curving plywood?

[mikehaugen]

So my boy (3 years old) is very fascinated with trains. I am planning a train themed bed to build and I have most of the plan done but for the "boiler" part I want some curved plywood. I am thinking the ends will be semi-circle plywood pieces and I want to bend another piece of plywood over the semi-circles. It will be 30" long and 20" radius. It's going to be painted so I will just use regular pine plywood.

I know it can be done by cutting a bunch of slits in the underside of the plywood but I have never done it. I think 1/2" plywood, but how deep should I make the grooves? I could technically figure the distance between the grooves by calculating the differences between the two circumferences and dividing by the saw kerf thickness, but I know stuff like that never comes out perfect. I would rather have more grooves to make it easier to bend and I don't care if the grooves close up completely underneath.

Any suggestions as far as groove depth or spacing? Is 1/2" a good thickness? Any particular grade of plywood more suitable for this? With a 30" length, are just 2 ends sufficient, or should I make an extra center piece?

Thanks in advance for any help.

 

[bcp]

Maybe you can get something like this in your area.

Radius

Curved Plywood Products form Roberts Plywood - Roberts Plywood 631.586.7700

Regular plywood:
http://schmeling.com/reference/pdf_files/plywood_bending_radius.pdf

Or use cardboard concrete tubes.

Bruce

 

[Billrog]

You can buy plywood that is made for the purpose of bending but I've never used any. I've bent plenty of plywood for building concrete forms with curves but always used 3/8" and cut it across the 4' width because the way it's made there's less resistance that way. Only for tight turns have I ever sliced the back approx. 3/16th" deep.

 

[mikehaugen]

Wow, thanks guys. BCP, according to your link it appears I could bend the 1/2" at a 20" radius without anything else... and that is regular plywood. Am I reading that right?

 

[bcp]

Looks to me like you could, but I didn't notice anything about the grade of plywood or number of plies. Yesterday I broke some long scraps of 5/8 plywood so it would fit in a trash can. Bending parallel to the grain, they broke far before those radiuses.

Bruce

 

[newbury]

Better yet buy the cheap PT stuff at Lowes in the 15/32 size when it's still wet from pressure treating. That stuff has so many voids on the inner layers it bends like cardboard. It bends itself!

 

[mjcountrybumpkin]

I vote get a sono tube (concrete form) and wrap it with a piece of 1/4 " Luan. Probably soak the Luan to make it more pliable ,get it to shape and let it dry.

 

[kevinwak]

I made a rocking horse for my boys. The body is a single piece of Baltic birch plywood and the bends from the sides of the body to the top are probably only 1-2" radius.

These bends were hard to make, since they are so tight. I made kerf cuts, and I don't remember now, but I think they only had 1/8" space between the kerfs. The kerf goes all the way through the plywood, except for the very last ply. Even with that I had to steam it to get this tight a bend. Once steamed and pliable all the kerfs were filled with hot melt glue from a glue gun and then it was bent around the form and clamped to cool.

If you are not familiar with Baltic birch plywood, it is made of solid birch with no voids between the laminations. Normal plywood has a core of poplar plies and then just a veneer of the desired wood. The plies on Baltic birch are very uniform, so it actually has an appealing edge grain. It comes in 5 by 5 sheets and most lumberyards stock a few sheets of it.

Your project is a much larger radius, so if it were me I would use 1/8" thick Baltic birch which should bend to that radius with no problem. I would make it out of four layers of the 1/8" ply to give me 1/2". The first layer I would bend around the form and attach. Then glue the next layers on. Use white glue, rolled on with a small paint roller, and put it on both surfaces, not just one. Once all the layers are glued and stacked on the form they can be clamped secure with 1" ratchet straps. Once the glue cures it should hold the shape. Maybe do a sample glue up first to get a handle of it before you do the final project.

Either way you chose to do it post some pics!

 

[kevinwak]

You know, rereading your post and comparing to my table to visualize, a 20" radius is pretty big. You could probably do that with two layers of 1/4" glued together.

 

[SPIKER]

I vote get a sono tube (concrete form) and wrap it with a piece of 1/4 " Luan. Probably soak the Luan to make it more pliable ,get it to shape and let it dry.

My suggestion as well the cardboard SONO tubes are pretty HD & painting with a good stain/paint will make them hold up well.!

You know, rereading your post and comparing to my table to visualize, a 20" radius is pretty big. You could probably do that with two layers of 1/4" glued together.

My other thoughts, was to also use several layers of thin plywood and glue them together in layers after wrapping them around the forms. from above.

Mark