Hot wire foam cutter

[suprsurfer]

I built this today to help with my foam molds only one problem! it doesn't cut.
everything is hooked up properly so there can only be two possible problems the way i see it.

1.I didn't use nicrome wire i used 19 guage steel wire (galvanized) from lowes so maybe it doesnt have enough resistance?

2.My 12V power source isn't producing? i bought it at radio shack check out the pics..

Thanks in advance

[aaaaaaagh]

way to build a cool tool. hope it works for you. what its for.

[Loki440]

I will give you the reason why it isnt working

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Your power supply dosent produce enough amprage

give me a bit and i will post what you need (need to do some calculations)

[suprsurfer]

loki would going to a smaller gauge wire work instead of more power? cause a smaller gauge wire=more resistance right

[lettucefresh]

I hook my **** up to a car battery.

[Loki440]

ok generally speaking if you wanted to continue to use 19 guage Steel wire, and wanted to use a 12v power suppy, you would have to find a power supply that puts out 120 amps for little over one foot of wire (or about 35 amps for a 4ft or so length of wire).

So in other terms its not gonna happen.


Now the two most common types of wire used in hot wire knifes are Ni-Chrome, and Stainless Steel. Now Nichrome has roughly 7 times the resistance of Stainless steel(dont know exact amount, this is just based on normal steel) but anyways inorder to heat up a wire hot enough you have to put enough current through it to heat it up (current x resistance = power or what we are looking for in this case "heat")
So in order to get the power output up, you can do two things increase the current(higher amperage power supply) or two increase the resistance(Higher resistance wire or decreasing the gauge or both)

So lets say you went to a 32 guage wire for about 1.5 feet of steel wire, that equates to about 2.34ohms for that length of wire, so 12v / 2.34ohms = 5.12 Amps (much better wouldnt you say), now for 4.5feet you would need 1.7 amps (even better)
Now lets compare to ni-chrome in 32 guage for 1.5 feet its resistance for that length is roughly 15ohms so 12v/15ohms= 0.8 amps (800 milliamps, and thats what your transformer produces ) And 4.5 feet equates to needing about 0.27 amps (270 milliamps)

So with that you can see whats the problem, so here is the solution to your problem, one get a smaller guage wire (32awg is good), in stainless steel or nichrome prefurebly (i will list pros and cons of each later), and two get a higher amperege supply 1 to 1.5 amp minimum going with a higher amperage supply means you can throw in a resistance circuit to bring down the amperage and adjust the power output based on what sized kot knife your using and what wire (think dimmer switch for a cheap easy resistance circuit). Or if you still want to use the power supply you already bought you can coil up a couple extra feet of wire thus effectively increasing the total resistance (but make sure none of the coils touch each other as then you just short circuiting it thus making all those winding actually only a couple inch of extra length added on.

Now for wire pros and cons

first ni-chrome higher resistance means that you need less amps to get it hot, so you can use the power supply you got, but its harder to find, more expensive and wears out more quickly(oxidization and such), and stretches a bit more than steel when heated.

Stainless steel easer to find , lasts longer(dosent really corrode like other metals when electricity is passed through it), usally cheaper, stronger and stretches less, but you will need more amperage to get it hot (but easily fixed with some extra coils added increasing the resistance.

As for where to find a better transformer, check out ebay for old train transformers in the 50 watt range, as they typically do not have a short circuit feature in them which most of all new transformers do and guess what your doing to make the wire hot, thats right creating a short circuit.

[Loki440]

loki would going to a smaller gauge wire work instead of more power? cause a smaller gauge wire=more resistance right

yes it was just taking me abit to do the calculations and write up my reply as i state above , a smaller guage does increase the resistance

[CRASS]

my brain hurts..

[Pinhead]

V=I x R

V=volts
I=amps
R=resistance

You could save some $$$$ and use your car battery.
Be careful not to get a short, Good idea to not wear
ring or metal watch band. If the wire is too hot add some
resistance. Also don't breath the fumes.

I like SS fishing leader better than nicrome wire.

