Since returning to the hobby I have been impressed with the number of layouts which feature turntables. I want one of those I thought...
An evening of eBay left me winning an auction for a secondhand Peco NB55 turntable and a ridiculously large motor and gearbox. It put the fear of God into me as it was a 30v monster. The frame was bent and when I checked the motor's vintage data plate, realised it required a 30V input, so this gearbox must have some ungodly amount of torque.
Total overkill IMHO so the mission was clear:
- Design and build a roundhouse to integrate with the turntable to store some engines.
- Find a suitable high torque low profile geared stepper motor,
- Hook it up to an Arduino
- Write some code
- Install it on the layout
- Design and build a roundhouse to integrate with the turntable to store some engines.
Roundhouse Design
I bought an old Builder Plus BPN24 Double Engine Roundhouse card kit to get some kind of idea of how big a roundhouse I would need and where I could place it on the layout. I will write up how that build went (spoiler alert: it was a bit of a disaster) and show some photos of the result.
However, it served its purpose, showing me that the roundhouse would need to be bigger in all dimensions and so I set about designing a new modular roundhouse kit based on the BPN24 design.
The next challenge was how do I plan this roundhouse so I decided on a version with 20 degree separation between tracks. This angle also determines how close the roundhouse will be to the turntable and I wanted something relatively close.
I cooked up a base ring in sketchup which would fit tightly around the Peco NB55 shell, this would help me to finalise the position of the turntable relative to the Roundhouse and the engine shed tracks.
I'm no expert in these things, but the ring was easy to model and with each track coming out at 20 degree that gave me 18 potential exit/entry points. More than enough for my purposes. I subsequently found out that prototypical roundhouse are between 10 and 15 degrees. What does 5 degrees mean between friends anyway... (Foreshadowing)
A late addition to the layout was a long siding. It was fairly evident during early test sessions that if I want to run long trains (e.g. a 6 x BR Mk1 + a loco) then I need to park these out of the way occasionally. This moved the roundhouse into the center of the layout a bit more.
I printed off a few Roundhouse baseplates and discovered that it was not long enough to accommodate my Japanese electrics so this needs to get lengthened.
The Turntable mechanics
I turned by attention to the turntable itself. The NB55 shell doesn't move, only the bridge section and having this and the base ring as two separate parts will allow me to clock the turntable for exact alignment. At least that is the theory.
Looking on AliExpress I found the ubiquitous 28BYJ-48 5v geared stepper motor and ULN2003 driver board combo which seems to be a popular choice for Arduino projects. This was quite the come down from the industrial sized DC motor which came with the NB55 from eBay.
My go-to 3d modelling software is sketchup which has a free browser version more than sufficient for my humble skills. I downloaded a model of the stepper motor from Sketchup Warehouse which is a trove of user generated content to make sure that clearances and bolt holes were accounted for and printed out a motor mount. I was amazed at how easy this was.
I measured the spigot that protrudes from the bottom of the bridge section and modeled a coupling with the correct shape for the stepper motor. After numerous iterations I managed to get print one which fit through the hole and mated with both bridge and stepper motor drive axle.
The outside diameter of the coupling might print out slightly oversized but can easily be shaved down to fit.
The NB55 does not have a flat outer shell, it follows the interior taper of the well at a constant thickness. I imagine that this is a manufacturing choice which allows the part to be ejected from the injection mould and saves on plastic, but it's a pain in the arse if you want to model a mating plate. Rather than go through all the hassle of measuring this I just opted for a 2mm thick ring which i can fix to the bottom of the plate .
The observant among you will spot that the ring has flat sides. This is a byproduct of how computers draw circles. In sketchup you define how many sides a circle has and it does the rest. Once printed these sides allow for perfect alignment between ring and motor mount.
The 3 parts printed out wonderfully on my Anycubic Kobra S1. God I love this machine.
