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foam cutting current ?

 

Hi all,

Is there a formula to help select the proper voltage and current for a
foam cutter ?

Any special wire ?

Dave
Graham Stabler
 

--- In CAD_CAM_EDM_DRO@..., "turbulatordude"
<dave_mucha@...> wrote:

Hi all,

Is there a formula to help select the proper voltage and current for a
foam cutter ?

Any special wire ?

Dave
I don't know of any formulas but power disipated is I^2.R and the R
will be a function of temperature itself so it might be a bit
complicated. I suspect it is a rule of thumb situation.

How long a wire do you need for cutting? A review of RC websites
might give you an idea of the currents, its not more than a few amps.
, lots of people seem to use 12v transformers driven from dimmers. A
current regulated supply might be better.

The best wire is nichrome, it has a high resistance and is very
strong. I do manual cutting with a 0.25mm wire which is rather thin
but reduces my "recast layer" meaning there is no melted surface on
the foam. In CNC I think they tend to use the wire in a non contact
mode so they always have this melted on layer which is fine if weight
is not the issue and it can give a nice finish.

Graham
 

The coefficient of resistance wire isn't as linear as platinum but it's
stable. You can design the current regulator to maintain a set temperature
once you've calibrated it.

Come to think of it, this will be an average over the length of the wire, so
if it's not all being used in cutting the foam, the temp wouldn't be
inaccurate. So if you string the wire on a tension mount with insulators,
then bring the current in with copper wire and clips. Then you put the
length of the resistance wire doing the cut in the controller, or the
controller could measure it if it was cold. Should get an even temperature
across the wire that way.

Just thinking out loud...

On 9/5/06, Graham Stabler <eexgs@...> wrote:

--- In CAD_CAM_EDM_DRO@..., "turbulatordude"
<dave_mucha@...> wrote:

Hi all,

Is there a formula to help select the proper voltage and current for a
foam cutter ?

Any special wire ?

Dave
I don't know of any formulas but power disipated is I^2.R and the R
will be a function of temperature itself so it might be a bit
complicated. I suspect it is a rule of thumb situation.

How long a wire do you need for cutting? A review of RC websites
might give you an idea of the currents, its not more than a few amps.
, lots of people seem to use 12v transformers driven from dimmers. A
current regulated supply might be better.

The best wire is nichrome, it has a high resistance and is very
strong. I do manual cutting with a 0.25mm wire which is rather thin
but reduces my "recast layer" meaning there is no melted surface on
the foam. In CNC I think they tend to use the wire in a non contact
mode so they always have this melted on layer which is fine if weight
is not the issue and it can give a nice finish.

Graham






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--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@...> wrote:

The coefficient of resistance wire isn't as linear as platinum but it's
stable. You can design the current regulator to maintain a set
temperature
once you've calibrated it.

Come to think of it, this will be an average over the length of the
wire, so
if it's not all being used in cutting the foam, the temp wouldn't be
inaccurate. So if you string the wire on a tension mount with
insulators,
then bring the current in with copper wire and clips. Then you put the
length of the resistance wire doing the cut in the controller, or the
controller could measure it if it was cold. Should get an even
temperature
across the wire that way.

Just thinking out loud...

If I follow, one could put some sensing leads with alagator clips, and
'store' them on the ends of the wire. for many cuts, measuring that
would be good enough, then on the heavy cuts, one can move them to
the edges of the part....

or, would one put the power on the clips ?

or, does it really matter ?
Phil Mattison
 

I use nichrome wire from the hobby store with spring tension, a 12v
transformer and a light dimmer. I've found that when cutting, the foam draws
heat from the wire so the temperature (and therefore current) is highly
dependent on foam density, thickness and cutting speed. The wire expands
quite a bit when hot so the spring is needed to maintain tension. Unless
you're setting up a production system where repeatable conditions and
results are needed, it may make more sense just to play it by ear. I just
turn up the power until the wire starts to glow, back it off a little, and
start cutting. How much to back it off depends on the thickness and density
of the foam.
--
Phil Mattison

----- Original Message -----
From: turbulatordude <dave_mucha@...>
To: <CAD_CAM_EDM_DRO@...>
Sent: Tuesday, September 05, 2006 7:36 AM
Subject: [CAD_CAM_EDM_DRO] Re: foam cutting current ?


--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@...> wrote:

The coefficient of resistance wire isn't as linear as platinum but it's
stable. You can design the current regulator to maintain a set
temperature
once you've calibrated it.

Come to think of it, this will be an average over the length of the
wire, so
if it's not all being used in cutting the foam, the temp wouldn't be
inaccurate. So if you string the wire on a tension mount with
insulators,
then bring the current in with copper wire and clips. Then you put the
length of the resistance wire doing the cut in the controller, or the
controller could measure it if it was cold. Should get an even
temperature
across the wire that way.

Just thinking out loud...

