Scratch Built Turntable

I am writing this article as a general explanation of the process of designing and constructing a scratch built railroad turntable for my Wildcat Valley Railroad with the hope that this will inspire others to attempt the journey. Much of the detail of actual design and construction is up to the reader since there are many methods and materials which can be used to attain the same result.

I found a couple of items on the internet for ideas and inspiration. The first is a photo that shows the power arch that I would use as a model for my turntable.

The drawing below shows detail for the decking and some general information.

The old WVR turntable was a Walthers 90′ Kit that I modified to operate with a stepper motor controlled with an Arduino processor. This worked pretty well but due to the construction did not turn as smoothly as I would have liked. The new turntable I designed is 115′ long and will accommodate larger engines. I would use ideas from the references above to design my free-lanced turntable. The first order of business was to assemble the components I would need for the build.

Rounding Up the Parts

I found a Nema14 stepper motor with 400 steps/revolution which would double the number of steps of the motor from the previous version. I found a 30:1 worm gear set with a worm the would fit the 5mm shaft on the stepper motor. The worm gear was bored to 6mm, so I found some 6mm diameter brass tubing with a 1mm wall to use to turn the bridge. To get power to the tracks I used a slip ring which has 6 leads rated for 2 Amps each. A slip ring allows an electrical connection using internal contacts between a stationary and a rotating member.

Other Items Purchased

Once I had identified the components needed, I began designing the turntable using FreeCad. I started with the turntable pit. Since plywood is never the stated thickness, I purchased this material in 2′ x 2′ sheets first so I could measure the thickness accurately since the height of the rail top on the bridge needs to be as close to the height of the top of the approach track as possible. I adjusted the height of the bearing mount accordingly. I also wanted to cut the diameter of the turntable pit first in order to get this dimension before modeling the pit walls which I planned to 3D Print.

Building the Pit

I fashioned a home-made circle cutting jig from Masonite and attached it to my router. I drilled holes for the radius that I wanted to cut. I screwed 1X4’s to the bottom of the plywood to hold the center piece in place until I was finished with the router. As you can see below, I needed to drill a few extra holes in the jig to get the diameter that I wanted. I used a 1/4″ bit to make the cut. I also cut a step around the outside to accept the lip of the wall.

Below you can see the rough pit in place on my layout. I cut a hole in the 1/4 plywood to make the shelf where the pit rail would be mounted. The 1/2″ plywood was drilled for the bearing mount and centered in the pit.

I created the 12 wall segments using Free CAD design software and printed them on my Mars 3 printer. It is hard to see, but there are block outlines and mortar grooves on surface of the inner diameter.

The photo interrupter protrudes through the wall. This will be used to detect a tab on the turntable bridge which will set the home position at start-up.

Below is the stepper motor and worm gear set assembled into the 3D printed mounting frame. At the end toward the camera is the slip ring. The wires for bridge rail power exit the tube at the opposite end. The four heavier gauge wires are the stepper motor power connection.

I attached the printed pit walls with Loctite Power-Grab and sprayed the walls and pit rail shelf with gray primer then added some color to the block wall with acrylics. I used a wash of India Ink to accentuate the mortar lines and dull the finish. I carefully cut two pieces of code 70 flex track ties to a length that would set against the pit wall to create a circle. I added a rail joiner to connect the two and soldered the joint to make a rail section long enough then trial fit the track and cut it to length. For the two ends that would need to be joined in place, I pre-bent about two inches of the rail to match the radius of the pit wall. Next the rail section was painted with camo brown and when dry I glued the rail in place with gel super glue.

Once the motor assembly was mounted underneath the pit, I tested the stepper drive and found it to be a bit noisy. It sounded as if there was a vibration but only in one direction of rotation. I deduced this was caused by the worm lifting the worm gear slightly. This was cured by the addition of two more flange bearings on the 6mm shaft with a spring to create some downward force on the worm gear.

I used vinyl spackling to create a sloped floor from the 1/4″ high track shelf down to center of the pit. I sanded and re-applied the spackle a couple of times until the floor was smooth. I cleaned up the mess I made on the ties of the pit rail and used a couple of colors of gray to cover the white.

The brass tube was cut to length and this 3D printed connector was glued in place. I fasioned the wipers from .008″ phosphor bronze and soldered the wires from the slip ring to them. There are two wipers for the track and two for lighting on the bridge if I choose to add some. The wipers fit in slots designed into this bridge connector. They wipe against circuit boards mounted in the mating component which makes up the center of the bridge. This piece has a flat washer glued to the underside which rides on the flange of the upper bearing of the motor assembly.

Underside shot of the turntable bridge showing the circuit boards that complete the connection. This center section fits tightly over the bridge connector to make it removable for cleaning and maintenance.

Building the Bridge

Below is the bridge assembly model in Free CAD with the wheelset in place

I laminated two thicknesses of .030″ X .156″ styrene to create the longer runners which would be used to mount the track ties. The center 3D printed section, the two cross braced 3D printed sections were glued in place with gel super glue. The two cross brace sections have pins designed to locate against the styrene runners for assembly. The four sides were cut from .060″ thick styrene and .080″ angle stock added to simulate vertical bracing and cut to length. These sides fit into slots in the center section and against the sides of the cross braces.

Here is a pic with the 3D printed power arch temporarily in place. I modeled the arch in Free CAD and printed it in one piece. The support structure was a bear to create and then remove without destroying something, but I got it done. I trimmed the excess from the runners on the bridge and painted both these pieces components with flat black. The wheelset bogies mount in the notches in each end of the bridge. Be careful to get the spacing and height correct in order to have the wheels ride on the pit rails correctly. I made a mistake with placement of one of the wheelsets and had to remove it and try again to get it right.

The 3D printed wheelset frames were painted then assembled, then I ran a drill through the holes for the bearings which was slightly over-size to make sure the centerlines of the bearings would line up and allow free rotation of the wheels. I also designed a motor assembly to mount under the control booth to give the appearance of a geared electric motor drive system.

I glued the arch to the center 3D printed section of the bridge with super glue. I stained the ties and decking pieces using India ink thinned with alcohol. The ties were glued in place with gel super glue and the remaining decking was assembled with Aleene’s Tacky Glue. I glued the upper platform in place with super glue.

The wire connection at the top of the arch was made from a length .032″ music wire inserted thru a piece of styrene rectangle. I bent the end slightly and gobbered on some super glue to resemble an electrical service head. Looking at my reference photo, it appeared that three wires were terminated on the horizontal piece and fed into this service head. I glued #38 magnet wire in place to simulate this.

The railings were made by 3D printing the posts with 2 – .020″ holes through for the railings. I painted the posts yellow, then drilled out the holes to make sure my .015″ dia. music wire would slide easily through. I glued the posts in position and carefully inserted the wire railings and glued them at one end. I then painted the wire yellow.

The control booth is made from scribed basswood sheet. The doors and windows are 3D printed with .005 styrene “glass”. The roof is corrugated aluminum from Northeastern Scale Lumber given a weathering treatment to remove the shine.

I mounted all the items shown in the diagram below on the underside of the turntable pit with the exception of the control unit which is mounted on the facia above my engine servicing area.