scudderfish
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 posted on 6/8/10 at 03:09 PM |
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Hot wire foam cutting
I'm trying to cut out a wing in one piece so I've made a bow with 30swg nichrome wire about 40" wide. How many volts/amps should I
use? 12v doesn't seem to get it anywhere near hot enough, I can still touch the wire 
Regards,
Dave
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auroan
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| posted on 6/8/10 at 03:22 PM |
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Isn't heat a function of the amount of amps going through it rather than voltage ?
[Edited on 6/8/10 by auroan]
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Miks15
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| posted on 6/8/10 at 03:24 PM |
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yes, so for a peice of wire which will give a certain resistance you will need to up the voltage to get more current flowing.
if you shorten the wire (if possible) you will decrease resistance and increase current, not sure youll be able to get it hot enough just by doing
that though.
quote:
Originally posted by auroan
Isn't heat a function of the amount of amps going through it rather than voltage ?
[Edited on 6/8/10 by auroan]
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scudderfish
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| posted on 6/8/10 at 03:29 PM |
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I've just measured the wire at 17ohms. I can't shorten it as I need to cut a single piece which is only a couple of inches narrower than
the bow.
V=IR
12=I17
I=12/17
I=0.7A
I think I need to head towards 24/36v. Any ideas? I only have one available car battery so I can't wire several in series.
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Miks15
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| posted on 6/8/10 at 03:37 PM |
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i would have thought you need more than that.
How about taking the battery of out the car (if youve got some handy?)
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CNHSS1
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| posted on 6/8/10 at 03:39 PM |
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when making model aicraft wings many moons ago we used to use 2 or 3 12v batteries in series and have the earth connection on a croc clip. If wing was
say max of 40", you'd have say 60" of nichrome wire. Click on furthest end of nichrome wire and monitor temp. If too cold, creep
croc clip closer until correct temp attained. If you get too close to the end of your wing, add another 12v batt and repeat process!.
Temp is a bit of any art. ensure wire is warmed up fully before cutting or else you get a hot central portion of the wire and cooler ends. Temp also
depends on foam, white expanded, blue foam or poly foam, each need differing temps (hence the adj croc clip idea).
ensure wire is hot enough to cut without dragging or you end up with an inconsistent wing section, but not too hot to burn the surface. Adjust your
cutting speed accordingly. Takes a bit of practice too
Model shops sell decent shiny ni-chrome wire (MFA used to be a brand name) or find an old leccy fire and unwind the element!
[Edited on 6/8/10 by CNHSS1]
"Racing is life, everything else, before or after, is just waiting"---Steve McQueen
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Daddylonglegs
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| posted on 6/8/10 at 03:46 PM |
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If I'm out of order tell me, but I assume you are using resistive wire? The only reason I ask is because that should work fine with 12V. I used
to use 3 D-type batteries in series years ago and tha worked fine.
It looks like the Midget is winning at the moment......
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scudderfish
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| posted on 6/8/10 at 04:01 PM |
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quote:
Originally posted by Daddylonglegs
If I'm out of order tell me, but I assume you are using resistive wire? The only reason I ask is because that should work fine with 12V. I used
to use 3 D-type batteries in series years ago and tha worked fine.
Yep, nichrome wire. My problem is down to my length (so my wife tells me! Boom! Boom!), that the resistance is too great for 12v to get enough amps
down it to heat it up.
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Daddylonglegs
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| posted on 6/8/10 at 04:16 PM |
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Oh I see.
I can't remember the wire I used but maybe the idea from CNHSS1 of trying different lengths (ooeer!).
Hope you get it sorted.
It looks like the Midget is winning at the moment......
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Bluemoon
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| posted on 6/8/10 at 04:35 PM |
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Well you could shorten it till it works at 12V you then know how long the wire can be for each 12V.. i.e. double it's length from what works at
12V then you need 24V etc.. I'd be careful if you need high voltages (say above 40V don't what to see you getting shocking results!!)..
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MikeRJ
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| posted on 6/8/10 at 05:15 PM |
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This chap suggest his 13" wire dissipating 13 Watts works well. Since you have
a longer wire you will need to dissipate more power to keep it at the same temperature - approximately 40 watts in this case.
P = (V*V) / R
V = SQRT(P * R)
V = SQRT( 40 * 17 ) = 26 volts.
Start with 24 volts and see how it goes.
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scudderfish
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| posted on 6/8/10 at 05:16 PM |
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Thanks all 
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BenB
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| posted on 6/8/10 at 06:04 PM |
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Personally I made my own little box of kit. I got a 24v transformer, put a dimmer switch across the primaries, then put it through a juicy rectifier
into a DC amp gauge. If you've got a high ampage AC guage you could use that and get rid of the rectifier. But I was using the box of a small
car battery charger which had a nice little 0-5A gauge on it so used what I had.
Works fine.
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scudderfish
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| posted on 8/8/10 at 09:35 AM |
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Hmmm, playing around with the 13w target and resistance of 17ohm/m, it is interestingly non-linear.
12v should power a wire 65cm long, where as 24v should power 2.61m! For 1m, according to my calcs, I only need a tad under 15v.
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