OK, let's start slow and at the bottom. This will take some time to get all the parts laid out with the accompanying explanations.
The 2 pic's I have enclosed are from our 3D design tool which is Design Spark (free version of Space Claim). It shows the foundation for the turntable and requires a little history to understand.
I started building my layout several years ago (after tooooo many years waiting). At that time I had a turntable that I purchased 25-30 years ago along with a 3-bay engine house. These two items dictated one end of my layout. The turntable came with a top section that was trapezoidal shaped (which explains the top section of what you see in these pic's). I had made a cutout to accommodate the turntable in a 4' x 8' piece of plywood. The turntable was DC motor driven with optical pickups mounted on the underside to stop the table at designated positions. In operation (and I had 9 positions for the stops) there was a problem when moving in the opposite direction from the setup direction (a condition known as hysteresis), which resulted in rotational misalignments. In addition, large engines would cause the track bed to dip enough to also cause derailments. So between misalignment and dipping problems, I was not happy with the old turntable.
About a year ago, my neighbor, who is a retired mechanical design genius, was bored and decided to help. He came up with the idea of using stepper motors and a lazy susan, the latter being a way to eliminate the tilting due to the small 'bearing' surface in the old turntable and the former providing great alignment precision.
The first step was to cut out the trapezoidal top piece followed by making a 24" diameter disc out of plywood using a router. The we ordered a 6-wire slipring to provide electrical connection to the rotating table. What you see in the pic's is a 3D simulation of what we have made (at this time no 3D printing involved).
I will take some pic's of the old turntable and of the actual top and bottom foundations and include at the next posting. One last item at this stage. My neighbor them took the stepper motor, added a 24VDC power supply and a stepper driver and designed and built a control box which I will show in the next posting.
All of this meant a new turntable and so emerged the need and use of 3D printers.