The Turntable, Part 2

Almost a full year ago, we took a look at the turntable project for Marceline. Which can be read here. Since then, I had a rethink about the plans surrounding this aspect of the project. We had tried a few alternatives around building the turntable bridge and making it work based on the Atlas turntable and it's construction. However, the drawbacks started to mount. I remember reading once that the value of a reviewer is to make mistakes so the reader doesn't have to.

Turntable MK1

To start with, the Atlas turntable has a Geneva drive that that translates the continuous rotation of the handle into intermittent rotary motion of the table, thus allowing it to automatically index itself with tracks. This drive is what moves the bridge based off the handle or motor kit that Atlas provides. When we pulled the large circular full deck bridge off, this aspect of it's motion was lost as was the drive. Which forced us to try and rig up a few alternatives.

First, we tried cutting the original deck apart into a bridge and running it off an Ho scale drive motor from MTH. This resulted in a gear reduction ratio that was near impossible to channel in for a realistic speed on the bridge. The end result being the bridge turning like a record player, or was too little and light to move the weight of locomotives. In the process, we destroyed one Atlas turntable.

Oh well, back to the drawing board.

Turntable MK2

One order from Atlas later, and our second turntable was under construction. Like the first, we cut the bridge out and this time tried to mount a handle and gear reduction to the outside of the module. So the turntable would be turned by hand. This worked for a while. That is until the turntable had a load put on it. In which case we sheered the drive shaft and cracked the gear that translated motion from the handle to the shaft.

Oh, and we destroyed that turntable in the process. 

Back to the drawing board...again.  

Having outright scrapped the original Marceline module in favor of a fresh start, the decision was made to abandon rebuilding the Atlas turntable. Instead, we would use the turntable as a foundation. An order from Atlas produced our third turntable. This time, however, it would not be the one the public saw. Looking over the problems, Tyler the Beguiler and I hashed out a pretty neat solution. 

The Atlas turntable and it's motor would be left stock. Instead they would both go at the bottom of the turntable pit. Which was dug deep enough to except them below the scale level of the pit floor. A new turntable bridge would be scratch-built to go atop the Atlas turntable. Taking it's power from wires soldered directly to the rails on the Atlas. A spacer would hold the new bridge at the correct level, and a false floor would go atop the Atlas to hide it's rotating deck from view. It was simple, elegant, and most of all, no longer required us to re-invent the wheel! 

The tabletop around the roundhouse was made from 2 inch pink insulation board. Which gave a great surface to cut the pit into. Once the pit was cut and the pit walls constructed from card-stock, the Atlas turntable was sunk into place. A foam-core top was built to sit around the turntable and over the motor pit. If for whatever reason the motor of the turntable needed to be accessed, it can be done so either by lifted up on this foam-core top, or by pulling the motor unit out from the bottom. 

 The design of the turntable bridge also was a big consideration. By the 1880s, wooden bridges such as the classic "gallows" style bridge were being replaced by ones made of iron. Many different designs were available for use. From iron girder style bridges that required manpower to turn, to electric and steam power. 

In 1890, John B. Collin was a mechanical engineer for the mighty Pennsylvania Railroad, based in Altoona at that road's engine shop. Plans had been developed in 1886 for a new shop, located in the Juniata section of Altoona, to be completed in 1890. Collin, who was a bit of a madlad, designed a turntable that was powered by a vertical boiler and steam engine. The engine was mounted on a separate operating platform with an arm that was attached to the main body of the turntable bridge. Powering a drive wheels underneath the working platform. While there is no evidence whatsoever of the historical W&A RR having owned such a turntable, it's design fit the bill for the fictional Marceline and was unique enough to warrant modeling. 

While this was going on, Tyler was doing an experiment related to another project. Building model railroad bench-work out of foam-core. While this is a topic for another post, one of the side results was that he had quite a bit of foam core left over. Thus, a usable material for building the turntable bridge became available. 

The bridge itself was made from foam core, with cardstock lamination to build up the plates of iron. Rivits where made by dipping a toothpick in white glue and 'dotting' it along the plate. The deck of the turntable was made by hand, using code 55 rail, and cutting lumber with a chop-it. The walkway boards where likewise hand cuts and stained individually before being glued to the deck. 

The space around the bridge was leveled using more foamcore. This will have to be painted and sealed prior to any scenery. Otherwise the foamcore will swell.

 The engine that drives the turntable was scratch built using a passenger car stove from a Bachmann On30 coach. Other

shapes where made from stryine and the cylinders and levers where from a Tichy crane kit I bought second hand as parts. It was only after I built the boiler, engine, and it's platfrom that I discovered that I already had a vertical boiler casting in my parts bin. Oh well, I actually like the current boiler a little better. 

A lot of work left to do. Yes sir. A lot of work.