The bridge is the actual moving part of a turntable. It needs to be strong and stable enough to support the locomotive as it is turned. The bridge core was made from the same MDF that was used to make the turntable base. I cut it about 3/32" shorter than the diameter in the base. However, I may need to file it down a bit more, because the turntable pit wall will be covered with styrene.
I shaped the bottom ends of the bridge to more or less match the prototype bridge profile. This clears the rail on which the bridge rides. I marked the center of the bridge on its bottom, and then drilled the hole to hold the metal sleeve shown in the photo. I also drilled the hole deep enough to clear the bridge shaft that will be installed. All these metal parts came from the original Diamond Scale turntable kit, but they can be found in your local hardware store.
The next step was to use an Allen wrench to install the sleeve on the shaft.
And finally, the shaft is installed on the bridge. For the time being I am not gluing it into the bridge yet. This will allow me to work on detailing the bridge itself without the shaft getting in the way.
After studying some prototype photos I realized that my bridge was too wide. I trimmed the MDF board down to 5/8" wide. This comes out to about the width of a standard tie (about a scale eight and a half feet). Next I painted the whole thing with a flat black acrylic paint. Not only does it absorb any light, it also protects the wood from glue.
The original Diamond Scale documentation came with a template for the bridge ties. The long version matched my turntable perfectly. I taped a long piece of Scotch tape upside down to the page (the tape's sticky side is up). Two small pieces of tape, at the ends, hold the long one to the page. I then laid standard ties on the template getting them to stick to the tape. There are also 13 18-foot long ties that will support the walkway. Finally, two 23.5-foot long ties will eventually hold the control cab.
I then carefully removed the tape from the page and placed them on the bridge, which I had covered with white glue.
After about 30 minutes I removed the tape from the ties. I let them dry some more before the ties were sanded flat, as shown in the next photo.
Next, the ties were stained with India ink/alcohol mixture, and then dry-brushed with dark gray, white, and rail-brown colors. In retrospect I probably should have used the longer "bridge" ties. The rails need to be powered to allow the locomotives to drive on and off the bridge. I plan on installing two wipers under the bridge that will pick up power from the rails placed on the pit base. I drilled one hole for each of the rails in the bridge for the wire.
This next close-up photo shows the wire soldered to the rail and getting ready to be glued down to the bridge. I cleared a bit in the next tie over to allow for the soldered wire.
Here's my set-up for gluing the rails down to the bridge. I used 5-minute epoxy glue, applied to the bottom of the rail with a toothpick. The wooden blocks are my templates for track spacing. Finally, metal weights holds the rail down while the glue dries.
This is what the bridge looks like after the glue has dried.
The next step is to install two pieces of brass that will act as wipers to conduct electricity from rails in the pit floor to the rails on the bridge. I didn't have any sheets of thin brass lying around, but I did have a set of Gold Metal Models Passenger Car Set Details. I trimmed some of the extra material from the sheet and shaped it. Next, after drilling a hole through the metal and into the wood bridge, I found the smallest wood screw I had in my parts box and installed the brass wiper. The last step was to shorten and strip the wire that connects to the rail and solder it in place. To get the best adhesion between the brass and the solder, I filed a portion of the brass so that it has a rough surface. The next photo shows one of the two wipers installed. Some final bending and shaping will be necessary to have the wipers make good contact with the rail in the turntable.
The photo below shows the wipers under the bridge. Although nothing is connected yet, I couldn't wait to see what one of these long steam engines looked like on the turntable bridge. The bridge is plenty long enough. The bridge sits too high in this photo because the wipers haven't been adjusted yet.