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Cutting Blue Foam

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Baz59921/01/2011 11:32:16
2 forum posts
I would like some advice about cutting blue foam with a hot wire bow I have made. I can cut the foam ok but the surface finish has ’waves’ on it. The peaks of the ‘waves’ at the height of the ‘line’ I want, but the troughs are below what should be the finished surface.
I have tried letting the bow’s own weight cut the foam as well as putting slight pressure on the bow but using both these methods has produced the same ‘wave’ effect (would have attached photo but not sure how).
I wonder if my nichrome wire is too thin, not exactly sure what gauge it is, but I don’t know what it should be.The supplier of the blue foam (Sheffield Insulations Ltd) assured me that this was what he supplied to all the modellers.

Any suggestions?

David Gilder21/01/2011 11:36:52
1859 forum posts
673 photos
I am only just getting into this foam cutting lark myself, but from what you have said about the peaks and troughs,, It sounds like you need to get the wire a little hotter (marginal) to enable the wire to move at a constant speed.
I believe the troughs are created as the wire heats up but as it cools (as it tries to cut) it cools and lifts a little.
Have a look on you tube for videos.. They are useful!!!
Eric Bray21/01/2011 12:49:01
6600 forum posts
2 photos
Likewise - the wire isn't hot enough, OR you are trying to push the bow too fast!
Check that your templates are perfectly smooth, and the wire isn't 'violin'ing' as it runs along them.
Also check that the wire tension is sufficient when the wire is hot! (Metal expands!) This can also give you a mis-shapen section, as the centre of the wire trails behind the ends, and cuts a different path if you have a long 'cut'.
Vecchio Austriaco21/01/2011 13:01:58
1498 forum posts
707 photos
Slight waves will always be there. I was cutting glider wings in the past together with my brother on the other end of the bow - with numbers on the stencils to asure same speed on both sides. But you cannot avoid a certain speed difference and also vibrations in the wire.
But before you put any planking on you can easily sand the waves away. Nobody will see it if you have some nice balsa or obechi sheets on top.
Baz59921/01/2011 16:18:46
2 forum posts
Tried adjusting the temperature of the wire but made no difference. My bow is 36inch and I am using Nichrome wire as it is easy for me to obtain.
I should point out that I am a Free Flight person (please don't hold it against me but I guessed you radio people use foam more than FF, hence posting it here). I am attempting to make the building jig for a F1A glider wing, using blue foam.
My power supply is a commercial power supply I use in my job and has an adjustable voltage of 0-40 volts with an adjustable max current of 2 amp. I found the best cutting 'speed' to be at 25 volts at 2 amp but of course the suffice result was still the same.
What should the thickness of the wire be?


Edited By Baz599 on 21/01/2011 16:22:08

Vecchio Austriaco22/01/2011 20:55:44
1498 forum posts
707 photos
I used 0,5mm diameter with approximately 5 ohms per meter. What the exact alloy was I have no idea. I just remember it was mechanically very strong and had a lot of thermal elongation. We always had to adjust he tension at the bow when it was hot.
Barrie Dav 223/01/2011 08:45:26
1012 forum posts
14 photos
I've made hundreds of white foam wings over the years for myself my fellow modellers when I lived in the UK and never had a problem with ripples on the surface. Perhaps blue foam reacts differently to the hot wire.
The main things is to use plenty of tension in the wire and the correct heat (at first, trial and error). I used a guitar string tensioning key at one end of the bow - which was made from 'H' section ali. bar with dural legs to accept the wire and I completed the tension of the wire after the wire had reached temperature.
I eventually designed a small machine which cut the profiles veryaccurately. Unfortunately I sold the outfit very cheaply when I left the UK. NowI wish I hadn't - C'est la Vie.
Tom Wright 227/01/2011 15:25:29
3908 forum posts
297 photos
One way around this problem can be found by searching u tube for toms blue monster
this is a 10 ft span ep rc model made from flat blue styro foam.
Ernie27/01/2011 16:32:38
2530 forum posts
21 photos
Hi Baz
Blue foam is easy to cut with a fine blade saw, like a hacksaw. Also, it sands well, not like white foam

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