# Hot Wire Foam Cutter CALCULATOR

video Hot Wire Foam Cutter CALCULATOR

I made this hot wire calculator because I got tired of frying ac adapters (transformers) or burning out hot wire's all the time and no way was I going to buy an industrial hot wire cutter!

Here (Excel Version)
Here (Open Office Version)

*if a different program (google drive) is used to run the spreadsheet it, the results can be off due to the VBA coding doesn't translate into google, fyi

Link to a sample problem using the hot wire calculator is here

If you want or need help or just want me to design it, I can. Just message me and let me know what kind of power supply you are using and what kind of wire you want to use for the cutter. Cheers! demolisionwolf [at] gmail.

*************************************ELECTRIC SHOCK WARNING or DEATH***************************

If you have breakers inside your house, and touch any exposed wire you'll get shocked. If you don't have breakers inside the place you are cutting and touch any exposed wire, you will die. If you want to be extra safe, plug into a power strip which has 'overload protection.' To identify if a power strip has overload protection, look for a 'reset' button on the power strip, this is a button in addition to the on-off switch. Overload protection power strips are like mini breakers, so using one will double your safety!  **********************************************************************************************************************************

When the calculator say's your design will not work

will be interpreted as one of the following results:
1) the ac adapter (transformer) will melt and stop working over time (you'll be able to smell it too).
2) the wire is too hot and will break from heat and not from normal wear-and-tear. You'll just need several wires on-hand.
3) the wire is cold, so you can't cut anything with it.

Design & Build Suggestions
1) find an ac adapter from a second hand store. I'd suggest finding something that outputs around 20 volts give or take.
2) enter in the ideal length, hot wire type, and diameter you would like to use into the calculator
3) adjust the hot wire inputs till the design works
4) build it

The Design of the Hot Wire Cutter
The calculator is based on the simplest design I could figure out-
1) an AC adapter which will convert AC current to DC current (like a lap top power supply or a video game console power supply). 2) a hot wire of your choice of steel (guitar string) or nichrome.

The Purpose of the Calculator
The use of this hot wire cutter calculator is to help prevent your design from burning out steel guitar strings left and right. Or, blowing your AC adapter, which i've done twice before making this calculator. You don't NEED this calculator to make a cutter, but if you want it to last, i'd suggest using the calculator.

Also, I'm usually against making videos or instrutables, but since i've learned so much from instructables and youtube videos I had to give back to the community when I've never seen anything online about what I came across. FIRST! lol

The Method of the Calculator
The idea behind the calculator is to first calculate the needed wattage to heat up the hot wire to cutting temperature given the inputs you've put in for the hot wire. Next, the allowed wattage of the AC adapter is analyzed. And finally, the analysis of the circuit is done (Table 3). Table 3 has the green and red boxes and is where you will look when making adjustments to your inputs of Table 2.

The Circuit Equations Used
W=VI                                                            <- Watt conversion
I=VR                                                             <- Ohms Law
R=(PL)/A                                                      <- Resistance Equation
P=P(1+alpha*change in temp)               <- Temperature Resistivity Equation
other calculations were made along the way for unit conversion and material properties.

Principles of DC Current
1) Current is a conclusion/output/product of voltage and resistance. Or, current is dependent on the independent variables of voltage and resistance.
2) As temperature of a wire increases, resistance of the warmed wire increases.

''''''''''''''''''''''''''''''''''''''''''''''''''July 2012 update '''''''''''''''''''''''''''''''''''''''''''''''''''
When using Open Office, ignore the colored cells in Table 3 and trust only on the "Meets Req." text for that row. The "Meets Req." text is a trusted result, the 'cell coloring' must be an excel function only thus can't be trusted in OpenOffice.

Also, the calculator only allows you to input your information, it does not let you change material property values. This is to prevent 'bad data' being mixed in with the calculator then it being re-posted. Message me if you want a material type not listed, and I'll put it in, thanks!

''''''''''''''''''''''''''''''''''''''''''''''''''Nov 2012 update ''''''''''''''''''''''''''''''''''''''''''''''''''''
The spreadsheet calculator is available in an OpenOffice Calc format and should fix all the bugs. Thank you for your patience and feed back.

'''''''''''''''''''''''''''''''''''''''''''''''''Jan 2013 update'''''''''''''''''''''''''''''''''''''''''''''''''''''
If the links are down for the spread sheets, try these ones or email me at demolisionwolf [at] gmail and I will email them to ya.
here (Excel version)
here (Open Office Calc version)

Can I be shocked by the hot wire cutter?
YES! As long as there are breakers in you house and the outlet you are connecting to is on a breaker, you'll be safe but will still be shocked.

With the hot wire cutter, there is risk of electric shock. The reason why its not a major issue are the breakers inside the house. They will kick off before any major harm can be done. Lets say the breaker fails to turn off, then one will probably die. The probability of a breaker failing is so very very small. But, to be on the safe side, use a power strip which has overload protection, not just surge protection. Surge protection monitors voltage only, not current or watts. Overload protection is a mini breaker inside the power strip and will behave as a breaker as discussed previously. To identify if a power strip has overload protection, look for a 'reset' button on the power strip, this is a button different from the on-off switch.

