Photo Essay: Making an Atlas Deck into a True Pit turntable

True Pit - The Saga of my Atlas Turntable Conversion

Author's Note:  This was originally posted in July of 2007.  It occasionally surfaces when someone adds a note or a question to the end of the thread.  Thanks to all of you for all the nice things you've said about the thread, and all the good questions which help us all understand the project better.

For a while, I was happy with my Atlas turntable.  It's a deck, and it indexes every 15 degrees.  I bought an Atlas roundhouse to go with it, and after putting the kit together, weathering it a bit and spending a lot of time detailing the interior, I was satisfied for a while.  Then, I started to see other turntables, and I just wasn't content any more.  I wanted a pit turntable, but my 5x12 foot layout table just didn't have room for one of the larger ones, and besides, I already had a turntable, motor unit and roundhouse.  So, after looking at a couple of conversions that others had done on-line, I decided to tackle a pit conversion.

My layout is built on 2-inch pink foam.  I took a marker pen and drew around the perimeter of the turntable, including the motor unit.  Then I used a drywall saw to cut out this keyhole-shaped opening.  It was fast, but it did make a mess.  I took a piece of Masonite and cut it in a rectangle, a few inches bigger than the opening.  I used 4 carriage bolts to hold the Masonite to the bottom of the layout, with the nuts on the bottom.  If this thing ever has to come out, it will be from below, not from above, as the stall tracks will overlap the turntable's fixed rim and keep it from being lifted out.

The one drawback of the conversions I'd seen was the rotating deck floor.  The standard solution is to attach a bridge to the deck, run wires up from the old deck rails to the new bridge rails, scenic the deck to look like a pit floor, and install the whole thing below the surface of the layout so the rails come out even with the lead and stall tracks.  Since the whole pit floor rotates with the bridge, the pit rail and bogies don't have to work, but I wanted to go one step further.  I figured that I could mount the bridge in the center of the deck only, and build a false floor which would attach to the outer rim of the table, leaving only enough space in the center for the hub on which the bridge was mounted.

First, though, I needed a bridge.  I picked up an Atlas deck bridge, which is the same 9-inch length as the deck track.  I loosely assembled it, and found that it was much too tall.  Here's the unmodified bridge sitting on the deck floor.  It's too high, even without allowing for the extra half-inch or so for the false floor.

So, I cut all of the diagonal and vertical parts and re-attached the top and bottom of the bridge, incidentally leaving a gap at each end for the bogies.  In this picture, I've painted the bridge with rust-colored primer, and attached the hub.

Next I needed that rotating hub.  For some unknown reason, I had an old 45-rpm record adaptor on my workroom windowsill.  The perfect hub - about an inch and a quarter across, and maybe half an inch high.  It also had a perfect center hole, which I didn't need for my design, but I realized it would help me line up the pieces when assembly time came.  The hub had to rest on a broader pedestal, which would also support the center of the false floor.  For this, I simply cut some squares of styrene, drilled a hole in the center for the wires, and glued them together.  These would be beneath the false floor, and wouldn't show.  I sprayed the bridge with a rust-red primer, and painted the hub with cheap black acrylic.  I soldered track wires to the deck rails and ran them up through the pedestal.  Then I glued the pedestal to the turntable deck, and the hub to the bridge.  Finally, I attached the bridge assembly to the pedestal, but I used tacky glue because I really needed to test the alignment.

Here, the bridge is sitting loosely on the styrene pedestal.  This is a rare shot from the far side of the layout, showing the turntable motor in its cutout.  I drilled holes in the Masonite for the drive and track power wires.

This was the trickiest part of the project.  I had to achieve positioning tolerances of well under a millimeter, or the engines wouldn't transition smoothly from the fixed rails to the bridge.  I had several false starts before I finally got it right, and then I was able to add CA to my mount to fix the bridge permanently in place.  I realized that the Atlas deck bridge gave me an unforeseen benefit.  The deck rails slide into the bridge and are held snugly by friction mounts, but they can be slid back and forth.  I used this to compensate for a slight lengthwise mis-positioning of the bridge.

Finally, I was ready to try an engine.  I rotated the turntable up to the lead track, and slowly brought an engine on.  It rolled smoothly on to the track, so I kept going to the stall directly across.  That's when my design flaw became apparent.  The small hub at the center, coupled to the long bridge, just wasn't stiff enough to support the weight of the engine, so the ends of the bridge would sag as the engine moved away from the center and unbalanced the whole thing.  So, Plan B went into effect.  The bogies, which I had planned to make ornamental, now had to be functional.

