Model railroading is a serious hobby for its enthusiasts. People who are involved in model railroading build elaborate displays based on geographic area or periods in history. The goal of a layout like this is to take reality, and scale it down to a miniature world. This world contains trains built and painted like their real-life counterparts, buildings, vehicles, people, even trees and mountains. One accessory you can add to a layout is a turntable. Turntables were used in the steam era to turn locomotives around when they were at the end of the line, as they couldn't efficiently be run in reverse.
Determine how big the turntable will need to be. In most cases, if you have an O-scale layout, it should be at least 15 inches; HO should be at least 10 inches; and N scale should be at least 6 inches.
Decide where in your layout you will put the turntable. It will have to be in an area where there are no support beams or other obstructions underneath the area, and also have enough room for a roundhouse if you are adding one to your layout. (A roundhouse is the building where maintenance work is done on locomotives.)
Draw a square box at least 4 inches longer than the length of the turntable bridge on the surface of the plywood sub-roadbed with a carpenter's square (the sub-roadbed is the plywood upon which your display is built).
Draw diagonal lines that intersect at the exact center of the square.
Use a compass to draw a circle inside the square, using the center of the square as the pivot point.
Drill small holes at each corner of the square, as well as in the center, using a drill bit that's 1/16 inch or smaller. These holes will act as guides when you're working underneath the table.
Cut a square of 1/4-inch plywood the same size as the square you drew in Step 3. This will be the floor of the turntable pit.
Using the guide holes, drill four mounting holes in the floor piece. Attach the floor underneath the sub-roadbed with mounting screws and mark a corner of the floor and the sub-roadbed with an X.
Remove the pit floor, then cut the square out of the sub-roadbed with a jigsaw or saber saw. Cut the circle out of the piece at a workbench.
Re-attach the pit floor to the bottom of the sub-roadbed and trace around the opening where the pit wall and floor meet. (The pit is the sunken area of the turntable in which the bridge rotates in order to turn the locomotive around; the pit wall, which is actually the hole that you cut into the sub-roadbed, is created when you attach the pit floor to the bottom of the sub-roadbed.)
Measure the diameter of the PVC pipe and drill a hole in the pit floor big enough to fit the pipe in, using the center hole as a guide.
Locate the center screw at the bottom of the Atlas turntable, and use it to draw a circle the same size as the PVC collar with the compass.
Cut a small notch in one end of the PVC collar that's about 1/8 inch square.
Cut two 6-inch pieces of No. 20 wire.
Solder one wire to each rail in the floor of the Atlas turntable.
Run the wires through the notch you cut in the PVC collar, and glue the collar to the floor of the Atlas turntable (placing it on the circle you drew on the bottom of the Atlas turntable).
Cut a 3-inch piece of PVC pipe. This will be used to connect the Atlas turntable and the turntable bridge. Run the wires through this pipe.
Re-attach the pit floor, and test-fit the setup by putting the PVC pipe into the collar on the Atlas turntable, through the hole in the pit floor, and put the bridge onto the PVC pipe. Don't worry that the bridge sits too high at this point.
Measure how much space is between the bottom of the bridge and the top of the sub-roadbed. Using scrap wood, make four spacers of the same width.
At your workbench, turn the whole assembly upside-down and glue the spacers between the sub-roadbed and the turntable.
Attach the turntable motor to the turntable, then use wood screws to attach the turntable to the spacers.
Remove the bridge and apply the surface you want to use to the pit wall. If you want, paint the pit wall to enhance its appearance.
Connect a power pack to the turntable motor, and test-fit the assembly again, making sure that the bridge can move inside the pit. To enhance the appearance, you can paint the pit floor gray and make black marks to simulate grease.
Test-fit the assembly again to make sure that the new surface on the pit floor doesn't prevent the bridge from moving freely. If everything fits, remove the bridge and glue the PVC pipe into the collars (after you pass the wires through the two small holes that are drilled next to the rails on the bridge). Let the glue dry.
To align the bridge, spin the turntable to line up the bridge with the approach track, and glue one end of the track to the floor of the bridge. Spin the turntable halfway around, and repeat the process.
Install the tracks that lead to the engine house.
Clean up the wires by trimming the excess length then solder them to the outside of the rails, and put the railings on the bridge deck.
Connect the power supply to the terminals on the Atlas turntable, and try it out to see if it works.