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Build a
for styrofoam, foam rubber &
Are you into modelling or upholstery? Then you
probably have often wanted a hot-wire cutter for
foam plastics – but didn’t know how to build one.
Wonder no more as we show you how to build a
very cheap hot-wire cutter from odds and ends.
By LEO SIMPSON
M
ANY PEOPLE have a need
to cut expanded polystyrene
foam (you probably know
it as “styrofoam” or “coolite”), foam
rubber (which isn’t rubber at all!) and
many other soft plastic materials.
Generally they resort to using a
Stanley knife, handsaw, bandsaw, jigsaw or even an electric carving knife
. . . and the result is generally a lot of
mess for not much cut!
The ideal way to fashion this material is with a hot-wire cutter. Actually,
that’s a bit of a misnomer. It should be
called a hot-wire melter because that’s
what it does – but invariably they’re
called hot-wire cutters so we’ll stick
to that name.
The cutter will go through most
types of soft plastic just like a hot
knife through butter (same principle really) and the result is a very
smooth cut with no debris to sweep
up afterwards. You also can do the
most intricate cuts which would be
impossible using any other method.
And the cuts can be angled.
In addition, you can also cut very
48 Silicon Chip
thick material. Our prototype cutter
would easily cut foam over 400mm
thick and it’s not hard to make a much
larger one if you wished.
A hot-wire cutter is made from a
length of resistance wire which is
held taut and heated to just below
red-heat. At this temperature you can
slowly feed the material through and
you will get a very smooth cut.
As you can see from the photos,
we made our hot-wire cutter from
a variety of materials we had lying
around the place. The baseboard was
made from Laminex-coated chipboard
left over from a kitchen installation.
The vertical element was made
from a scrap of 16mm Formply while
the horizontal 5/16-inch threaded rod
came from a cable reel. The hinge,
screw eyes, spring and other bits were
also hauled out of the junk box.
The important point to note about
this project is that it does not have to
look good; it just has to work.
For example, we could just as easily
have used some raw chipboard for the
base and a piece of hardwood decking
for the vertical element. Or we could
have used a piece of 3mm steel strap
bent at rightangles and hinged from
the base to carry the vertical cutter
wire.
No doubt you have other bits and
pieces which could be pressed into
service just as effectively. But where
do you get the resistance wire?
Fortunately, that is easily answered
as it comes in small packs of 28 B &
S Nichrome or Cuprothal from Dick
Smith Electronics and Jaycar. Three
types are available and just which
type you use will depend on what
power supplies you have available
and how big you want the cutter to be.
We made our cutter quite big
because we envisaged using it to
cut quite thick styrofoam for use in
scenery for a model railway layout. If
you want yours to cut thinner sheet
materials then you may opt for something smaller.
The active length of wire used in
our cutter is 430mm and is probably
just a bit longer than we need. But
let’s say you want a similar length,
other plastics
400mm. What we found is that you
need about 40-45 watts to heat the
wire adequately. More than 50 watts
will make the wire glow brightly and
that is not want you want as it could
set fire to some materials.
Anyway, if you have a power supply capable of around 50 watts you
are in business.
Your power supply could be a
conventional adjustable DC power
supply such as the 40V 3A supply we
described in the January & February
1994 issues of SILICON CHIP.
Alternatively, if you have a computer power supply capable of 200W or
more, it can probably be pressed into
service. Or you could even use a 12V
car battery. Either way, if the supply
you use is not adjustable, you will
need a means of adjusting it. After
all, the amount of heat for effective
cutting will depend on the type and
thickness of material so you do need
to be able to adjust the available voltage over a small range.
Now you need to consider how
much voltage and current your power supply can provide because that
determines what type of resistance
wire you need to use.
OK. Let’s consider the easy approach first and that involves using an
adjustable power supply such as the
40V 3A unit referred to above. Since
this unit can only supply a maximum
of 3A, that meant that the resistance
would have to be reasonably high.
OK, so it’s not pretty – but it works! We scrounged the baseboard from an old kitchen cupboard, the spring from an old
bed, the screw eyes and hinge from the junk box . . . we’re sure you get the picture. Here we’re about to cut through this
thick block of polystyrene foam in just a second or so. The result: a beautifully clean, straight cut with no mess!
April 2000 49
Like a hot knife through butter . . . that’s exactly how our hot wire cutter works. On the left we’re cutting a complex shape
from a sheet of polystyrene foam, with a hunk of foam rubber waiting its turn. To prove the point, on the right is that same
hunk of foam rubber being cut. Notice how straight, clean and mess-free the cut is? You can’t do that with a Stanley knife!
