[Epic] Cutting foam with Hot Wire

From: Mike Bowen <mbowen_at_...>
Date: Wed, 3 Dec 1997 14:10:44 -0600 (CST)

On Wed, 3 Dec 1997, Chris Salvato wrote:

> I'm going to be constructing my own hot wire foam cutter, but I need a
> little information and the net is a bit scarce. Specifically, I'd like to
> use a wall socket to power the thing and use the thicker variety of
> NiChrome wire which I found at a local hobby store, but I'm not exactly
> sure how to control the power going into it to make sure I don't blow a
> circuit breaker or set myself on fire. can anybody suggest some
> technical-sounding stuff I should look for at my local Radio Shack that
> could do the trick? I imagine a variable resistor of some sort would allow
> me to fine-tune the amount of current going through it, but just what kind
> of animal should I be looking for?
>
> Please respond to me personally, I don;t think this post is really on-topic...

Sure its on topic-- cutting foam for buildings/hills/etc is a big part of
the hobby. True, you don't want to use 110VAC for a cutter. Bad.

Use a 50 amp 12V car Battery Charger to provide safety. Its much harder to
kill yourself with 12 volts. 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.

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, "only" a couple amps, compared
to the 12v side, which can do 50 amps(which needs expensive MOSFETs to
limit). You can check the output on the 12v side to see if the dimmer
switch can control the voltage. 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). 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

Next, ypu 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.


**legal stuff**disclaimer**

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

** Build at your own risk**

>
> Chris Salvato
> shidara_at_...
>

**
It is by caffeine alone I set my mind in motion, it is by the beans of
Java that thoughts acquire speed, the hands acquire shaking, the shaking
becomes a warning, it is by caffeine alone I set my mind in motion.

mike
**
Received on Wed Dec 03 1997 - 20:10:44 UTC