Well wikipedia gives a melting point value of 240 °C for polystyrene.
Can anybody here at sways confirm or deny?
I converted 240 C to 464 F and decided to play with the table.
I don’t like that you can’t use voltage, with amps fixed, to calculate length at various gauges. So I set length = 10 inches and changed gauge from 20-26 to calculate voltage with the calculator table.
Then I used the boxes below the calculator table to get the total watts of power needed. This allowed me to calculate watts per inch of nichrome wire needed to generate 460 F at 4 different gauges:
20 gauge = 0.96 watts/inch
22 = 0.87
24 = 0.81
26 = 0.74
Watts = volts x amps. Calculate the inches for the gauge of nichrome wire used/needed for your power source’s fixed volts x amps. This should get you pretty close to what you will need.
Can anybody give me the desired temperature for and EPS hotwire?
I’m guessing it has to be a bit warmer than the melting temperature so the foam doesn’t cool the wire too much.
Please…
You're not getting an answer because most people don't approach the problem that way. Instead they use a variac and adjust power to suit whatever wire/length. There's a fairly wide range of temps that will cut, from just barely up to just under where the wire breaks. Hotter, of course, is faster. Different foam densities and bead sizes will cut slightly different also.
For those who may come seeking hotwire information in the future, this appears to be an excellent website about nichrome hotwire cutters:
Follow this link for a good introduction to hotwire cutters:
http://hotwirefoamcutterinfo.com/Introduction.html
I have condensed the information presented in this thread and posted it at this link:
http://bgboard.blogspot.com/2011/01/i-spent-over-8-weeks-trying-to-find-all.html
There seems to be a greater amount of useful information about nichrome wire.
Use the information you learn in this nichrome wire calculator
http://www.jacobs-online.biz/nichrome/NichromeCalc.html
as well as with these technical data specs for nichrome:
http://www.wiretron.com/nicrdat.html
After more careful sifting through the Hotwire Cutter Site above, I found this pearl about nichrome cutting wire temperature:
“600 F (316 C) is normal foam cutting temperature.”
These Ohm’s Law Formulas may be useful too:
Volts = Amperes X Ohms
Amperes = Volts/Ohms
Ohms = Volts/Amperes
Watts = Volts X Amperes
Watts = (Volts X Volts) / Ohms
Watts = Amperes X Amperes X Ohms
Ohms = Watts / (Amperes X Amperes)
The thing that was not clear to me before reading the above information, but is now, is that the amperage required to heat a piece of nichrome wire, to a specific temperature, for any given gauge is constant for that gauge – regardless of length. And voltage must change as wire length varies.
Almost forgot, this seems to be one of the simplest hotwire cutter bow plans – found right here at swaylocks:
http://www.swaylocks.com/comment/164400#comment-164400
Good luck to those who come seeking information next. Hope you find this information helpful/useful.
I understand. However, with what I have gleaned so far, a Variac changes voltage with a fixed amperage. This would mean those who use Variacs must use a specific wire gauge for their amperage.
For example a 3 amp Variac (2.9 amps needed) would need 10.5 volts and 26 inches of 24 gauge nichrome to generate a 600 F wire temp.
Similarly a 3 amp Variac (2.9 amps needed) would need 6.6 volts and 26 inches of 22 gauge nichrome to only generate a 400 F wire temp (limited by gauge and amperage).
However for 22 gauge nichrome wire; 3.7 amps, 8.5 volts and 26-inch length are needed to produce 600 F. A 3 amp Variac will not work.
It seems a bit of research would be required to purchase the correct Variac. Wire gauge is limiting. A Variac is not quite as simple as some here have implied.
[quote="$1"]
I understand. However, with what I have gleaned so far, a Variac changes voltage with a fixed amperage. This would mean those who use Variacs must use a specific wire gauge for their amperage.
