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Old Apr 01, 2012, 08:44 PM
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Data
Hot Wire Foam Cutter Calculator (UPDATED 7-31-2012)

Update:

After much user input of cutting data (requested originally in this thread), I've written a spreadsheet calculator for hot wire foam cutters so that people can use it to predict the wire size needed for any power supply type and bow combination. Or if you have a particular wire you want to use, you can predict what power supply will work with it. If you have both a supply and wire, you can predict what the current draw will be, and determine whether your supply will be sufficient. It should work with all kinds of supplies -- battery, AC, DC, fixed and variable types.

************************************************** *************************************
(Note 7/31/2012: the latest version of this beta calculator spreadsheet is now attached to this post below. If you try it out, please let me know how it worked for you. I will update and correct it based on any feedback I receive. Please remember this is a beta test version of the program, and it may contain errors or produce erroneous results. This program is not intended to tell you how to build a foam cutter, or how to make it a safe one. If you do use the information it provides please make sure your power supply is properly fused on both input and output to prevent power supply damage, or wire overheating. Responsibility for producing a safe cutter and using it safely is the builder's responsibility.
************************************************** ****************************************





Original First Post:

Request for user wire cutting data in order to write the hot wire calculator:


General Cutting data needed for all wire cutters: It would be helpful to writing the hot wire calculator if people could post here the voltage and amperage measured that you use when you are cutting foam, and your hot wire length.

What I can't use is what is commonly given -- in most people's descriptions of their setups -- power supply rated voltage and amperage, and bow length. I need the actual cutting voltage, and amperage, after you've adjusted your supply to cutting heat. So it will take a meter to measure those things. And I can't use the physical bow length, but only the measured length of wire that's actually heated -- the length between contacts. If the calculator I'm writing works accurately after testing, I will post it here on the forum.

Thanks for your help!
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Last edited by vtdiy; Sep 05, 2012 at 09:09 PM. Reason: Updated Calculator to version 1.4 beta (attached)
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Old Apr 01, 2012, 09:03 PM
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Vtdiy I cannot help but feel a little responsible for this thread
from what I have learned is that diffrent types of wire require higher or lower output voltage and amps than other types of wires.
like Nichrome would need a higher output than stanless
so should the type of wire and thickness be included in the data?

I just started building a new hot wire box using a 12.6v 3a transformer, large project box banans plugs and indicator light from radio shack

and a push on dimmer from wally world.
I will try to post some data once i get it wired up and in use.
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Old Apr 01, 2012, 09:44 PM
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Steve. I wonder if a watt meter can be used for this. I am in the process of putting a new cutter together, two actually if you count a hot wire scroll saw that I'm also making the power supply for, so I may be in a good position to measure the voltage and amps while actually cutting as long as it doesn't take electrical engineering level skill

My two wires that I plan on using are stainless jewelers wire from Michaels, and .010-.021 guitar strings. I don't use nichrome to cut with but I am curious as to other types of suitable wire. I hope that the info that you get back from others includes some other nonconventional wire types so that we can actually see what readily available options we have out there.

If you think of a way to easily measure the V and A let me know and I will be glad to test and post my numbers

Thanks

Mike
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Old Apr 01, 2012, 09:45 PM
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Rogue, it makes a difference what the wire composition is to what the required voltage is, but (I believe) NOT the wattage per unit length. That is the figure I'm actually trying to get by asking for V, A, and heated length.

This is hard to explain.

If you switch from say stainless to nichrome wire, using the same diameter and same power supply, you would need to adjust the voltage higher. BUT, I believe, the amperage would drop, and the actual wattage would stay the same -- that's my theory, anyway, and I'd like to verify that.

If it's true I can build a calculator that can indeed give you results for different compositions, different wire thicknesses, different power supplies, and bow lengths. A unified wire cutting calculator. No more guessing.

