Plywood Tobbogan Slide, or How to Build With Plywood and (almost) Without Fasteners

A friend challenged me to build a tobbogan slide that packs away in the summer with minimum fuss and space requirements.

I had built one with screws and lumber before, but that system doesn't lend itself to easy disassembly. Even using bolts you would have to keep the hardware. So, I figured I'd build one that holds itself together, kind of like a big jigsaw puzzle.

This projects uses 5 sheets of 3/4" outdoor grade plywood, a 2' 1/2" threaded rod (aka tie rod), and some hinges and screws to hold on the hinges.

Most of the structure is held together by tabs and holes. Usually I just draw stuff out before I build it, but I had no idea how well this would work, so I built a model first. I don't have too many pictures from cutting the real thing because my camera battery died, but I hope the model will make the point.

As for most things, modify this to what you have or need. Some points to consider are:
- the bigger the attached area of the tab, the stronger the tab
- the bigger the attached area of the tab, the bigger the receiving hole has to be
- the bigger the receiving hole, the more weakened the part with the hole
--> so, keep both reasonably similar, unless one part gets more stress.

- given the 4ftx8ft size of a pice of plywood, you will have to join parts together to make anything more than 8ft long
- seams should not line up - like LEGO or brickwork, keep them spread out along the length of the final product
- if the final product will be suspended from both ends, like the slide, the most stress will be in the middle, so pull the seams away from there when possible
--> consider both how you can fit things best onto your sheets and how to put it together for the best strength

- a triangle is one of the strongest arrangements you can build in because angles are set by the sides and vice versa
- if you build a triangle, figuring out the changed size of the slots is a pain in the neck (remember your hypotenuseses... :-) )

- have tabs at the ends of things, so you can fit them in a hold in the middle of another piece to secure the seam together.

Check out the pictures for some sketches/plans and of the layout of the pieces on the plywood for the tower. The model is based partly on these sketches and a 10ft slide, but the actual thing ended up with more, smaller tabs and holes and a 12ft slide.
Sorry about the pictures of drawings but I don't have a scanner set up. Probably a good thing lest someone take these as final plans to scale, which they are not. :-)

I used a circular saw to cut the pieces to size (minus the tabs), and then a plunge router with a jig and a spiral bit for the holes and the tabs.
I started out using a jigsaw, drill, and drop-cuts from the circular saw, but found progress kind of slow. So, I ended up using a router which made things way faster. If you don't have a router, it will just take a little longer.

Routers can be quite hard to use free-hand because the spin of the bit keeps trying to pull them out of line. The problem is fixed if you run the router against a fence in the correct direction - play with it, "left" or "right" don't describe it well. The problem with a fence is that you have to secure it to the workpiece at the proper distance to run the edge of the router against the fence to cut the hole where you want it. The operation becomes much faster if the fence could show you where the hole will be. Add one more idea of using the jig to set at least one end point of your cut, and a set of points sticking out to hold it in place, and you have the jig I used.

THE JIG
Cut a right angle out of a scrap piece of plywood and screwed it onto a thin sheet of hard board (the back of an old shelf in this case).
Run the router along the plywood with the bit you are going to use, and it will cut the hard board exactly where you will want to line it up with the marking for your hole or slot.
Screw several screws just longer than the thickness of the jig through the jig so they stick out by about 1/8in.
I usually work on the floor over some 2x4s, so with this, you can just align the jig, step on the edge of it out of the way of any handles or other parts of the router to hold it in place.
Try cutting some holes and slots into some scrap before you get started, you still have to figure out under what circumstances the router will try out it's free will :-)
I found the easiest way to get the router to stick to the edge is to rotate it asif you are rolling it against the edge.

One problem using a router is that the corners of the holes won't be square. In this case, since beauty is secondary, I just cut the edges of the slots 1/8in longer to get the 1/4in bit's edge to the point where the corners would be. If you want to be more accurate, you could jig-saw the corners square.

I used a 45 degree chamfer bit with a pilot tip on the slot on the slide. I am pretty sure a wider slot would have worked if I had not had a bit for it.

I wanted to be sure all the holes line up and the angles work out, so I built a model of the final slide. I also wanted to see if the construction would actually be stable at all, without any further braces.

Cutting these pieces took rather longer than I expected. Some of it was a question of re-working too many things. Some of the things I found helpful were:
- measure the distance from your circular saw blade to the edge of the sole of the saw, then use the edge of another piece of plywood as a fence positioned that distance away from lines you want to cut. If you have clamps handy, clamp down the "fence", btu make sure the clamps won't get in the way of your cut. Run your saw against this setup rather than following a line by eye. The result is almost as straight as if you had used a tablesaw.
To eliminate yet another source of measurement error, you can cut a scrap piece of wood to the distance of the sole of your saw from blade to edge and use that to line up with any layout markings.
- use a cut-off piece to make sure the holes will indeed fit the plywood.
- make sure the holes for anything mating at an angle are sufficiently wider to still fit.
- holes that are wider than the plywood is thick are not really a problem, this is not where the design gets it's strength. The length of the holes is more critical.
- those staggered holes can get confusing - I cut one side of each mating set of pieces, and then used the cut edge to check against the layout markings on the next piece before cutting, both for accuracy and to make sure none of the holes and tabs got mixed up.

Put the pieces together. If you got this far, I probably won't tell you anything new here.

At this point, I have the tower and slide all assembled. I have suspended the edges of the 12ft slide I ended up building over some boxes and jumped on the center of it, and it's even more solid than I had hoped. I expected to have to add a support or two along the length, but I don't think that will be necessary.

The slide is not attached to the tower yet because the trajectory would take you onto a pond whose ice is not to be trusted yet. I am planning to add some more pictures when the slide is attached, but figured I should post this while winter lasts :-)