I started work on the pit. The finished dimensions will be 13 5/8" diameter and 1 7/8" deep. I have a piece of 15/32" plywood that's 2' x 4' in size. That's just about right for three 16" square pieces. I'm planning to cut the main hole to 14" diameter and line the side of the pit with 1/8" thick hardboard. That brings the hole down to 13 3/4". I want to leave 1/8" clearance on each end of the bridge and that brings it down to 13 1/2" and that will be the finished size of the bridge. Perfect.
For the three pieces of plywood I drew two diagonal lines from corner to corner, crossing in the center. Where they crossed I drilled a 1/8" hole in all three pieces while they were clamped together. I made up a board to draw circles, a compass I guess, that has a screw hole in one end and holes for a pencil at 6" and 7". The screw goes in the center hole of the plywood. With the pencil in the 7" hole I drew a 14" circle for the pit on one piece of plywood and the 6" hole for a 12" circle that will give me the ring rail support on another. The third piece of plywood goes under the other two and forms the base. With the three pieces of plywood stacked the height is 30/32" from the upper surface of the base. I need a finished height of 1 7/8" or 1 28/32". I plan to use 1/2" Homasote over the sub-base of plywood. That will give me 45/32" or 1 13/32". To get the last 15/32" I cut some of the left over plywood into shims and glued them between the top layer with the 14" hole and the middle layer with the 12" hole. Then I cut a strip of 1/8" hardboard (Masonite) 1 9/16" wide and glued it in to form the pit wall. It sticks up 1/8" above the Homasote which is 6" in O scale which is the thickness of a narrow gauge tie. Perfect. I'll be able to run stall tracks up to the edge of the pit and the rail will meet the bridge rail just above the lip.
Update (1-9-18): I took the plywood underlay into the shop and cut the recesses for the turntable, ash pit and coaling tower. I mounted the turntable using corner braces glued in place beneath the plywood so the turntable can set flush in place and still be easily removed for maintenance. Four screws in the corners hold it in place. I can't do any more assembly on the pit until I get the bridge trucks in place because all final dimensions are tied into them including the depth of the circular rail and the height of the pit wall.
Update (2-13-18): With the dolly truck parts delivered and assembled the turntable bridge is finished. I can now use that to come up with final dimensions for depth and rail height in the pit.
I started by assembling the ring rail. Originally I was going to buy some code 70 rail and 5" x 7" ties and hand lay everything. I've had such bad luck getting parts delivered for this project that I didn't want to wait another month and a half for the rail to be delivered. I had some code 70 Micro Engineering flex track laying around and it even had weathered rail. I used some for the bridge rail and another piece for the ring rail. The flex track was 36" in length and my ring rail is 37.75" in circumference. I cut the flex track lengthwise down the center of the ties and then soldered two pieces end-to-end to give me the right length. I used rail joiners and just soldered over them for a stronger joint and that really helped when soldering the rail into a ring.
I took the time to file the rough edges of each tie and then painted everything with rattle cans, grey and darker grey. I had drawn the rail location on the piece of plywood before cutting out the center so now all I had to do was glue the rail along the line. I used wire nails to hold the rail in place and Gorilla Glue to hold it down. I then glued in 1/8" hardboard strips to dress up the edges of the plywood that showed inside the pit. I also glued a piece of Homasote to the top piece of plywood to bring everything up to the correct level.
Final assembly and detailing coming up soon.
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