Dome build: Getting serious.

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After what seems a ridiculous amount of time I have finally cut out four arcs of 3/8" [9mm] birch plywood to make two ribs. Two ribs and the cross struts makes a segment. One of sixteen. The vertical struts are not vital to the segment but will be cut anyway.

As usual, I started with a beam compass made from a stiff length of box section alloy tube. The pencil was fixed with two nuts. One above and the other below the beam and simply screwed onto the pencil by hand. The compass pivot point was a long screw fixed into a spare length of timber clamped to the 5x5' sheet of birch plywood.

For cutting the arcs I used a fine toothed, metal cutting blade in a jigsaw. The fine teeth reduce ragged edges and splintering to a minimum. No doubt taping over the cut line would help but I had no suitable, clear tape to spare. After sawing outside the arc lines I sanded the edges with an angle grinder with a coarse flap wheel. This is a remarkably quick and easy way to smooth the cut line.

I then set the overlong arcs up against the shed to check I had a full quarter dome. I could then cut off the bottom of the arc to match the floor using a bit of timber packing as a guide.

A plumbline gave me a vertical where I could measure up from a stiff, level batten supporting the squared off base. For scale, the level is 4' long. The rib is 162cm wide x 162cm high.


The arcs are overlong with considerable overlaps. As can be seen by the number of spring clamps I managed to use to hold each rib together. I haven't decided whether to half lap them like I did with the dome's base ring.  The rib plywood is quite a bit thinner than the base ring.

Then I drew the flats on the ribs for the vertical struts. Each line was 63.6cm long and the included angles between them 162°. By coincidence the height and width of the full rib is 162cm.

Click on any image for an enlargement.

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