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Making Your Own Turntable?Click to see larger image

Why the DIY Approach isn't always a good idea

By Craig Bisgeier

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As an early-period modeler (pre-1900) I knew early on I was going to need several small turntables on my layout.  Most of the commercial kits were too large, but there was one commercially available choice – Diamond Scale Models makes a nice kit for a 65’ turntable that would work well.  The problem was it was expensive, and I figured I could scratch-build my own for less money.  Like many decisions reviewed in the glaringly bright contrast of hindsight, I was to find out this was not the case.

Shopping

I started by trying to figure out what mechanical parts I’d need to move the turntable.  Diamond Scale sells the mechanical parts for their turntables separately, but they don’t make it easy to figure out what you will need if you don’t already have their turntable kit.  Plus, buying their mechanical components separately is almost as expensive as buying the whole turntable kit – without the turntable!  I decided I needed to find another source for mechanical parts. 

I was able to get almost everything I needed (bearings, shaft collars) from Small Parts, Inc.  I recommend that you buy your shaft stock (round steel rod for driveshafts) locally at a hardware or home improvement store because it costs a lot to ship.  Figure out everything you need and buy it all at once to save on shipping costs.  It stinks to have to order two 39 cent parts and pay $8 in shipping to get them. I bought several extra parts as well for just in case.  I also figured I might build another turntable later so it would be best to have the parts available.

I didn’t buy gears there from Small Parts, they were very expensive.  In fact, everywhere I went the gears were really expensive!  I eventually found a very inexpensive helical (worm) gear and a matching flat gear from a robotics supply house on the internet.  I soon learned why they were cheap – they were cheap.  The parts were resin castings instead of metal or high impact plastic, and the quality wasn’t very good, but I was able to make the parts work. 

The Pit

The turntable pit was fairly easy to build with my collection of woodworking tools.  I glued up two sheets of ½” Medium Density Fiberboard (MDF) 3” wider than the finished pit diameter face to face and let it dry.  Then I screwed it to a wide cleat across the bottom, and marked the top of the board from corner to corner, making a large “x” through the center.  I drilled a small hole at the center for the next step. 

I used a simple circle-cutting jig for my router, which uses a pin on one end that rests in the small hole I drilled at the center of the MDF.  I installed a ¼” straight bit, Then I made several cuts at different radii.  The first cut was made at 4-1/8” radius circle and cut 7/8” down, which defined the pit wall and the raised step that supports the pit rail.  Another cut was then made at 3-7/8” radius, and this cut went all the way through the 1” sheet. 

The basic turntable pit, cut with a router from laminated boards of MDF and masonite.

The cleat across the bottom held all the parts together even after the router cut all the way through the MDF.  Finally I made a third cut at 4-5/8” radius all the way through which left me with a ½” wide ring with a ½” x 1/8” tall step on the inside edge. 

For the pit floor I used a leftover piece of ¼” Masonite I had lying around.  I drew a big X on it before mounting the pit ring to it, and used the marks on the pit ring to align the floor as I glued it down.  Then I drilled out the center of the floor and installed a ¼” I.D. bronze shoulder bearing for the turntable shaft.  I painted the pit a couple of times trying to find a good neutral concrete color.  I settled on a tan / gray color that looked like early concrete to me.

When I went to install the pit rail, I realized the shelf I left to lay ties on was too narrow.  I was able to fix that quickly by taking some 1/8” x 1/8” stripwood cut into scale 8’ lengths and laying them in front of the shelf. 

The shelf for the pit rail wasn't wide enough, so I had to add ties to the edge to widen it.

Then I set out to install the pit rail.  I started by trying to use Micro-Engineering flextrack cut down the middle, but the ties wouldn’t stay on.  I looked at Central Valley tie strips but they were WAY too big for my 1890’s layout.  I decided to use the wood ties I’m using for the rest of my RR.  I dyed them with brown shoe dye, including the stripwood in the pit.  These were glued in easily and I threw some cinder ballast over them to cover the gaps between the MDF shelf and the stripwood.

