16.01.2021 Arc templates and ring segments.

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Saturday 16th Jan. A hard overnight frost. 20F / -7C. Followed by sunny periods in the morning. Thin snow still lying in the shadows.

I laid a sheet of plywood across the trailer sides to start marking out some arcs. Because of the 2.1m radius I needed a very long board to act as a stable centre line for my radius bar. The dimensions of the 2.1m radius bar + 1.5m plywood sheet length have to be added together. A bit of extra length also helps. 

Image [Right] taken from the top of a 10' stepladder. My TZ7 camera has a giant "dust bunny" in the centre of the frame.

I rested the free end of a very long board on the sack truck and screwed the other end to the plywood. This would ensure nothing moved during the marking out. I normally use G-cramps/C-clamps to hold the board but the radius bar strikes them as I scribe the arcs. Countersunk screws offer no hindrance to swinging the radius bar but means a permanent hole in the plywood.

The long board has to be on the exact centre line of the plywood or the arcs would be skewed. The ends of the arcs need to be perpendicular. Or the arcs won't fit accurately end to end to form a true circle. 

Which means that using the radius bar as an end marker requires extra care. I used both sides of the radius bar for marking the ends of the arcs. Then bisected the offset angles between the two lines. A better arrangement would be a radius bar with a straight edge dead on the centreline. i.e.Half the nominal width of the radius bar.

I decided to forego making such a device. Any inaccuracy in aligning the straight edge on the pivot centre line would make the extra effort pointless. A pause for lunch before I start sawing. I may make the arcs slightly deeper. I used 22cm for the breadth but could just squeeze them to 23cm deep if I want to minimize material wastage. In fact I ran over by 5mm on the last arc using 23cm.

As mentioned in an earlier post. I am scribing the arcs to the same radius inside and out. As I don't have the dome available, to confirm its true dimensions, I must make do and guess. Based on the manufacturer's own specifications. If the radius needs to be changed slightly, later on, then I can return to marking and sawing. For the moment I need an accurate, radius template. For placing the foundation blocks on a true circle. 

I want the building slightly smaller than the dome but not by too much. 4.3/2 = 2.15m. The radius of the dome. I chose 2.1m for the radius bar to ensure my [ground] base circle is slightly smaller. Only a couple of centimetres difference though. Perhaps I should make it smaller still? The dome materials must have some thickness. While the manufacturer's would tend to quote OD. You can see why I'm leaving the sawing until after lunch. Measure twice. Cut only once. 

The posts have depth outside their centre line. Say 100/2 = 5cm. That's 10cm extra on diameter. 4.2m. The thickness of the GRP? Say 1cm. That's another 2cm subtracted to ensure the building is smaller. 5cm clearance on radius for rainwater? That's another 10cm on diameter. 22cm in total? 4.3 - 0.22m suggests I should really aim for 2m radius on my building arcs. Make the arcs deeper too at 23cm. 

The sun has come out and it has reached 31F. I had better start all over again. The 2m radius is more demanding of sagitta than 2.1m. Which is why I overshot slightly on the last arc. No problem provided I keep an eye on which arc it was. I don't want to use that one for rapid arc marking on new sheets of plywood.

I eventually cut out 9 arcs x 1.52m wide at 2m radius, inside and out, to close the circle. [Image above left] It needed the arcs to be spread out to achieve 4m inside. I used a fine tooth, [metal] hacksaw blade in the Bosch jigsaw. Running at high speed with enough blade rocking to ensure rapid cutting. Yet still able to follow the scribed arc. Reducing the rocking setting slows the cutting to a crawl. With the risk of overheating the blade. 

It was odd how skewed my end of arc markings appeared to the naked eye. They should have been perfect with my method. Unless the radius bar is bent at the "business end?" I shall have to check with a taut line and centre peg when I have the ring laid out again in daylight tomorrow. Though it may need a masonry drill to get a centre peg into the hard frozen ground!

I may be able to use the laser level's vertical beam for this. I haven't cut any ends off the arcs yet. To avoid wasting arc length by repeated trial and error cutting. The arc ends should all lie on a diameter passing through the centre of the circle. 

The edges of the plywood sheet leave unusable triangles on each end of the arcs. [Image above right] There is no way to join the arcs end to end using these. Because all the "points" overlap. Straight and accurate radii on the ends of the arcs allow them to be butted together with low alignment error.

The weight of 12mm Baltic birch ply is considerable. 21kg or 43lbs per sheet according to online plywood suppliers. Even half a sheet of cut arcs [3] feels very heavy to put away in the shed. It never rose above freezing today but I was pleasantly warm as I moved about. The cloud often hid the sun and it became dark quite early.

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