14.12.2020 Day 2 of budget 4.4m dome project. Setting-out templates.

 *

Monday 14th Dec. No ill effects from yesterday's "groundworks." I thought I'd be aching all over today after the unaccustomed exercise.

The image [Right] is a scale plan of template arcs for setting out the larger building boundary. The idea is to maximise the dimensions of each arc. While keeping material usage [and wastage] to a minimum. Only four arcs can be safely cut [lengthways] from a metric 4'x8' sheet. [1220x2440cm]  

All of the arcs are drawn at the same radius inside and out. So they can be used as positioning templates both inside and outside of the desired post/foundation ring. Whichever is most appropriate should a tree stump get in the way. Several saplings have had to be chopped off since the original observatory building was erected. Rather than dig them out and badly disturb the well compacted gravel I just left the stumps to rot.

Are only four arcs enough to accurately set out the ground base ring of concrete anchor blocks? Cutting the arcs across the sheet might seem more economical due to reduced wastage. Though short arcs have to be placed end to end to achieve the same overall circumference. Each joint cheerfully adding its own potential for inaccuracy. My own experience suggests that longer arcs are very much easier to align at each joint by eye. Fewer joints = fewer errors of alignment.

No attempt should be made to fix the templates together permanently. They do not have the strength to support each other. They could be drilled at each end for a single, large, alignment nail. [To avoid repeated losses] To join them loosely together after laying them all out on a flat surface. This will speed repeated alignment in use.

The image [Left] shows the original foundations [May 2017]  for the octagonal observatory building. Showing the concrete foundation blocks in their holes. Some of the large boulders unearthed during the [entirely manual] groundworks are also shown.

The overlapping template joints should then be clearly marked 1 1, 2 2, 3 3, etc. I used pencil on the last dome project and it was almost pointless. Clear markings with a magic marker will save hours of searching for newly invisible scrawls. You wouldn't use a marker on visible surfaces but it is excellent for hidden ones. Or where it doesn't really matter.

The next decision is how thick the plywood [or other sheet material] wants to be for adequate strength and stiffness in all planes. No point in making thin arcs if they are too weak to support their own weight. No point in thin material if they snap under their own weight when lifted from the ground. Or are too weak to hold the curve when flexed along their length.

Can these arcs be incorporated into a dome base ring? Or observation slit ribs at a later date? In order to save the considerable material costs of [say] 3/4" thick plywood. They will already have the correct outer radius. So choosing suitable materials could more easily make them dual purpose.

*