For this example, we decided to
use some 28 B&S Nichrome from Dick
Smith Electronics. This comes in a
small pack with a few metres of wire
(Cat. W-3205). This has a nominal
resistance of 13.4Ω ±5% per metre
and so a 400mm length will be 5.4Ω.
We jury-rigged up a 400mm length
of this wire under tension and found
that we needed about 40W to get it to
satisfactorily cut a range of styrofoam
in various thicknesses.
That translated to a voltage setting
of around 15V at 2.7A, comfortably
within the 3A limit of the power
supply under discussion.
If you don’t have an adjustable
power supply of sufficient current
capacity, you might consider using a
computer power supply or perhaps
even a 12V car battery. Either way, you
will need some means of adjusting the
voltage fed to the cutting wire. We
have a simple solution for that problem too and we’ll discuss that later.
Diving into our junk box again,
we came up with a 200W PC power
supply that could deliver +12V at up
to 8A and +5V at up to 20A. Such
power supplies can be picked up
very cheaply these days or salvaged
from computers tossed out for council
Fig.1: modified from the Glow-Plug Driver last month, this “power supply”
enables you to heat the wire to just below red heat.
50 Silicon Chip
cleanups.
If we elect to use the 12V option
(from a computer supply or car battery), it is appropriate to use the 28
B&S Cuprothal resistance wire pack
from Dick Smith Electronics (Cat.
W-3200). This has a nominal resistance of 6.09Ω per metre and so a
400mm length will be just under 2.5Ω
(2.44Ω to be more precise).
With 10V DC applied, the current
will be just over 4A and again we have
the right result of between 40W and
45W to achieve a clean cut with this
length of wire. How do we get 10V
from a 12V supply? Patience, now;
we’ll come to that in a moment.
But perhaps the computer supply
you have scrounged cannot supply 4A
from the +12V – some of them are a
bit skimpy for this rail. The answer
is to go to the 5V rail which even in a
fairly modest machine will typically
be able to supply 12A or more.
So if we’re going to use the +5V rail,
we need lower resistance wire again
and in this case the 28 B&S wire from
Jaycar could be the answer (Jaycar Cat.
WW-4040). This has a nominal resistance of 3.77Ω per metre. A 400mm
length will have a resistance of 1.5Ω.
In this case we are in trouble because 5V across 1.5Ω will result in a
current of only 3.33A and a power dissipation of 16.7W; not enough cutting
power for a 400mm length. Clearly,
we have to make other arrangements.
One possibility is to double up, or
better still, triple the wire. With three
400mm lengths paralleled up, we get
Fig.2: the modified PC board component layout with the PC
board itself at right for comparison. Both are reproduced
same size.
a total resistance of 0.5Ω. With 5V
applied we’ll get 10A (in theory), or a
power dissipation of 50W, more than
enough for the job.
Shorter cutting wire
Alternatively, you could always
compromise and go for a unit with a
shorter cutting wire. We know that we
need around 40W for adequate cutting
from a 400mm length of resistance
wire. That translates to 1W per centimetre. So if we decide on a 250mm
cutting wire, we’ll only need 25 watts.
Going back to that 28 B&S wire from
Jaycar, a 250mm length will have a
resistance of just under 1Ω (0.94Ω
to be more precise) and when 5V is
applied across it, the current will be
around 10A and the power dissipation
around 25W, right on the money for a
25cm cutting length.
We could also repeat the exercise
for the higher resistance wire. Using a
25cm length of the 28 B&S Nichrome
wire from Dick Smith Electronics, we
get a resistance of 3.35Ω. With 9.5V
applied, we get just over 2.8A and a
total power dissipation of 27W, which
is OK for this cutting length.
By now you should see how you can
choose the length of the cutting wire
and its resistance to suit the capabilities of your power supply.
Of course, if you want a 1-metre
cutting wire, you will need a cutting power of 100W and your power
supply will need to be beefed up
accordingly.
(Hint: if you used the 6.1Ω/m
Cuprothal wire, you would need a
supply capable of about 25V and just
over 4A).
computer supply. It just so happens
that we published a suitable circuit in
last month’s issue under the guise of
a “GlowPlug Driver” (see SILICON
CHIP, March 2000, page 72). With
a few minor changes, that circuit is
ideal for our purpose. Fig.1 shows the
modified circuit while Fig.2 shows the
component overlay.
If you compare the circuit of Fig.1
with the GlowPlug Driver circuit on
page 73 of the March 2000 issue, you
will notice that we have made three
modifications. First (and second), we
Adjusting the voltage
We mentioned the need to adjust
the voltage to the cutting wire if you
are using the +12V or +5V rail from a
This photo gives a good idea of the construction and in particular the tensioning
method. The resistance wire “cutter” must be kept under tension to achieve a good
straight cut. A healthy “twang” when plucked means the tension is about right!
