I first drew up a scale template to determine how to get the most sections out of 1 sheet of plywood. This template is for 0-54 and 0-42 tracks, with app 2 5/8 inches extra on inside and 1 7/8 extra on the outside for clearance (overall size was 35 inch diam inside, and 58 inch diam outside). I worked with another individual on a fast track helix, and he simply laid the track on the plywood, and used a small compass to draw the lines. http://www.blogger.com/post-edit.g?blogID=5640420068638743046&postID=5389337055289979752
I decided to use external wooden support to try to minimize the overall weight. If I had used Threaded Rods, I would have had to extend the inside and outside dimension of the plywood roadbed at least an inch so the plywood would not warp.
Next I created a home-made, large size compass out of a couple of old yard-sticks
To assemble the pieces, I worked from the top to the bottom of the helix. This allowed me to glue and screw plywood splices, without having to keep the layers separated. The problem with this is you have work backwards on you entry and exit tracks and grade. Before attaching the second layer, make sure you're grade is in the intended direction, and you top exit point is correct. Also, the top and bottom of the helix should have a transistion grade. Make sure to not have any splices near the end of the top exit, or near the end of the level directly above the bottom exit. This gives an extra 1/2 inch clearance to allow the plywood to flex for the transition grade. The next 2 pictures show the final result from the bottom (where you can see the splices), and top (track side). Notice that is is about 5 feet in diameter
Next we put on the legs. Putting this together is like handling a 5 foot wide slinky, so get some help. I used 2x2 legs. Since the rise is 6 inches between layers, that meant that each 1/4 of the turn would be 1 1/2 inches higher than the last one. I laid 4 legs together on a table and offset each one by 1 1/2 inches from the previous one, and clamped them together. Then it was just a matter of drawing a straight line across all 4 legs at 6 inch intervals. Since the inside legs were going to be offset 1/2 way between the outside legs, I repeated the process, but started the first line at 3/4 inch, then used 6 inch intervals. I put lag bolts for levelers at the bottom of each leg to compensate for any inaccuracies in leg placement. The legs were attached using 1 and 2 screw L Brackets and using only screw in each until all brackets were attached, so the brackets could be set a a slight angle. The other major problem when attaching the legs is that there is no level vertical or horizontal reference to help align the legs. I finally ended up using a level on each leg to make sure it was vertically plumb, because visually they all look like they are leaning.
Nest came the carpet roadbed, track, and of course a test train. The carpet was the cheapest grade gray carpet with rubber backing that Lowes carries. My wife came up with a paper template that we pinned to the carpet. and then cut it with had scissors. I have since learned that Black-and-Decker has an electric scissors that works fantastically, and you can put the track down and then trim around it using the track as a guide.
The helix was first set up on the floor, with a temporary turnaround at the top and bottom. Trains of various lengths were run to ensure no problems. One problem I discovered while using the carpet roadbed was that it was hard to keep the tracks level. Track wires were run down the inside of each inner leg. Tacks were wired every 1/4 section (overkill) and I tried to keep all runs the same length. The reason I ran so many feeders was I wanted to minimize any pin corrosion over time, since power was being fed from both sides of 3 tracks maximum.
One of the hardest parts of the project was painting the blessed thing. No matter how many times I touched it up, I would sill find places I missed. The whole helix was painted with Lowes Primer tinted gray. I ended up not putting a top coat on it.
Finally the time came to put it on the layout. First I had to make sure the bench work underneath was built sufficiently robust that it would not only support the helix (which was 2 sheets of plywood plus 2x2 legs and track), but also support myself if I had to stand on it. Then I invited 4 friends over, and it took all five of use to install it, since it was in such an awkward position. Notice also, the supporting bench work also has a whole in the center, so the complete inside of the helix can be accessed. There is even enough room so that a step ladder can be used inside to reach the top level.
Picture of final result, showing upper and lower level connections. The upper level is attached to a shelf layout containing 2 suspended suspension style bridges (see later posts). The lower level attaches to a Lionel Graduated Trestle set which then has a loop back when it reaches the bottom level The wiring on the inside legs was hidden using inexpensive foam pipe insolation. It is split the entire length, so it just slips over the leg. It is also easy to cut, so you can notch where wires and other obstructions occur. The edges of the foam split have glue on them with a protective plastic cover. The plastic can be removed for perminent installation if desired, but all of mine are just held on by the friction.
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