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Help with foam hot wire cutter

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GSM

New Member
Hi guys,

Newbie's first post so I need you to keep this dead simple for me!

I've done some research into hot wire cutters but I'm a bit vacant with electronics.

I've bought this power supply unit 3V-15V Adjustable 25/30A Max Power Supply : CB Radio Accessories : Maplin

Now for my question. My cutting bow is 2ft long. If I place a thicker nichrome wire than I get sufficient current draw and the wire heats up and cuts foam.

However, when I put in a thinner wire, It heats up but not enough to cut well.

Now my power unit only has variable volts. What would I need to do to get the thinner wire hotter (assuming changing the length of wire isn't a viable solution).

Can you give me any simple formulea I can use so I know what I'm doing in future regarding lenghts of wires, diameters, volts, ohms, amps etc?

I'll be honest and say I thought the thinner wire would get hotter than the thicker one so I'm just left confused!

Signed a girl playing with boys things :rolleyes:
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
You need to understand ohms law

V = I x R

R is the resistance of the wire, the thinner the wire (of the same type) the higher it's resistance - which is why the thinner wire doesn't get as hot.

So rearranging that formula the current (I) = V/R - so to increase current you either make the resistance lower (thicker wire), or make the voltage higher.

What's wrong with using the thicker wire anyway?.
 

GSM

New Member
Ta Nigel,

OK Ohm's law understood I now see why the smaller wire is not getting as hot.

The thick wire gives a wide 'kerf' this is the bit of foam that is vaporised whilst the cut is made which reduces the cutting accuracy. Also during cutting I will need templates at either end as guides. I can get 'dipping' between the templates with a thicker wire, where the foam has melted away too much, which means the piece is unfortunately scrap.

So what I need to know is how can I 'push' more current down the thinner wire whilst still using this power control unit? Can that be done? If so what piece of electronics would I need to add to control current in the thinner wire and get it to heat up more?

I have an Amp dial on the unit, which shows the very low amp draw of the thin wire. From the Ohm's law it seems the only way would be to increase the voltage or shorten the wire?
 
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Chippie

Member
For the thinner wire you need to wind the volts up...

I use nichrome wire on my cutting bow and run at about 18 v AC, this is supplied from a transformer whose input voltage is adjusted using a variac to alter the wire temperature for different foams...White EPS cuts differently to Pink and Blue foam...
 

GSM

New Member
Thanks Chippie,

Can I use the power supply unit I've purchased with a variac? Any suggestions which variac I would need and how I use it?
 

Chippie

Member
Thanks Chippie,

Can I use the power supply unit I've purchased with a variac? Any suggestions which variac I would need and how I use it?
Hmm..not sure where you could buy a variac these days, mine is left over from my business of many years ago when I was in the tv repair game..

But basically it is a transformer with a single primary winding which has a contact that moves up and down the winding to give a variable output between 0v and 240v ac...
So it ahs a cable with a plug on that goes into the mains socket, then the transformer is wired to the output terminals...( I had a short extension lead wired to mine for plugging in tv sets when repairing power suplies etc)..

Dont think using one with your Maplin psu is gonna help...You need to get hold of a mains transformer with a suitable secondary rating..( Rapid Electronics ?)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks Chippie,

Can I use the power supply unit I've purchased with a variac? Any suggestions which variac I would need and how I use it?
A variac wouldn't really help, as your supply is probably a regulated one, which you don't really need for your use.

Using your existing supply you have a number of options, all of which involve either increasing the voltage, or decreasing the resistance.

Probably the easiest way would be to get some different wire, that is thin but has a lower resistance.
 

Hero999

Banned
You could use an 24V transformer with a phase controlled lamp dimmer style circuit to vary the RMS voltage and therefore the temperature.

The trouble is off the shelf lamp dimmers don't like transformers and require a minimum load so you might have to build your own dimmer circuit and connect it tot the secondary side.
 

GSM

New Member
Thanks guys, realised now that variac would be a whole different approach.

I have a catalogue on order from a resistance wire company which will give me the option of choosing then wires with lower resistance so that might give me a quick solution.

I also want to build a bench mounted cutter with a shorter wire, so my PSU may will be up to that job.


I have found a variac on the Maplin site:


3V-15V Adjustable 25/30A Max Power Supply : CB Radio Accessories : Maplin

Would that be the sort of thing Chippie?


Though I'm currently just playing about cutting foam I'm in the process of starting a new company. I'm on a steep learning curve with controlling the wire cutters as you can see but I will eventually need a versatile system.

Is it worth me persuing the variac above to build a more versatile PSU?

(I'm rubbish with theory but can solder etc so building circuits isn't impossible, but I might be asking for a lot of help!)
 

Boncuk

New Member
Use the G-string of a Gibson guitar string set. It works perfectly. :)
 

Chippie

Member
That would do......

But there are better safer solutions..see the previous posts about a transformer with the variable bit on the secondary..Much safer!! :)
 

Hero999

Banned
There is nothing unsafe about using a variac providing it's followed by an isolating transformer.

For safety reasons keep the transformer's voltage rating below 24V.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
There is nothing unsafe about using a variac providing it's followed by an isolating transformer.

For safety reasons keep the transformer's voltage rating below 24V.
No reason to be that low, and it might be too low for what she needs - I would suggest a 30V or 36V transformer after a variac.
 

Hero999

Banned
If you go over 24VAC then expect to feel a shock if you touch the output.
 

tcmtech

Banned
Most Helpful Member
My brother and I built a simple hot wire foam cutter last year and all we use is a single strand off of a 3/32" aircraft grade steel cable. Its about 22 gauge in thickness.
At 30 inches its still has a low enough resistance to work well running on a 12v power supply/battery charger.
 

GSM

New Member
OK sounds nice and simple so far with the two options which I can follow.

As Chippie mentioned earlier different foams like different settings. The easiest way I know to get the right settings is to have a volt meter and amp meter so I can set pre-tested settings before cutting.

Would it be relatively easy to say get the variac, the transformer (24volt or greater) and then wire in some meters so I can see what is happening? Sorry for being dim about this stuff, but I've learnt a lot already from you guys in a few hours.
 
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