an approximation of what is required, efficient use of materials and fewer mistakes will reduce the required amount. You will need to visit here dome calc for the exact dimensions and a printout of the arrangement of the 130 triangles needed to build this, all dimensions are calculated on whatever system you prefer to work with metric of imperial, I tend to use both. Being of the old school. The resulting dimensions of the triangle , and its important to note are the radius of the dome, from the apex of each triangle, and not the flat section between.

The Dome Ring

The first step is to build the Ring that supports the dome structure on its castors, as this is supported in 8 places and has the geodesic fixed on the other side, It need not be too heavy, and indeed will be quite flexible initially. I went for a laminated ring of 3 layers of 20mm Ply.

Make yourself a large compass out of long length of light timber a nail in one end and a hole for a pencil at the radius of you dome plus say 5mm extra for a bit of a lip to work with and another hole for a pencil 105 mm closer to the nail end, I made my two lower rings in the lamination 105 mm deep.

Step 1
Lay a couple of sheets on the ground perpendicular to each other and mark off 105 mm increments on you nail end sheet, and use these to scribe in the pencil end sheet, , An outer scribe line and an inner scribe line for each increment on the nail end of the compass. You will get around 8 or 9 arcs out of a sheet.
Cut out all the Arcs with a jigsaw out of one sheet only.
The tapered Arcs that are left over, put in a pile, these are very handy later, as are the semi triangles out of the corners of the sheet.

Step 2
Use the same compass to mark out a circle on the ground at the outer diameter of the ring, and arrange the arcs in a way that no two joins are near each other. Mitre the but joins at an angle of say 45 degrees pre drill and countersink holes at 300 centres staggering from inside to outside as you go with extra fixings at joins. Witness all the pieces so you know where they came from, and disassemble. Reassemble with a generous coating of construction adhesive, and screw off.

Now you have the first two layers of the dome ring.
The third layer is slightly smaller in outside diameter, as I mentioned earlier the radius of the dome is calculated on Apex to Apex of the individual triangle, scribing in a 24 or so faceted ring is not necessary and too difficult so we make another round one slightly smaller to accommodate the flat sides of the bottom layer of triangles, Now this radius can be calculated by math, but if your not up to it, skip ahead to making triangles and then come back here to use the first layer of triangles as a guide to calculating this radius. My radius worked out at 24mm less than the overall radius.

The Triangles

Depending on you size and choice of Dome, 3V 4V 10 foot 20 foot, the resulting quantity and size of the triangles will obviously vary, for mine 130 triangles were needed 6 different types. At this stage we are building an entire dome the slit will be cut in later. This approach ensures a faithful shape.

Whilst 130 triangles sound like a lot of work, when you do them five at a time it does not take long to have several piles of triangles each labelled and sides identified by letters or numbers.

Step 4
Lay the sheets of 4.5mm ply five deep on dunnage, mark out your triangles on the top sheet, trying to maximise sheet use. The use of a couple of clamps on the sheets will minimise drifting as you cut, I used a 9 ¼ Circular for this job, Safety glasses are a must plywood throws splinters everywhere. Accuracy within a couple of mm is fine, geodesics are quite forgiving when constructed this way, Its important to Err on the too small side rather than the too large side,

Don't forget to identify all the triangles and the orientation, as most are NOT equilateral. You need to be able to grab the right triangle when you need it and know which way round it goes.

Step 5
Arrange the first layer of triangles around the Base ring and check that the correct amount type and orientation fit without gaps or overlaps around the circumference of the ring if you've used maths to calculate the top rings outer diameter, If you didn't use math you are going lay the triangles on the ring and measure the top ring outer radius and make it to suit, so its back to step 3 for you. :>)

All things being equal you should be able to make them fit pretty good without to much fuss, Index them now with a mark on the base ring, this will help while fixing. If they are way out, double check orientation and type its easy to make a mistake, and a mistake on the first few layers will be disastrous. Once your happy, and got a friend a to check as well. Apply a liberal dose of construction adhesive along the lower edge of the triangles one at a time and fix with one self tapping trumpet head screw into the top lamination of the base ring leaning the triangle inwards at around 10 degrees. Countersinking or predrilling not necessary. Work your way around the circumference until your back where you started, again try not to put any joins between triangles directly over joins in the dome ring lamination assembly, Check your index marks.

First half of Layer One Done.

Step 6
PVC Moulding
Important Note, If you using say 4.5mm ply don't try to use 4.5 mm moulding due to the slight angle that triangles meet at , you will find it extremely difficult to assemble and also the tight fit displaces the glue you will apply to both sides of the moulding, and that defeats the point of gluing it in the first place…Go ½ mm larger.

