I thought about making one of these (to get 3mm foam) but was thinking of a different approach. Let gravity do the work; it is a constant after all. I use gravity to cut square ends on foam blocks when making core blanks, and also use gravity to cut tapered core wings with a home made feather cut I made years ago. So why not use it to rip foam sheets.

I was thinking of a platform at almost 90 degrees to vertical. Then attach weights to the foam to have it pull through the heated wire that is 3mm off of the vertical platform. Once the weight was figured out for a given foam type, it SHOULD be the same for all.

I made an adjustable power source for my foam cutting ( 24 volt step down xformer with a dimmer switch on the inpit, and an amp meter on the output. As long as I adjust the current for a given foam, and length of cutting wire, it cuts about the same every time. I have a chart at home, but I think it is something like 2.3 Amps for blue foam.

For the vertical cuts on foam blanks, I just set the "cold" wire where I want the cut.... then gradually turn up the heat until the wire starts to cut through the foam. Not only is it good for that cut, but now I have the amp setting I need for that type of foam....at least for the wire I use so it may be different with different wire.

Well I haven't been building much with foam, so I never really got around to making this even for a proof of concept. I imagine it will be rather large (slightly larger than a sheet of FFF) and would prove a pain to store.

 

Forgot to mention. The pic on the right looks like impurities in the foam. Hard to tell for sure, but I occasionally get sections like this, and someone told me they were impurities in the foam. I seem to get this more, and to a greater degree, in pink foam as opposed to blue for what ever reason, but I haven;t see it as bad as you have shown.
Not sure about the un even thickness.... could be the wavyness on the original, or wavyness created when tension is released when it is sliced.? Maybe moving the wire closer to the roller?

Hopefully Ken's foam will be better with impurities, and it sure does appear to be better about being flat.

 

What a cool Mod Yoram!

It looks like the wire may be walking up and down. I would try to put the wire between the two rollers so then you know that the foam is held on both sides and make sure that spring is really tight so it does not have the chance to move up and down.
You may also try to use a flat 'knife' like strip of metal instead of the wire? Or perhaps a thicker wire that resist moving even when heated.
Just some ideas that I hope will help.
Very cool stuff Yoram, thanks for sharing.
Mark and Trish

 

The idea with hot wire is for the hot wire to melt foam just ahead of the wire so the wire never really touches the foam. Well that's the theory, but with that said in practice I've never had that happen. But if set up right (feed rate, wire temp, and tension ) thin wire is not a problem. I forget what size I run, but it is VERY small stainless wire. It's the smallest size aircraft safety wire. I've cut 4 foot panels with my homemade drop bar cutter, and the wire in the middle of the bow exists the chuck within a fraction of a second after the ends, so even though there is some drag, it is apparently very little. If memory serves me correctly the last 4 foot core I did took right around a minute to cut. Root cord was around 12-13 inches, and tip was around 6. My bow's are home made out of scrap PVC, so certainly not hi-tech. I can post pics if you want.

Cutting with a hot wire is a lot like CNC... Speed of cut, wire tension, wire temperature , density of the foam all work together to make a nice cut. As the actual kerf is larger than the wire itself, having a bigger wire will make a much bigger kerf. Well that and needing quite a bit more power to heat it up as it is now less resistance for a given length so you will need more current, which will mean a bigger power supply. Think C ratings on a Lipo. If a 10C works fine for small wire, you will need a 30C for the bigger stuff.
Although I don't cut all that much foam, my gravity test method appears to work very well for finding the right current setting for a given foam.

I'm still thinking the internal stress of the foam that was released once cut is causing it to ride up after it exists the roller. Maybe add an additional roller, right after the wire, or somehow get the wire behind the existing roller. May make it more difficult to start the cut if it is there though. At the very least get the wire closer to the roller.

Just my $0.02

Just found a pic of my drop bar setup... 1st post in this thread: http://www.wattflyer.com/forums/showthread.php?t=13025 7th picture

And 7th picture in this post shows my gravity end cut: http://www.wattflyer.com/forums/showpos ... stcount=28
2nd pic shows my "slicer"

 

Found a CU shot of it cutting and the end result. This is a 4 foot panel cut as described in above post.
http://www.wattflyer.com/forums/showpos ... stcount=33

Keep us posted. I am still interested in making one of these... Depending on your testing, I may go the PP route.. No need to store it like with my gravity one.

Would be nice to cut a plane out of 6mm foam, and split it so you'll have two 3mm planes

 

Yea figured that, was just having some fun...

 

I agree this would be great to have I am thinking for thin skins around the ribs and former's
Mark and Trish