10.04.2021 Shutter width.

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Saturday 10th 30-50F, white frost, calm and sunny. Still no cladding plywood in the nearest DIY outlet. There is a discount from Sunday but still no stock. Still no sign of despatch of the router bits I ordered either. 

BTW: The Surform "plane" cut the GRP edges at first but seemed to blunt quite quickly. After only a minute or two it was no longer attacking the edge with the same aggression. More like scraping than cutting. 

I have a number of areas of the build where I would like to be working but feel hindered and rudderless.

The observatory building needs lots more cladding sheets. Should I compromise and use the 9mm 3-ply instead of the 5-ply 12mm? I'd really rather not. Strength is vital in a gale. The holding power of CSK screws on the 9mm plywood would be much lower with fewer plies to resist head penetration.

The 12mm, birch plywood, dome arcs should to be well under way but aren't. I'd still prefer the accuracy of the router on a trammel. To the wavy lines offered by the Bosch jigsaw. The plan was to rough out the arcs with the jigsaw to within a few millimetres. Then trim back to the line with a router. Nothing is happening with my present [blunt] router cutters. Without the arcs I can't proceed with the shutters.

The upside is that I get plenty of time to think through each step and rehearse it in my mind. New and better ideas often pop up that way. Perhaps I'm just thinking more slowly?  

I had a look at the shutters on the old dome:
Slit is 80cm wide. 90cm wide over both shutters.
40cm + [4x12mm] ribs = 45cm per shutter.
2x single thickness shutter ribs. Double thickness slit ribs.

New dome, slit width = 98cm.
Assume double thickness ribs for both shutters and slit.
6x12mm = 7.2cm. 49 + 7.2 = 56.2cm per shutter.
98+14.4 = 112.4cm for both shutters.
[Make some allowance for clearance and/or rib warping.]

Now I have a minimum width for the shutters. It would be a disaster to have cut them out and found them wanting. So I settled on 58cm width for each shutter. One can always trim off an excess. Gluing extra where there is a distinct shortage of vital width is rather more difficult. So I drew the cut lines at 58cm from the centre line. Then followed up with a permanent marker.

Which brought me right back to measuring the dome [again] and deciding not to decide how much overlap I need at the zenith. If I cut them too long the shutters may waste valuable material for closing the second, spherical triangle at the side of the doorway. The shutters are very likely to be very flexible until the stiffening ribs are added. Making their true length, in place, difficult to guess at.

A shutter overlap at the zenith is highly desirable to protect the drawer slides from the weather. An overlap can also protect the interior of the dome from wind blown rain and snow. One can never guarantee that the dome is left, turned towards the weather. Nor monitor for the unlikely.

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