Model Forum / Radio Controlled / Air Models / February 2007

30 inches = 2.5 feet, and at 2.5 ohms per foot, you've got a 2.5*2.5 ohm resistor.
That 6.25 ohms. Let's call it 6 to make it easier.

Running 12V through it will result in a current of 12V / 6 ohms = 2
amps, using Ohm's law (voltage = current * resistance, in volts, amps,
and ohms respectively).

Using the other rule (power = current-squared x resistance, in watts,
amps, and ohms), we get

Power in watts = 4 * 2.5 = 10watts

That's sort of lukewarm (you can hold a 10W bulb in your hand quite
comfortably). Not hot enough.

If you get a 50Amp 12V supply...you'll get the same 10Watts. The
supply voltage is typically regulated -- the supply puts out that many
volts into just about any load...UNTIL the current gets too high, when
either (a) the voltage drops off, or (b) the power supply melts, or
(c) a fuse/breaker goes. So changing the amps of the supply is just
not the right way to go. You can hook up a car battery, but it STILL won't
get hotter.

What you need to do is either

* Get a lower-resistance wire OR
* Get a higher-voltage source.

Combining the two formulas gets you something useful:

Power = I^2 R = (V/R)^2 R = V^2 / R.

So if you double the voltage, you get 4 times the power. And 40W
starts to sound like it'd make that wire fairly warm. So if you had
TWO 12V batteries (in SERIES), you might do OK.

But I think that I might follow some other suggestions and consider
going with fishing-wire leader rather than nichrome wire -- almsot
certainly lower resistance.

-John

The message <N1ush.261$jA.46@bignews1.bellsouth.net>
from Randy Maheux <rmaheuxr@bellsouth.net> contains these words:

I have used a 12v DC power supply rated at 7 amps which originally
powered a CB radio, works fine but I suppose it all depends on the
thickness/resistance of the wire,

                     regards,              Terry

Remember two things:

NiChrome wire needs a certain amount of initial current to get it past
the 'warm-up' phase - for you rlength of wire, you'd probably want
somewhere between 18-20 volts run voltage (at load) w/ a 20-25V initial
voltage.

Instead of a 'dimmer', which just chops the AC waveform (it's no longer
a sine wave, so the transformer can't work efficiently), try a variale
autotransformer (Variac is a good brand name).  You can find them online
or on Ebay readily.  I got my 10 AMPO unit for $15, and that was a
'consumer' product - already encased in metal w/ cord, outlet, power
switch / light etc.  By keeping a sine wave, you'll gain efficiency.

   You can use them in two modes:

1) 0-120V - this is standard.

2) 0-140V - this allows a 20 volt 'boost' to provide a little extra
power when you need it.

So you'd install a variac in line with your 12V (or 18V) transformer to
allow variance in the supply voltage.  This effects current flow,
therefore watts etc.

Remember the wire in open air will take more energy to maintain its
heat.  Once it starts cutting through the foam, there's no fresh, cool
air to absorb the enrgy, so the wire temperature may rise dramatically
if you don't keep the foam moving!

                    -Steve

I believe all the replies so far are making this more difficult and
expensive than it has to be.

Look in your garage (or shop the garage sales) for a battery charger
with a capacity of at least 8 amps.  This provides around 14 volts
isolated DC.  Mine is an old NON-solid-state unit.  I don't know if
the newer solid state types will work.

Plug it into a socket wired to a ceiling fan speed control, or, if you
don't feel comfortable messing with house current wiring, purchase a
cheap router speed control from Harbor Freight.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=43060
$12.49...on sale right now.

Obtain some .032" stainless steel aircraft safety wire from your local
aircraft mechanic, or buy your own through Aircraft Spruce & Specialty
http://www.aircraftspruce.com/catalog/appages/safetywire.php  or other
supplier of aircraft tools and supplies.  $7.95 for a 1 lb. spool.

The SS safety wire will not be as brittle as nichrome, will not take
as much current to heat it up, and is much easier...and cheaper...to
obtain...AND in long lengths

I made a light, simple bow from a length of 2-1/2" X 3/4" plywood with
two 12" long pieces of 1/4" dia. music wire rods(from the hobby store)
on the ends. On each rod, grind a notch around the diameter on the
"wire" end, about 1/8" from the end to allow the cutting wire to be
looped around it.  Drill a hole in the edge of the plywood at each
end, so that the rods can be pressed into the wood and will be 2
inches or so "out" from parallel. I used an approximate 15 degree
angle from the perpendicular to the length of the plywood.  This
allows the rods to be bowed-in to keep tension on the cutting wire,
and not slip out of the wood.  Plan ahead so that the distance between
the notched ends of the rods will be at least 1 or 2 inches longer
than the longest wing core you want to cut. As you will see, this
assembles and disassembles easily, so you can make many different
length bows simply by making different length plywood supports, or by
drilling extra holes in a long one.  Once constructed to your
satisfaction, you can make it even lighter by cutting lightening holes
in the plywood with a hole saw.

