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Old Oct 31, 2003, 09:40 AM
S55
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How to position templates for hot wire cutting of flat bottom wings?

How do you position your templates for hot wire cutting flat bottom wings? Is it A or B to take advantage of the flatness of the foam block? Pros and cons?

The foam block is a little warped, but so little that I would almost ignore it. Is method A preferred just to eliminate this?

Thank you,
S55
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Last edited by S55; Oct 31, 2003 at 03:20 PM.
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Old Oct 31, 2003, 09:48 AM
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I don't think that sollution A will solve the fact the foam is not flat. When cutting you'll put some weight on the foam, then cut and when removing the weight I think the foam will 'jump' back to be warped.

I face the same problem, and think I'll slide the foam over my table - while cutting a small piece from the bottom with the wire cutter. then the surface should be flat.

with option B you might have a problem getting your wire so close to the work surfase that you can actually cut out the front of the wing. At least my bow cannot work so close to the surfase of my table.

Patrick
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Old Oct 31, 2003, 03:33 PM
S55
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Patrick, you’re right. When you remove the weight, you’ll see the warp is still there. Your idea of simply pushing the board through the cutter may not work either because the board will tend to flatten under its own weight.
Now this concerns me. How do you deal with a badly warped board? Set the table at a very steep angle? Hang the board and let it go vertically through the cutter?

S55
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Old Nov 01, 2003, 02:18 PM
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Oke,

I tried sliding the foam over the table today... not a great success.

I stopped, and opted to glue a spar in the wing. I think that should straighen it out.

Patrick
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Old Nov 01, 2003, 03:25 PM
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I don't think you want to put the template at the edge of the foam because that's were the most stress is in the foam.

I like to put the templates in the middle of the foam (or offset from the middle if I think I can get two cores out of a thickness of foam). I then use the following technique:

I put the hot wire bow in a vise and hold the foam in my hands.

To make the first cut, I hold the wing with the leading edge toward me, the wire above the wing. I cut down toward the wing and have the wire hit the template about an inch or so back from the leading edge of the wing. It hits at a bit of an angle because I want it to be moving along the wing when it hits so that i don't burn extra deep as I stop and change directions.

I slow down when I get to the trailing edge, but keep moving. After falling off the trailing edge of the template, I cut down, then pull out of the block at the rear.

For the next cut, I keep it the same side up, but hold the trailing edge toward me. I again make a flying start and I hit the template about 3/16" from where I hit the template the last time. This leaves a pointy ridge of material on the top of the wing that I have to sand off later, but it allows the wire to go through some foam before getting to the template, cooling the wire a little.

I continue cutting toward leading edge of the wing (with the wire on top of the wing) and go around the leading edge and start cutting the bottom of the wing with the wire under the wing. After I get an inch or so back from the leading edge, I flip the wing while still cutting without stalling the wire so that I can finish again with the wire on top, pulling the core toward me as it exits at the trailing edge again.

This technique allows me to make thin trailing edges without melting things because I don't stall at the trailing edge.

I hope this helps.

- Brad
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Old Nov 03, 2003, 09:43 AM
S55
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Brad,
Your way is too tricky. You need a lot of care and sure hands.
My bow needs templates at both ends of the board and the wire can move up and down to follow the templates, but cannot move back and forth. The foam board sits on a wood board that slides on wheels. The board can be either pushed by hand or pulled by a motor. In this case it truly is hands free cutting and the cut is very smooth.
S55
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Old Nov 03, 2003, 01:53 PM
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First, I'd make sure the foam is flat when I cut the wings.

Second, I'd use the leftover foam beds when it comes time to sheet the wings. And I'd make sure they were weighted until flat. If everything is flat at that point I'd expect straight wings when the epoxy cures.

I've made 6' long, triple taper glider wings from pink foam that had all kinds of curves when bare. But were straight and rock solid when sheeted with balsa. I also used a substantail CF spar.

Not sure about utilizing an uncut surface for the bottom. Can't see that it would be a problem, but it probably has all kinds of "rash" from handling damage, no?

Also, consider that the center of the wire will lag behind your cuts ... you'll want a means to continue cutting past the end of the trailing edge for a bit.

tn
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Old Nov 10, 2003, 01:40 PM
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HI Tom you wouldn't happen to have plans for su 27 this is a nice plane i first seen it in a QF mag and then seen your video thats great id love to build one
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Old Nov 10, 2003, 06:14 PM
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I'll shoot you a pm, daydreamer
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