With all the bugs worked out of the program and the model completed it was time to get the thing off the bench and onto the layout. This should be a simple matter of dropping the module into place and plugging in the power. It turned out to be sooooo much more.
The physical positioning was the first thing. I had recently added the final top layer of Homasote for the yard section of the layout. When I went to drop in the turntable module I found two problems. The biggest was that the framework for the layout was in the way and had to be modified. The second was that I wanted to rotate the module to fit the gear hanging under it for the easiest access for maintenance. When I rotated the module it wouldn't fit in the hole. I ended up running the base through the table saw again to square it up. I also had to clean up the edges of the hole where the Homasote didn't match well.
With that completed I got the module in place and the turntable bridge reassembled. I now had to deal with several inaccuracies I had ignored during construction. One end of the bridge was 1/16" higher than the other. I thought this might be caused by the dolly truck installation where I had added some shims to get the rail up to the right height. I ran the wood buck that the bridge is built around through the table saw and squared that up as accurately as I could. The I used my digital calipers to measure the thickness of the bridge ends and found a 0.030" difference there. It turned out that the dolly trucks had a large variation in the thickness of the castings. Rather than trying to correct this I used a hand file on my shims to make up for the difference. The bridge now sits flat and is square with the world.
My next problem was that the pit wall isn't perfectly round in one spot and one end of the bridge is too close when driving past this area. I want to have 1/16" clearance all around. I took another look at the bridge and found that the wood framework extends just a hair further on one end. I filed and sanded that and now have those problems taken care of.
My next issue was getting the bridge to go through a full rotation smoothly. I wanted to have as little weight as possible on the dolly trucks to prevent unnecessary loads on the stepper motor due to small variations in the ring track height. I added a second bearing at the base of the turntable shaft and this solved the problem by removing any side-to-side movement.
Now the turntable operates smoothly and the module can be taken out and replaced easily. My next problem was with repeat-ability when going from one track to another. I found there was an error of about 0.030" in alignment when reversing direction of rotation. The drive belt had too much slop in it but if I tightened the idler too much it would load down the stepper motor. I found that I could add two more idler pulleys between the stepper motor and the turntable shaft pulley. These can be adjusted to add just enough tension to remove any slop in the belt.
To finish off the turntable I painted the pit walls a concrete color and used Bragdon weathering powders to add streaks and stains to the concrete. In the floor of the pit I added a layer of dirt and then a layer of cinders that will also be used on the rest of the yard area. The dirt I got from my sister's back yard in Pennsylvania. I couldn't use local dirt around my home because I live in Florida and what they call dirt is mostly sand. The cinders I got from the Arizona Rock and Mineral Company (click here). These guys have a great selection of scenery material. I used HO and O scale cinders because I wanted both fines and granular. To finish it off I added small clumps of weeds here and there.
With these these things taken care of the operation of the turntable is reliable, repeatable and really enjoyable.
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