The subroadbed for this helix, and a common hobby standard, is ½ inch AC grade plywood. Helices can be notorious for consuming massive amounts of materials. And laying smooth radius curves can always be a challenge. Given the eight foot diameter of the helix seen here, the easy way would have been to simply lay out half a turn on a sheet of plywood, cut it out and install. That would have created a lot of waste however. By creating smaller arc segments, two complete levels could be completed out of three sheets of plywood with very little scrap. At nearly $30 / sheet, that’s a lot of savings on a 9 turn helix.
Start by laying out the curves. These can be laid out with a string or yardstick like conventional curves.
- Make a straight line down the center of the plywood sheet lengthwise.
- Determine the center of the arc by measuring in from the edge of the plywood the distance of your greatest radius (the outside of the helix) and put a nail at that point. This is your new center.
- I use a drywall square to lay out curves up to a 48" radius. Placing the corner of the square on the nail and a pencil at the appropriate radius, you can mark an exact centerline. String or a yardstick will also work.
- Continue marking radii until you scribe the inside edge of the panel. You’ll want to add enough clearance beyond the radius for the centerline of your track(s) to accommodate supports and clearance and layout the outer and inner circumferences in addition to centerlines for every track.
- Next, make a solid line from the outer to the inner radii at the sides of the plywood to mark the ends of each arc.
- Move the centerpoint down the plywood and you can lay the next arc section out below the first. Continue until you have enough arcs to build the helix.
The number of arcs that can be laid on a single sheet will be determined by your radius and the depth of each arc. To maximize lumber, a second sheet of material butted against the first will allow you to extend the arcs all the way across the sheet.
Only a thin crescent in between panels and the fin-shaped scraps at the edges will be wasted. The end scraps may even be saved and used for splices or other small jobs later.
Cut out the curved inner and outer circumferences with a jigsaw. The straight edges at the ends of the arc are the most critical. A circular saw with a straight edge guide will be helpful for keeping these cuts as straight as possible. The straighter the cuts, the better the joints between arcs. Clamp a straight edge parallel to the cut and guide the circular saw carefully.