Hot Wire Foam Cutter Calculator (UPDATED 7-31-2012) - Page 2 - RC Groups
Thread Tools
Apr 02, 2012, 05:31 PM
low tech high tech
vtdiy's Avatar
I just brought my automotive battery charger in and connected it to a router speed controller, and then across 9" of stainless steel fishing leader measuring .011" in diameter.

I cut a small piece of foam after adjusting and measured the specs afterwards. I like a heat level that is fairly low -- not nearly red in a dark room as Harry D prefers.

I measured 1 amp at 3.5 volts (DC) across 9". That works out to 0.4 watts per inch -- considerably less than Harry's data of 1.5 watts per inch.

So far we gave a wattage per inch range of 0.4 - 1.5 watts per inch.
Sign up now
to remove ads between posts
Apr 02, 2012, 05:43 PM
low tech high tech
vtdiy's Avatar
Another test:

4.5 volts at 2.25 amps over 23" with .015" guitar string

That works out to 0.44 watts per inch -- very close to my other result -- which makes sense since I probably have a fairly consistent heat preference.

Notice that the wire length and diameter do not affect the watts per inch I prefer for cutting. And I'm guessing that the wire composition also does not affect it. I have some steel wire I will try later this evening to try out that theory.
Last edited by vtdiy; Apr 02, 2012 at 06:21 PM.
Apr 02, 2012, 06:35 PM
low tech high tech
vtdiy's Avatar
Plain wire test:

5 volts at 2.66 amps over 21 inches of 28 ga. steel (not stainless) wire -- .015" dia., measured.

That works out to about .63 watts per inch. Still pretty close -- it was a little kinked and that might account for my increasing the heat a little to slide it through the foam.
Last edited by vtdiy; Apr 02, 2012 at 07:02 PM.
Apr 02, 2012, 06:51 PM
low tech high tech
vtdiy's Avatar
One more;

3.4 volts at 3 amps over 20 inches of stainless aircraft safety wire -- .032" diameter.

That works out to .51 watts per inch. Again right in the ballpark for my own heat preference.

This one was quite a bit thicker than anything else we've tried, by at least double. Since resistance varies with the square of the diameter, this wire had less than 1/4 the resistance of the .015 wire. Yet the wattage per inch remains substantially the same.
Last edited by vtdiy; Apr 02, 2012 at 06:57 PM.
Apr 03, 2012, 10:45 AM
low tech high tech
vtdiy's Avatar
(Original version of Hot Wire Calculator was posted here, now removed -- latest version for download now available at post 50)

I've written and attached a preliminary calculator in Excel format. I'd appreciate any test results you get in relation to it. Please realize that it may not be accurate at this point, and should be considered simply a test of the ideas.


I'd like to expand this program to include other wire compositions than single strand stainless steel, and also solve for other kinds of questions. For instance, given a specified wire diameter available, "what size power supply do I need," etc.
Last edited by vtdiy; Jun 10, 2012 at 08:56 PM.
Apr 05, 2012, 11:47 PM
If I build it, it will fly
I LOVE this thread!

Here is another question for all you hot wire users.

Why do you prefer the W p/IN that you do? What arguments are there for .5w p/in vs 1.5w p/in? (or if this is too off topic can someone send me to a better thread?)

Apr 06, 2012, 02:47 AM
Crashing in Melbourne
Just a though.....

Would be practical to use a Brushed ESC connected to a 3 cell lipo and use a servo tester to drive the ESC to run a hot wire cutter????
I have a spare 10 amp ESC from a brushed kit.
I figure it's cheaper than buying a power supply.

Apr 06, 2012, 04:44 AM
Registered User
Originally Posted by pghern
Would be practical to use a Brushed ESC connected to a 3 cell lipo and use a servo tester to drive the ESC to run a hot wire cutter????
Yes, been doing it for years. Though I usually use a 12V lead-acid battery rather than a Lipo but either work. All it needs is electricity, it doesn't care where it comes from .

Apr 06, 2012, 04:54 AM
Do it Right, the first time!
CoolerByTheLake's Avatar
Sure looks like a lot of work! I just use this, much easier!!!
Apr 06, 2012, 02:56 PM
low tech high tech
vtdiy's Avatar
What looks like a lot of work, filling in your power supply output and your desired heat?

That site calculator is nice, but only applies to nichrome wire and:

"The calculator is based on experimental values for straight oxidized nichrome wire in stagnant (not moving) air and not in contact with any other material. Any other configuaration will result in different values from what the calculator will show."

The one we've created here is based on ACTUAL performance CUTTING foam with stainless steel wire -- and will be expanded to other materials if people can provide the data.
Apr 06, 2012, 05:11 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
I am sure your chart has its merit, but for me, I use the KISS method. I usually use Owens-Corning Fomular 250 (pink, 25# density) for my projects. I have a home brew PS, 4A,28V, has volt meter, and variable contol, and for me, it works. I use a test piece, the same length or thickness, adjust accordingly, and cut!
Apr 06, 2012, 06:00 PM
low tech high tech
vtdiy's Avatar
Yes Cooler, if you have an existing variable high wattage system and have worked out your wire type and diameter, it won't do anything for you that you haven't done already.