[/quote]
not exactly.
a variac regulates voltage.
ohms law (v=i*r ) still dictates the amperage of the circuit,
perhaps the variac does not have the capacity to deliver the amps if it is too small (like the battery charger for your cell phone or a AAA battery ) . however the variac will not try to 'fix' amps.
otherwise, amps will always be v/r.
the big kids really do nothing more than dial up the voltage until the wire gets hot.
-the variac make the volts,
-resistance determines the amps.
-amps determines the temperature.
more or less.
regards,
-bill
I believe that’s what I said. Changes = varies = regulates.
The maximum amperage delivered is fixed.
And resistance is increased by increasing wire length of the cutter.
My physics is rusty. But I believe:
Amperage is the number of electrons flowing through the conductor in a given unit of time. Voltage indicates the magnitude of force pushing the electrons through a metal conductor. Resistance determines the number of electrons that can pass through the conductor.
But I thought the heat energy dissipated the electrical energy. Heat energy is generated by friction of the electrons moving through a metal conductor. Energy can be neither created nor destroyed, just converted from one form to another.
I think watts figures in. Watts = the work done.
W = VI
V = IR
W/V = V/R
Eventually the Heat Energy melts or vaporizes the conductor.
Increasing wire length increases resistance. With wire length fixed and maximum amperage fixed, increasing voltage increases the heat energy produced.
Again those who use Variacs have to determine the correct wire gauge for the maximum amperage of their power source.
Regardless, I calculated the length and gauge of nichrome cutter wire needed for my 12v 2a, 6v 6a, and 12v 6a battery charger outputs. I have arrived at relatively accurate estimates without empirical testing.
For the many past swaylocks posts from people using fixed output power supplies, it is very simple to calculate the wire gauge and length needed with the table provided above – of course they could sit down and use the electrical equations and data sheets for nichrome wire. Nonetheless, no guesswork or circuitous answers were ever needed to provide accurate estimates for nichrome wire.
Buy some wire. Heat the wire with a little juice. Cut the foam. Wear safety glasses.
Stop over thinking the process.
While all the Smart people were thinking about watts and ohms and volts........and wire lenght and computer programs...
and spelling......
The Dumb people already cut their foam....it's really not that complex......
I've cut foam with a battery charger and a guitar string.
....Veriac....Nicrome wire or Surfding's wire...20-24 gauage...12 to 24 volts...done deal......
Stingray
Appreciate the input Stingray… serious. You have offered ball park estimates. The kind of info I was looking for all along.
No worries though. I know exactly what I need for now. A few dollars worth of 22 gauge nichrome, some screw eyes, and a spring to go with my plywood, extension cord, kill switch, alligator clips and battery charger… I’m ready to rock.
I will most certainly buy a variac for my surfboard blank hotwire bow sometime this year.
Agreed. Even the equations are not that complex.
hang loose
Bluegrass
I have concluded that with my 12v 6-amp and 6v 6-amp charger settings I can put together a hotwire cutter that is as versatile as a Variac powered unit.
After reading the information at the Jacobs site further, I saw that you do not want to choose a wire gauge that requires max amperage from your power supply. I chose a guage (22) and length that would need no more than about 62% max amperage – in my case 3.7 amps at 600 F and 12v.
In the NIchrome Calculator, I set voltage = the max available for both amperages, i.e. 12v. I then left length as the variable and determined the lengths needed for various temperatures in 22 guage nichrome wire. This way temp can not exceed the value I have chosen in the calculator.
I found that I should get temperatures between 600-800 F by using 22-gauge Nichrome wire, with alligator clips placed from 37 - 30 inches apart (shorter = hotter) with 3.7-4.5 amps respectively. Voltage at 12. A 7-inch difference in length gives a 200 degree temperature range.
For the 6v 6-amp setting, 600-800 F, 22-gauge Nichrome wire, with alligator clips placed from 18.4 - 15.2 inches apart for the same 600-800 F temp range. Amperage from 3.7- 4.5. Voltage at 6. A 3-inch difference in length gives a 200 degree temperature range.
