14.10.2021 Mounting slides on curved shutters:

 *

Thursday 14th 37F a cloudy day. 

I was too busy elsewhere to do much on the dome yesterday. Though I did make a 2"x6" board to fit one of the shutters. It was immediately obvious that I was completely off track. [sic] 

Any overlap of the board on the slit ribs would need the slit ribs notched to match. The drawer slide thickness would be added to the depth of the notch. So I need to keep the front profile but make the board much thinner in the horizontal plane. Which raises the serious issue of the freedom of the drawer slides on the shutters.

Now I have made a simple drawing to get my head around the problem of siting the bottom drawer slides. For simplicity the shutters are shown as tapered across their width. Rather than curved. [Which are much harder to draw with simple software.] 

The problem with the real shutters is rotation over the length of the shutter. There is nothing for the eye to fix on. Two highly asymmetric ribs are joined by a heavy, but flexible, GRP cover. There is no sense of straight or parallel.

What is clear, is that the slides must be brought well inboard [with curved shutters] compared with [normal] straight [rectangular] shutters. The slides must clear the slit ribs and remain within the highly asymmetric shutter footprint. This ensures a full width mounting board can be used for strength and stability. The lower slides must be both level and parallel to the upper slides. How best to arrange this on the circumference of 4.3m diameter circle? The difference in height of the slides is around 2m. 

It now occurs to me that I could project the laser's vertical line to be parallel with the zenith board. I can set reference markers at base ring level as far apart as the slit width. Then measure from the reference points to the slit rib beam. It sounds simple but in anything but in reality.

Until now I have been using the slit ribs as reference points. When they are, in reality, rather flexible in all planes. They were fitted into the slit cut-out. Itself a crime against geometry in drawing [and cutting] parallel lines on a the surface of a sphere. [With additional decorative facets!]

Dropping plumb lines from the zenith board was complicated by the very uneven floor level. Though reasonably flat, it slopes heavily towards the northwest. A difference of 200mm or 8" in level across the width of the dome. The dome is supported [and leveled] on highly variable thicknesses of timber off-cuts.

I need to thin the horizontal thickness of the slide mounting boards on the bottoms of the shutters. The slide supporting surface on the dome itself must lie between the slit ribs. This slit rib, cross brace, can be made as sturdy as desired. It also serves structural needs. In reinforcing the base ring across the width of the open observation slit. Once established, this horizontal beam can force the alignment of the shutters via the slides. Provided, it is accurately aligned.

The rest of the base ring is well supported by the dome itself. There is no such beneficial structure in the entire slit area from horizontal to slightly beyond the zenith. The huge gap of missing dome material must be replaced with structural elements wherever possible. Without obstructing the opening. 

The slit ribs add [beam] stiffness thanks to their depth. Provided, of course, that they are well attached to the dome. Here I used resin and glass mat lay-up. [inside and out] To ensure a bond and to provide a weather seal at the meters-long butt joint between the slit ribs and cut-out. I also used silicone sealer in the narrow gap.  More for sealing than any hope of increased adhesion.

*