20.03.2021 The dome is round.

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Saturday 20th 36-42F, overcast, light breeze and feeling rather cold. Misty rain.

First I levelled the dome skirt within a centimetre on wooden blocks using the 360° laser level. Then I measured the diameter at the skirt with my laser rangefinder. Using the flat base of the meter as a reference on the dome surface to [hopefully] point straight across a diameter.

No problem. It measured within a centimetre of 4.3m all round. The steel reinforcing arch helps to keep the doorway to the correct curve and to avoid sag. It will not be used once the observation slit is cut. Or it would cut across the field of view. 

Lifting the dome onto blocks can obviously cause some spring at the skirt. So I had gone around lifting it and lowering it again to take out any tension. At one point there was only a pair of opposed stacks of blocks across the "equator". The dome rocked stiffly without sag. There were several other blocks with a little clearance for safety. Just in case the dome fell off the loaded blocks.

Later I moved the spare [shutter material] segment over and hid it behind the dome. Using long boards on the ground makes segment movement quite manageable. I have already developed the knack. So it took only about ten minutes. I use an F-clamp with plastic feet to get a firm grip on the dome edge.

It rained very lightly but hardly enough to have it running down the dome. So not a real test of the weatherproofing. Sheltering inside the dome reminded me not to cut the observation slit until the shutters are completed. No point in making a hole in the roof until it can be quickly and safely closed!

I keep wondering how high to make the observation slit. I rarely look directly overhead but the slit could be made slightly more waterproof if it is past the zenith. Which would automatically provide a fall [slope] away from the zenith board. Anything less than reaching the zenith would make a fall towards the zenith board. With the risk of rain collecting and dripping in. It is true that the curvature at the top of the dome would provide a lateral fall away from the zenith.

I haven't felt any moisture on the inside of the dome yet. Despite the high humidity and low temperatures today. The ground is now soft, almost liquid mud. Made worse by my frequent wandering about and melting frost. It is lethal if you get into a slide by being off balance!

The reinforcing rib inside the dome skirt provides an opportunity to resist the base ring from lifting. Without needing mechanical fixing in the form of [the planned] lots of brackets. The upward thrust would hold the ring firmly in place. A second ring could then be added above the moulded rib. Providing a much deeper, double ring. A narrower ring, or series of blocks, could join both rings to make a very solid structure. 

The underside of the lowest ring runs on the support/rotation rollers. The top of one of the rings under-hangs the anti-lift devices. While the vertical, steering rollers need a smoothly circular ring to run against. 

I ought to arrange an accurate, judder free, radius bar for the router. To obtain smoothly curved, plywood rings [arcs]. The jigsaw does not produce truly round arcs. At least, not in my impatient hands. The radius bar for a router must be very stiff not to dig in or chatter. 

I had that problem with the last dome. My original radius bar was simply too flexible. So I made a triangular frame in the end. Too late to rescue the damage I had already caused by judder. I need a new, small diameter, router bit [cutter] too. Plywood rapidly dulls router bits. Even though most are carbide tipped these days. The price of bits varies enormously.

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