Big Foam Cutter wasRe: [Epic] Hot wire

From: Mike Bowen <mbowen_at_...>
Date: Sat, 20 Jun 1998 12:01:45 -0500 (CDT)

On Fri, 19 Jun 1998, Rick Biesik wrote:

> Does any one know about building a home made hot wire for cutting
> Styrofoam? I have the wire which is typically used in the store bought
> models, as far as a power supply (ex. transformer) what is needed. I would
> prefer that over a battery. Also any tips on the construction.

This is a repost of my "Hot Wire Cutter from Hell" No jobs too big for
this baby. Battery powered cutters don't have to guts to do a fast job on
really thick foam. Plus, you go broke buying D cells or wait while nicads
recharge with those tiny craftstore batt-op cutter.

<safety nag>
 Remember this hotwire cutter can do some burns to the unwary.
Be careful, don't build this unless you know what you're doing
</safety nag>

Use a 12V car Battery Charger to provide safety. Its much harder to kill
yourself with 12 volts DC. To provide a variable source of voltage, the
cheap way is to get a 110 volt Dimmer switch, like is used to control
lighting for your house, or speed control for a ceiling fan. It uses a SCR
controller to "clip" the voltage, making it lower "average" voltage.
Lower voltage means less heat on the wire.

By changing the heat, you can get just the right amount for the cutting
job. too much, foam melts and fumes. too little- the foam won't cut.

you don't need a full 50 amps charger, but I like to run power supplies
(12v batt. charger ) at much less than their Max current rate. They last
longer that way, and won't overheat. the 50 amp setting puts out a little
more voltage than the standard 10 amp. This extra voltage can help if you
use a wire with higher than normal resistance. Besides, with the wire you
use for cutting, you won't be anywhere near 50 amps, more like under 6
amps with typical cutting wire

so, so far, you have this dimmer switch controlling the input voltage to
the battery charger. It is easier to limit the voltage on the input side
of the transformer that is in the 'charger, as it's "only" a couple amps
go thru the 110V side of the transformer ,compared to the 12v side, which
can do 50 amps(which needs expensive MOSFETs to limit). You can check the
output with a voltmeter on the 12v side to see if the dimmer switch can
control the voltage, or hook up the modified charger to a car
battery and see by turning the dimmer switch that you can reduce the
charge rate (watch the amp meter on the charger)If you do not have a
voltmeter . some battery chargers have a slide switch that is set to
control amps. like a 2,10,50 amp setting. This switch works by tapping off
the transformer on a different winding,-- this reduces the voltage, which,
in turn, reduces the amps. the slide switch is good for coarse
adjustments.

Most battery chargers have plenty of room to mount this dimmer switch
inside of the battery charger case, drilling a hole in the case to allow
the adjustment lever or shaft to go thru the case. This way, all that 110v
stuff is inside the case. Much Safer. If you feel the need, add in a fuse
or circuit breaker to the 110v side while you have the case open.

Next, you need to make a wood or fiberglass bow to hold the Nicrome wire
in place. I have made several different size bows, from large bows for
cutting R/C wing cores, to a small one for making "epic" scale hills.
 I drill holes thru the end of the bows, and place a nut/bolt/washer deal
to hold the wire in place on one end, and use a nut/bolt/washer with a
spring added to provide tension on the wire, so it wont sag.

bad ascii art follows

|--[bb]--[]SSSSS[-]********************[]--[bb]--|
    bb bb
    bb bb
    bb bb
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

bb= wood or fiberglass bow
[ = nut
] = nut
-- = bolt(threaded part)
ss = spring
| = bolthead

you will need to get the spring from a hardware store-- it took a few
to find the right length and "springyness"

attach a 12v lead to each bolt head, and turn on battery charger to 10
amps and adjust the dimmer switch so it is hot enough to cut foam.
depending on the thickness of the Nichrome wire, you may need to set the
'charger to 50 amps.

remember, you don't need the wire red hot to cut the foam.

sometimes it helps to clamp the bow to a table and move the foam thru the
wire with both hands, instead of one hand holding the bow, and the other
holding the foam.

A note about wire--
you can use Nicrome wire, or for tougher foam you can use steel wire. This
charger can handle the increased current needed for that. Steel wire tends
to sag less when going thru a big sheet of foam. I use the steel wire to
cut plastic sheet, but watch for the fumes.


**legal stuff**disclaimer**

YMMV. works for me, but I worked in electronics, so remember, BE CAREFUL!
110v can hurt you, the 12v wire can burn you, so you are on your own here.

** Build at your own risk**


**
mike
**
Received on Sat Jun 20 1998 - 17:01:45 UTC