www.TK560.com

[jegner]

I know this topic has come up before, but I thought a new thread would be in order. I got this in an email From a board member:

"I myself use Mig wire for cutting sheets of extruded polystyrene in a jig that I made for exactly that. You just hook a 36" long straight pice of mig wire up to oppsite ends of a Car battery charger, and with about 7 amps, you can slice thru foam like a hotknife thru butter! "

-- ThorsgaardFoundry

Sounds like a cheap way of cutting plastic. I'm wondering about using it to cut acrylic or PETG? Up the AMPS maybe?

The hobby store sells a $10 hand held foam cutter, but I'm wanting one for use as a tool in my shop. Something with a bit more punch. The Car battery charger sounds like an option. And the MiG wire, thats something I have a lot of!

Does anyone else have any ideas on making a hot wire cutter? Plans, diagrams, schematics?

Google reveals a few hits:

http://www.hhhh.org/~joeboy/resources/hotwire_foam_cutter/hotwire_foam_cutter.html

http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx this one uses a model railroad transformer as a control.

http://wolfstone.halloweenhost.com/HalloweenTech/fotmak_MakingFoamTools.html shows some interesting options.

http://www.dansworkshop.com/Hot%20wire%20foam%20cutting.shtml another one. this one talks about using a dimmer and a door bell transformer.

Sounds like it's time to make one!

Jim

[drcrash]

Cutting solid plastic is harder than cutting foam. You'll need a better power supply that does low volts and lots of amps.

For foam, you generally want a transformer that can do 2 amps, and 4 is better. (Depending on the foam, and other variables.)

Stainless steel fishing leader makes a good resistance wire for cutting foam. For manual cutting, it's better than nichrome because it's stronger and less likely to break.

Don't use regular steel. Stainless has several times the resistance, and requires several times less amps to get hot.

Most people use stainless wire (fishing leader or "safety wire") in the .012" to .024" diameter range. .020 is pretty common.

The longer a bow you want, the bigger diameter of wire you want, to keep it from breaking, but that takes more amps.

A good, easy way to make a hot wire foam cutter power supply is to plug a high-amp Class II AC power transformer (wall wart or brick) into a router speed controller from harbor freight. The router speed control gives you a box, a fuse, a switch, and a knob, and you're in business in 5 minutes.

You can also use a dimmer, but you want a regular wall dimmer (600w), not a lamp dimmer (300w). A lamp dimmer can't handle the amps.

An old-style car battery charger will work if you don't need a long wire (more than about 3 feet). The longer a wire you have, the more volts you need. The thicker the wire, the more amps you need.

Whatever kind of transformer/power supply you use, it should be a real magnetic transformer---with a heavy iron core and windings---not one of the lightweight switch-mode devices. (Like most computer power supplies these days.) If you put a switch-mode "transformer" on a dimmer or router speed control, it will not work and you may fry it.

You can wire 2 transformers in parallel to get 2x the amps, but only if they're the same make and model. (Not just the same voltage & amp ratings; there are subtleties you don't want to know about.)

For cutting solid plastic sheets, you probably want a hot knife rather than a hot wire.

If the hot knife blade is the resistance element, you need a bunch of amps because it has a lot less resistance than a thin wire.

I made a funky hot knife out of a 100-watt pistol-style soldering iron, using
thin stainless strips as shapeable blades, in place of the usual soldering iron tip. It delivers about 1v at about 100 amps. I mostly use it for cutting foam, scooping channels and things out of blocks.

I also have a storebought hot knife that's basically a pencil-style 25w soldering iron with an exacto knife blade chucked into the tip. It works fine for cutting foam, but is disappointing for cutting solid sheets.
(Better for HIPS than acrylic.)

Here's a post from RCgroups that summarizes a bunch of stuff about making hot wire power supplies:

http://www.rcgroups.com/forums/showpost.php?p=6769158&postcount=22

Here's the thread it's from:

http://www.rcgroups.com/forums/showthread.php?t=628704

Most of that stuff is too complicated. For a simple manual cutter, you don't want to build a regulated DC power supply or add a rectifier or capacitors. A dumb transformer or two on a router speed control or dimmer switch is fine.

If you go with the router speed control, I'd consider using 2 24 volt wall warts from allelectronics.com, and wiring them in parallel. That's really easy. (You don't really have to wire the wall plug sides---just plug them into a cube tap and plug that into the speed control.) See that posting above for how... you need to do a polarity check when you wire the low-voltage sides in parallel.

If you go with a dimmer, be careful about the wall-side wiring. If you're buying a transformer, I'd suggest the 24V 5A one from www.allelectronics.com.

You can also use the 24V 2A one from Radio Shack, with a thin wire. If you decide you want more amps for a thicker, you can wire another one in parallel.
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com