Making The Rings

Since I elected to make my own wheel apparatus to make the dome capable of being rotated, here is my design.

The plywood I selected is brand named "Baltic Birch". In 3/4" thickness, it has about 12 plys - not 5. And, the wood used in these layers is birch hardwood, not softwood filler. Having so many plies will give it the strength and flatness I desired. It is expensive tho.

I will not get into too much of the math, but here are some overall details you might find interesting.

I found the outside circumference of the dome, at the top of the lip where the rings will be placed inside to be 306.5". This gives us an Outside Diameter (OD) of 97.5 ". The wall thickness is about 1/4" and I allowed for 1/4" of extra clearance and that gives us an OD of the upper ring to be 96.5". (306.5 / 3.14159 - (.25 x 2) - (.25 x 2)

I thought long and hard of the room I will have inside an 8 ft dome and every inch counts. That is why I went to twelve sides, instead of eight.

The Upper Ring will be three layers for strength and to help keep the dome flat around the edges. I made a decision and a compromise that 3 3/4" of width would be adequate. Doing the math, this gives us an Inside Diameter to the Upper Ring of 89".

The Lower Ring has to be slightly smaller in diameter than the Upper Ring to allow for clearance from the dome skirt. I I allowed a half inch for that. The math gives us 95.5" OD and 88" ID for the Lower Ring.

This also means the Lower Ring will protrude into the interior by 1/2" from the Upper Ring. This will be used to advantage later for securing the dome to the base.

The circumference of the Upper Ring is 303+ inches. The least waste in plywood would make each arc to the ring to be about 50-51" or so. Dividing 303 by 6 gives us an outer measurement to each arc to be 50.5". A convenient measurement.

A router will be the best tool to make the arcs.

 

We, using whatever means we can, fasten the router to a length of plywood. The method is not fussy. Having it mounted firmly is.

 

A closeup.

I chose a 1/4" carbide bit with 1" shoulders.

 

We rotate the bit until the carbide face is as far out as it will go to obtain a true measure.

 

From that, we measure one half the OD and ID of the Upper RIng.

48.25" and 44.5".

Note: The OD is measured from the near side of the bit. The ID is measured from the far side of the bit. Because we are cutting the outside edge and the -inside- edge of the segment.

 

We plop the plywood on the Asylum's Sacrificial Altar.

 

Approx 49" from the edge, we drill a hole in the middle of the sheet and put a drill bit the same size into it.

 

We start the cut. I found several small incremental passes to be more effective than a single pass.

 

Done.

 

We need a jig saw to cut into the end, just a bit.

 

You can barely see the knifeline I scribed on the wood. How I did it will be explained in a bit. And of course, the cut we made with the jig saw.

 

We place the end of our tape measure into the cut we made.

 

And, pulling tightly and without any kinks in the tape, we measure off 50.5".

 

And we scribe a knifeline. I was taught in shop class, this is the most accurate way of drawing a line in wood. Assuming the edge we use is suitable.

 

What it looks like.

Note my extremely detailed and elaborate plans.

 

And we cut the inner arc and here is the finished segment.

 

3.75" exactly. I was actually surprised - and impressed.

 

A sheet of ply is usually 48" x 96". A std carpenter square is 16x24". I wanted to use an easy way to drill the pilot hole for the next segment.

I thought why not measure down from the edge on the left and use the square to indicate the center of the sheet.

 

An earlier experiment indicated I need 5" of horizontal width per segment. Here we have 39.25 - 44.25 and we need a new hole at 49.25"

 

And we drill the new pilot hole at the corner of the square which will get us close enough to the center of the sheet.

 

A stack of six completed segments.

Next, I will clip the ends of each along the knifeline and temprarily join these together and make certain the ring will fit properly inside the lip of the dome. If so, I will make another twelve segments for the Upper RIng, six for the Intermediate Ring and twelve for the Lower Ring.

(To Be Continued)