But I need the data....
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Old Apr 01, 2012, 09:51 PM
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One other question Steve. Do you want the data to include the extra wire wound around the cutting bow, acting as a resistor, or would you rather have figures that are clean, like the heated wire between two alligator clips? I could just use my larger bow and adjust the clips to get the right heat and measure from there.
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Old Apr 01, 2012, 09:57 PM
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Quote:
Originally Posted by gmwahl View Post
Steve. I wonder if a watt meter can be used for this. I am in the process of putting a new cutter together, two actually if you count a hot wire scroll saw that I'm also making the power supply for, so I may be in a good position to measure the voltage and amps while actually cutting as long as it doesn't take electrical engineering level skill

My two wires that I plan on using are stainless jewelers wire from Michaels, and .010-.021 guitar strings. I don't use nichrome to cut with but I am curious as to other types of suitable wire. I hope that the info that you get back from others includes some other nonconventional wire types so that we can actually see what readily available options we have out there.

If you think of a way to easily measure the V and A let me know and I will be glad to test and post my numbers

Thanks

Mike
Hi Mike!

Well a wattmeter would work, too if it's accurate enough and it's in line with the hot wire . The longer the wire, the more accurate it's going to be, so a short scroll saw type might not register much on the wattmeter, since it's designed to measure a lot of watts in a motor system.

But in a battery powered brushless ESC type power supply (like I think you have) it is often placed between the battery and ESC, and that won't give as good an indication of what the wire is getting because of losses in the ESC.

But any data would be appreciated!

Another way to measure the voltage and amperage is to just check the voltage across the cutting wire contacts with a multimeter. Record that. Then switch the meter over to amps mode (assuming the meter has at least a ten amp scale), disconnect one of the power supply lines to the cutting wire, and insert the meter in series with the power supply and the cutting wire (ie between them). This will then read the amperage figure. Record that. Done.

BTW when I said I wanted these figures while cutting -- I should have said when it is set for being able to cut. You don't actually have to be cutting foam! I just want those figures for when the wire is properly heated at cutting temperature.

Oh also, when making and breaking connections to meters, be sure to turn your power supply off. Natch.

Thanks for asking Mike!
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Old Apr 01, 2012, 10:07 PM
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Quote:
Originally Posted by gmwahl View Post
One other question Steve. Do you want the data to include the extra wire wound around the cutting bow, acting as a resistor, or would you rather have figures that are clean, like the heated wire between two alligator clips? I could just use my larger bow and adjust the clips to get the right heat and measure from there.
If you include a resistor wire in your measuring points, it's okay if you can tell me the total length of resistor wire plus cutting wire (ie the total length between the power supply contacts to the wire. You would have to unwind and measure the resistor for that.

Or you could give me the total amperage, and just measure the voltage across the cutting portion of the wire. In that case I need to know the distance between those measurement points.



EDIT well thinking about that, they are related by the following rule:

1.) Amperage can be measured anywhere in that circuit.

2.) Voltage can be measured between any two points, BUT I need to know the wire length exactly between those two points.
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Old Apr 01, 2012, 10:07 PM
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Ok, I think I can handle that. I actually have an ESC\ battery type of power supply, and a PC power supply. I'm building two more power units from a 12 volt 3 amp radio shack transformer and a 25 volt 2 amp transformer. Having never made one with the transformers before, I am very interested in recording this info with them.

So just to make sure we are on the same page, once I get my wire heated to the optimum temp for what I'm cutting, and I'm assuming make a cut or two just to be sure that I'm at the best setting that I can be at, then take the measurements. Correct?
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Old Apr 01, 2012, 10:19 PM
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Originally Posted by gmwahl View Post

So just to make sure we are on the same page, once I get my wire heated to the optimum temp for what I'm cutting, and I'm assuming make a cut or two just to be sure that I'm at the best setting that I can be at, then take the measurements. Correct?
That's right. it could be a small scrap of foam. Or if you are actually cutting a part, check the V and A after the cut is over. That will be sure to be your preferred setting since you will have adjusted it..
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Old Apr 01, 2012, 10:26 PM
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Got it! Stay tuned, I'll cut something tomorrow hopefully and get back on it. Gives me a good excuse to make a wing anyway
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Old Apr 01, 2012, 10:33 PM
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Thanks Mike.

Hope you caught some fish, too, over the weekend!
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Old Apr 01, 2012, 10:38 PM
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Thanks Mike.