Warning!  Use caution when working with safety wire, as the ends are
sharp and the wire may snap under tension.  Safety goggles should be
used!   You have been warned!

Twist a loop in one end of your cutting wire and place it over the
notch on one of the music wires. At this point you can straighten any
kinks that may be in your cutting wire by chucking the loose end in an
electric drill and, under tension, spinning the wire along its length.
This stiffens (work-hardens) the wire somewhat, and also lessens the
tendency for the wire to stretch under tension.  Don't spin it so much
that it gets brittle, or breaks….A dozen or so revolutions on a 36"
wire should be sufficient!  Now spring (bend) the other music wire in
about an inch and use this length to twist a loop in the other end of
the cutting wire. Slip this on the other music wire and it will hold
sufficient tension in the wire for cutting cores.

The power supply leads can be connected to either the rods or on
opposite ends of the cutting wire using alligator clips.

With the power supply turned OFF, set the cutting wire on the lead-ins
of your airfoil templates, against the leading edge foam. Connect the
power supply leads to each end of the bow.

Using just enough force to hold the wire against the foam of the
leading edge, start dialing up the variable power supply until you see
the wire slowly move into the foam. Use the variable supply as a speed
control for the cutting speed.

If you get in a hurry and use too much heat, the wire will cut a curf
much wider than the wire diameter, and will cause ripples in the wing
core surface.

If you hold the heat constant, but try to force the wire through the
core, the center of the wire will lag behind the ends, and you will
produce a core that is thinner in the center than at the ends.  This
will be most pronounced when cutting over the top of a high-camber
airfoil.

I have a .pdf covering all this plus diagrams, sources, etc.  I'll try
to get it posted somewhere.

Randy,
I use my Sears auto battery charger.  It gives 10 amps at the charge
setting and that seems enough to heat the wire I am using (.012 control
line flying wire).   To control the temp I use small alligator clips
the attach the power cord to the  cutting wire and move one in our out
on the wire.   Move it too far in and the wire becomes a flash bulb!
I have used this setup for over 10 years and cut hundreds of wings for
control line combat and stunt models.
Bob Furr

PS   If you are interested I sell a video on cutting foam wings through
ebay.   The buy it now option is only $9.99 and postage.

Here's my summary of wire & power supply issues for hot wire foa
cutters:

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

Comments welcome.

Some highlights:

Most people seem to go for a 2- to 4-amp transformer heating a .016" t
.020" stainless wire at roughly two amps and one to a few watts pe
inch.  12 volts is good up to 2 feet or so, maybe more with thin wire.
24 volts seems to be preferred, for up to 4 feet or so.

Doorbell transformers can't cut it;  they're only rated for about a
amp, or less.

Train transformers generally can't really do the amps either.  New one
with overload protection will shut down, old ones will run beyond thei
ratings, but it's iffy

The easiest way to get a variable heat control is to plug a hefty $
wall-wart into a $12.50 router speed control.

Or if you don't mind connecting the wall-side AC bits yourself, you ca
wire a regular power transformer to a dimmer.  A regular dimmer isn'
really designed for inductive loads, but the amps you're drawing ar
not much on that side of the transformer.  (E.g., a 4-amp 24-vol
transformer full draws less than amp on the wall side.)

To get more amps, you can wire two transformers of the same make an
model in parallel, but you should know how to check that you've got th
polarity right.  (See the link above.)  This works for wall warts
too---to "wire" the wall side, just plug them into the same power stri
or multi-outlet extension cord, and wire the secondaries in parallel.

www.allelectronics.com has good prices on reasonably beef
transformers.

www.altex.com has good prices on fairly beefy wall warts.

You can get 30-foot lengths of 304 stainless steel wire for $1.80 t
$2.60 in the size range we're talking about from Small Parts Inc.

Cabela's sells big spools of stainless wire for (salwater fishing line
for about $9.  (You get two or three hundred feet.)

A lot of people prefer stainless to nichrome because it's physicall
stronger, and has about three quarters the resistance.  Regular stee
wire like guitar strings has much lower resistance, meaning you nee
much thinner wire to avoid running too many amps for a reasonabl
transformer.  (Use a really THIN guitar string if you must use a guita
string.)

If you want thin nichrome, maybe for a setup with a marginal (low amps
power supply, you can get 30 feet of 26 gauge nichrome (2.67 ohms/ft
from www.aeroconsystems.com for $9.

