Bent Wood Laminate Observatory Dome Page

Bent Wood Laminate Dome

My second dome was completed in 2015 and is a birdcage type design that uses bent laminate arches instead of the traditional plywood arch construction (below photos).  I wanted to try a different construction by bending thin wood strips around a frame and laminating them together to arches. The goal was to produce a very strong arch that was thinner and lighter than a traditional birdcage type plywood arch. The laminate arch frame was covered with oriented strand board and exterior roofing materials, so it is a heavier construction. I also replaced the original dome door, that slid up and over the dome, with a double door design.

Dome Ring

The dome rests on the dome ring. The dome ring is 3.36 m outer diameter, 12.5 cm wide, and constructed from 3 layers of overlapping arcs cut from 18 mm thick plywood. The 3 layer dome ring weighs approx. 42 kg. Each dome ring layer requires 8.12 arcs (128 cm) and a standard 122 cm x 144 cm 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 diameter 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 arcs were cut out with a jig saw.

 

The below left photo shows a test fit of the first dome ring layer and the below right photo shows the completed 3 layer dome ring. The black tape marks the joints that are left unglued so that the ring can be separated for storage. These joints were glued prior to final assembly.



I used a belt sander to correct any uneven areas. The dome ring top and inside received two coats of black paint, and the dome ring bottom was treated with two coats of oil based wood sealer followed by three coats of clear polyurethane (below left photo). To layout the arch placement, I began by marking a diameter that ran between the two central arches. To get the diameter exactly through the ring center, I marked the starting point and then wrapped a large tape measure around the dome circumference. I placed marks every 1/4 of the circumference. A nylon string was streatched across the diameter running between the central arches. I placed a tape measure across the peripheral arches perpendicular to the central arches . The tape measure and the string intersected at their respective middle points (below right photo). I next marked the positions for the two central arches (50 cm to either side of the diameter between the central arches). I calculated the circle arc length on each side of the central arches and divided this by 10, giving the distance between each peripheral arch. I marked each peripheral arch placement along the circumference and then streached a string between each pair of opposite peripheral arches; this gave the proper angle to make the peripheral arches intersect above the dome ring center. 


Bent Wood Laminate Arches

The dome frame supports the arches and sheet materials that clad the outer dome surface. Most traditional  birdcage type designs use plywood arches, identical to those used for the dome ring arches (3 layers and 2 layers thick for the central and peripheral arches, respectively).  I wanted to try a different construction by bending thin wood strips around a frame and laminating them together to arches. The goal was to produce a very strong arch that was thinner and lighter than a traditional birdcage type plywood arch.

A table saw was used to cut 7 mm thick and 4 cm wide strips from pine boards. Five pine boards were coated with glue and bent around a frame to dry for approximately 4 hours. The arches were secured inside the bending frame using small wood wedges. Temporary screws held the laminate together at positions in between the wood blocks (these screws were later removed). After 4 hours drying, the arch was removed from the bending frame, placed in a vertical frame to dry overnight, and the next arch was glued up in the bending frame; this allowed 3-4 arches to be produced each day.

The bending frame is a series of wood blocks secured to scrap plates and planks (below left and right photos):




The laminate is pressed against the bending 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 frame (left side) is bent between wood blocks to retain the proper curvature during overnight drying. The other arches on the drying frame (right side) are completly cured and hold their bend.



The 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).



Dome Frame Construction

The two central arches and the middle peripheral arches were installed first (below left photo). Before I began cutting the peripheral arches, I 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.


It 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 right angle supports on the base ring. The peripheral arches screw directly onto the central arches at the top mitered cuts. 


Oriented 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 how 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. I 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 frame, so 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 onto 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.


Dome Slit Shutter

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 not shown in the below photos; the T-configuration can be seen in the last two photos of the completed observatory). Aluminum strips along the track edges prevent 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).

  

Dome Exterior Covering

The dome is covered in modified bitumen roofing sheets that were open-flame seam welded with a gas torch (below photos).


 

Dome Installation

The dome was loaded onto a trailer and transported to the base structure (below left photo). An extra set of wheels was added to the base structure to accomodate the heavier dome, giving 20 wheels capable of supporting a total of 450 kg.  I later replaced these 20 small wheels with 10 industrial wheels, capable of supporting a total load of 560 kg (see the Observatory Base Structure Page for additional information regarding the dome and centering bearings). Scaffold supports were placed inside and next to the base structure (below right photo). The dome was positioned on two aluminum ladders and a wood cross brace. The ladders supporting the dome were slid onto the scaffold supports to position the dome over the base structure.



The dome was positioned next to the outer scaffold support and the trailer was depressed; this lifted the ladder over the outer scaffold and the dome was pushed until the ladder was well over the outer scaffold support (below left photo). The dome was pushed until the center of gravity was between the inner and outer scaffold supports, then it was tipped onto the two scaffold supports (below right photo). 




The trailer was removed and wood blocks were positioned under the dome. The scaffold supports and ladders were removed next
(below photo), so that the dome rested on the wood blocks. The wood blocks were removed to lower the dome onto the base structure.



The below photos shows the completed dome. The upper left photo was after replacement of the plywood sheets with vinyl covered metal siding (summer 2016). 


                                                                                                                                         
   
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