If I follow, one could put some sensing leads with alagator clips, and
'store' them on the ends of the wire. for many cuts, measuring that
would be good enough, then on the heavy cuts, one can move them to
the edges of the part....

or, would one put the power on the clips ?

or, does it really matter ?










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URL to this group:

OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.


I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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--- In CAD_CAM_EDM_DRO@..., "Phil Mattison"
<mattison20@...> wrote:

I use nichrome wire from the hobby store with spring tension, a 12v
transformer and a light dimmer. I've found that when cutting, the
foam draws
heat from the wire so the temperature (and therefore current) is highly
dependent on foam density, thickness and cutting speed. The wire expands
quite a bit when hot so the spring is needed to maintain tension. Unless
you're setting up a production system where repeatable conditions and
results are needed, it may make more sense just to play it by ear. I
just
turn up the power until the wire starts to glow, back it off a
little, and
start cutting. How much to back it off depends on the thickness and
density
of the foam.
--
Phil Mattison
I am starting to think that is how it is done. just heat the wire and
watch the cut.

I don't suppose it is anything difficult as I am trying to make it.

I was envisioning using a feedback so the cutting would slow when the
wire cooled, and/or adjust the current automatically.

Dave







----- Original Message -----
From: turbulatordude <dave_mucha@...>
To: <CAD_CAM_EDM_DRO@...>
Sent: Tuesday, September 05, 2006 7:36 AM
Subject: [CAD_CAM_EDM_DRO] Re: foam cutting current ?


--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@> wrote:

The coefficient of resistance wire isn't as linear as platinum
but it's
stable. You can design the current regulator to maintain a set
temperature
once you've calibrated it.

Come to think of it, this will be an average over the length of the
wire, so
if it's not all being used in cutting the foam, the temp wouldn't be
inaccurate. So if you string the wire on a tension mount with
insulators,
then bring the current in with copper wire and clips. Then you
put the
length of the resistance wire doing the cut in the controller,
or the
controller could measure it if it was cold. Should get an even
temperature
across the wire that way.

Just thinking out loud...

If I follow, one could put some sensing leads with alagator clips, and
'store' them on the ends of the wire. for many cuts, measuring that
would be good enough, then on the heavy cuts, one can move them to
the edges of the part....

or, would one put the power on the clips ?

or, does it really matter ?










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[Moderators]
URL to this group:

OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to
reach it if
you have trouble.


I consider this to
be a
sister site to the CCED group, as many of the same members are
there, for OT
subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
bill
List Mom
List Owner


Yahoo! Groups Links
 

--- In CAD_CAM_EDM_DRO@..., "turbulatordude"
<dave_mucha@...> wrote:
I am starting to think that is how it is done. just heat the wire
and watch the cut.

I don't suppose it is anything difficult as I am trying to make it.

I was envisioning using a feedback so the cutting would slow when the
wire cooled, and/or adjust the current automatically.

I used to cut out lots of foam wings for RC airplanes way back yonder,
and I did it exactly like Phil describes. Worked just fine.

Irby
 

I was thinking to put power on the clips and use a heavy enough gauge copper
to make the resistance much less than the resistance wire.

The control circuit would pump current through the wire and monitor the
voltage drop. Divide to get the resistance, which tracks temperature. If the
temperature drops as you feed material, the controller would compensate with
more power.

Anyway, that's the idea. If you have a PIC controlling it, it would be
pretty easy. So if you're running narrow material, the PIC would measure the
length of the resistance wire while it was still cold by a resistance
measurement.

It would have the advantage of not burning your foam by ever getting too hot
and auto compensate for feed rate to an extent.

On 9/5/06, turbulatordude <dave_mucha@...> wrote:

--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@...> wrote:

The coefficient of resistance wire isn't as linear as platinum but it's
stable. You can design the current regulator to maintain a set
temperature
once you've calibrated it.

Come to think of it, this will be an average over the length of the
wire, so
if it's not all being used in cutting the foam, the temp wouldn't be
inaccurate. So if you string the wire on a tension mount with
insulators,
then bring the current in with copper wire and clips. Then you put the
length of the resistance wire doing the cut in the controller, or the
controller could measure it if it was cold. Should get an even
temperature
across the wire that way.

Just thinking out loud...
If I follow, one could put some sensing leads with alagator clips, and
'store' them on the ends of the wire. for many cuts, measuring that
would be good enough, then on the heavy cuts, one can move them to
the edges of the part....

or, would one put the power on the clips ?

or, does it really matter ?
Graham Stabler
 

--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@...> wrote:

I was thinking to put power on the clips and use a heavy enough
gauge copper
to make the resistance much less than the resistance wire.

The control circuit would pump current through the wire and monitor the
voltage drop. Divide to get the resistance, which tracks
temperature. If the
temperature drops as you feed material, the controller would
compensate with
more power.
That sounds feasable as long as you are cutting something of constant
width so you can get your clips next to the material.