Not everybody in the world is in that situation. Many people have a smaller power supply and wonder what size bow they can use it for. For instance a 12 V 2 amp system. Or a 6 volt system or a 5,or a 9, or 16. Yes they can all be used but with different capacities, depending on current output, bow length etc.

Some might only have a battery. Some might want to know how long a bow they can drive, some might wonder what size wire leader or guitar string to get.

Or a person might want to build a hand held form cutter with D cells -- like a servo cutout tool, or a fuselage hollower, and wonder how thick a wire they can drive and how long it should be for one or two cells or more.

Anyway, yes if you don't need this, then you don't need it. Agreed.
Last edited by vtdiy; Apr 06, 2012 at 06:07 PM.
Apr 07, 2012, 01:26 AM
Registered User
Hey Steve, just got a wireless 30" setup finished tonight. It uses a 20 amp brushed esc, receiver, 11.1v 3300mah battery, all mounted to the handle of the bow. No getting tangled in wires for me anymore! Anyway, I'm trying out some .023 carbon steel welding wire from harbor freight to cut with, and tried it out tonight on a couple of cheap white foam wings. Worked great! I only needed half throttle on my cheapo Chinese transmitter(finally found a good use for it), and it cut just the way I like it, about 3/8" per second.

It's late tonight now so I won't setup to get some numbers, but tomorrow I should have some for you to add to the bag. Also, I'm sure that I could power a 36-40" bow without a problem with this power setup and I'm anxious to find out how to compensate for different voltages with different amps and watts. I wonder if I can throw on a 2 cell lipo and give more amps and get the same results. I will try and test this too for ya to see if your wattage hypothesis is accurate.

Last edited by gmwahl; Apr 07, 2012 at 01:31 AM.
Apr 07, 2012, 11:31 AM
low tech high tech
vtdiy's Avatar
Hi mike! Any setup of over 100 watts with a variable power supply should be able to put out as much heat as anyone needs to cut normal sized wings, (or as little).

Battery power supplies can put out tremendous amounts of power compared with transformers commonly available. Technically your supply can put out a max of the rated current times the rated voltage -- say 200+ watts! (20 amp ESC x 11.1 V batt).

However be very careful with this type of system to check your amperage. There is no limit on it. It is possible for the stick to be set so high that you draw more than 20 amps through the wire. This would blow your ESC. Or if grossly mismatched and accidentally set at high throttle cause a Lipo fire.

The best way is to have an ammeter in line as you gradually increase throttle from zero, and see how the amps are rising. If it looks like you are under about 8 amps at half throttle, you can assume that your rig wouldn't exceed your ESC's max 20 amp rating at full throttle. Likewise it wouldn't likely be able to exceed your batt's 3300 mah 10C (conservative) limit of 33 amps.

Two other concerns I have with Lipos is overdrawing them (if it has an ESC, then it should however have a cutoff, but some might be tempted to have a non-variable system without an ESC -- not a good idea with Lipos) and radio interference. It might be a better idea to use a servo tester with the ESC than a TX/RX, since it's hard wired. They can be purchased quite cheaply.

With a 2 cell system, you are still capable of 140+ watts (20 amp ESC x 7.4 V batt). However it will likely draw more amps -- so do check with an ammmeter to make sure you won't likely exceed the ESC rating at full throttle to keep your system safe. If you don't get enough heat at full throttle, however, you will need a thicker wire. Or if too many amps at full throttle, reduce wire size. It will be nice to predict these things for steel wire in the future, so I look forward to adding your data, and a steel wire category (besides the current stainless steel wire type).

As a guess, if you were only running 1/2 throttle before, you will probably need 2/3 throttle for the same wire on the same bow using your 2 cell batt.

Thanks for the info!

ps. Another smart thing would be to wire an automotive type fuse of 20 amps in line with your bow.
Apr 07, 2012, 05:36 PM
Registered User
Ok Steve, add this to your figures.

.023 carbon steel solid welding wire

28.5" between contact points

Set to cut at 3/8" per second with no wire lag

Battery voltage @ 12.08 volts with 3300 mah 20c

Results: It uses 5.3 volts to the wire. Draws 3.7 amps. Gives me 19.61 watts for .68 watts per inch.

I'm really liking the carbon steel welding wire for what it's worth. It's only 5.50 at harbor freight for a two pound spool and comes in a variety of diameters from .023-.035. Heats fast and seems pretty strong so far after cutting 6 wing cores. Next I want to try some braided low diameter wire and see if it works too. It would be nice to know what options are available to people when the wire of choice is just not available at the moment. Very useful
Last edited by gmwahl; Apr 07, 2012 at 05:51 PM. Reason: add pics

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Drivers and Steppers for CNC Hot wire foam cutter sscherin CAD/CAM 1 Feb 27, 2012 04:08 PM
Wanted CNC Hot Wire Foam Cutter (4 axis) -Snowfly- Aircraft - General - Miscellaneous (FS/W) 0 Dec 07, 2011 05:13 PM
Discussion Need plans for hot wire foam cutter chuckinnc Foamies (Scratchbuilt) 2 May 17, 2009 01:04 PM
Discussion Need advise making hot wire foam cutter Fuelman DIY Electronics 7 Nov 22, 2008 01:04 PM
HELP: Need info on building hot wire foam cutter Chic The Builders Workshop 4 Jan 11, 2003 12:13 PM