18.05.2021 Measuring the radius of the shutters' sloping shoulders.

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Tuesday 18th 51F, bright overcast. Some sunshine expected.

My next task is to confirm the vertical radius of the "lower" side of the shutters. This is not a simple case of holding up an arc to the edge. Because the shoulder is quite steeply sloping. The GRP shutter material having been cut from an extra, spherical, dome segment. 

I need one shutter to be evenly supported on the central "high" side. With the shutter following the curve of the dome. Then I can measure the radius with the test template vertical.

I'll probably have to make some smaller and lighter template arcs from disposable plywood scraps. To measure the shutter's radius while the plywood is vertical. NOT perpendicular to the shutter's sloping shoulder. Any error in the outer rib's radius will twist the whole shutter. The top edge of the outer shutter ribs will need to be bevelled with a suitable router bit to match the shutters' lateral curvature.

Thinking about it again it is obvious that the outer shutter rib will have the same radius as the dome plus its clearance above the dome. So 2.15m + 10cm = 2.25m. Exactly the same radius as the inner, shutter ribs. The shutters are flexible enough to faithfully follow the ribs for curvature. The angle of the top edge of the outer rib is formed by the tangent of the dome's curvature at that point. Easily checked with a level and mitre gauge. Or, much easier, my level with a rotatable protractor bubble. 10° will do nicely.

It would have been so much easier to have made normal, thin plywood, bi-parting shutters. They would have been much lighter and straightforward to build. The temptation to use matching fibreglass in the same colour, texture and curvature, as the dome, was too much for me to resist. I just hope the final result will have been worth it. 

I keep trying to make the job more complicated than it is. The sphericity is the clue. The shutter strips match the dome curvature exactly. They just need to be raised high enough for clearance and to provide a seal between parallel ribs. 

Talking of which: I may still have to make a plywood weather strip to cover the centres of the closed shutters. This is fixed to one shutter and overlaps the other when they are both closed. The rain runs preferentially down the strip. Rather than trying to enter the inevitable gap between the central ribs.

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