I will start with 600 F first (longest length).
Simple, fairly accurate and cost effective.
The cheapest and easiest way is an old computer PSU (Power Supply Unit)
You can find it anywhere, I have three here for free. They have 5V and 12V DC outputs (always check the maximum power). This is very easy and delivers enough power for sure. It depends on the type but the weakest I have can deliver 180W, wich is more than you need.
I only use two different wire lengths. So it is an easy calculation (ohms law) which wire resistance you need. I use 45W/m. If you can’t use ohms law, please stay away from electricity.
A variac is a nice investment if you use your cutter on a regular basis. But there is NO gain in comfort when you only use a few different wire lengths. So for amateur boardbuilders a PSU is the best choice!
So like I said above 45W/m works very good!
I hope this is the answer you are looking for.
Regulating the temperature is impossible without some sort of feedback loop. The temperature depends on all the heattransfercoefficients wich vary during use. But that’s not a problem polystyrene also melts at 500°C 
i use a DC lab power supply a little like a battery charger but you can vary the amperage. i use alligator clips and nichrome wire which is the best. a little off topic but there are these plating units they use in the jewellery industry for gold and rhodium plating of rings and such. the units cost up to a grand. i found i could get the same result with a 20$ switchable wall wart. other jewellers used to say it was crap or whatever but they worked fine as it was the same voltage and amps. so i guess there are always alternatives. i think a few dudes use car battery and they are fine. i personally would stay away from using ac power as we are using exposed wires and AC power kinda kills you pretty much
Actually DC kills you faster (needs lower voltage to kill you) than AC! Just never use a higher voltage than 24V and you will be fine no matter if it is AC or DC.
Thanks for the feedback gents.
What gauge Nichrome wire are you using?
I search on ebay, I look for the correct resistance to give +/-60W/m with 12V. There is only one thickness that is the closest
So that’s the thickness I use. (don’t know it by haert, I buy enough at once and do the math over every time)
Any chance you have a link to a table that provides specific data about nichrome wire heat properties relative to gauge and resistance?
Sorry made a little mistake! It is 45W/m that you should use! (i’ll edit my previous posts)
You can derive it from the text under here. It is what a salesman of nichrome wire wrote:
You probably need one of these two:
swg 24, 0.559mm, 0.022inch, 4.563Ohm/m, 1.391Ohm/ft
swg 26, 0.457mm, 0.018inch, 6.828Ohm/m, 2.081Ohm/ft
(I use the swg26 on a 45cm bow and that one works fine!)
I spent over 8 weeks trying to find all the information I needed on the internet to build a hotwire foam cutter. I consolidated the information I found into the table below. There was more useful information available about using Nichrome wire than any other wire. So, this is what is addressed in the table below (Chromel C Nichrome.)
V = Voltage (Max for power source)
I = Current or Amperage, Amperes, Amps
R = Resistance or Ohms
V = IR
V/I = R
Divide Voltage by the Amperes needed to heat the wire to your desired foam cutting temperature. This gives you the total Resistance needed in Ohms. Then divide total Ohms of resistance by Ohms/ft to get the length of Nichrome (Chromel C) wire needed:
I found small quantities of Nichrome wire available online, for reasonable prices, at this site:
http://www.jacobs-online.biz/nichrome_wire.htm )
The table below should help you estimate the gauge and length of wire needed for a hotwire foam cutter. The temperature needed will depend on the type and density of foam being cut, and the desired cutting speed. The internet information I found suggested that 600 F is a common temperature for cutting foam with a hotwire. This would be a good starting point – temperature should be adjusted according to testing results and observations from experience. Be cautious, wires are very hot and can break if the wrong gauge has been chosen. Wear appropriate protective/safety equipment, especially goggles.
The data in this table was generated using the Nichrome Calculator at this site:
The table can be found at this link:
