Dome building: With perfect hindsight.

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Monday: It is 66F, overcast and windy today so I am pottering on steadily. Today I added more 6mm [1/4"] screws, oversized washers and nuts to bring the tally to 9 per paired rib. Another screw will hold the upper ribs together when they are completed to bring the total to 10. 

The ribs are 9mm thick birch ply so become 18mm [3/4"] when bolted and bonded together. The vertical struts reinforce the ribs to make a remarkably strong framework. The horizontal ribs add stiffness by preventing the ribs from bending to left or right. 

By duplicating the ribs, the gores were entirely independent of each other. Building a dome with ribs alone relies on their accuracy and the covering "skin" for strength. Once started, the typical hemispherical dome is a full size model and most of its weight  already present. My [insane] method is more of a "belts and braces" approach. Though at the expense of much greater complexity and hand fitted parts throughout. 

However, my dome can be easily separated into gores, base ring and slit frame as desired for transport, detailed work or shelter from bad weather. The individual gores can also be easily managed by a single person. Even carried 'upstairs' for final assembly. The smallest completed dome requires a gang of willing helpers or a crane for lifting into place. My dome is still manageable [by a single person] until the ribs/gores are finally glued, bolted and sealed together, in place, upstairs, on top of the rotation wheels, 15' above the [made up] ground.

This image  from the inside shows the smaller gores on either side of the observation slit ribs. My choice of ribs + struts construction allowed me to use the ribs for guidance as to the spherical shape of the gores.

In retrospect I should have relied more heavily on identical vertical strut lengths instead of cutting each to size. That would have ensured greater accuracy of the individual gores. Instead of which I had to make the gores to match the next one along. Whenever I re-cut the horizontal struts [to unwanted close gaps] it would change the shape of the dome. Causing the top to sink or rise unexpectedly. The dome itself was so stiff it would not allow me to cheat anywhere.

The horizontal struts were a nightmare to cut because of the compound miters and length being so critical. When a row of horizontal struts was the wrong length they forced gaps between the plywood ribs at that level, above or below. To add to the misery, the plywood ribs were much too flexible to be fully trusted.

I had to cut the ribs across the width of the 4'x8' plywood sheets or the waste would have been horrendous. Not to mention the difficulty of cutting them "out of the solid." Again, with the perfect vision of  hindsight I should have used slightly thicker ply and cut the ribs to full length along the stiffer direction of the sheet.

I had hoped that the ribs would meet the slit frame before they ran out. They didn't. Not by 18" or so. Adding rib extensions has been a real struggle and is still far from over. I have not enjoyed this stage of the build at all.

With stiffer ribs I could have cut them all accurately and identically to shape including the "flats" for the facets.  If I was doing it again I would use a very stiff radius arm and plunge router to cut out the ribs along the sheet. The cost of the ribs would probably have been doubled and birch ply is not remotely a cheap material! Hundreds of pounds/dollars equivalent in extra material for the ribs alone!

An image of the dome drying after a heavy shower when treated with mineral powder wood protector. Cosmetically I could almost live with this "weathered" finish as it darkens further with exposure to the light. It still has to be properly sealed or it will leak like a sieve and is very likely to rot and fall to pieces. The discussion on a suitable sealer is ongoing.

I have spent weeks experimenting [and struggling] with different strut lengths and angles to reach this point. Only the miter saw has made this possible. The ability to shave small increments allowed me to succeed at the cost of wasted time and endless frustration. Then the table saw, with its vital DIY sled, allowed me to shape the panels to the actual dome skeleton. When in reality they should all have been identical on each tier!

Now I have the gores bolted together, but still not glued together, I can run an angle grinder over the inside of the rib pairs to match the curves to a single arc. The ribs were each roughly cut out using a handheld jig saw rather than a router on a stiff radius arm. Or even the bandsaw out of doors where there was room to swing a full rib arc.

Due to the covering panels there isn't room now for a router to trim back the inside arcs of the ribs to perfect uniformity. Though it could still be done by separating the gores. It made plenty of noise but I finally evened out and matched the inner arcs of the ribs. A 24 grit carbide disk in the angle grinder followed by a 36 coarse sanding disk and then an orbital sander to round the shoulders off.  No more sharp edges or protrusions to catch long telescope dewshields. The ribs will probably be used as handles for shoving the dome around. So rough edges and splinters are best avoided. Many domes have handles to aid rotation but they have to be found in the dark. Ribs are within reach everywhere.

Click on any image for an enlargement.

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