You're overthinking it. IF you have 2.7Ω per foot and you have X number of feet you multiply the ohms times the length. So if you have 6,5 feet you multiply 2.7 x 6.5. Which equals 17.5Ω.

You have a fixed voltage supply at 24 volts (assumed). I (current or amps) is calculated by dividing E (the voltage supply) by Rt (the total resistance). In this case, 24V ÷ 17.5Ω = 1.4A. Wattage is Volts times Amps. So 24 x 1.4 = 33.6W. As @MrChips was alluding to - you'll need to change the voltage to control the wattage. And wattage is the amount of heat you need to cut through the foam. Actually, there's more to it than simply calculating the wattage. There's also the amount of time it takes to do the work. But lets not go there just yet.

 

If it were me, I would start by trying to figure out what type of wire you should be using. The first step is probably figuring out how tight you're going to have to pull the wire to keep it strait, and how hot it needs to be to cut effectively. It's OK if you don't have an exact answer, do your best to estimate. Now with these numbers, try to find the type of wire that will be strong enough to withstand the required tension while at the required temperature. Also keep in mind that some wire types will corrode faster when hot. The strength calculations will also tell you what diameter you will need. Now you will have the length, diameter and type of wire, and from there you can calculate the total resistance of the length of wire. And that will help you choose the power supply.

Hot wire cutters have been around for a while so my guess is there is a type of wire that is commonly used, maybe this is the first thing to research.

 

@MrChips OK, comma versus decimal point. However, @paradauz used a decimal point in the 0.4mm wire size. So I was thinking maybe he was confusing the two and that may have been introducing error into his calculations. He could have been thinking about milliamps or milliwatts. I just don't know for sure how he was coming up with his numbers (gender assumed to be "he").

 

@MrSoftware It's been over 10 years since I did my insulation work, but I believe I had some scrap ChroMolly welding wire. I was working for a welding shop where they used that kind of wire in some cases. Typically when a spool gets short on wire they'll just pull it off the feeder and toss it in the can. So I took probably about 30 feet of the stuff and connected a length slightly longer than 10 feet to my welder and it got hot enough to cut through 2 inches of foam at a length of 8 feet in about 10 seconds. Without keeping the tension the wire would sag way too much. But I used my welder which was a fixed 24 volt source.

Entering into the equation @paradauz is trying to solve is the cut rate (speed) at which he cuts through his foam. That's one part that always confuses the heck out of me. it's probably a simple solution but it's scary to a boy who graduated high school from the #4 from the bottom school in the Los Angeles Unified School District. My math skills were considered remedial, and they were poor at that. I've had to teach myself some algebra. But because I never use it I find myself still struggling with numbers. Electronics has not been a good hobby for me. Still, I like making sparks and smoke. And sometimes things that actually work.

 

@MrChips OK, comma versus decimal point. However, @paradauz used a decimal point in the 0.4mm wire size. So I was thinking maybe he was confusing the two and that may have been introducing error into his calculations. He could have been thinking about milliamps or milliwatts. I just don't know for sure how he was coming up with his numbers (gender assumed to be "he").

Yeah, I'm from Europe, so we use a comma, but since a lot of literature and guides are American, I confused and mixed them, but It didn't influence my calculations. It's just the fact, that It's hard for me to wrap my head around all the things I need to account for.

 

If it were me, I would start by trying to figure out what type of wire you should be using. The first step is probably figuring out how tight you're going to have to pull the wire to keep it strait, and how hot it needs to be to cut effectively. It's OK if you don't have an exact answer, do your best to estimate. Now with these numbers, try to find the type of wire that will be strong enough to withstand the required tension while at the required temperature. Also keep in mind that some wire types will corrode faster when hot. The strength calculations will also tell you what diameter you will need. Now you will have the length, diameter and type of wire, and from there you can calculate the total resistance of the length of wire. And that will help you choose the power supply.

Hot wire cutters have been around for a while so my guess is there is a type of wire that is commonly used, maybe this is the first thing to research.

I totally agree, I most likely will be using Nichrome 60 wire, as it's the most common for hot wire cutters, and I was thinking about 0.4 mm thickness, but I'm really not sure what tension do I have to apply in order for it to not sag, as well as will it be able to withstand those kinds of forces.

 

Remember my post about cutting foam panels and how my hot wire stretched when hot, and how I needed to keep tension on it. I used a small dishwasher door spring on a 10 inch fulcrum with the spring mounted about 5 inches from the fulcrum point. It kept enough tension to keep the wire straight while making a straight cut through two inches of foam. And it took about 10 seconds to make that cut.