April 2000 51
swapped the positions of resistors R1
& R2 to change the output duty cycle.
Whereas before the desired duty cycle
was around 17% to obtain around
2V from a 12V supply, the modified
circuit will give a range of duty cycles
from around 75% to 85%.
This is about right, if you want to
use the examples quoted above and
want around 9.5V to 10V from a 12V
PC power supply or car battery.
Our third modification was to remove the 0.1Ω 5W wirewound resistor
R5 and replace it with a link. This
resistor will otherwise cause too much
voltage drop when you are using it
from a 12V or 5V supply. Note that
if you are using it on the 5V supply
you probably will need to fit a small
flag heatsink to the BUK453 Mosfet.
You may also want to replace the
10kΩ trimpot with a conventional
potentiometer if you want to easily
adjust the wire temperature from time
to time. It would also be a good idea to
fit an in-line 5A fuse if you are going
to power your hot-wire cutter from a
12V car battery.
Foot-operated switch
Another refinement to our circuit
could be the inclusion of a suitable
foot-operated switch to apply power
to the hot wire at the appropriate time,
leaving both hands free to guide the
work. We’ll leave that part to you –
just make sure any switch you use has
high enough ratings (say 10A at 30V
WATCH THE FUMES!
When heating or melting any
type of plastic (eg, with a hotwire cutter!), beware of the fumes
which are given off. Always use
the cutter in a well-ventilated
area (preferably fan assisted)
and avoid breathing the fumes.
DC) and use heavy-duty connecting
cables (again, at least 10A).
Building the cutter
While you can see the construction
details from the photos, there are few
points that need to be covered so we’ll
briefly describe how our prototype
was made.
First, we made the baseboard from
a piece of Laminex coated chipboard
measuring about 700 x 300 x 18mm
thick. The precise measurements are
not important but ours was quite
large so that it would have a large
“throat” for cutting big slabs of material. Laminex or melamine coated
pyneboard is ideal as it easily cleaned
and suitable for sliding the material
through the cut. It is also fairy heavy
which means that the cutter does not
move about when you are pushing
material against the wire.
We used an ordinary 100mm steel
butt hinge for the vertical support
which was made of 16mm Formply
measuring 450 x 70mm. For the hori-
zontal wire support, we used a 500mm
length of 5/16-inch threaded rod. This
has the advantage that it is easy to
make the wire connections to it.
The wire connection to the baseboard can be via a countersunk screw
with the external wire connection
underneath the board but we took the
simpler approach with our prototype,
as can be seen in the photos.
The 28 B&S wire is held under
quite a bit of tension by the small
spring attached to the screw eyes on
the baseboard and vertical support.
Our spring came from an old wire
bed frame.
The vertical support needs to be
hinged and under spring tension for
two reasons. One, you need a fair
amount of tension so that the cutting
wire is not deflected as you push the
foam onto it. Second, the resistance
wire expands by about 10mm from
cold to hot and the spring tension
needs to take this up.
A look at the photos will show
that the threaded rod is under a fair
amount of tension and can be seen to
be noticeably bent against the load.
Mind you, the wire should not
be too tight otherwise it will tend
to break. Ours made a pronounced
“twang” when it was plucked.
Inevitably though, the wire can be
expected to break from time to time,
so make sure you keep the leftover
resistance wire in a safe place. Incidentally, the hotter you run the wire,
the more likely it is to break.
Finally, by using the threaded rod
and the hinged vertical support, the
hot-wire cutter can be easily dismantled and stored as a flat package.
Where do you get it?
It mightn’t look like the best cut in the world but hey, it was our first-ever attempt
– and it’s a darned sight better than you could get with a knife! One trap for young
players we found was to cut too slowly or leave the work in the one place too long.
If you do this, the polystyrene starts to melt (as you can see happening near the hot
wire). Adding a foot-switch to turn the power on and off could help prevent this.
52 Silicon Chip
We’ve told you where to scrounge
all the bits from in the cutter itself but
so far haven’t said where to get the
power controller. As we mentioned,
this was published last month as a
Glow-Plug Driver and the kit is exclusive to Oatley Electronics.
So if you want to build the controller, simply buy the Glow-Plug Driver
kit from Oatley Electronics (it sells
for $14.95 including a case). They can
be contacted on (02) 9584 3563, fax
(02) 9584 3561, email sales<at>oatleyelectronics.com or visit their website:
www.oatleyelectronics.com.au
Just don’t forget to swap resistors
R1 & R2 and leave replace R5 with a
SC
wire link.
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