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I spent the morning marking and cutting out arcs in 15mm birch multiply form a 5'x5' sheet. These arcs will be joined to make the dome rotation/base ring. I decided to use a heavy brass plate to hold the pivot point board in place. Spring clamps just got in the way of the 5' long beam compass. Experts would probably have used double sided tape to hold the scrap of ply used for the pivot line. I used a fine toothed blade in the electric jigsaw to cut close to the lines. Then smoothed the curve with an angle grinder fitted with a coarse flap wheel. It was actually easier to remove an 1/8" with the sanding disk rather than cut too close to the line and then have no material to wear away into a nice, smooth curve. This also ensured there were no ragged edges left from the saw. Birch tends to fray upwards when sawn. Ear defenders and an industrial dust mask were worn for protection.
The second image shows the result of laying all seven arcs together. All the arcs are of the same, 5' linear length but naturally are slightly longer in circumference. D = 10'4" x Pi = 31.75ft.
It has reached a tropical 70F outside so I was glad when the lunch gong sounded. So I could go indoors and cool off.
With so many joints it will probably be a good idea to make a slot between the plies with the table saw. Then glue tongues of 4mm ply between skewed butt joints as reinforcement and to spread the loads more evenly. Or, I could make half lap joints. Moving the huge ring 'upstairs,' with just a butt joint between arcs, would have quickly found any weaknesses. I have no desire to cut two whole rings with overlaps between the joints just to make it safe. The weight of the seven, bare arcs is 26.2 lbs or just under 12kg.
My plan is to cut more arcs so that that shorter lengths can be fixed to the bottoms of the vertical dome segments. Hopefully no overlapping joints will occur between the base ring and the bases of the segments. There may be a rotation point [phase?] where this is possible. It is odd how large the complete ring seems down on the ground. I took an arc upstairs to check it against the octagon posts in case I had made a serious mistake in my radius measurements. It fitted nicely.
Joining the arcs into a full circle will require joints. I shall use half lap joints which will consume some of the length of each arc.
The diameter of the dome is 320cm x Pi = 1005.4cm circumference.
My seven arcs are each 160cm long in outer circumference.
7 x 160 = 1120cm. 1120 - 1005.4 = 114.6 / 7 = 16.4 / 2 = 8.2 cm
Checking my sums: Required ring circumference = 1005.4cm.
Arc length = 160 - 16.4 [overlap] = 143.6 x 7 arcs = 1005.2. Near enough!
16.4cm is the length of maximum circumferential overlap per arc.
So 8.2cm [3.2"] is the maximum length of joint overlap allowed at each end of the arcs to provide the full circle.
This assumes all arcs are intended to be equal in circumferential length. The seven arcs raises the question of whether I ought to use eight, shorter arcs? This would help to ensure all joints overlap when the 16 dome segment bases are added on top. Yet another thing to worry about?
With so many joints it will probably be a good idea to make a slot between the plies with the table saw. Then glue tongues of 4mm ply between skewed butt joints as reinforcement and to spread the loads more evenly. Or, I could make half lap joints. Moving the huge ring 'upstairs,' with just a butt joint between arcs, would have quickly found any weaknesses. I have no desire to cut two whole rings with overlaps between the joints just to make it safe. The weight of the seven, bare arcs is 26.2 lbs or just under 12kg.
My plan is to cut more arcs so that that shorter lengths can be fixed to the bottoms of the vertical dome segments. Hopefully no overlapping joints will occur between the base ring and the bases of the segments. There may be a rotation point [phase?] where this is possible. It is odd how large the complete ring seems down on the ground. I took an arc upstairs to check it against the octagon posts in case I had made a serious mistake in my radius measurements. It fitted nicely.
Joining the arcs into a full circle will require joints. I shall use half lap joints which will consume some of the length of each arc.
The diameter of the dome is 320cm x Pi = 1005.4cm circumference.
My seven arcs are each 160cm long in outer circumference.
7 x 160 = 1120cm. 1120 - 1005.4 = 114.6 / 7 = 16.4 / 2 = 8.2 cm
Checking my sums: Required ring circumference = 1005.4cm.
Arc length = 160 - 16.4 [overlap] = 143.6 x 7 arcs = 1005.2. Near enough!
16.4cm is the length of maximum circumferential overlap per arc.
So 8.2cm [3.2"] is the maximum length of joint overlap allowed at each end of the arcs to provide the full circle.
This assumes all arcs are intended to be equal in circumferential length. The seven arcs raises the question of whether I ought to use eight, shorter arcs? This would help to ensure all joints overlap when the 16 dome segment bases are added on top. Yet another thing to worry about?
Click on any image for an enlargement.
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