Foamie hybrid thoughts...

[Fred Trout]

I think I get what you are saying. You are going to steam-bend/kerf the front curve but don't want to stress the steam-bend/kerf when you cut the window (unclear if before or after steam-bend/kerf) so that the panel snaps. You still can steam or kerf the curve. Making the curve will mess up any window cutout measurement anyway, so the curve has to be done 1st, right ? Those strips on the window margin are not what you were worried about ?

Could steam curve, put the canvas/ply on one side before you cut out the window. Will need to remove canvas with razor (before routing cutout) then it will pull away easily enough. Then apply canvas/ply on other side. Same issue with cutting out door on side leaving a tiny piece of foam holding the panel together; so everybody tapes the cutout door in place to reinforce it all. Just for info, the guy who did the compression glued cardboard on the inside after compression bending and the panel held shape fine without any other support. He said he did the bending on large panels too, btw. See kerf sticky:
http://tnttt.com/viewtopic.php?f=55&t=44081&start=60

I put wood around my window unit because it mounted using a nailer strip rather than a compression ring. Anyway, something like a heavy window unit could shake loose as you travel so you want to spread the load out - therefore wood or flashing. But a simple sheet of plexi would be fine without a support frame.

[GPW]

Gentlemen , having bent foam for the past 12 years on small planes , it all comes down to Tension and compression forces ... A simple backing , covering , skin , board , sheet, whatever to relieve the Tension forces of the outer curve , and HEAT to aid the COMPRESSION of the inner part of the curve... Foam is Mostly AIR , heat allows the foam to lose it’s inner air and compress ... With Proper heat (190F) you don’t need a tension backing , and the material once cooled in place (form suggested) it will keep that shape forever... No springback .. So there is never any tensions on the newly bent part that would cause it to crack on it’s own . The Bent shape has NEUTRAL stresses ... and even when heated does not try to return to straightness...
Once again I’m posting a device we came up with to bend thinner foam sheets .... suitably sized up it Could be use to bend thick sheets ... http://www.rcgroups.com/forums/showthre ... d+Foam+gpw

[mikeschn]

I worked on the first wall a little bit today... Can you guess where the spars go?



Mike...

[Fred Trout]

Nice router work - mine had edges that looked like a jigsaw puzzle

What did you use for glue wood to foam ?

[mikeschn]

Hi Fred,

It's a quiet weekend here, albeit busy, with the traditional 4th of July stuff, so there won't be any teardrop construction this weekend.

But to answer your question, I used GG.

As for the foamie routing, it's pretty easy actually. CNC makes quick work of it. I've never cut foam before, and I was concerned that it would bead and break off large chunks at a time. But that wasn't the issue at all. Instead the bigger issue was the deflection of the EPS due to the lack of structure prior to gluing in the lower threshold of the door. And to this moment I don't know if the door is square, er I mean rectangular, or trapezoidal, or parallelgramish.. you get the idea.

Mike...

[Fred Trout]

Thanks ! Trapezoidal, or parallelgramish seem to be my favorite end results If I end up with something actually square and true, it was certainly accidental Which is why Great Stuff is my fav since it fixes all. Must be nice to have access to a CNC. I used to program one back in 79-82 - it read punch-tape

I suppose you could mount the foam onto some rigid substrate like coroplast or OSB with temporary nylon dry-wall-screws. I really like em for mounting light stuff on foam, preventing walls from moving during glue-up, etc.

[GPW]

Just guessing you could get bigger ones if you needed ...

[Fred Trout]

The nylon inserts ? Yeah those are the smallest they have at Walmart, with about 3 others longer and zillions of other types around. They seem to hold pretty well considering how weak foam is. Lateral strength as would be needed on a X-Y axis CNC should be pretty good.

[GPW]

The one absolute thing we can count on is that Foam is Weak .... Which is why we skin it .... Then it’s Much STRONGER !!! Best never consider the foam by itself ...