Cut mouldings for the two upward pointing sides of the triangles you have fixed to the base ring, metering of the joins is not necessary square cut a little short of the corners is fine. Apply a bead of construction adhesive down ONE side only "the side that's on the fixed triangles of course" and press all of the mouldings onto the triangles. Leave to cure for a few hours, not completely cured but not fresh. Whilst waiting arrange the other half of the first layer Apex down in the vees where they are supposed to go, and check type and orientation, contrary to observation here a mistake my not necessarily be obvious.

Step 7
Apply a bead of adhesive down both exposed groves in the mouldings one triangle at a time and press triangle down into moulding, the corners should meet up reasonably well, "Do not be Tempted to trim a triangle" the error you introduce will haunt you for the remainder of the project. Check Type and orientation "Twice" ….
If the triangle tries to ride up out of its place just drill a small hole thru the moulding and the ply and slide a small brad into the hole to hold it down while the glue sets, in the corners is effective sometimes in both triangles if you get a stubborn one. You can remove these with your fingers later. This drilling holes and popping a nail in will happen a lot. A small tack or brad in a 1mm hole is more than adequate…
You will find that as you work around the ring the second lot of triangles will force the first into shape, "AS LONG AS YOU DON'T TRIM ANY".

Step 3
Make another hole in your compass for a pencil at the slightly smaller outside radius but the same inside radius as mentioned above. And mark out enough rings to go one more layer, Pre drill , mitre and stagger joints as before, glue and screw off.

If your building this in a place where it will be difficult to move later, you can put breakpoints in the laminations, for temporary dismantle for relocation, as I had to. See pictures.

That it Dome Ring Finished.

Put the compass away safely you will need it for the shutter rails. :>)

It's a good Idea to let the first layer set properly before continuing on. As the next layer is done slightly differently.

Cut all the moulding for layer 2 in preparation glue one side and fix to top of first layer, you will notice that the top of the first layer undulates around the ring, this is the nature of a geodesic and should be that way.

Also cut all the moulding for both sides of every triangle on the second layer and arrange them where they will be needed.

Step 8

The Second and subsequent layers are a little more challenging in that they do not have a screw in them to hold them up, they rely on each other for support, and therefore must be fixed 2 at a time, a spare set of hands here goes a long way to maintaining your sanity. This is where the drilling of small holes at the apexes and the little nails are extremely helpful if you don't have 6 hands.

Check your first two triangles for type and orientation, One apex up the other apex down glue both sides of a piece of moulding and the tops of the two the triangles where they will sit, and slide the lot together, support the triangle with its apex down whilst the third along is slipped in with its apex up, moulding glued both sides and in place and glue in the top moulding of the first layer. If necessary hole and nail where needed, but they should be self supporting. That's the tricky bit. The rest is straight forward, pointy but down pointy bit up two at a time, they support each other, holes and nails for additional holding where necessary. Work around till your back where you started. The undulation will now be more apparent and some sagging at out of round may also be apparent, don't worry, the next layer corrects the previous, DO NOT TRIM,

Important Note
There will be occasions where you think the triangle that goes in here will not even be close to fitting, but if it's the right one then you have a sag somewhere in that layer or an out of round that requires force and nails to pull into shape.
As you work up the dome layer after layer, using the same method the correction made by each subsequent layer may need more and more persuasion, the Off cut tapered arcs I suggested you keep can be very handy, by screwing them to the inside of the dome with trumpet heads to hold it up as you work. As the radius of these arcs is pretty close to what the inside radius of your dome should be, The closer you get to the top the stronger the structure gets, and the nails and temporary supports can be removed. Again DO NOT TRIM to make a fit…

A Layer a day is a good target, If you try to work faster the glue does not hold the previous layer together well enough to apply stress to. Longer than a day and the glue sets too hard and is difficult to manipulate.

Hope your following so far. :>)

Step 10
Fibre Glass
Glass Mat and resin are expensive, so we do not need to glass where the slit is. Deciding where to put the slit in relation to the geometry of the dome is not really important, however if you study the shape and consider where the shutter will slide up and over you will notice there is a flatter area in the geometry that allows for a lower profile on the shutter itself. So putting the slit opposite this is most practical and geometrically pleasing to the eye.

Roughly mark out this area allowing for a generous over the meridian for the aperture of the scope it will house. Think ahead, I made mine 36 inches, "yeah dream on" LOL, there is no need to waste mat and resin in this area.

Hint
Heres a few more things that will make this job easier
A box of a hundred latex gloves
A Really sharp pair of shears
Lots of clean containers "Disposable"
A litre of Acetone
A Resin Roller.

Another hint
Resin Rollers are a pricey little item for something you will probably never need again. You can make one out of an old 2 inch paint roller with the roller smashed off and replaced with alternately sized washers held on with a nut. All the same hole size just alternating outside diameters, they work just as well as the real McCoy.

