Making The Rings
Since I elected to make
my own wheel apparatus to make the dome capable of being rotated, here is
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.
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
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.
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
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
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)