September 29, 2010

Turntable - Part Four

Time to install the turntable and see how it works.

I made a new shaft for turntable Mark II. The bands came out a bit nicer than the earlier version as I learned from that experience. I tinned the brass shim stock before wrapping it around the shaft, then a quick touch with the soldering iron secured it in place without melting the plastic shaft.  Note that I have not soldered the wire leads to the brass bands at this point. They are dangling from the center of the turntable.

Before attaching the shaft, I placed the turntable on the terrain surface at its intended site and carefully marked the center line of the pivot point. Then I drilled out the center with a 1/2 inch bit to create the pivot hole in the base plywood. The foam here is 3/4 inch thick.

Next I measured the center of the turntable and installed the shaft, taking care to keep it square and plumb. I epoxied the shaft and a laser cut collar on the bottom of the turntable ties. I could not laser cut everything to exact center due to the design of this turntable, so I had to measure the center by hand.

 Once the 5 minute epoxy was cured, I soldered the wires to the bands in holes that I drilled in the shaft.

The turntable pit after hogging out the foam. 
Shaft and collar before the wiper wires are installed.
I marked the perimeter of the turntable on the foam  with a pencil on the end of the turntable as it rotated. Then I hogged out the foam and removed any traces of hot glue that held the foam in place. Luckily there wasn't too much hot glue, but there was a glob right where the shaft went.

I tried two different height collars to see which one gave the proper rail height. Then I glued the appropriate collar in place. I  temporarily placed the turntable in the collar and kept it spinning while the glue on the collar set. I again used 5 minute epoxy as it sets faster than yellow glue, yet gives time to make adjustments and is very strong when dried. When that epoxy was cured, I again checked the turntable and made some minor adjustments. Then I went underneath and glued a second set of collars on the bottom of the plywood. These two collars are providing the precision placement of the turntable, not the hole drilled in the plywood.


Once everything was cured, I tested the turntable again. The spin is nicely centered and smooth. It is easy to line it up by eye, a good thing as this is a manual operated turntable.

The shaft and collars not strong enough by themselves  to prevent the turntable from rocking as the engines roll on them. These engines are quite heavy, perhaps three or fours pounds, each with the weight concentrated in the tender. In the prototype the turntable wheels control the rocking, but I am not sure the model wheels will be strong enough.  I think I will install two rub plates under the ends of the turntable that will lightly rub along the pit rail. These should prevent any excess rocking and thus the wheels will be mostly decorative. The rub plates will be almost invisible, so they will not ruin the look of the model. Unlike the prototype, I have essentially unlimited torque to spin the turntable, so the rub plates friction shouldn't be a problem.




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    Record Player

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