An ammeter is handy for setting heat levels.  If you can read the amp
reasonably consistently, you can find a "good" current level for
given type/diameter of wire, and then dial up to that fo
different-length wires.

You can get a multimeter that reads up to 10 amps as an AC ammeter fo
about $4 from Harbor Freight.  To use it with a transformer on a dimme
or router speed control, you must rectify the output to DC.  You can ge
a rectifier from RadioShack for $3, and the wiring is trivial

--
drcras

Take a look at the foam cutter plans at www.utahflyers.org.  We have cu
400+ wings with the power supply and wire as shown

--
Le

If I find a better wire I will use it.  

I tried about 20 different wires before recommending the 20# fishin
wire.  I had to find something that everyone could get.  I had to fin
something that was reliable and strong.  I have five foam cutters an
use the same wire on all of them.

EPP wants a hotter wire and almost all of my cuts now are EPP.  Th
1.9# EPP wants a hotter wire than the 1.3# EPP foam.  I used to cut
lot of the Dow blue foam.  The cutter needs to work on all of thes
foams and still cut at a comfortable and controllable speed.

I still have rolls of all kinds of stuff around here that I don't use.
One of the best cutting wires was a multi strand fishing wire but i
liked to run with higher amps.  I have stainless steel wire, nichrom
wire and even have wire I got that is recommended by Burt Rutan.  
have tried anything I could get my hands on.  

I found a good match with the transformer and the 40" bow.  The trut
is that the bow and wire are so well matched that the cutter will wor
without the dimmer in line and still stay within the 4 amp range.  

I say the proof of the pudding is in the 400 planes we have cut and th
low error rate of the cutting which is probably around 5-10% and ha
been at 0% at times on certain cuts of up to 100 wing halves.  

Some wires that are strong have too much overcut.  Some require to
many amps to get hot enough.  Other wires break or stretch too easil
when they get up to temperature.  

Finding a wire that will work is one of the most important parts o
building a foam cutter.

How long is your wire?
How many amps are you using?
How many cuts will a wire make before it breaks?
Where do you buy your wire and what are it's specifications?

Here is my power supply set up.

http://www.utahflyers.org/FCSetup.php

I actually have 5 amp 24 volt transformers now not just the 3 amp show
in the picture.  The 3 amp seemed to work as well as the 5 amp in thi
application but I was over the suggested amp limit.

I had so many questions from the local flyers that I poste
instructions on the web.  Maybe it will be helpful to some of you.  

I started with a 72 inch cutter and used a rheostat and long befor
that I cut with 120 volts and lived.  I never did get shocked but
realize death was just a mistake away.  The transformer will heat th
72 inc wire enough to cut but very slowly.  To make this long of a cu
you need a different wire or preferably more voltage.

I had to experiment to find what worked.  I could easy get the heat bu
I was using too many amps for the transformer.

I tried two different train transformers as a power supply with
variety of cutting wires and neither worked well.  I tried my ca
battery charger but it hummed so bad I thought I would melt it down.

The set up as shown is the best transformer set up I tried

--
Le

The biggest danger is when the cutting wire breaks.  All of the powe
apparently goes off and it would be easy to forget the cutter is stil
charged and waiting like a rat trap to zap you.  

When a cutting wire breaks I turn everything off and unplug the powe
supply too just to make sure.  The light I use on my power suppl
reminds me of the power status of the power supply.

On one of the automatic cutters years ago I used insulated wire to pul
the bow through the foam because I was having trouble with stretching o
the strings to the weight.  If you watch the foam cutting video on ou
web page you can see the cutter.  Some how these wires  shorted ou
with use and I had live power to the front of the automatic mechanis
that I didn't know about.  the wires got so hot they melted th
insulation.  My first indication of a problem was a melting plasti
smell.  There is always something to watch out for.  

The only injury I have had is when hot plastic driped off the cuttin
wire onto the back of my hand.

When I cut with the 120 volts I wore welding gloves and rubber sole
shoes and made sure I didn't stand on bare concrete. I am extremel
safety conscious and made sure that my life insurance is double fo
accidental death to take care of my family when I am gone

--
Le

Back to foam cutting.

All cutters have a learning curve.  There are a bunch of small problem
that pop up that make you keep thinking.  The CNC cutter have their ow
set of problems because of more moving parts, more programing, les
ability to react during the cut, more parts to fail.  CNCs are also ar
bigger to store.

Don't get me wrong I think CNC cutters are the way to go in some case
but if you are a weekend builder and want a replacement wing for a ki
or want to design your own plane you can get great cuts withou
spending all of the money.  CNC cutters at their best when they can b
fine tuned to cut the same cut over and over

--
Le