spring back using 1/4 MDF in lamination project

[chris fox]

I am working on a project at work, were looking at different ways to create 1.5 thick curved band (aka wing). This band will be approx 40" long x 18" tall and is based on a 98" diam. We will be fabricating about 40 of these for a project.
One of our ideas is laminating .25" MDF in 6 layers. 4 layers will represent the overall shap and 2 layers will be 3 inch strips. The idea is for running cable through the wing along its total length. So 2 strips will yeild .5" channel inside this wing. The final shape will be routered to size and the finish will be paint(sprayed).
So my question is, figuring the spring back so we can achieve this 98" dia/49" rad. wing? Is there any way/thought for determining this i.e. make a fixture based on a 97" dia.?
I remember reading about this in one my FWW issues but couldnt remember where it was.

thanks,

Chris

[Jamie Buxton]

There are formulas around which purport to predict the springback in bent laminations. I've not found one which works. I'd build a form and try it. With six layers, there won't be much springback. My first try on the form would be something like 48" radius, shooting for 49" after springback.

For big-area laminations like this, vacuum-bagging is the way to go. Look up vacuum veneer press. www.joewoodworker.com is a good online source. Use a non-creeping glue -- either urea-formaldehyde or epoxy.

You can make the bending form kinda like traditional wings were built. You bandsaw "ribs" with the desired profile, and skin them with thin plywood, using glue and staples to hold it in place.

You might also see if you can find "bending plywood" in your town. This is plywood intended for bending, just like you're doing. It is considerably nicer to work with than MDF.

[chris fox]

Jamie,

Thanks for the reply. We looked at kerfed mdf but the edges will need to be finished so this was ruled out. Making rails at 48 vs 49" looks like a good first try. Vacuum bagging is great idea, over 40 times this would really speed up the process.

thanks

[Jamie Buxton]

...We looked at kerfed mdf but the edges will need to be finished so this was ruled out....

Bending plywood is different from kerfed MDF. It really is plywood. It is three-ply, and the middle ply isn't very thick. It bends quite well in one direction, and not very well at all in the other. My dealer has it in "4x8" sheets or "8x4" sheets, depending on which direction the face grain runs. They call it "Italian bending poplar", if that helps you track it down.

Spellman Hardwood's web site says they stock the stuff.

[Neil Lamens]

Chris:

Here's a video link to using the material Jamie is talking about http://blip.tv/file/471819


There are 4 episodes that go through the procedure in a vacuum bag, its not a wing, but its the process.

Also there is a woodworker Earl Kelly http://www.earlkelly.com/ he does a "leaf table" he may be a resource.