Now it was time for the pit floor and pit rail.  The floor was simple, just a disk of styrene cut to the full size of the outer rim of the turntable unit, about 10 inches across.  I cut a wedge out of one side so I could slip it around the hub, and then a circle in the center for the hub itself.  I built up a wall of foam pieces so the pit floor would sit evenly between the wall and the pedestal.  I glued the outer edge of the pit floor to the rim wall, and let the inner edge sit loosely on the pedestal.  For the rail, I took a piece of flex track about 27 inches long and cut it in half the long way, so I had two pieces of single-rail track.  I put one away as a spare, and carefully bent the other into a circle.  You really have to bend the whole thing smoothly by hand, because this is too much flex for the track to take without complaining.  Once bent properly, it should hold together with a rail joiner and not need solder.

I agonized for months over the bogies, literally.  Finally, I took a set of good Branchline trucks and removed the good metal wheelsets, replacing them with cheap plastic ones.  I removed the wheels from the axles, turned them inside-out and pushed them to the center of each axle, so I had a double-width wheel in the center with a flange on the outsides of the wheel.  I mounted these in the trucks, and built a frame from some scrap angle aluminum and styrene.

The false floor is nothing more than a disk of styrene, which rests on the foam blocks around the outside of the turntable rim, and on the square styrene pedestal in the center.  Since the bridge was already glued on, I cut a wedge out of one side so I could slip the floor in, and then replace the wedge.  I glued the floor to the outer rim blocks, but the styrene rotates with the deck so the floor just sits loosely on that. 

I loosely glued these assemblies to the turntable bridge, and powered up the turntable.  I marked where each wheel ended up at each of the indexed positions of the turntable, giving me a pair of circles on the pit floor.  They were close enough to the same circle to proceed.

I glued the pit rail to the floor with CA, starting at the center of the rail and working around in both directions.  When I had about a quarter-circle to go, I did a final measurement and clipped the excess rail length.  Then I joined the rail ends and applied the last of the CA, adding a couple of extra half-ties just like any other tracklaying at a rail joiner.

At this point, I was ready to attach the bogies permanently, after checking to make sure they went smoothly all the way around, and didn't leave the pit rail either horizontally or vertically.  The design of the bogies was to build them up from small strips of styrene, so adjusting the height was a simple matter of adding or removing strips.  After attaching wires to the rails, I brought in my trusty S1 (let's not get too carried away with steamers just yet) and ran it directly across the bridge from the lead track into a roundhouse stall.

Mechanically, the turntable project was done, but the scenery is important, too.  Since the roundhouse is an integral part of the scene, I had to finish the detailing and interior lighting before I could permanently attach it to the layout.  I actually ended up re-mortaring and re-weathering the exterior walls, while adding lights to both inside and out.

I'd been looking for an excuse to get some more of Dave Frary's rock wall molds from http://www.mrscenery.com/, and this was it.  I used his "Weathered Rock Wall" mold for the pit wall.  I cut off a small section of that keyhole-shaped foam chunk that came out of the layout base a couple of pages ago, and attached a small sheet of styrene around the curve.  Then I mixed up a batch of Hydrocal and poured the casting.  After about 10 minutes, Hydrocal sets up to a thick, semi-solid state, but it can still be formed around a gentle curve.  I made 3 of these curved castings. 

The styrene floor was not very interesting, so I covered it with Gypsolite.  I squirt a bit of brown acrylic paint into my Gypsolite to give it a light tan color.

Once they were hard, I cut the castings to the right height for the pit wall.  I got two good sections of wall from each casting, carefully cutting with a Dremel.  I used the nicely-formed outer edges for the top of the wall, and the flat Dremel-cut edges for the bottom.  I sprayed the wall sections with a Rustoleum speckled spray can, and did additional weathering with an India-Ink-in-water wash.

While I was doing the castings and waiting for them to set, I applied paint to the pit floor, using washes of green and brown.  I ballasted the pit rail, after painting it.  The rail doesn't carry power, and any leftover scenic material won't get on your car or engine wheels, so go to town on this one.

Once the pit floor paint dried, I added turf, ground foam and talus to the floor.  I loosely fitted the raw castings for the wall.

I sprayed the pit wall castings with Rustoleum speckled paint, and weathered it with an India-Ink-In-Water wash.  Once they were dry, I put the castings in place around the pit.

Finally, I made final adjustments with everything in place, and fastened the turntable base to the Masonite with some small styrene shims and tacky glue.  It's not going anywhere, but I just wanted to keep it from vibrating out of position.  Then, I glued in the pit wall sections and added scenery around the whole thing.

Ready to roll.  I've got to tweak the alignment a bit, but this is a project I was pretty happy with.