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Old Feb 03, 2012, 07:21 PM
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Polycrylic by Minwax is a water based acrylic finish. There is some sticktion to it but I don't think that it was meant as a glue.
It is possible to glue wood to foam with water based glues like white or yellow carpenters glue. The problem is that the moisture has to evaporate and can take a long time if there is foam in the core and foam covering the wood in the shucks. Vacuum bagging has the same problem. That is why a glue like epoxy or polyurethane (Gorilla Glue) will work. The epoxy catalyses itself. The polyurethane has cyanoacrylinate in it and will cure with moisture.

Chris
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Old Feb 04, 2012, 02:53 AM
I don't like your altitude
Stupot46's Avatar
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Have you tried UHUpor? I've always had good results foam/foam,foam/wood with it but haven't tried it on a sharp curve.May be worth putting on your "experiments"list.The way I would use it would be as a contact glue on the first 1/2"of the leading edge,then when you have made a good bond there do the rest of the panel
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Old Feb 04, 2012, 08:24 AM
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Epoxy works best for me.... I don't want moisture getting into the balsa skins.

Vern
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Old Feb 22, 2012, 09:36 AM
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Templates

With my last round of laser cut parts, I threw in some lite ply templates for this wing. Of course lite ply would not stand up to a hot wire very well or ever be smooth enough, so I incorporated 1/64" plywood edge facing to the cut edge. With the grain running lengthwise, it should cut quite smooth.

Attached with contact cement.

Kent
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Old Feb 22, 2012, 11:33 AM
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Kent,

It's actually better to have a narrow ridge for the wire to ride on.

Vern
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Old Feb 22, 2012, 11:52 AM
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I trust that you are saying narrower than 1/8", such as 5/32" birch ply.

Gotcha.
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Old Feb 22, 2012, 03:04 PM
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Kent,
I used 3/32" for the Colibri 110 templates, they worked fine. You have to get them very smooth with no catchy things the wire can get hung up on.

Vern
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Old Feb 22, 2012, 04:37 PM
Deniable plausibility
Shedofdread's Avatar
Derbyshire, UK
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FWIW, I usually use 3mm birch ply. Like Vern says, sand the surface the wire runs on very carefully (I go down to 400 grit). Things that make the wire snag and catch are the enemy!

Or if it's a 'posh job', aluminium's really nice 16g in a reasonably hard grade.
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Old Feb 22, 2012, 09:16 PM
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Test Panel

Vern: My typo. Meant 3/32" plywood. I know that is what you just used.

I have not used the hot wire in years, so a test panel was in order.

A few quick swipes of 320 grit on the template edge and they where ready. Since the face grain of the 1/64" plywood is parallel to the wire movement, it hardly needs sanding at all.

Now to test the PU glue.

Kent

Hot wire foam cutting (0 min 25 sec)
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Old Feb 23, 2012, 01:29 AM
I don't like your altitude
Stupot46's Avatar
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That's a nice set up,puts the wood bow idea to shame.What are the arms made of?
btw,did you decide on a trailing edge solution?
Regards Stuart
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Old Feb 23, 2012, 02:15 AM
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looks like a near perfect cut from where I'm sitting Kent
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Old Feb 23, 2012, 04:14 AM
Deniable plausibility
Shedofdread's Avatar
Derbyshire, UK
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Don't know if it's just me but does anyone else find foam cutting very satisfying? I suppose it's because very quickly, you get something aeroplane-like...?

Anyway, test panel looking good there Kent. Will you be skinning it in something to test the PU glue?
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Old Feb 23, 2012, 08:00 AM
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Kent,

Nicely done!

Vern
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Old Feb 23, 2012, 10:09 AM
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PU test

Thanks for the comments.

My thought with using PU glue is that it is expanding glue and as such will fill tiny voids and completely bond the skins, even without a perfect press job. I did 2 panels. One with all dry components and one with dampened surfaces. The water really kicks the PU into high gear for super gap filling.

Results were a little surprising. The wet one "walked". A few well placed pins or blocks could have restrained it, but the dry one worked well, so there is no need to go damp.

The pressing method used left some gaps in the skin. But those gaps were fully bonded due to the expanding glue.

The TE was laid up with same PU glue as used for the skins to core. No pre-sanding for this test. It bonded well and it sanded out well. Some thing of an external bevel.

I'll do another "proper" mock up.

Stuart: You noticed my tube steel bow. Seemed like a good idea. Easy for me to build. I simply strung it up with wire and put the juice to it. Then I noticed something odd. The wire was glowing red hot outside of the electrical clips. So the lesson here is that the bow must not be an electrical conductor, or it will short circuit the hot wire. That first bottom cut was a fiasco. I'm back with the wooden bow. It is shown below the last shot in post #54.

Kent
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Old Feb 23, 2012, 12:18 PM
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I'm sure the foaming (when adding water) also produces a 'fluffier' PU foam (ie less strong and dense). The foamed PU without additional moisture seems to be more 'plasticy' when cured in my experience.
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