# Hot Wire Cutter

Discussion in 'General Electronics Chat' started by dnb_p, Nov 21, 2016.

1. ### dnb_p Thread Starter New Member

Nov 21, 2016
3
0
Hi all,

I'm an architecture student and would be extremely grateful for some advice with a tool/machine that im making. Basically i'm creating a large hot wire cutter and need to pass enough power through to heat a 1.6mm, (or 2mm gauge) 1m long nichrome wire to a temperature suitable for cutting. This gauge increases the stiffness which is necessary when dragging through the foam.

This link below displays a hand tool that works in a similar way. This tool uses 0.8mm nichrome wire and comes with an adjustable transformer that can supply 2 Amps @ 16 volts

https://images-na.ssl-images-amazon.com/images/I/817vqvmvMML._SL1500_.jpg

I've tried using an adjustable power supply at 3 amps, 30 volts with crocodile clips but had no success. I obviously don't have much knowledge in this sector and need to find a way to heat the wire, any help would be great!

2. ### cmartinez AAC Fanatic!

Jan 17, 2007
5,892
7,325
Wow... one meter long? I'm guessing you want to manufacture mattresses or something similar. Anyway, I've seen many machines that do this already, and at such thicknesses the wire needs to be monitored for its temperature, which is normally done by alternatively powering it and measuring its resistance using a Wheatstone bridge circuit.
I'll try to help you with this project, but keep in mind that maybe using a special-edge band saw blade would be a better option.

Now for some basic math. The tool you've mentioned uses 2 amps @16 volts = 32 Watts, and it seems that it's heating a wire length of about 10 cm. So in order to heat the same gauge wire, but with 1m length, you'd need 10 times as much power. That would be 320 watts... but then you've mentioned that you want to power up a wire that's twice the thickness of the original tool (1.6mm in diameter), so it's cross section is four times the original wire, and so you'd need four times as much power to heat it up. That would be at least 1,280 watts. (I could be wrong on this last assumption, so I'll be back later to confirm)

And even if you were able to properly heat the wire, the moment you started cutting the material with it lots of fumes would be produced, and they will all travel upwards and through the groove that the cut is creating... and so you have the real serious risk of igniting the material and staring a fire.

But there are people in this forum more versed than I in this matter. Let's just wait and hear what they have to say.

Last edited: Nov 21, 2016
Bernard likes this.

Jan 17, 2007
5,892
7,325

4. ### ronv AAC Fanatic!

Nov 12, 2008
3,693
2,837
More power.
Some wires are different than others, but this should give you an idea.

Your wire is pretty big! So it will take a lot of current. If my conversion is right it is like #12 wire. So to get it to 1000F will take 26.5 amps. From another page the resistance is .1 ohm /foot. so about .325 ohms. = 8.6 volts.
If you can use a smaller diameter wire it might be cheaper to use a higher voltage and lower current. Maybe shoot for 12 volts.

cmartinez likes this.
5. ### gramps Member

Dec 8, 2014
54
20
What kind of foam are you cutting?
I have used hot wire cutters 48" long to cut Styrofoam. The wire size isn't anywhere near 2mm, more like 0.4mm (26 gauge). You don't "drag" the hot wire through the foam. What you do is allow the wire to find it's own speed to melt its way through. This keeps the wire straight. Move slowly, perhaps 2-3 seconds per inch. On 26 gauge nichrome wire, I find that 3-4 amps is about right. YMMV on other kinds of foam.

6. ### ronv AAC Fanatic!

Nov 12, 2008
3,693
2,837
Being impatient, when I built mine (much smaller than yours) I added a spring to keep tension on the wire.

Bernard likes this.
7. ### Bernard Expert

Aug 7, 2008
5,073
577
My long one is a 1 X 3 X 38 in wood bar with 2- 1/8 in. X 12 in. spring steel rods at each end supporting .77 mm nichrome wire under about 5 lb. tension. Power is a 6A Variac.

8. ### dnb_p Thread Starter New Member

Nov 21, 2016
3
0
Thanks all for your help so far!

I’m cutting Styrofoam and the reason for wire thickness is that I need it to hold it’s preformed/bent shape for cutting out funky but controlled shapes. This is opposed to linear forms with thin, tensioned hot wire cutters a few of you have mentioned.

Okay so i have no idea on how to start building this kind of circuit could anyone offer any advice, or is there an adjustable power supply i could get hold of that would do the trick?

Cheers!

9. ### ronv AAC Fanatic!

Nov 12, 2008
3,693
2,837
This is probably to cheap to be real good, but you could give it a try.
What temperature do you need?
http://www.ebay.com/itm/MFJ-4230MV-...990208?hash=item1ea0373000:g:SO0AAOSwLzdWTo6V

Jul 18, 2013
17,746
5,401
Many use the simple 'dimmer' style controller on the primary of a transformer, I preferred to go the other route and wire it on the secondary.
There are multitudes of Triac style dimmer circuits out there.
Max.

