Convert Atlas Turntable to 90 ' TT

[Les]

Åny thoughts/ideas re converting an Atlas TT to a 90 ' TT - would like to hear of results of those that have done so and how it was accomplished as well as problems with such a conversion

[Jim Banner]

I haven't done mine yet but I do have a plan.  The first step would be to cut a whole in my table top the size of the present turntable plus the ring outside of it.  That is to say, a hole that the whole thing would drop through.

I intend to leave  the present bridge rails in place and build up the turntable, complete with pit decking, right over top of them.    Because I want to use real wood for the pit decking, I will probably do the building up with 1/8" Baltic Birch plywood.  Strips of 1/8" sheet styrene would be a better choice if the finished deck was to be styrene instead.  My first layer will be a 9" circle with a strip cut out of the middle to clear the old rails.  This will prevent the rails from pushing up the center of this plywood and will leave a space for wires from the present rails up to the new bridge rails.  At the same time, I will add 1/8" styrene tabs to the tops of the spacers between the original approach tracks, letting these tabs extend out to the size of my finished turn table.

Now it would be time to mount the turntable in place.  Because those styrene tabs stick out past the original unit, they will support the modified turntable 1/8" above my table top.  That 9" circle of 1/8" plywood will give me a base on which to attach a 12" circle of 1/8" plywood.  You would need a circle slightly larger for 90 scale feet, and might want to make it out of styrene.  My circular second layer will extend out to the ends of the styrene tabs and will slide on them.  To ensure a long life, I will round all the corners of the styrene tabs, first with a file or scraper, then smooth them with 320, then 400 grit wet and dry paper used wet.  Finally, I will rub a generous amount of graphite (Kadee Greasem) into the bottom of the 12" ply wood circle where it rides on the styrene tabs and a small amount into the top of each styrene tab.  If you used styrene for you second layer, rubbing some graphite into the bottom of that would also be appropriate.  Just before gluing that top layer in place would be a good time to solder a pair of wires to the old bridge rails and bring them up through the top layer.

Next I intend to add a 12" length of rail to the top of my 12" circle.  Having built turn tables before, I know to attach only one end of this track to the circle, leaving the other end free.  And in order to make use of the automatic polarity change as the turntable turns, I will align the new bridge track with the old track beneath it.  That attached end gives me something to align one of the new approach tracks to both horizontally and vertically.  I plan on keeping the new approach track flat with the bottoms of the ties 1/4" above the table top.  This just happens to be the thickness of the two layers of the turntable above the table top.

With the turntable assembly and that one approach track firmly in place and aligned, the next step is to turn the bridge 180^o and align and glue down the free end of the bridge track.  With both ends aligned to the one approach track, both ends of the bridge track will automatically align will all the other approach tracks that I align to either end of the bridge track.  Needless to say I will be using approach tracks only at the indexed positions of the bridge.

Next step is to do something about the drive mechanism. The easiest thing it to reverse the motor mounting and mount it right on the table top with its worm gear engaging the turntable's pinion.  I may have to do a bit of filing and fitting and will probably have to glue on some styrene tabs to run some screws through, but it looks like it will all work out.

The last step would be cosmetically restoring the bridge decking, building up some fill or a wall around the outside of the table, and so forth.

This photo should help make things clearer.



The yellow measurement indicates the maximum size of the turntable with the motor mount reversed.  That 6-3/4" is about 98' in H0 scale.

The blue arrow indicates the squared end of the pinion shaft where the crank normally attaches.  This will likely have to be trimmed down to clear the larger circle.

The green shows one of the 1/8" thick styrene tabs on one of the raised spacers between the original approach tracks.

One last tip, if you have a new turntable: install the crank and run the table completely around.  When I did this with mine, I found there were some tight spots that required disassembly and a bit of attention with a file to remove moulding artifacts.  Later I acquired a used turntable (the one shown) and it runs perfectly smoothly.  It is possible that using the motor drive to break in a new turntable would avoid having to take it apart.  Note that the plan of attack above does NOT require you to take the turntable apart and I would avoid doing so if at all possible.

I hope this gives you some ideas.  Writing this has certainly motivated me to get busy and get building on mine.

Jim

[Les]

Jim:  Thank you for your prompt reply and detailed modification plan - I will painstakingly review your plan and trust my limited abilities will result in a 90' turntable

[Jim Banner]

Les,

You are welcome.  Bottom line on abilities - we all have limited abilities.  The best any of us can do is to stretch those limits by practice and each time going a little farther and a little farther again.  If you follow this site regularly, you have probably seen some of the amazing work that people are doing.  But I am pretty sure that what they are showing are not their first attempts.  And as perfect as we may find their latest attempts, they are probably already thinking about improvements for their next attempts.

Jim