Turntable Thoughts

Dodging through traffic on my ride home is perhaps not the best time to be thinking about how to make the Pembroke turntable work.  Indeed, usually when I’m riding my bike I can’t think about much of anything.  Yet tonight I had quite a good flow of ideas.  Thanks to Mayor Gregor Robertson and all the bike lanes his council have pushed through, I managed to survive the experience.

It all started when I was tossing around ideas for indexing.  I had been thinking of tiny wires sticking out through the pit wall, engaging in a hole in the end of the turntable.  Thoughts of a disk beneath the benchwork were discarded because of the difficulty of aligning the turntable and the disk.

But tossing around that idea a little lead me to the idea of using another lazy susan base.  This was very successful for the staging turntable, so why not do it again?  The only difference is that now, instead of having a minimal height turntable, I am looking to build a representation of the bridge above the lazy susan.

IMG_2562.JPG

Having a disk under the layout that rotates exactly with the bridge above simplifies all the mechanics substantially. For power collection, I envision two rails soldered to a PC board core with phosphor bronze wipers feeding electricity to the disk. The turntable can be soldered directly to the PC board, making it not only strong but electrically live.

The disk also makes indexing much simpler than the fine wire poking into the end of the turntable. As the sketch illustrates, I’m still undecided as to how many indexing rods to make. Should it be one for each track, or a single one. Despite the drawing, I’m now leaning toward a single rod, for which the semantics are “lock”

This will be a manual turntable. The real engineer and fireman had to jump down six times a day and heave the engine around in all sorts of weather. I don’t see why you should get to stand in the comfort of my basement and simply push a button. You’ll have to work, or at least turn a crank if you want to turn your engine around!

I’m thinking about using a Lego gear box for the mechanism. As long as someone doesn’t try to turn the bridge when it is locked, I think Lego should be more than sufficiently robust.

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4 thoughts on “Turntable Thoughts

  1. Do you have any thoughts about how you will build and secure the cosmetic pit wall? The dirt that turns – is it sloped approximately like the bridge?

    AH

    1. In my imagination, the pit wall is styrene, glued to the plywood to which the lazy susan is screwed.

      The pit floor doesn’t turn, but is also connected to the pit wall. I think the bridge core needs to plug into the base somehow in order to enable me to get the base in after the pit floor has been built and scenicked.

      The pit floor would be a shallow ‘W’ in cross-section, where the bridge is a shallow ‘V’. In snowy climes, they did this to allow some accumulation before the pit needed to be dug out.

      1. Yeah I understand better now. I guess the lazy susan gives you have lots of options to retain the bridge from the middle of its base.

        AH

      2. I think the key for a P87 turntable is to tightly control the indexing. The staging turntable works well because the indexing is controlled at the edge – far from the axis. A pin in the end of the turntable itself would be ideal, but figuring out the mechanism, and hiding the pin in the pit wall are both difficult. A disk under the layout is better cosmetically, but then you want to minimize the number of joints between the indexing disk and the bridge because every join could work loose with time, and is likely not to line up properly ever.

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