11. ### shortbus AAC Fanatic!

Sep 30, 2009
6,409
3,737
May be instead of round wire you could use nichrome ribbon wire for the shapes. http://jacobs-online.biz/nichrome_ribbon_wire.htm But according to this video from the how it's made show, they use Kanthal(chrome/nickel) not nichrome wire for cutting (around 4:20 into the video).
http://www.kanthal.com/en/products/...wire-and-resistance-wire/list-of-cuni-alloys/ Like the video shows the shapes are cut with a straight wire not a formed one.

The CuNi wires have a lower resistance so less amperage will be needed to heat them to the same degree. Here's a good chart showing the different resistances for different types of resistance wire. http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_2.html

12. ### jpanhalt Expert

Jan 18, 2008
7,059
1,508
@shortbus I think you may mean "less voltage." Power = (I^2)*R

John

13. ### dnb_p Thread Starter New Member

Nov 21, 2016
3
0
Upto 700 degrees Fahrenheit would be ideal. This one looks good are there any other similar products on the market as quite hard to get hold of in the uk/long wait. I'd need to be pretty sure that it could work if i purchase. Do you reckon its a goer?

Thanks!

Nov 12, 2008
3,693
2,837
15. ### #12 Expert

Nov 30, 2010
18,076
9,680
No, "to heat it up at the same rate of temperature change".
I like your 320 watt number but the increased thickness of the wire only amounts to thermal inertia (time) and more radiating area. When the cutting wire touches the Styrofoam, the heat conducted into the Styrofoam far surpasses the heat that can radiate off the wire into air, so thickness of the wire is insignificant compared to the thermal conductivity and latent heat of melting of the intended victim.

The limit I envision is about applying less power than is required to melt the wire in free air, but enough power to melt Styrofoam. A red hot wire is more than enough to cut Styrofoam, but it will go black almost instantly when it touches the foam because the melting point of Styrofoam is 240 C. (Does Ni-Chrome glow at 240 C? I don't think so.) This is where a power pulse followed by a temperature measurement is ideal, but expensive compared to just finding how much power one can inflict on the cutting wire without melting the wire, and how fast it will fall through the Styrofoam. And, yes, I do mean, "fall through" the Styrofoam. Any pushing will distort the shape of the cutting wire, so the practical effect is to gently touch the wire to the foam and watch how fast it falls through the block of foam.

The amps and volts depend entirely on the resistance of whatever wire substance is used. What matters is watts.
My first opinion is to choose a high melting temperature wire, bring it up to cherry red, and see how fast it falls through the Styrofoam. If this fails, adding more watts during cutting is the answer. Risky, but possible. Downright practical if the pulse and measure method is available.

Edit: Further Googling reveals the threshold of glowing temperature to be 400 C.

cmartinez likes this.
16. ### shortbus AAC Fanatic!

Sep 30, 2009
6,409
3,737
#12 likes this.
17. ### #12 Expert

Nov 30, 2010
18,076
9,680
I have confirmed that you don't have to glow to cut Styrofoam, but I've never seen a "hot wire" Styrofoam cutter.

18. ### cmartinez AAC Fanatic!

Jan 17, 2007
5,892
7,325
You're right. Now that you mention it, it does make sense. So maybe what really counts is the external area, given by pi*D*H . This is because all the heat will be radiated from that surface.

I've built foam cutters before. And I'm almost sure that monitoring the wire temperature while cutting is essential for an application this large.

Too little current and it won't cut, too much and the wire will break. Just right, and it will cut, but if the cutting process stops suddenly, the wire can easily overheat and break.

#12 likes this.
19. ### #12 Expert

Nov 30, 2010
18,076
9,680
That's why you tagged me. I'm pretty good with the basics.
Takes me a while to get the specifics right.
Especially on something I've never done in real life.

"flash ignition temperature is given as 345-360°C and the self ignition temperature as 488-496°C. [APME “Fire Behaviour of Expanded Polystyrene (EPS) Foam”. Rev. 1 18/12/1992.]"
http://blog.cheapwallinsulation.org/2011/03/polystyrene-ignition-test.html

This suggests that the target range is 240 C to 310 C, which does not include glowing, and the safe way to do this is with the, "pulse and measure" method. Pulse some power into the heater, then measure its resistance to find its temperature...quickly. Maybe a microprocessor would be the easy way to do this.

cmartinez likes this.
20. ### djsfantasi AAC Fanatic!

Apr 11, 2010
4,548
1,686
I suspect that it is likely that the OP is not cutting expanded polystyrene (EPS), but instead is cutting XPS or extruded polystyrene. "Funky but controlled shaped" suggests to me a modeling (e.g. architectural perhaps) or artistic objective. Only the OP can clarify this.

XPS is the preferred foam in the latter situations. I bring this up, because I suspect the thermal properties of the two materials may differ. I have yet to confirm my suspicions. Better to know up front and avoid designing for the wrong scenario.

cmartinez and #12 like this.