[suprsurfer]

thanks a bunch for the info loki. im probably gonna go with SS wire because of the durability/hassle factor of changing the wire. So i guess i need a new power source.

BTW where did you find the information to make those calculations (very interesting)

Thanks again, I'll post my final setup once i get it working!

[CrisVA]

V=I x R

V=volts
I=amps
R=resistance

You could save some $$$$ and use your car battery.
Be careful not to get a short, Good idea to not wear
ring or metal watch band. If the wire is too hot add some
resistance. Also don't breath the fumes.

I like SS fishing leader better than nicrome wire.

Or he could butcher a plug from something, and have 120v straight from his house. That would be enough.

Of course then he gets the fun of seeing is he could blow fuses in his house.

[lettucefresh]

All those calculations are good and such... you chould know how to do them. but just get a really, preferably overly powerful source and go from there.

[onetwotapas]

i didnt read this whole thing, have to run out of the door, but try making the wire shorter. theres a lot of wire there, so resistance will be low (and wont produce the heat needed). get a better power supply, a smaller wire, and maybe a material that has a higher resistance.

edit: i just read it all, listen to loki, he knows his sh#*

[Loki440]

BTW where did you find the information to make those calculations (very interesting)

There common electrical equations based on wire circular diameter, and conductor properties, (in a previous time i took electrical engineering technologist courses, but blah to that) as for the amount of power needed to heat up the wire i based that apon nichrome resistance and what it typically needs.


But as others said also nad i did to just get a big enough powersupply and then you can bring it down for your application, thats why i suggested the old train transformers as they have a built in power adjustment(ie speed control)
ones like these are decent http://cgi.ebay.ca/MARX-TRAIN-TRANSF...QQcmdZViewItem

[grafgulch]

I hope this isn't a double post but if you make a foam mold to vacuum bag over when making a board using the cap construction method do you need to glass the top of the mold and apply a release agent such as wax or can you apply the wax directly to the foam mold so the layup doesnt stick to the mold?

PG

[suprsurfer]

On second thought it's easier to just buy some nichrome wire. so im gonna pick up some 28-32 gauge stuff. By the way in case anyone is interested i found this table giving ohms/ft for different gauges of nichrome http://www.wiretron.com/nicrdat.html.

[holikujak]

way to build a cool tool. hope it works for you. what its for.

are you serious?

[NLC]

lokis right on target with the whole thing . I spent a week making as many different homemade hot-wire set ups that I could find on the net . After a week I had three fires , two nasty finger burns and a price tag of just over $400 for the experience . If you don't follow what lokis pointy towards , save the suffering and just buy the kits available on the net or find an alternative to the task . Maybe take a drive to somebody that has a good set up and have them cut it for you . Oh yeah , and don't forget to wear a respirator to protect you from those very toxic fumes generated by hot styrofoam .

[akary]

i'm trying to make a little business to open this summer. so far everything has been PERFECT. be on the lookout for maverick custom longboards.

http://i19.photobucket.com/albums/b1...K/KIF_1579.jpg
this shouldn't have ****, but rather Amuthaf*ckinK with a u type that in as a URL

this is one of my molds getting a layer of fiberglass (without fiberglass the foam is softish, porous, and quite simply doesn't work) i gave it a wet layup, and because at that time i didn't have a vacuum pump (i now have a scavenged hospital pump with a drip-can. they tossed it while it was in perfect working order because they got blood on it, and then protocol doesn't let them use contaminated pumps) in lieu of pump, i stretched the bagging film over the glass & epxy, then stapled it down to keep it taught and smooth.

http://www.youtube.com/?v=8xzw5Cyq-ps

her i am using the bow i made. i made the variable transformer too. the bow has an iron wire (just about anything will do, though ni-chrome is what the pros use) the higher the voltage, the higher the resistance. the higher the resistance, the hotter your bow will get. it's probably dangerous, but i use 75 volts X 20 amps.

[Pinhead]

Or he could butcher a plug from something, and have 120v straight from his house. That would be enough.

Of course then he gets the fun of seeing is he could blow fuses in his house.[/quote]

120V AC can kill you dead!!!!!!!!!