Dome build: Treating the segment as a gore.

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Friday is a still, sunny day with 60F, so I am cutting out more ribs. A job I can't do in the shed for lack of room to handle full sheets of plywood. Reached 11 ribs in total. Both edges of the rib have to be cut twice because of the difference in radius. Leaving a long thin and curved, tapered piece of waste.

Saturday is another fine, still day and I just finished the last 5 of the 16 ribs required before morning coffee. Which makes me realise that I had better start building something soon before it starts snowing!

Sunday: I rebuilt the first segment using two ribs and new, shorter strut lengths. It was immediately obvious that the angles and lengths were all screwed up. As were the ribs which had become corkscrews. Something obviously needed to be done.

I decided to build a complete segment [or gore] reaching to the dome's pole or zenith. Easy enough with lots of spare plywood ribs and plenty of spring clamps.

I set up a vertical pole and adjusted the rib overlaps until the ground radius was correct at 1.52 meters. The dome height obviously had to be the same too. So I clamped a bit of scrap plywood to the vertical pole to support the dome ribs at the correct height.

 I have been finding a 'speed' square very useful for transferring the scribed miter angles to the very ends of the struts for trimming on the compound miter saw. It is much easier to use than a clear, drawing protractor.

These images shows the full size, complete dome gore mock-up. The long straight edge [image top right] is checking the angle of the rib at each horizontal strut. With the nearest edge centered on the upright pole I could easily see if the rib matched the angle of the straight edge. It didn't.

There followed some pencil marking and trimming of the struts to get the ribs flush with the straight edge. The ribs are certainly much straighter than before. Though still not perfect. This suggest that the ribs are no longer the correct length.

The answer here is to fit just the lowest horizontal strut and clamp the ribs at the pole. Just like a hemispherical gore. Then the intermediate struts can be measured for correct length by measuring between the ribs and newly trimmed struts tried for fit.

The ribs must be relaxed, perfectly straight and flat for this fitting. Or each segment won't lie flat against the next. Symmetry is also important. Sandwiching the 'real' ribs with more spare ribs and clamping them all together should ensure the ribs are nicely straight and flat.

The image [above left] shows the reinforced ribs and the carefully spaced extension ribs meeting the upright pole. I used the 4' level to ensure the 'real' ribs were perfectly straight and properly spaced. Spare, 15mm thick, dome ring arcs ensured the ribs were stiff enough not to flex sideways.

The final image [right] shows the complete segment with the struts fitted. The segment now lies evenly on either rib against a flat surface. I have noted the angles I cut on the struts and their length ready for building the next segment tomorrow.

Later I dragged the segment up onto the platform and propped it on one of the octagon's posts. The dome curve is slightly too low to clear the telescope but this is without any base rings or support wheels. It was now too dark for photography so I shall try again tomorrow.

Click on any image for an enlargement.

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