I made my rig like a chop saw. Put the foam on a table overhanging the edge then brought the 10 foot 2X4 down chopping through the foam. If I moved too fast the spring would flex and allow the wire to stretch. I couldn't cut too fast. I also had to have someone hold the other end of the piece being cut off. Otherwise it could influence the cut. I'm assuming you're cutting to a rigid pattern, so your cut speed will likely have to be slow enough so as to not introduce sag (or lag) in cutting while following a contour. I had a friend who was into model airplanes a good 40 plus years ago. He used NiChrome wire to cut his foam. And if he cut too fast the contour of the wing would be distorted. So he had to cut very slow. That presented a problem for his rigid patterns because of the excess dwell time with the heat. And back then neither of us knew much about current or watts or anything else. In fact, his rig would be banned from this forum because he used a full 120 VAC. The wire he was using was out of a wall heater. And yes, it was extremely dangerous from two stances - first it was using a potentially lethal voltage, and second it was an extreme fire hazard. He was lucky enough to not be hurt by his rig. Mine - just little over 10 years ago - was powered from a welder. Wire voltage was around 24 volts. Also, I used a ChroMolly wire. It never got red hot, but it was hot enough to cut through the foam without making all kinds of mess and without the risks associated with high voltages and high temperatures.

However you eventually solve your problem - be careful. Using 24 volts is highly recommended. The amperage needed will depend on the wire resistance and the time it will take to make your cut.

Good luck.

[edit] one last point about my rig - when I tried to cut outdoors the breeze would cool the wire so much that it wouldn't cut. Had to cut inside the garage to avoid the cooling issue. I'd have to imagine that ambient temperature will also need to be factored into your final project. OR you could just go with trial and error. Learn what works through trying different methods.
[end edit]

 

Thank you for sharing. I found out that I need 2.6 A to heat up the wire till 420 Celcius, meaning I will need to have V=2.6x18 = 46.8 V to heat up the wire. The power needed in this case could be 130W. I'm planning to use a 48V 15A power supply, as I would also need to supply current to motors and drivers.

 

Evidently many of the commercial CNC cutters of significant cut length use Titanium Alloy wire in place of the NiCr versions over 1.5metres.
This helps in mainlining tension.
Also here is an example of one manual machine of up to 1.5 metres .


Max.

 

I need 2.6 A to heat up the wire till 420 Celcius

I accept your answer. I'm wondering how you arrived at 420˚C. Seems rather hot.(788˚F)

 

I accept your answer. I'm wondering how you arrived at 420˚C. Seems rather hot.(788˚F)

I looked up the datasheet of the wire, and in the table, it states that it has to get 2.1 A for 316 C and 2.6 A for 417 C, I decided to go with the higher value, to maybe account for the cooling of the wire, due to its length. Is it a concern, can excessive temperature negatively affect the cutting process?

 

The hotter the wire the faster it will cut and less lag. But also, the hotter the wire the faster it wants to cool, meaning it will lose thermal energy faster at a higher temperature than it would at a lower temperature. However, hotter being the key factor, it will cut faster with less wandering of the wire.

 

The hotter the wire the faster it will cut and less lag. But also, the hotter the wire the faster it wants to cool, meaning it will lose thermal energy faster at a higher temperature than it would at a lower temperature. However, hotter being the key factor, it will cut faster with less wandering of the wire.

Do you think the 420 C will be a sufficient temperature?

 

If you've read somewhere that 316˚C is sufficient then 420 will be faster. I can't add (or detract) from those figures.

 

If you've read somewhere that 316˚C is sufficient then 420 will be faster. I can't add (or detract) from those figures.

Alright, thanks for your help

 

Why not make the wire length the max thickness of the material. Cut the other way. I too used a spring and Kanthal wire to cut glass (mirror tile).
I tried cutting an L-shaped section and it mostly worked.

Sag will be a major problem. I doubt you want to add the complication of a tension controller?

I, sort, of built one of them to for another project. Open loop. Turned out we didnt need it. I know what you have to do at low tensions. You have to allow the clutch current to go slightly negative.

 

Why not make the wire length the max thickness of the material

Not sure what you mean by that. I know that sag will be a major problem, but I'm not sure how to calculate it in advance, so I guess we will need to try out different springs or tension pivots.
I also considered using a titanium alloy wire, which will sag less and be more durable.

 

Your cutter length needs to be greater than the thickness you are cutting. I hope I didn't misunderstand something,

You can always move the work instead of the tool too.

 

Guitar string works nice.

 

So far I have not seen any mention of how hot the wire needs to be. Certainly the required current will depend a whole lot on that requirement. A nichrome wire in air will lose a lot of heat and so that also needs to be included. So first you need to determine how hot the wire needs to be, and then you need to determine the required tension to keep it straight enough to make the cuts that you require. Then with that tension requirement you can consider the strength that the wire must have to not stretch at that temperature. After that is found you can look up the resistance for the diameter of wire that you will have, and you can easily look up the resistance per meter and know the resistance of your setup. But you still will not know the heat loss of the wire in your setup, so you will need to have a variable voltage supply so that you can set the required temperature for the cutting speed you have chosen.
Thus finding the best wire size will be more complex than just asking a bunch of folks who do not know the rest of those details.
Now I see that as I replied, there was a comment about the temperature desired. And while titanium wire would be nice, it is rather more expensive. And steel wire would be the cheapest but it will not last as long.