Installing the ties that support the pit rail.

At this point I had to think about powering the bridge.  I had several options but I chose to use a split pit rail design because it would be simple to wire.  This means the rails are electrically split apart evenly, positive on one side and negative on the other.  You have a gap and a ‘dead area’ between the two rails.  Contacts on the ends of the bridge wipe the pit rail, and provide power to each of the bridge rails to ensure the bridge won’t cause a short.  This turned out to be a decent solution.  I formed the pit rails (code 55) using a pair of pliers to bend the rails, and soldered feeder wires to their undersides.

From there I went to spike in the rails.  Guess what?  I learned MDF won’t take spikes.  Not at all.  So I spent I spent a LOT of time using a #74 drill, making holes through the ties into the MDF before I could spike the pit rails down into it.  I had to reinforce many of the joints with CA glue, but in the end it worked out all right.

The ring rail, finally installed.

The Bridge

I started the bridge by figuring out the dimensions it would need to be, about 2/3” wide by ¾” deep and 8-1/4” long for a 60’ long span.  I cut a form out of maple hardwood and then trimmed it to match the shape of the bridge I wanted to build, which was deep in the middle but narrower at the ends.  I had some Micro-Engineering bridge girders I’d bought a while back for this purpose.  A little creative kitbashing and I had a pair of girders the right shape and size.  It was quickly evident that it was going to be deeper that I wanted it to be (over ¾”), but I felt I was already committed by the depth of the pit.

I carved two slots in the bridge, one on each side at an end from top to bottom to allow the bridge rail feeder wires to reach the underside of the bridge.  I found the exact center of the bottom of the bridge and drilled a 1/4” hole in the bottom perpendicular to the bridge surface (OK -- I had to do it 3 times before I got it right) and epoxied a ¼” metal rod into the hole, being very careful to hold it perfectly vertical until the glue set up.  This metal rod is the pivot and drive shaft for the bridge.  Finally I painted the wood form black. 

The turntable bridge under construction.

When the paint dried I used epoxy to glue the bridge girders to the wood form, being very careful to keep each of the girders properly aligned as they dried.  I sanded the top of the bridge down a bit when it was all dry, and then decked it with heavy square bridge ties held down with carpenter’s glue.  Once again, I sanded the ties down a bit when the glue dried.

Ties down on the Bridge.

I did get one important detail part from Diamond Scale, their small bridge bogies.  This is a white-metal kit with free-rolling brass wheels you put together yourself.  Since it’s metal it can be used as an electrical pick-up against the pit rail.  I did an all-right job with assembling it; I think I got a little epoxy in one of the wheel wells because it didn’t turn when I was done.  Balancing the bogie on the pit rail, I laid the bridge down on it and taped it in place so the wheels straddled the rail evenly, then used the provided screws to attach it to the bridge.  Slight adjustment was necessary (my pit rail didn’t turn out too even) but after a short while they worked well enough.  The bogies have elongated screw slots to make the alignment process very easy.

Diamond Scale bridge bogies riding on the pit rail.

It was time to get the rails down on the bridge.  I cut the rails extra-long to ensure they wouldn’t end up short, then soldered a feeder wire to the underside of each.  I threaded the wires through the bridge and spiked the rails down.  Well, I tried to -- the plastic bridge girders proved difficult to spike through too.  I had to make use of the #74 drill again.  They turned out a little wavy but they were in gauge, which as far as I was concerned was fine for now.

Rails spiked down to the bridge.

Flipping the bridge over, I took out some .005” shim brass and fashioned a pair of track wipers to place at either end.  I didn’t trust the bridge bogies to pick up the current reliably.  These wipers straddle the underside of the bogies and are the actual power pickups for the bridge from the pit rail.  I screwed them down to the bottom and wrapped the feeder wire around the screw, which completed the connection.  A test of the system, with a multimeter attached to the pit rail feeders and the bridge rails shorted showed a complete circuit.  Hooray, it works!

