Internal Support Ribs

Although geodesic structures are inherently strong I wanted to strengthen the Hex/Pent panels around their periphery with a 2" wide strip of 1/2" ply. This would also make final assembly much easier (i.e. possible!) when it came to joining the panels together. The ribs also allow an inner skin to be fixed to the inside of the dome giving a nice contrast in geometry - outside you have 360 triangles - inside you have 55-odd hexagons and pentagons! More importantly the ribs offer something for the shutter-opening brackets/hinges/widgetry and motors to be bolted to, without going through the outer skin.

Each rib consists of a 2" wide strip of 1/2" ply. Both long edges must be cut at an angle unique to the triangle edge that it is supporting (I cheated here and fudged the angle at 4 degrees as the actual difference in this angle for all the different triangle edges only varies by about 0.6 of a degree. Life's waaaaay too short, so sod it!) I used a borrowed table saw for this job, cutting up around four sheets of 4'x8'x1/2" ply into 8' strips. The saw blade was set to 4 degrees off verticle.

The ends of each rib must be cut at TWO specific angles simultaneously if they are to fit together accurately and offer maximum support to the panel as a whole. A mitre saw, a drill-bit and a little trigenometry can overcome this minor headache reasonably well. Doing this by hand, however, was a major pain in the bum, so after struggling through about half of the ribs with the hand mitre saw I invested in a power mitre saw on which I could set the angle and tilt of the blade. The blade on this saw did not cut as neatly as the hand saw, but it made short work of the remaining ribs.

The ribs were then glued and nailed to the inside edges of the HexPent panels and clamped tight around their periphery with a ratchet strap while the glue cured. The ribs remove a lot of the flexability in the panels and add a lot of strength to each panel and the whole dome assembly.