Learn me: Building a hot wire foam cutter

I have a bunch of EPS foam:

I need to create fiberglass molds for wide body for race car:

Cutting with a hacksaw doesn't work so hot:

Don't have a big bandsaw. Anybody built their own hot wire cutter? Tell me everything you know.

We made one for my brothers house its post and beam with "stress skin" walls. 3/8 chip board glued to 8 inches of foam glued to 3/8 sheet rock. We made a "C" out of 2x4 used 2 sheet roock screws in the end and then used piano wire wraped around the screws and a 50amp battery charger to slice the foam. Amps have to be adjusted the longer the wire.

I've not tried it yet, but when the time comes, I've planned to follow (loosely) the plans from Rocket Team Vatsaas (unless I turn up something better beforehand):

http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx

In reply to 44Dwarf:

This foam came from a company that builds stress skin super insulated structures. Very cool group. Almost all of them in the warehouse were deaf, which makes communication a little challenging. Really good guys though.

Does this only work with an older "dumb" battery charger? My charger is "smart" and adjusts power based on resistance.

How many axes do you need to work? I made one years ago out of MDF when I was an architecture student. Mine was shaped similar to a bandsaw, where there was a flat base, and the wire was fixed perpendicular to the level plane in all directions. When thinking about the type of wire I used, piano wire comes to mind, but I may be wrong. Heat was generated by simply connecting a transformer between the two ends of the wire; the transformer I used had alligator clips.

Important things to note:

The armature should be extremely strong, and shouldn't flex, so that the piano wire can be wound very tight. I sandwiched together a few layers of MDF and laminated them together with screws and wood glue. When you have a wire that is both vertical in all directions and tight, you can approach the wire from any direction, and your edges will be very crisp.

The hotwire could easily cut through a foot and a half of EPS like it was butter. I hate to think of all of the polystyrene fumes that I inhaled, though.

Single axis, but I need it to cut through 2' of EPS in a pass so I can split the form for each side.I was going to use guitar strings, but sourcing a transformer had me stymied.

Nichrome wire

I think I picked mine up at radio shack. Something like this: http://m.radioshack.com/radioshack/product/detail.do?itemId=2103960&categoryId=&cust_categoryId=&path=

A lot of model railroad landscapes get built out of foam, and the way I've seen a lot of those guys cut it is to rig a wire over a wooden "C" frame and simply power the wire with a 12 volt model railroad transformer, adjusting the speed control to change the heat.

MadScientistMatt wrote: A lot of model railroad landscapes get built out of foam, and the way I've seen a lot of those guys cut it is to rig a wire over a wooden "C" frame and simply power the wire with a 12 volt model railroad transformer, adjusting the speed control to change the heat.

This is a good idea.

Mitchell wrote: I think I picked mine up at radio shack. Something like this: http://m.radioshack.com/radioshack/product/detail.do?itemId=2103960&categoryId=&cust_categoryId=&path=

Could you use a used ATX power supply from a PC instead?

The 12v rails on it should be good for more than 15 watts and you could probably find one for free. You could wire a momentary switch between the green wire and ground for a power button. Yellow is +12v

Googles and Youtube, that's where ya oughta be lookin'

Go

DaveEstey wrote: I was going to use guitar strings, but sourcing a transformer had me stymied.

If they can do the job, this makes sense....easier to get than piano strings.

Couldn't you just strip an old toaster?

DaveEstey wrote: In reply to 44Dwarf: This foam came from a company that builds stress skin super insulated structures. Very cool group. Almost all of them in the warehouse were deaf, which makes communication a little challenging. Really good guys though. Does this only work with an older "dumb" battery charger? My charger is "smart" and adjusts power based on resistance.

Yes the older DUMB type with 3 settings something like low, high, jump start.

Ordered an old model train transformer. 50 watts with a rheostat for $20.

http://www.skycraftsurplus.com/ni-chromeresistancewire21awg.aspx?gclid=CK6A2cj6sbgCFUui4AodUzoAWQ

Ni-Chrome Resistance Wire

Intended for hot wire applications. Heaters, foam cutters, etc.

0.836 Ohms / Ft.

3.3 Amps produces a wire temperature of 400 Deg F / 205 Deg C 4.3 A = 600 Deg F /315 Deg C 5.3 A = 800 Deg F / 427 Deg C 6.5 A = 1,000 Deg F / 538 Deg C

Sold by the foot with a 20 Ft. minimum.

DaveEstey wrote: Ordered an old model train transformer. 50 watts with a rheostat for $20.

I was going to suggest this xformer but it looks like you got a good deal, with the rheostat too.

http://www.radioshack.com/product/index.jsp?productId=2102702&filterName=Type&filterValue=Transformers

JoeyM wrote:

DaveEstey wrote: I was going to use guitar strings, but sourcing a transformer had me stymied.

If they can do the job, this makes sense....easier to get than piano strings.

And I'd send you some old guitar strings, but it's not worth the postage. You can buy good, new ones for less than $10/set locally.

Ni-Chrome Resistance Wire will be much better than Piano Wire or Guitar Strings. It is made to be heated and will last much longer than either of those.

I don't need it to last a LONG time, but I'll consider ordering some up.

I'm thinking work table with a hole cut in the top - arm up over it about 2' up. Spring load the wire underneath to maintain tension when it heats up. When not in use I can always pull the wire and have a perfectly usable table (always in demand).

A battery charger works too.

I've seen a hand-held sort of armature made out of some Hockey sticks so you can span enough to create RC plane wings.

What about an electric carving knife?

That would work for smaller cuts, but you're not going to cut through 2 feet of foam at once and have a straight line.

A lot of homebuilt aircraft are now made of a foam material. I can bet you that cottage industry has something.