The electrical wiper underneath the bridge bogies.

My excitement about the electrical system working was quickly tempered by the discovery that the bridge didn’t rotate in the pit well.  In fact, it tended to bind up badly through more than half its rotation.  How could this have happened?  I had been so careful through each step of the process!  I was heartbroken, as there did not appear to be any way to fix the problem.  At that point, I gave up on the project and stored the parts away, on the chance that someday I would figure out how to fix it.  Reluctantly I admitted defeat and ordered a Diamond Scale turntable kit.

The stillborn turntable, after developing a fatal design flaw that I did not know how to fix.

Breakthroughs

Well, it turned out that buying and building a Diamond Scale kit was the best thing I could have done.  By going through the process of building their kit, I learned all the answers to the mistakes I had made on my first attempt.  The Diamond Scale kit came out great, and now works flawlessly at Wilson Point on my Housatonic Railroad layout.  Got to give them credit, they have really sweated the details and make a product that builds up well with moderate skills, and operates great. 

The Diamond Scale 65' turntable pit - a work of art

When I was finished building the Diamond Scale kit, I was psyched again to pull out my original turntable and upgrade it based on all the new tricks I had learned.  The first thing I went to work on was the bridge.  I had quickly learned the secret to keeping the bridge from binding as it rotated – the driveshaft mechanism and bridge should be separate parts that are not rigidly connected.  I disassembled the old bridge down to the hardwood form and the metal rod, and cut away most of the form leaving a block much shorter and narrower than the original.  I then rebuilt the rest of the bridge leaving the center section hollow, the hole being large enough to fit loosely around the drive block. 

The small wood block in the center of the pit is used to push the bridge around, without a secure connection between the two parts. The bridge core rests on the turning block in the pit.

Success!  The bridge is now free to rest on the pit rails and the drive block can flex around inside the hollow center, pushing the bridge along but not causing any binding problems.  I glued new ties down on the bridge deck, and then used Pliobond to fix the bridge rails to the ties.  This went sooooo much easier then the last time, and looked better too.  I used the same method to electrify it as I had before.

I didn’t do much to the pit because if I were going to do anything I would probably just cut a new one to fix all the problems I had.  As I already knew the pit rail isn’t completely level and that caused some issues getting the bridge to align correctly, but it isn’t critical.  I’ll apply the lessons I learned to the next turntable I build – make the pit shallower, level the ties supporting the pit rail to the top edge, use wood to make the pit rail ledge, and leave the top of the pit assembly square so there is a mounting flange.

Animation

The last major step before installation was to put in the turning mechanism.  Before I’d given up on the turntable the first time I had started to build a drive mechanism.  It had worked fairly well, and I decided to re-use it.  I’d made it from one of the resin worm gears, some 1/8” steel rod, some small metal corner braces, a couple of 1/8” bronze shoulder bearings and a few 1/8” shaft collars, all mounted of a small chunk of plywood.  It was angled so the face of the worm gear would line up with a circle gear.  I reasoned since I still had it I would try to make it work.

So I drilled out a matching 3” circle gear to fit on the ¼” driveshaft sticking down through the bottom of the turntable floor.  I also drilled a small hole through the gear and into a ¼” shaft collar and epoxied them together with a short piece of wire to ensure the gear would not turn on its own.

The Diamond Scale turntable had an open assembly beneath the turntable that held the driveshaft in place, so I made one like it out of wood scraps and mounted it to the underside of my pit.  It had a hole through it and a bronze shoulder bearing to keep the driveshaft aligned at right angles to the bottom of the turntable.  Inside this assembly I mounted the circle gear on the shaft and put a shaft collar on the bottom of the driveshaft to hold everything together.

