Raspberryfisher's Blog

notes on fishing & travel

Hot Wire Foam Cutter

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My electrical notes to create a hot wire foam-plastics cutter.

Many DYI sites talk about using piano wire, guitar wire, heater wire, hair-dryer wire, et cetera, but there is a lack of control associated with this, so I procured (bought) a Nickel-Chrome 60/16 Alloy Wire from Omega Heaters were datasheets were available.  I went with a small roll of 22 AWG wire, which from initial work has proven to have the right blend of heat and rigidity. The datasheet lists the typical resistance of 22 AWG NI-60 as 1.055 ohms per foot, with the actual roll received had a listed resistance of 1.076 ohms per foot.

It is noted, the actual temperature of the wire is a function of current.  Then, acknowledging a tool is to have a constant length of wire, thus constant resistance, the method to control heat-temperature of the wire is by voltage. So the power delivery to the resistance wire is

  • 120 VAC to an
  • On-Off Switch – Consider including an On Light for safety – to a
  • Light Dimmer to a
  • Transformer to the
  • Hot Cutting Wire

The light dimmer controls the voltage, thus limiting the current and defining the temperature of the wire. So whats left is defining the size of the transformer.

Using Omega’s datasheets, we plot the current-voltage curves,  relative to temperature. (Note, the anomaly of 22 AWG at 875C reflects the actual data posted by Omega, and I suspect is an error).

NI60 Current-Voltage to Temperature Performance

Based on these curves, and a cutter designed with a 1.5′ blade, I am looking for a transformer delivering a maximum of 10V with a VA 31, and should be able to provide a hot wire at ~600C. (No, I have not included the resistance drop of the hook-up wire).

What if I want a cutter, at other lengths, what size of a transformer would be appropriate?

  • 9″ Blade-Wire of 22 AWG, R is 0.807 ohms, a 5V 31VA Trfmr would provide ~600C.
  • 18″ Blade-Wire of 22 AWG, R is 1.614 ohms, a 10V 31VA Trfmr would provide ~600C.
  • 36″ Blade-Wire of 22 AWG, R is 3.228 ohms, a 24V 38VA Trfmr would provide  ~650C.
  • .
  • Notice Wire Change to 20 AWG for Larger Blade-Wire
  • 36″ Blade-Wire of 20 AWG, R is 1.977 ohms, a 24V 120VA Trfmr would provide ~800C.
  • 48″ Blade Wire of 20 AWG, R is 2.637 ohms, a 24V 90VA Trfmr would provide ~600C.
  • 60″ Blade Wire of 20 AWG, R is 3.296 ohms, a 36V 110VA Trfmr would provide ~700C.
  • 72″ Blade Wire of 20 AWG, R is 3.955 ohms, a 36V 90VA Trfmr would provide ~600C.

To date, I have not needed a wire hotter than 600C (though, if necessary, I do have the parts to do so – Larger VA Transformer, use large (lower resistance) wire at same length).

Given this, what transformers am I using?

  • Triad Magnetics VPS10-4300 – Digi-Key –  for smaller (36″ or less) blades – 5V or 10V Output, 43VA
  • Triad Magnetics VPS36-3600 – Digi-Key – for larger blades – 18V or 36V Output, 130 VA
  • .
  • Also have 175 VA variants to crank up the temperature, if ever necessary.

Final Notes

  • Why a hot wire cutter?  I have limited use (need) for one, but I do use it to cut blocks for other tools, such as oven box for setting epoxy or varnish for custom built rods or insulation side panels for the bamboo rod oven. It is my wife, in her custom upholstery who really uses it, but it is part of my tools for custom rods.
  • As the wire heats up, it expands – of course! Use a spring at one end to maintain tension.
  • The wire heats and cools rapidly, and the cutter is used for brief periods.  Thus, I have not provide any significant thermal isolation, such as ceramic isolators, at the electrical hookup. Always know where your fire extinguisher is.
  • An AC Current Clamp meter is a helpful tool to validate performance, but also in setting the apporximate temperature of the wire. Of course, a voltmeter can be used.
  • For smaller work, such as fly-tying, cleaning up frayed or nuisance ends, use a medical cauterizer.
  • It is a DYI – This is what I have done as a reference. Anybody who uses this data needs to apply their own application notes, double-check their work, assemble and test their solution is a methodical and safe manner. Be safe.

Pictures added 2014-07-04




Alternate source for lower cost wire:    http://jacobs-online.biz/power_supply_design.htm    I am not affliated with Jacob’s Online nor have I purchased or received anything from them, so the usual caveat emptor applies.



Written by raspberryfisher

2009/10/04 at 16:21

Posted in Tools

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