I suspect the thermal properties of the two materials may differ.

I believe the melting and ignition temperatures would be the same, but the density could dramatically change the power input required.

 

I believe the melting and ignition temperatures would be the same, but the density could dramatically change the power input required.

You suspect and I believe. We need to discover the properties of XPS.

 

The temperature does not have to be "red hot" to very quickly cut the foam. What is very important is the way you support the foam and cutter while moving it. Careful even movement is necessary because the wire goes without force in any way you let it! We cut model airplane wing shapes using pieces of formed coat hanger! (plain steel!) We also use piano wire but have found that the old coat hangers work just as well. We use a 10 amp variac driving a 4:1 transformer (26v @30 amps open circuit) to heat the wire. The transformer gives the current boost so the smaller variac can be used, plus it isolates the cutting wire from the line voltage. The heat load of the foam is such that complicated measurement and feedback schemes are not necessary. The variac and transformer are quite expensive (over 100 bucks) so the power supply listed above may be a lower cost way to get the needed current. With switchers, be sure the supply will start up with a heavy load like that connected. The cold resistance of the wire is such that the starting current can be easily 50 amps for a second or two! It will take 20 amps (hot) so be sure the power supply is capable. Use #12 flexible wire to connect the transformer or power supply to the cutting wire. Screw clamps are necessary. No alligator clips or simple twisting the wire!

 

I cut both EPS and XPS foam with hot wires and they cut very much the same. In order to create a smooth cut, it is essential to keep the speed of the wire through the foam constant. Within reason, a hotter wire will simply cut faster.

 

I based my 4ft span one from 1/2" PVC pipe and tension'd the top by threaded method based on the old Bow saw design.

Max.

 

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!

Constant current would help - a few years back; EPE magazine published a foam cutter project based on PWM current control.

Most magazines will supply photocopies of old articles for a fee.

 

Constant current would help

The cold resistance of the wire is such that the starting current can be easily 50 amps for a second or two!

I don't think constant current will help. The reduction in resistance as the wire cools will help regulate power to the proper level. Probably not enough, but it's going in the right direction.

 

I don't think constant current will help. The reduction in resistance as the wire cools will help regulate power to the proper level. Probably not enough, but it's going in the right direction.

I think that constant temp would help. And that could be determined by measuring the wire's resistance during dead time when applying PWM.

 

I think that constant temp would help. And that could be determined by measuring the wire's resistance during dead time when applying PWM.

The heat load of the foam is such that complicated measurement and feedback schemes are not necessary.

I hope this guy is correct because he has actually done it.

 

I don't think constant current will help. The reduction in resistance as the wire cools will help regulate power to the proper level. Probably not enough, but it's going in the right direction.

Depends on the tempco of the wire used.

The wire from WW power resistors is as close to neutral as the manufacturers can make it. Some E-cigarette wire I heated with a blowtorch didn't change much either. OTOH: there is such thing as WW NTC surge limiting resistors - I have some.

Its mostly bulb filaments and electric heating elements that are PTC.

 

When I insulated my basement I just took some welding wire, spool fed type and stretched it between two points, using a 2 x 4 x 10 feet. At one end was a fixed arm. At the other was a spring tensioned arm. As the wire got hot it expanded and slackened, that's why I used the hinged spring tensioned arm, to keep the wire taught. I then used my welder (24 VDC) as a power supply. Please don't ask me what the power was, I just know that I could cut accurate straight lines without having all those pesky static charged snow balls sticking to my socks and hair. The panels I was cutting were two inches by four feet by eight feet. And the wire never got glowing hot. The ten foot length of welding wire (don't ask what type) got hot enough to cut through two inches by eight feet in about 5 seconds.

[edit] I think the wire I used was Chromolly. A bunch of it was scrapped and I managed to grab about a quarter spool. Probably 0.050 inch thick.

 

Depends on the tempco of the wire used.
Its mostly bulb filaments and electric heating elements that are PTC.

Now why would anybody be careful to choose a special wire to avoid a naturally occurring tempco that is right for the job?
ps, this IS a heating element job.

 

Now why would anybody be careful to choose a special wire to avoid a naturally occurring tempco that is right for the job?
ps, this IS a heating element job.

I guess that leaves us with two choices:

1. Simple, practical and boring
2. Complex, nuanced and fun

Mmmhhh... why do I have the feeling you'll be partial to option one?

 

why do I have the feeling you'll be partial to option one?

Because you know me.
I was all for the pulse and measure method in post #15 but then we got advice from ronsoy2 based on real experience.
One experimental result is worth a dozen theories.

 

One experimental result is worth a dozen theories.

I think you are undervaluing experimental results.