I mounted the drive mechanism to the bottom of the turntable with glue, and positioned it so the worm and circle gear met up and meshed snugly.  When the glue had dried I adjusted the circle gear to the optimal height and tightened down the shaft collar to hold it in place on the driveshaft.  That completed major construction on the turntable and it was now ready to install.

Installation

We decided to install this turntable in Dock Yard to turn locomotives that terminated there.  There had been a small turntable here in the 19th century so it was reasonable to do that, even though it had not been planned into the design.  We found a spot for it in the back, squeezed between the backdrop and the main yard ladder track.  We were able to install a turnout of the little yard lead I have that lined up with the turntable, so it seemed like a good place to use it.

The completed DIY turntable at Dock Yard in South Norwalk on the 1892 Housatonic Railroad layout.

To be honest, I did not perform the installation of this turntable, a couple of my friends did.  So I can’t give any details about how it went, except to say that they were not happy the pit did not have a mounting flange.  That made it much harder to put in, they tell me.  I will certainly take that into account the next time I build a turntable.  That said, they did get it installed and did a pretty good job too.

I came in when it was time to install the drive mechanism.  I have a lot of experience installing the Diamond Scale drive mechanism for the Wilson Point turntable, and was able to use that for this one too.  The Diamond Scale kit comes with a nice plastic hand crank, universal joints and bearings.  I had to make my own crank from wood and buy U-joints from a model airplane outfit.  I was able to use the bearings and shaft collars I already had around from my initial purchase. 

My Do-It-Yourself turntable hand crank.  It works all right!

My crank wheel was epoxied to a ¼” steel shaft that extended through the benchwork and supporting girders behind it.  From there it attached to a ¼” to 1/8” U-joint, and an intermediate 1/8” shaft and 1/8” – 1/8” U-joint completed the connection to the mechanism attached to the turntable.  This is the same method Diamond Scale uses to mount their crank mechanism.  When all the connections were made and the epoxy cured, we turned the wheel – and the turntable rotated!  It was a great moment for me, I was very proud.  It’s not as smooth as the Diamond Scale model but I’m just happy it works after all the money and effort that went into it.

We hooked the turntable up to the same auto-reverser / DCC circuit breaker that runs the turntable at Wilson Point, so there is no noticeable delay or hiccup when the loco changes direction / polarity.  There is an electrical gap in the pit rail though so sound units do turn off and back on again as they get turned.  All future turntables will be wired so one contact runs through the driveshaft and the other through the pit rail, which avoids this problem.

Reflections

In the end, I did what I set out to do, scratch-build a working turntable for myself.  But as you can see from my experiences, the adventure was fraught with mistakes, frustration, dead ends, and wasted time and materials.  The necessary mechanical parts were difficult to find and expensive to buy, and where expensive precision parts were substituted for, quality was lost. 

On the other hand, purchasing and building the Diamond Scale kit was a pleasure.  It was expensive, but in the end only a bit more than what I paid for all the parts and materials I needed for the scratch-building project.  The kit came with everything I needed; gears, bearings, shaft collars, all precision parts included except for some metal rod I had to buy separately, and the pit came partly assembled to ensure good alignment with the driveshaft. 

But perhaps the most valuable part of the Diamond Scale kit was the instructions and experience gained in building the kit.  It was an eye-opening process that taught me how to build a good working turntable.  Had I bought and built this kit first, I could have saved a lot of time, money and frustration when I went to go and build my own. 

So if you are considering trying to build your own turntable, my advice to you is “Don’t do it”, until you have built one of these great kits.  If you only need one, bite the bullet and buy the kit.  If you will need more than one, buy and build the kit and when you are done, you will know how to build all the turntables you want.  If you simply must try it, I strongly suggest trying to get your hands on some pictures of a Diamond Scale turntable in kit form and copies of the instructions.  You may not have as hard a time of it this way, but you will still need to locate all the needed mechanical parts.  You’ll spend as much as you would have on the kit and it probably won’t go together nearly as nice.

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