Dome building: Procrastination.

With the dome sitting there in the sunshine I had to decide how best to incorporate the observing slit frame. This is an important component because it seriously weakens the dome by inserting a big section of fresh air. Where the rest has lots of seriously stiff woodwork. 

I found some remaining 16mm birch plywood which was left over from making the base ring. Doubled, it will bring the thickness to almost 1.5". Which should be enough in a 15cm, 6" deep rib to safely support all that fresh air. The joints will need to be offset for strength and the two rib arcs well screwed and glued together.

Meanwhile some alternative views of the dome's progress:  

The ribs are now nicely reinforced by the horizontal and vertical struts. With greatest depth of rib at the strut joints where it is most needed for strength.

The dome is self-supporting just resting on the base ring. Each pair of gores is held by a single spring clamp at the top of the ribs.

Wednesday 18th and I only worked for a couple of hours on the dome. I cut new slit framing ribs in 16mm birch plywood. The arcs had to be drawn with a home made beam compass for accuracy. I allowed 2" external overlap and four inches extra depth to bring the inside radius to match all the other ribs. Which meant 6" overall depth. I should have just enough 16mm ply left to produce two doubled arcs for 32mm overall thickness. The joints will all be offset to avoid local weakness.

The next step is to cut the full height gores to allow the slit frame arcs to drop down to the correct height at the top. I am trying to make the shutter doors as low profile as possible. While maintaining the same faceted appearance of the rest of the dome for a uniform look.

I kept cutting back the tallest gores to allow the slit frame arcs to just drop between them without distortion. Then I added extra props to stabilize the frame. Eventually the frame was resting centrally on a 2"x2" marking the end of the frame 6" beyond the zenith. I could then screw the partial arcs of the slit frame together. A long level was placed across the top batten to check it really was level.

Earlier I had unwound the "corkscrew effect" on the upper ribs from adding the vertical struts. These had lifted the original horizontal struts just enough art the third tier to make them slightly overlong relative to their latitude ring on the dome. The 1/3 tier of horizontal struts were removed and run through the miter saw to bring them all to a slightly shorter length. It was easy to match the compound miters by pressing a pre-cut strut end against the saw blade and then adjusting the miter saw settings to close any air gaps.

Once reassembled the ribs were now all upright without the previous kinks at the third tier. I continue to use Torx screws, without using any glue, so I can step backwards to undo any mistakes. The Torx screws only rarely fail to undo provided the driver bit matches the socket and is unworn.

The last image shows the slit frame and ribs stabilized with struts down to the ground.  Further struts lift the longer gores to match the slit ribs. I could then clamp extensions of the dome's plywood ribs up to the slit framework.The 2x2 batten is supposed to represent a 2" lift above the dome but looks far too high.

Click on any image for an enlargement.

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