8.10.2021 Chopping off the slit ribs.

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Friday 8th 51F, thick mist again. 

What to do next? I have slept on the problem without any new ideas popping up. The forecast is for brighter weather when the mist clears. Which will probably be mid afternoon.

I set up the 360° laser and checked that the base ring was still dead flat. It was. I set up a 4' level to confirm the laser was accurate and vice versa. Then marked the bottoms of the slit ribs to match the top of the base ring. 

The ribs obviously could not extend down into the path of the rollers. So it was safe to match them to the height of the base ring on which they would eventually rest. 

I then released the temporary rib props and was thankful that the dome did not sag. The unsupported, slit ribs did not move even a tiny fraction against the laser line. I could then use the multi-tool with a semicircular blade. To cut off the carefully marked ribs. Measure three times, cut only once and then do it slowly.

A very noisy and slow procedure in 24mm Baltic birch ply it was too. I had clamped battens below the cuts to give me accurate lines to follow. The result was satisfyingly straight and level. Which is precisely [sic] why I didn't use any other kind of saw. Then it was lunch time.

Next I have to join up the remaining arcs of the base ring under the slit and shutters. Once the dome is completed the base ring will be evenly supported by fourteen 7" Ø rollers. These will be spaced on a 4m Ø ring at about 80cm apart. The triple laminated, base ring will be bolted to the dome in over 30 places. So everything should be nicely level and stiff. Ensuring free dome rotation. 

I can no longer remember the weight of the plywood dome. Though I imagine the GRP one will be heavier. My friction wheel drive will hopefully be well up to the task. Getting a bigger and heavier dome moving from a standstill is still a slight worry. Assuming 500kg all up weight, that's half a metric ton. 

Low rolling resistance is assured by lots of large, hard nylon, industrial rollers with needle roller bearings. The underside of the plywood base ring is hard, flat and smooth. I can easily gear down my chain drive to the friction roller if needed. The plywood dome can really belt around if I put a bit of effort into the drive crank. So there should be no problem rotating the new dome.

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