To support the lower rim of the dome, lend it more rigidity, and provide a hard flat surface for the dome transport wheels, I cut out out 16 x 45 degree annular sectors of 3/4" marine ply that will be glued and screwed together to form a double thickness (1.5") ring. This Dome Ring has an outside diameter of 2500mm, exactly matching the outside radius of the dome itself, and is 150mm wide.
Another, double thickness ring was similarly cut and will be attached to the top edge of the octagonal observatory walls. This Wall Ring is 50mm smaller in diameter than the upper dome ring, allowing the dome ring to overhang the wall ring by one inch all round. The wall ring is also wider than the dome ring, at 330mm, so that the eight outside corners of the walls are aligned with the outer circumference of the ring and the inner corners with the inner circumference.
At a later stage a thin, 6" wide metal strip will be attached to the outer edge of the dome ring. This strip will hang down below the wall ring and prevent rain from being blown into the gap between the rings. A draft-excluder brush strip, as used on the bottom of doors, will also be attached to the outer edge of the wall ring with the brush pointing up and gently rubbing against the dome ring to keep out insects and dust.
Using graph paper and compass I experimented with how many sectors I could cut out of each sheet of ply. The best fit was achieved with two dome ring sectors and two wall ring sectors nested tightly together. My 10:1 scale drawing suggested that I should have 30mm to spare... however, my pencil lines and slightly springy compass meant that my error factor was a little over 30mm! Cutting the first sheet was going to be an act of faith!
10:1 scale drawing of how two dome rings and two wall rings
fit onto a single sheet of 4' x 8'
Eight sheets of 3/4" x 4' x 8' Hardwood Marine Ply are required to cut out the 32 sectors required for the two rings. For those who haven't had the joy of trying to handle this stuff, a single sheet is humongously heavy even for two men. Dragging this stuff around my yard and into the garage single-handedly for cutting nearly cost me my life on numerous occasions! My arms hurt. My back hurts. Everything hurts!!
Before cutting each sector, the two ends of the sectors were carefully defined so that they would be accurate 45 degree sectors. This was done using simple geometry... on a large scale!
I knocked up a long radius arm for the router out of a length of skirting board, using a screw as the pivot point. The plywood sheets are raised off the ground by some lengths of scrap chipboard to prevent damaging the cutting bit. Three passes with the router were necessary to cut all the way through the very hard 3/4" ply.
Slow progress... as you can see, there is precious little space between the cuts
every millimetre is precious!
And the proof of the pudding is in the cutting...
Full house - they all fit!!!