In the beginning.....

Over the years I have been the recipient of two Atlas 9" turntables.  The intermittent rotary motion of the Geneva Movement is not prototypical, but the indexing is perfect.  Another issue was that the 2-8-0 Consolidation loco was too long for the Atlas deck.  The initial plan was to build a 10" pit and bridge with the Atlas underneath providing rotation and indexing. 

Last October Pelsea started a thread about Turntable Wishlist and Development which got me to rethinking this project.  In that thread Jim at BSME provided a link to a thread on RMWeb about a DCC controlled Peco turntable using an Arduino.  I liked the concept that a stepper motor could be controlled by an Arduino to power the turntable so the Atlas drive concept was abandoned.  Based on the RMWeb thread a new turntable was drawn up using the stepper drive and an MDF pit.  The project is detailed in the following posts.

The Bridge

As it turns out, ExactRail makes a 72 ft deck plate girder bridge with the Rio Grande logo which is the perfect size for my 2-8-0.  Here is the bridge as received sitting on an Atlas turntable.  Just noticed both Atlas TT's are shown.

To power the bridge I sourced a 0.9° stepper motor from Stepperonline.  This motor has 400 positions, and when microstepped, it jumps to 6,400 steps per revolution.  Some pretty fine resolution.  From Servocity.com I acquired a hub, 1/4" D-shaft,  pillow block bearing, and a 1/4" to 5mm coupler.

The bridge was cut down and a 1/4" MDF baseboard was fabricated with 10-32 tee nuts installed.  I took a spare pair of trucks, put the wheels face-to-face on one end of each axle and mounted to the baseboard.  A brass plate was drilled for the hub and two oversized holes to match the tee nuts.  The large holes will allow the bridge to be adjusted both laterally and lengthwise.

And here is the bridge with the parts assembled. 

The Arduino will require some way to know where in rotation the bridge is located.  Most of the discussions on the RMWeb thread and Pelsea's thread involved rotating past a sensor (IR or magnetic) to find the start position.  In one post Pelsea mentioned an absolute encoder.  Attached to the lower (short) shaft of the stepper is a CUI  AMT203-V absolute encoder available from Digi-Key.  It requires a very small 14 pin connector with really, really teeny pins crimped to a flat ribbon cable.  

The Arduino Mega

I had a spare (actually several but that's another story) Arduino Mega on hand to control the turntable.  It has 70 I/O pins so there is pleny of room for push switches to control the TT.  The CUI absolute encoder cable uses pins 50-53 plus 5v and gnd on the Mega.  To connect the Arduino to the stepper requires a motor driver.  I selected the Adafruit motor shield v2 per the RMWeb thread.

I contacted CUI and Jason Kelly sent a sketch for the CUI encoder that reads the encoder and prints the position on the Arduino Serial Monitor in 4096 steps.  (Takes some math to convert the CUI 4096 steps to the steppers 6400 steps)

For controlling the stepper I started with the sketch by Cambriancoaster on RMWeb page-8, post #194. I commented out the lines with Astepper1.setCurrentPosition(0); and experimented with different values in the lines that say "...moveTo(xxxx);.  I also experimented with the values for AFMS.getStepper to adjust the acceleration and speed.  The acceleration/deceleration allow a smooth start and stop of the rotation.  Each of the four push-buttons rotates the bridge to the hard-coded position.

Key elements of the two sketches have been combined to create the following sketch.  I haven't yet added the location initialization in this sketch, and don't plan to spend time there until the turntable is installed.  I am posting this draft sketch for the benefit of any interested parties until such time as the final sketch is available.

[CODE][/CODE]

The Pit

In the March issue of Model Railroad Hobbyist Magazine was an article by Rob Clark about building a manual turntable.  His approach to building the pit using a router and MDF inspired me to move forward with my pit.

Thanks Rob!!!

I looked at some router circle cutting jigs on the web and decided to build my own.  I started with a 6"x24" piece of 1/2" plywood and used a hole saw to drill a hole for the bit.  I used a 1/4" panel bit in the router to cut the long slot, then a 3/4" bit to counter-sink the slot.  I drilled holes for the router base screws and counter-sunk them with a 1/2" Forstner bit.  I used the table saw and sabre saw to trim it to final shape.  

For a pivot I cut the head off a 1/4"x 2-1/2" bolt, added a couple of washers and a knob from the local hardware store.

And here it is assembled.

The pit is comprised of three layers of MDF.  The base is 1/2".  The ring is a 1/2" glued to a 3/4".  The two pieces for the ring were glued together then screwed to a piece of scarp wood per Rob's instructions.  A 1/4" hole was drilled in the center for the pivot.  The shelf for the rim track is exactly 1" below the top and 5/8" wide.  A shelf was routed on the outside diameter 0.785" deep so that the turntable can be inserted from below.  Finally the outside diameter was cut with the router.  The pivot was left in that position to cut the bottom.  I didn't take any pictures of the ring but you can see the shape in the drawing in the first post above.

The center of the base was marked and a 5/8" Forstner bit used to counter-sink the hole.  The center was drilled to 3/8" to allow for clearance around the shaft, pilot holes were drilled and the pillow block bearing was installed.  The router was then used to cut the outside diameter, same as the ring.  The ring and base were then glued together and painted with a light gray paint.

I chose to go with Atlas code 83 flex track for the rim track.  I used a 1/4" wide pinstrip tape to mark the desired width, removed the sliding rail and cut the ties to length.  The track was formed into a circle and cut to length to fit snugly into the pit.

Blocks were glued to the bottom of the pit to attach the stepper.  The stepper is mounted to the 1/4" MDF base with rubber mounts to absorb and vibration and smooth any jerkiness in rotation.  That base is screwed to the blocks thru over-sized holes to allow adjustment should any be required.  The router was used to cut the hole in for the TT in the benchwork.  Here the turntable is assembled and ready to install.  Glue joints between the layers of the pit are visible.

And here is the turntable temporarily mounted in the benchwork plywood with the cork roadbed and track to show how it all aligns.  The rails and electric pickup are not yet installed on the bridge and the control booth still needs to be built. 

The turntable project will probably now be set aside while I finish wiring the staging yard and build the helix.  A special thanks to all those who have helped and from whom I have borrowed ideas.

Nice start Dave

Nice Start Dave, looking good so far.

I will be watching this thread so I can automate my turntables.

Thanks, Will......

....Looking forward to seeing the progress on your turntables as well.