I'd just go with the TLAR approach at least to begin with and start to
think technical only if you find you need to. The cnc aspect removes
most of the problems associated with manual (non gravity assist)
cutting such as non constant feedrate and snagging formers. A CNC
wire cutter has been on my list for as long as I have been interested
in CNC, one day.

Graham
 

As other people in the group the current to give the proper cut vary with the foam density and also with the speed of the cutting so I use to determine it experimentally.

I use a 12v / 24v transformer with double pole double side switch so I can select to send to the wire 12 or 24 volts and in the primare I feed with a dimer I use one that the knob is a on/off switch when I push it so the settins are keep and i make a trial cut with the foam and the speed and in that way I can get the settings if I put to much heat the with of the cut is to wide (as the foam melts) so I put the settings so the with of the cut is about 1/8" since I use this as the with of the too when I calculate the Gcode of the path.

As altenative of the nicromel wire you can use also steel wire (piano wire) that have better mecanical properties or if you had dificult time you can use metal guitar strings the first or the second with good results ( I already try them).

Have a nice day

Jose L Sanchez
Stanley A. Lewis, Jr.
 

I'm usually a lurker, but I'll chime in on this topic. I've built
three cnc foam cutters (before that manual systems).

Factors at play: foam density(some crappy foam changes density
within itself, esp. expanded ps), wire lenght, wire material, wire
diameter, method of motion, method of wire attachment, template
material, just to name a few.

Shoot even ambient temperature can in some instances affect the wire
heat.

Now to cut a really nice piece of foam, slow movement and low heat
down to a point. Hot wire cutters don't cut they melt ahead of the
wire. Lower heat (slower motion) equals finer cut, but more tedious
in moving it.

The specialized CNC software drives specialized controllers to heat
the wire. Basically, they are mosfets that switch the power off/on
at a specified rate controlled by the PC/PIC. Calibrated by the
operator.

Now for some thinking out loud... What this means is this using the
spindle speed control or other "user definable" signal (yes, even
step clock) could switch/trigger a mosfet(s) to switch the power to
the wire on/off. Trial and error on the calibration and you are
done.

But, change the wire type, wire diameter, wire lenght, foam
composition and density and..... another calibration.

check out cncfoamcutters@...

stan
Phil Mattison
 

I haven't done any rigorous experiments, but observation suggests that the
temperature of the wire is lower inside the cut than outside in the case
where the material is thinner than the length of the wire. I have seen the
wire glowing outside the cut but not inside. If you are depending on the
wire to act like a thermistor for current regulation, you will be regulating
based on the average temperature, so the actual temperature where it counts
will be lower than calibrated where it matters. The material density and
feed rate will also affect that temperature differential. I have used my
cutter to cut foam rubber, which absorbs much more power from the wire. If
you want really accurate regulation you will need to account for those
variables. Probably not simple. If you use adjustable taps you will need to
account for length. If you don't need really accurate regulation then "TLAR"
is a lot easier and cheaper. I guess it depends on whether the thing is an
end in itself (fun) or a means to some other end.
--
Phil Mattison

----- Original Message -----
From: Graham Stabler <eexgs@...>
To: <CAD_CAM_EDM_DRO@...>
Sent: Wednesday, September 06, 2006 1:23 AM
Subject: [CAD_CAM_EDM_DRO] Re: foam cutting current ?


--- In CAD_CAM_EDM_DRO@..., "Dennis Schmitz"
<denschmitz@...> wrote:

I was thinking to put power on the clips and use a heavy enough
gauge copper
to make the resistance much less than the resistance wire.

The control circuit would pump current through the wire and monitor the
voltage drop. Divide to get the resistance, which tracks
temperature. If the
temperature drops as you feed material, the controller would
compensate with
more power.
That sounds feasable as long as you are cutting something of constant
width so you can get your clips next to the material.

I'd just go with the TLAR approach at least to begin with and start to
think technical only if you find you need to. The cnc aspect removes
most of the problems associated with manual (non gravity assist)
cutting such as non constant feedrate and snagging formers. A CNC
wire cutter has been on my list for as long as I have been interested
in CNC, one day.

Graham





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OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.


I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
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Graham Stabler
 

Just got this through from a member:

"Since the heat (energy) is not used for cutting, but for
overcoming the latent heat of fusion of the foam, all of your
observations are right on the mark.

I'd suggest a force feedback arrangement, since it will ultimately
be wire tension that will detect whether or not the foam is
solid or molten.

Tried to post this from my newsreader, but I'm registered
under a different ID and I'm too lazy to go to the website / yahoo
group to post (also too slow on dialup).

Feel free to forward to the yahoo group if you wish / mark in BC
Canada"

So I did, sounds like a really nice idea. Three pulleys and a load
cell would be one way to do it.

Graham

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