Hot wire foam cutter

[Mr Ed]

I need to make some stuff out of polystyrene so Im gonna knock one of these up hot wire cutters tonight. Theyre a pretty simple thing but Im trying to make it using stuff I already have lying about.

Ive got a bunch of bits of wood, steel, plenty of spare electrical wire and guitar strings to use as the cutting wire.....what Im trying to figure out is what to use as the power supply.

I have a bunch of random transformers from random old appliances (the black box type that plugs into the wall with a 12v lead connecting to the printer/wateva)....but Im guessing these might not supply enough current for the job.

I have a bunch of car batteries that could be used, but Im guessing that these might supply too much current. (Maybe I could whack a variable resistor in series with a battery??)

I know a bit about electronics but I have no idea what range of voltage/current I should be aiming for. Any tips?

Or am I best to just to go buy a transformer from jaycar thats more suited to this sort of task?

[oldcorollas]

http://www.rcgroups.com/forums/showthread.php?t=292102

battery charger?

[Mr Ed]

Gonna give this a go with a car battery and see how it goes. I ducked out and bought some nichrome wire ($5 for 4m at jaycar) at lunch time and the idea is that I just hook the power up the the wire at different points to vary the current.

Will post up how I go.

[thechuckster]

jaycar power supply would be better - you'll want to vary the amps somehow.

Edit: a power brick for a printer/scanner will toast itself pretty quickly - most are under 1amp output rated.

[Mr Ed]

If theres too much current I guess I could loop the cutting wire round so theres two long resistors in paralell, then move the position at which they join for finer adjustment.

My physics is a little shaky these days though.

Some guy on PF says he uses and battery and it works. I figure its worth giving it a go. I could put a 10amp fuse or something on it if Im really worried.

[oldcorollas]

got the resistance for the nichrome?

Power = voltage x current.

but yeah, hook it up at different lengths and see when it melts

when makign furnaces, i'd use abotu 10metres of kanthal wire (similar.. ish.. sorta..) and with a 100V/10A supply, it would toast itself if the power was not controlled (i think 1400-1500C was highest accidental temp...

hmm... got an injector or fuel pump resistor lying around? that will reduce current.

[thechuckster]

find the Race magazine issue that had Nero and mate doing the foam core for a rear wing. It talked bout the volts/amps used by his form cutter

[Mr Ed]

It should be 13.77ohms/m, the bow will be roughly 1300mm and I have 4m of wire so that gives me a resistance range of 18-55ohms.

Ok so here my attempt at some physics on the thing, I used to know this stuff a bit better....but I havent really thought about it in 10yrs.

Say my battery is 12v and 400amp.... this gives me P = 12x400 = 4800w

I=√(P/R)

Max current = √ (4800/18) = 16.3amps
Min current = √ (4800/55) = 9.3amps

This seems a bit high, say I want to aim for a maximum of 6amps.....R=P/I²

Max current (6amp); R = 4800/6² = 133.3 ohms
minus resistance of bow (18ohms)
= 115ohm resistor req'd

Before I get too carried away....how do those figures sound?

[Mr Ed]

find the Race magazine issue that had Nero and mate doing the foam core for a rear wing. It talked bout the volts/amps used by his form cutter

Yeah Ive actually got a copy of that issue but I lost it somewhere in the midst of moving from Brisbane.

[oldcorollas]

umm V=IR=12

if you want 6 amps, you need 2 ohms total

133ohm and 6 amps... = 800V battery

battery is voltage fixed, not power fixed

[banana_socks]

How much power do you need to cut through the foam? Need that first before you can really work anything out. Also you'll have trouble finding an accurate value for the internal resistance of a battery, so it could be significantly out. I wouldn't really bother too much trying to calculate it.

Personally I'd avoid looping back the wire, as you'll make cleaner cuts using a single piece. If you need to reduce the current, use a resistor.

Battery charger is probably a decent start. 10amps isn't going to do too much damage but should be enough to cut. If it's not enough, then you've got a baseline to work from.

[Mr Ed]

Cheers...I thought I must be doing something wrong.

Well Ive got minimum resistance of 18ohms for a 1.3m bow, which will only give me 0.667amps. From what Ive read I dont think that will be enough.

Sounds like I'll either need to find a different wire or use a smaller bow. Perhaps this is why people recommend using guitar strings?

Sounds like I should make my bow only 150mm or so wide, which I can get away with this time. I'll make a bigger one another day.

[Mr Ed]

How much power do you need to cut through the foam? Need that first before you can really work anything out. Also you'll have trouble finding an accurate value for the internal resistance of a battery, so it could be significantly out. I wouldn't really bother too much trying to calculate it.

Personally I'd avoid looping back the wire, as you'll make cleaner cuts using a single piece. If you need to reduce the current, use a resistor.

Battery charger is probably a decent start. 10amps isn't going to do too much damage but should be enough to cut. If it's not enough, then you've got a baseline to work from.

By loop the wire, I meant having the loop out of the way of the cutting wire, but it sounds like I wont need to bother.

I need to do some testing but google seems to suggest somewhere between 2 and 4amps will work.

[thechuckster]

post a query on the race magazine forums?

[Mr Ed]

I didnt realise there was one. Will check it out.