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Anybody with hot wire foam cutting experience near Savannah?

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edwisch

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Aug 11, 2020
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Our small team made a hot wire cutter and bought some foam from home depot. We made templates, put two guys on the hot wire cutter and the results were... inconsistent, to be gracious. I've looked for online videos about hot wire cutting but didn't find any relevant to airplane parts, only to smaller, ornamental parts with a vertical wire desktop cutter.

I did take the EAA composite workshop years ago and got good results with their setup, but we're obviously missing the secret sauce.

Possible error souces:
1. We don't know what we're doing!
2. Bad foam
3. Bad templates

If anybody is in the vicinity, we'd love some coaching!

Thanks!

Ed
 

bebedriver

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Sep 7, 2020
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Excellent book which gives info on technique etc. 'Radio Control Foam Modelling' by David Thomas. I have it and it covers the subject very well from the point of view of building hot wire rigs, making templates, right type of foam etc. Got mine s/h from Amazon.
 

Protech Racing

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You need smooth edged templates for starters . They should have an entry and exit point , not simply the shape that you desire.
 

12notes

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Mike Arnold's AR-5 videos contain a good instructional example of hot wire cutting of a wing core.


It's probably worth it to watch all of his videos. The AR-5 held the world speed record for planes under 300kg for 18 years, and I think still has the record for lowest drag despite having fixed gear.
 

Vigilant1

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The "foundational" Burt Rutan video on "moldless" composite construction shows Mike Melville and him doing hotwire cuts. I'm sure you've seen it, but it might be worth a review to make sure you have all the bases covered.

From about 10+00 to 25+00. Burt used wood templates, but they can hang up the wire and cause trouble if the edges are/get rough.

It is here, among other places:
 
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cblink.007

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Also this video. Not as good as the Mike Arnold or Burt Rutan videos at all, but same techniques...and an epic moustasche and chops!

 
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tr7v8

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I've cut quite a few model wings myself. What issues are you getting. One point is you need virgin foam, some is recycled & has hard lumps in it. As others have said the templates need to be very smooth, otherwise the wire will drag & could break. Run your thumbnail over it there should be no roughness. The wire should cut with virtually no pressure, you shouldn't need to pull or push. You need a fairly high amp supply for it. On a 36inch bow I used an 8amp battery charger. What is the cutting wire, nichrome is best although some use stainless steel.
 

berridos

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I always watch how people produce great eps hotwire cuts. Mine were all very poor until i build a cnc styro cutter. Its worth the investment. The first one was 1,4m cutting area, the current one (in progress) goes up to 2m.
Its not easy to keep a constant cutting speed per hand that allows to keep the heat to a minimum.
 

TLAR

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Gravity feed is as low dollar as it gets and produces awesome results.
It does eat up some foam, to get feed and temp set but the results speak for themselves.
You can also cut very large parts, perfectly
 

berridos

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The problem is that at the end you need a cnc cutter to cut the wood templates. The fuselage of my last aborted project had 240 slices. In spain wood is fairly expensive. The investment in wood would have been much higher than building a cnc and cutting without templates and instead relying on cnc.
The same happens if you have tapered wings. You also need a lot of templates as i wouldnt recommend cutting sections of more than 1m. Even 1m could be to much in order to keep constant wire temperture. If you also have highly tapered cuts on a 4 axis cnc, the shorter side will burn much deeper groove than the longer pathed side.
Guess its a matter of the precision you require.
 

TLAR

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I will look in my shed for the name of the wire I use, and the power supply. It will cut 60 inches plus or minus 1mm (guessing), but the only way to do it is with gravity
Sorry for the price of wood in Spain, it’s cheap here.
I should state I use Dow Dock foam, tried the others and they are inferior.
I do everything I can to bond without sanding. This means an exact template and I will try to cut any mismatch rather than sand.
 
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edwisch

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We're using nicrome wire, a store-bought transformer and a light dimmer. We turn the dimmer up high, then turn it down to the desired voltage.
 

TLAR

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When I started I was using Home Depot blue Dow foam. The cells are a lot smaller than dock foam and it does make a difference. I can’t think of the name of the wire I use, it is not nichrome. I have used it in the past.
Are you making critical parts?
Watch some utube videos
 

edwisch

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We're converting foam into scrap, ain't nothing critical about what we're doing.
 

cblink.007

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The problem is that at the end you need a cnc cutter to cut the wood templates. The fuselage of my last aborted project had 240 slices. In spain wood is fairly expensive. The investment in wood would have been much higher than building a cnc and cutting without templates and instead relying on cnc.
The same happens if you have tapered wings. You also need a lot of templates as i wouldnt recommend cutting sections of more than 1m. Even 1m could be to much in order to keep constant wire temperture. If you also have highly tapered cuts on a 4 axis cnc, the shorter side will burn much deeper groove than the longer pathed side.
Guess its a matter of the precision you require.
For our test model, we had our templates cnc cutted by a guy who does custom RC models, and they turned out perfect. The templates for the full scale prototype will be done with stanless steel, as we learned that hotwires tend to beat up wooden templates pretty good!
20180618_104233.jpg
20191124_141400.jpg
20200826_091111.jpg
 

edwisch

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Aug 11, 2020
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Sir, it takes practice, with exactly does the part surface look like?
Occasionally smooth, more frequently span-wise grooves, i.e., as if the wire had dug in to the foam. Based on comments above, our problem might be the foam. We have noticed pre-existing cracks in the foam, sort of like wood that has dried out and cracked.
 

TLAR

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Occasionally smooth, more frequently span-wise grooves, i.e., as if the wire had dug in to the foam. Based on comments above, our problem might be the foam. We have noticed pre-existing cracks in the foam, sort of like wood that has dried out and cracked.
Utube has some good videos, whatcha building?
 

cblink.007

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Occasionally smooth, more frequently span-wise grooves, i.e., as if the wire had dug in to the foam. Based on comments above, our problem might be the foam. We have noticed pre-existing cracks in the foam, sort of like wood that has dried out and cracked.
Do your templates have proportional counting numbers on each of them, or is one of the templates smaller than the other? This is the result of such a size delta; are you having the same issue?
20160710_132946.jpg
This was a small freeflight model in grad school back in 2008, made with beadboard foam and too few templates...
 
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