23.8.17

Marking out dome rotation rings and ribs.

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The photo, of  a scale drawing, alongside shows how many arcs can be sawn out of a 5'x5' sheet with a 10'6" outer diameter. The arcs will be joined end to end and laminated to produce a complete ring to support the dome ribs.

My careful scale drawing suggests that I should reduce the width of the arcs by a small amount to allow more full widths and also for the saw cuts themselves. It should also be remembered that the inner arc should be of slightly shorter radius than the outer so that they are concentric with each other. My method of moving the pivot a fixed distance along a line produces identical curves. The two radii could be marked by adjusting the radius arm pivot each time. Or using two radius arms. [Oversized beam compasses] Or the sharper curve could be produced on the inside curve by the smoothing router which follows the jigsaw.  Jigsaws don't work well on radius arms in my own experience. It's hard enough to get them to follow a straight edge fence.

One saw kerf may not seem much but it soon adds up with multiple parallel cuts. For example: 9 x 1/16 = is over half an inch. Taking this into account saves wasted material when the absolute width of the ring is not [usually] critical. Some dome builders cut their arcs with a router. Obviously the width of the router bit takes its toll on the consumption of expensive materials. Some builders have set up two routers, side by side, to ensure perfect concentricity, the correct radius for both inner and outer arcs and a fine finish. Which probably doubles the kerf wastage and dramatically reduces the number of arcs per sheet.

I am thinking of saving money on birch ply by placing the rollers on top of the octagon posts. Timber plates between the posts can reinforce the arrangement and give a very good, bolt fixing. Simply wood screwing the roller support plates into end grain on the tops of the posts is not very clever.

Note that , line of pivot points, for the radius arm, must be perpendicular and well fixed to the sheet to be marked and cut. Any sloppiness or misalignment will badly throw off the arcs. With birch ply costing as much as it does in the thicker sizes it should not be wasted! The router arm pivot can be placed on each corner of the sheet to be marked and arcs struck off along the material providing the pivot line. Just as you learned to do in geometry class to draw a perpendicular to a line, now so many years ago. The router arm must be rigid. If it flexes or rocks it will not draw an accurate curve.

I took the trailer to the builder's merchants and bought 4 x 4mm, 2 x 9mm and 2 x 15mm birch multiply, all in 5'x5' sheets. Just to give myself some initial materials to experiment with. Plus half a dozen, plastic, 'clothes peg', spring clamps for when I am gluing plywood ribs together. Swift and firm clamping is essential when working high above the ground. There is usually no time to be messing about with heavy G-cramps, C or F clamps when working alone. I should really have bought more spring clamps as they are relatively inexpensive. I'll see how well the ones I bought perform before investing more funds. It's hard enough to find consistently good clothes pegs.


Click on any image for an enlargement. 
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