HOTWIRE FOAM CUTTER POWER SOURCE / SUPPLY & WIRE 

The material below is intended as information for learning.  It is not a recommendation to build your own hotwire power source/cutter.  If you do not understand the physics of electricity, and the associated risks/danger, it might not be a good idea to build your own hotwire cutter.

I spent over 8 weeks looking for information on the internet that I would need to build a hotwire foam cutter, using a 6v/12v car battery charger. I have consolidated the useful information I learned below.  I have updated some of the information and links since I first wrote this blog post.  If you plan to use an automobile battery charger, it must be a "manual" charger.

There is more information online about using nichrome wire for hotwires -- this is what I discuss below.  I found small quantities of Nichrome wire available online, for reasonable prices, at this site:

http://www.jacobs-online.biz/nichrome_wire.htm

This is a good link for an introduction to hotwire cutters:

http://hotwirefoamcutterinfo.com/Introduction.html

The table at the bottom of this post was generated using the Jacobs Online nichrome caculator.  It provides specific resistance values and temperature/amperage values about various nichrome wire gauges for making general wire length estimates.  The online nichrome calculator gives more detailed, and I will assume, more precise information:

Nichrome Calculator link:

http://www.jacobs-online.biz/nichrome/NichromeCalc.html

These Ohm's Law Formulas may be useful:

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)

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 gauge you have chosen to achieve the 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 concluded that with my 12-volt (6-amp) and 6-volt (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 Online site further, I saw that you do not want to choose a wire gauge that requires the maximum 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 12 volts.  The thing that was not clear to me before visiting the Jacobs Online site is that the amperage required to heat a piece of nichrome wire to an exact temperature, for a specific gauge, is constant for that gauge -- regardless of length.  So voltage must change as wire length varies to achieve an exact temperature (e.g. 600 F).

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 (voltage = 12).  A 7-inch difference in length gives a 200 degree temperature range.

For the 6-volt 6-amp setting, 22-gauge Nichrome wire, with alligator clips placed from 18.4 - 15.2 inches apart and the same 600-800 F temp range -- Amperage from 3.7- 4.5 (voltage = 6) -- a 3-inch difference in length gives a 200 degree temperature range.   These values were determined using the Jacobs Online calculator, not the table at the bottom of this post.

The nichrome wire 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.  Make sure the electrical components (switches, clips, wire etc.) are rated for the amperage your hotwire cutter will be using.

Simple DIY plans for a "low-voltage" variable-output hotwire power supply:

http://www.jacobs-online.biz/power_supply_design.htm

http://hotwirefoamcutterinfo.com/_DIYPSDesign.html

Information about variable rate power supplies:

http://hotwirefoamcutterinfo.com/__Variable.html

Variac power supply:

http://www.circuitspecialists.com/variacs

Hotwire cutter video links (foam press mold cutting):

http://vimeo.com/17137988

http://www.youtube.com/watch?v=m-R__S6U9J0

Simple plans for a hotwire bow or harp courtesy of airframe at Swayocks.com:
(click on image of plans for higher resolution)

http://www.swaylocks.com/comment/164400#comment-164400

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Information about Polystyrene (XPS, EPS) foam types/specifications:

http://bgboard.blogspot.com/2013/12/...types-and.html

The data in the table below was generated using the Nichrome Calculator at Jacobs Online:

http://www.jacobs-online.biz/nichrome/NichromeCalc.html

The resistance and amperage values/requirements in this table appear to be accurate.  But using these values to calculate needed wire lengths mathematically, as described previously, will give slightly longer estimates than the Jacobs Online nichrome calculator.  I will assume the Jacobs Online calculator is more precise and factors in wire length and resistance increases that occur when wire temperatures increase.