The dome rests on the dome
ring. The dome
ring is 3.36 m outer diameter, 12.5 cm wide, and
from 3 layers of overlapping arcs cut from 18 mm
The 3 layer dome ring weighs approx. 42 kg. Each
layer requires 8.12 arcs (128 cm) and a standard 122 cm x 144
plywood sheet is enough for approximately 15 arcs. I
fabricated a simple swing arm by attaching a 1.7 m
long dowel to a board (below photos). I drilled 7.5 mm
holes at positions 1.55 m and 1.67 m from the swivel point
and inserted a pencil into the holes to mark out the
first arc: the first arc was cut out with a jig saw and
used as a
template to lay out the remaining arcs. A total of 27 plywood
were cut out
with a jig saw.
laminate is pressed against
frame with small wood wedges (below left
photo). The wood wedges worked just as well as screw clamps and were
considerably less expensive. The below right photo shows the vertical
drying frame. The small arch on the drying
side) is bent between wood blocks to retain the proper curvature during
overnight drying. The other arches on the drying
side) are completly cured and hold their bend.
below left photo shows the
final arches after planing and course sanding. The below right photo
shows a first test fit of half the dome arches (before cutting them to
the proper length).
The two central arches and
peripheral arches were installed first
(below left photo). Before I began cutting the peripheral arches,
test fit everything to make sure the curvature fit and all
peripheral arches intersected above the base ring center (below right
photo). The dome frame contains: two central arches, 18 peripheral side
arches, and 1 back peripheral arch.
took several hours to do all
the compound miter cuts, but the
result was that everything fits tightly together and is very strong
(below left and right photos). The arches are screwed into galvanized
supports on the base ring. The peripheral arches screw directly onto
the central arches at the top mitered cuts.
Strand Board Dome Covering
The dome is covered
with 9 mm thick
oriented strand boards (OSB). I considered using 6 mm plywood, but a
local carpenter advised that OSB can better withstand dampness and
water penetration that plywood. There are 6 different gore shapes, with
a total of 22 gores around the dome circumference. To estimate
many OSB boards would be required, I traced the 5 different
side gore shapes onto clear plastic
sheet (below left photo), then cut out the plastic templates.
arranged the templates to find the best configuration to minimize the
required number of OSB boards: 5 boards for the 20 side gores,
1 board for the 2 rear gores, and 2 boards for the dome slit
cover. I used a total of 8 OSB boards to cover the dome
the estimate was correct. I used the plastic template to trace out the
first gore on the OSB
board, cut the gore out with a skill
saw and clamped it in place (below middle photo). The 9 mm OSB was easy
to bend and clamp around the dome curvature.
The best procedue I found for
fitting the remaining gores was to
rough cut each new gore to slightly larger than required and then clamp
it against the previous gore. Because the OSB board gave little
resistance to the skill saw, there were areas where the cut got away
from me and deviated from a straight line. To get a snug fit, I
clamped the new gore, then scribed the previous gores outline
the new gore with a compas (below left photo). I cut this new scribed
line and refit the
gore. Next I scribed the support arch onto the underside of the gore
(opposite edge from the compas scribed line). I drew a new line
parallel to the underside line, but offset by half the arch suport
width, and cut this line with the skill saw. Using this procedure, each
new gore fitted tightly against the previous gore and overlapped the
arch support by approx. 50% of the arch width. The below center and
right photos show the dome frame almost completely covered.
The dome slit is 1
meter wide and covered by a double shutter
that opens to
each side. The shutters move on roller blade wheels that ride
along the top and bottom tracks; these tracks are heavy wood boards,
stiffened with 1 cm thick aluminum edging (top track) and two boards
joined into a "T-configuration" (bottom track, T-configuration
shown in the below photos; the T-configuration can be seen in the last
two photos of the completed observatory). Aluminum strips along the
roller blade wheels from rolling off of the tracks. The
motion is very smooth and the doors are very easy to open and close.
Below are several photos prior to applying the outer dome covering.
The observatory shutter doors are held together by 4 compression
latches, equally spaced along the opening edges (small latch at upper
center, below left photo). Two larger compression latches secure the
top of the doors to the dome frame (below left and right photos).
Rubber weatherstripping between the doors gives a water tight seal.
Two large compression latches
secure the bottom of the doors to
the dome frame (below left and right photos).
The dome is covered
sheets that were open-flame seam welded with a gas torch (below photos).