Dome build: Trial Segments.

Now I need to cut the miters on the cross struts to make up my first dome segment. If it proves satisfactory I can start mass producing struts to length with the correct, miter angles on each end.

The ends needed to be compound mitered. Not only do the gores shrink in width as they rise but the ribs are splayed. They should all lie on radii from the center of the dome. The compound angles I used were [incorrectly] between 5 and 6 degrees both ways. I pre-drilled the plywood and then screwed into the end grain of the struts.

Being a fairly sensible chap I decided to test the segment by laying it on each side on a flat surface. The ribs wouldn't lie flat!  So I trimmed the middle strut and all looked well. Satisfied I was on the right track I made another segment using the new strut measurements. Whoops! Oh, deary, deary me! They certainly fitted together but wouldn't stand evenly on a flat floor.  There was a difference of 1" in level on both outside ribs!

Image of the two segments inverted and tested against the base ring. Wrong! There is a four inch error in the horizontal plane and a 1" error in the vertical plane.

My choice of 5 degrees for the compound angles must have been half that required. Watch this space. I shall certainly have to increase the horizontal miter but the other should not need to be increased. The middle struts will also have to be lengthened to match. Or, rather, a new strut cut to the correct length as suggested by Geo-Dome's trapezium dome calculator.

Later, I set the miter saw to 11° and re-cut all the strut ends. This sharpened the dihedral angle of the battens to much better match the base ring. Though the top struts now seemed to be too tight an angle and this put a slight twist in the ribs. That  said, the strut lengths were now far from accurate in length having been cut and re-cut numerous times.

Having confirmed the dihedral angle is much closer I now need to make up new struts to the calculated length. Just to check the full sized model is correct. The exact miter angles are still an unknown so some adjustment will be necessary. Though these angles may be obtainable from the Geo-Dome trapezium dome articles if I do a bit of simple maths.

At this point I am really just building a normal hemispherical dome with gores. Only when I cut away the flats and apply panels will the form be changed to a tiered and faceted design.Which means I can use a dome design calculator to check my strut lengths and miter angles.

Click on any image for an enlargement.

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