Principles of electric shock & prevention
From what i understand 1mA can tingle, 10mA(0.01A) will contract your muscles so you can't let go of the wires, and 100mA (0.1A) can kill a person. The duration of exposure to the current also plays a big impact in the damage done. The amps are far more dangerous than the volts.

Inside the US, there are strict electrical regulations & codes which will prevent an individual from being killed inside their home.  breakers in the electrical panel are these life-savers.

The amp count for the breaker is selected based on the intended use of the circuit. Wall outlets have 15 amp breakers, in the kitchen & outlets located outside its 20amps, 30, 50 & 60 amp breakers are designated for ovens and mechanical equipment which heat and cool.

DemolisionWolf (author) says: Mar 17, 2013. 1:12 PM
Its been 1yr since i posted this instructable; the calculator has been download over 840 times through SendSpace and viewed over 4,000 times this year. Thanks to everyone who has given constructive feed back on how to improve it.
BigBlue85 says: Jan 27, 2013. 6:40 PM
Hey there, the document won't let me enter values. on mac or pc. not sure why....
Can you calculate it for me?
I am trying to use a laptop power supply that gives 18.5v at 4.6amp and I need to keep the wire under 18". I was planning on using a guitar string, but will use what ever you suggest, It just can't be longer than 18".
DemolisionWolf (author) in reply to BigBlue85Jan 31, 2013. 11:03 AM
I'll look into why its having issues, thanks for the heads up.
If you use the 18.5v 4.6amp supply and cutting typical styrafoam, here are the combinations i've found which should work great for you;

Option 1) Nichrome wire gauge 26 with a length between 14"- 18"
Option 2) Nichrome wire gauge 28 with a length between 14"- 18"

If you insist on using a guitar string the set up would be:
Option 3) 0.011" diameter & 40" length or longer
Option 4) 0.010" diameter with a length no shorter than 33"
These lengths are longer but its the set up i use; I just wrap the guitar string down the sides of the frame wrapping them around screw-posts at each corner. Check out my other video here which goes over my frame set-up:
http://youtu.be/3mPOUFpAbaY
This has worked great for me. I made mine a year ago and used it a ton, never broke a string and put it in storage, and have pulled it out once or twice since then and works like a charm.
frazzlemydrive says: Apr 7, 2012. 11:11 AM
Could you say what the password is to unprotect the spreadsheet? The material of my resistance wire isn't there, but it'd be really easy to just modify yours, it looks really great apart from that!
DemolisionWolf (author) in reply to frazzlemydriveApr 9, 2012. 10:17 PM
Thank you, It was quite a bit of work!

The idea of protecting the worksheet is to prevent people from adding or manipulating the worksheet then spreading it online.

But, tell me the material and I'll put it in! And if you got other suggestions I'd like to hear them, thanks
diy_bloke in reply to DemolisionWolfNov 5, 2012. 4:06 PM
i understand that but the problem is that i cannot enter anything in open office, nothing, I have to fill out table 1 and 2 right? not possible, protected
DemolisionWolf (author) in reply to diy_blokeNov 9, 2012. 8:38 AM
diy_bloke you are correct. To fix the issue, I'm offering two download types for the calculator; Microsoft Excel and OpenOffice Calc. Thank you for bringing this to my attention and i'm sorry for getting back to you so late.
diy_bloke in reply to DemolisionWolfNov 9, 2012. 12:02 PM
thats ok, tnx. dont worry abt it :-)
davidnfreeman says: Aug 3, 2012. 6:51 AM
Hi DemolisionWolf, Nice Calculator just what I wanted!!

But it won't let me change the values says "the cell or chart you are trying to change is protected and is therefore read only". I am using a Mac with Microsoft Excel 2008.

Thanks in advance for a solution.
DemolisionWolf (author) in reply to davidnfreemanNov 9, 2012. 8:41 AM
davidnfreeman, are you still experiencing problems with the calculator? i'm sorry for getting back to you so late. Only the violet colored cells are allowed to be changed, is which cells are you reffering to? I've recently added an additional download for the calculator specific for OpenOffice if that may help.
Tazo says: Apr 5, 2012. 7:58 PM
EXCELLENT CALCULATOR!
DemolisionWolf (author) in reply to TazoApr 9, 2012. 10:16 PM
Thanks, I hope people find it useful!
Apolo8 says: Mar 24, 2012. 8:08 AM
Hi...
May you please make it for metric measures or explain what changes need to be done?
very nice 'ible...thx for posting!
Apolo8 in reply to Apolo8Mar 24, 2012. 8:57 AM
Sorry...Found the conversion tool and understand it...
ToyMaker says: Mar 24, 2012. 8:50 AM
Since it works in \$excel\$ it will also work in OpenOffice.org Calc, which is free open source and available here > http://www.openoffice.org/download/