Hope you caught some fish, too, over the weekend!
I did!!! Felt dang good too! The weather sucks now for a few days, so no fishing for me and more building
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Old Apr 02, 2012, 02:31 PM
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Originally Posted by vtdiy View Post
What I need is the voltage and amperage measured that you use when you are cutting, and your hot wire length.
Here are some measurements I took from a bow that I've been using successfully for years:

Wire material: stainless steel, 0.016" diameter (see photo)
Power supply: transformer / dimmer type
Actual heated wire length: 34.2"
Voltage across heated length of wire: 19.1 VAC
Current: 2.62 amps

I adjust the current to get the best cutting action. Generally, this is just below the point where the wire starts to glow a dull red (as observed in a dark room).

I've found stainless steel to be much better than nichrome, and a fraction of the price. The 600' roll I bought (less than $20) will last me several dozen lifetimes. A bit thicker than the 0.016" I have would probably be better.

Just a note to those who may not be aware of it. The voltage and current from dimmer type controllers operating at anything less than full output cannot be accurately measured by most inexpensive, general-purpose multimeters. At the risk of oversimplifying, this is because these dimmers work by cutting off the electrical waveform part way through each cycle. Most multimeters cannot properly read the resulting "irregular" waveform, and the errors involved can be very significant. Accurate measurement needs a meter with what is called RMS capability (see photo). The measurements shown above were taken with a meter that has this.

Hope that helps. Good luck with your project.
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Old Apr 02, 2012, 03:22 PM
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Thank you very much, Harry D, that's great.

And thanks also for the information that a chopper can mess up the voltage measurement, makes sense, and I hadn't considered that.

Folks using a DC source would find it a lot simpler to measure accurately with a normal meter on the DC setting.

That would include an old style auto battery charger being controlled by a router speed controller, or an unregulated battery source using a length of the cutting wire wound as a resistor. Also a computer power supply used as a power source with a wound resistor.

So looking at the data you gave us you are using about 50 watts to heat 34 inches of wire, or about 1.5 watts per inch. And thank you for also giving us a description of how hot you tend to run your wire.

Now I hope we can get some reports from other people to compare to this wattage per inch figure. I think we will see a range, but not a very big range -- small enough to be useful for estimating what is needed for any power supply in the calculator.

I also believe this wattage per unit length range won't vary greatly for different wire compositions or even wire thicknesses. I think the variation will be greater for different cutting styles than for the other factors. For instance -- some people like the wire hotter than others. And as I know from my CNC wire cutter, I use a very low heat there because I run the wire very slowly and cut by radiant heat, not contact. I run about 1 mm per second.

Again, this doesn't present a problem for a calculator. The output of that calculator can take into account the use of the cutter and the relative amount of heat a person wants to use before recommending an approximate wire diameter. Likewise it can take into account the wire composition.
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Old Apr 02, 2012, 03:42 PM
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BTW, this data from Harry D already shows us something to answer a lot of questions often asked.

Example:

"My power supply is a 12 volt supply rated at 2 amps -- will it be adequate for a hot wire cutter?"

Calculation:

If a bow for foam requires about 1.5 watts per inch of cutting wire, and you can supply about (for conservative sake) about 75% of your rated supply in watts:

75% x 12V x 2A = 18 watts

18 watts / 1.5 watts per inch = 12 inches.

Your power supply can supply a wire cutter of about 12" in length -- this can cut 12" wing panels. (if you glue these together as required for a dihedral wing, your simple wing capacity is 24" -- or you can glue multiple panels end to end to make a wing of any length).

"Yes but I want a wire cutter that can cut a 24" panel. What do I need for a power supply for that?

Answer, well you could go with either a 4 amp 12 volt system, or a 24 volt 2 amp system, or any other combination that gives you about 48 watts output.

"Yes but what is the difference between a 48 watt system that runs on 12 volts and one that runs on 24 volts?"

Answer:

The 12 volt system will use thicker cutting wire to achieve 48 watts, while the 24 volt system will require thinner wire. Or, the 12 volt system needs a wire composition of lower resistance than the 24 volt system to achieve 48 watts. You can go with either alternative, or even a combination of the two. A calculator can help you decide which combination is best for you

Of course all of the above requires a little more user data to confirm what the useful wattage per unit length range is.
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