Tip

Firstly put on five layers off latex gloves, Its easier just to shed a layer every now and again rather than struggle putting on a new set without getting covered in resin.

How much Resin at a time, ?.

I found that a two square meter patch was pushing my luck to do a tidy job quick enough before the resin started to go off, and that was mixed at between 1 to 2% catalyst, just a few drops in a litre of resin or so it seemed. Shallow trays rather than pots also keep the heat from the catalytic effect from making it go of faster. "Resin can combust if mixed and left in a deep pot, One of my over mixes smouldered away quite nicely. Start with one square and see how you get on,

Caution
"Catalyst" ie: Organic Peroxides are dangerous, read safety precautions…

Method,
Thoroughly mix and quickly paint the area with a thick coat , quickly apply the mat with a good overlap onto the base ring or previously laid mat. Use your roller to press the mat against the ply, as the mat starts to melt paint over thin spots where air is caught or there is insufficient resin, and roll thoroughly paying attention to the moulding joins.

No doubt this is not the ideal way and professionals would approach it differently but this was my first go at it, with no instruction given, and I'm happy with the result. Took three afternoons to completely glass the dome complete with small round patches around 200mm in diameter on all the apex joins on the inside as well.

Step 11
Marking out and cutting the Slit.

This is where a laser level that can project a vertical line is really a must have, I have no idea how it could be marked out otherwise.

After deciding on the width of the slit you need to project a vertical line up one side and partially over the top offset by half the width of the slit, the two lines each side of centre must be perpendicular to the dome ring and more importantly parallel , so I put a piece of wood across the dome far enough back away from where the slit would be to project the lines, marked the timber to the width of the slit each side of centre
Marked the Dome where the line should be each side of centre, followed the laser lines with a pencil, I went from 600 above the dome ring to 400 past straight up and horizontal between the ends of these two lines.

Consider here that the top portion of the shutter must have enough room to slide completely back before running out of travel, hence a two part shutter, the bottom part would rarely be lowered anyway as its window is too close to the horizon.

Check for parallel and cut out with a jigsaw.

Step 12
Reinforcing the Slit and general assembly.

Now that a sizable chunk of the dome has been removed the inherent strength in a geodesic is severely compromised, Merely framing up a window for the shutter is not good enough. Although this is the next step.

Back to our oversized compass and another sheet of 20mm Ply. We need two quarter circumference arcs approximately 50mm greater radius than the dome and 2 more slightly larger than the dome ring top lamination. Cut these in the same way as the dome ring itself , the two larger rings will be for the shutter rails, so cut these as close to a true curve as you can, the two smaller rings will have the same inside diameter as the larger ones, but the outside will be scribed to fit nicely on the inside of the dome, ie: many facets. Get someone to hold these in place while you mark and the cut with jigsaw. Laminate and screw the two together the same way as before, trim to length. Leave to fully cure..

Cut a bottom sill and a top sill to the length required to push the rings firmly into the corners of the cut-out, plus a couple of extras at the required length to spread the opening to parallel while the glue dries. A good bead of glue on the outside of the inner ring and screw at 300 centres from the outside of the dome in, making sure that the spreaders are holding the slit parallel. And glue is oozing out of the join along the whole length of the rail.

Use Glass Resin and Tape on the outside over the screw heads across the join between the dome and the guides and over the top of guide rail and around the top and bottom sill.

Leave the Spreaders in for the time being as the sides will still want to collapse inward a little, to stop this I found that re-enforcing the bottom of the Dome slit did the trick as this is where all the stresses caused by the missing portion are focussed.
Building in a set of handles to transfer the stress down onto the dome ring, was a simple solution.

Glass mat and resin has been applied to this area on the inside as well.

Further re-enforcing

Now the only thing holding the Dome onto the dome ring is glue and a handful of screws, lets not overlook that. Here is the finished Item. An appropriately sized disk cut from the wasted corners of the dome ring sheets of 20mm ply has been glued and driven into the pockets between the first layer of triangles and the top lamination. Then a large stainless screw is screwed in from the outside through the disc into the centre lamination, all glassed over from the inside as well.

Step 9

The Last triangle, We don't even put that one in for two reasons.
1 You cant
2 Its where the slit will be

But now the dome will be rigid enough to climb all over.

The Strength of a geodesic is in its form therefore i recommend the slit be cut in after glassing, as the dome will be temporarily and severely weakened by removing a large portion. You can see that this is not exactly how I did it, It would have been better if I had. I cut the slit in after the first 1 meter of glass was applied.

This is a very brief run down of the procedures I used to construct a Geodesic Observatory Dome, I am happy to fill in blanks should antbody require further information, However I will not give out me email address for spammers to find. Any questions can be posted here, registration is easy and there are many other benefits. See for yourself.

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