Large Bendable Cutter

Another question received from the website. I figured I'd post this as an FAQ. Been getting a lot of questions about power supplies lately, hopefully this will help some of you out!

Question ::

I would like to cut large (4'-8') curved parts accurately out of 8' blocks of foam. Therefore I want pretty stiff wire, probably in the 20ga range or possibly even 18ga. Can you recommend a bench top power supply sufficient for this type of cutting and also a wire recommendation? I'm assuming that with the variability of the power supply it would also allow me to build shapeable wire routers for specific tasks, correct? 

Response ::

Sounds like quite the project, a 4-8' bendable wire!

The good news is that with a large gauge wire (16, 18, 20ga wire) you only really have to worry about finding a power supply with enough amperage. The voltage requirement for large gauge wire is pretty low in comparison to smaller gauge wires. So almost all of your power supplies will have enough voltage, but not always enough rated amperage. 

For what you're describing, 18 gauge wire will probably be the best. But the AWG scale applies across all wires. Meaning… you can go to the hardware store and pick up some copper wire, and test it to your liking. See what works best across your cutting length, bendability, rigidity, etc etc. That usually works well for most people. But honestly, NiChrome Wire is pretty damn cheap through jacobs-online.biz, you can always just pick up 16-20 gauge and see what you like best. 

Do you have a limit on power supply cost? The Bench top power supply section of our website has a few great online stores to shop prices on units and shipping. I've had the best success with Circuit Specialists and MastechPowerSupply.com. Both are reputable, and focus on customer service. They may be able to answer finer details about each power supply. 

This bench top power supply is pretty nice, 30v10a for $140 - great price, great range of amperage and should easily be able to handle 16-20ga. This power supply would be able to get 20g wire hotter by comparison, because 16 takes a little more amperage. 

One other option is to go with something called a "Variac" power supply which is a variable AC transformer. I always tell people to use these with caution, because anything above 45 volts, will penetrate dry human skin. Most variacs can output 120 volts, so just be cautious about touching the metal components. BUT, with a power supply like this one it has an output of 20 amps for only $110! You'll have to rig a 3-prong wire on your cutter, but thats not a problem - just be sure no one gets confused and plugs it into a real live wall socket…! Generally speaking, Variacs are nice because they're cheap, and can produce a lot of power. Downside is it gets a BIT more dangerous as you use more voltage. For an application like this you're never really going to need more than 30-40 volts, so you should be safe to operate it at your cutting temperatures. But as always, just have a healthy respect for electrical currents and watch out for unintentional wire groundings. 

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Okay guys...! For those of you that routinely read my blog, some of this is probably starting to sound repetitive. So I hope you don't mind! Sometimes it helps to see the wide range of applications to really understand how to apply the electrical principles of hot wire foam cutting.

Take care!
-carlo

Wide Hot Wire Foam Cutter

Here is another question I received today from user just like you!

"Looking for best benchtop variabe powersupply to cut 2lb eps foam into surfboard blanks. The 26 g nicrome in bowcutter is 5ft in length. Thank you"

Anyone people use cutters longer than 2 feet, I typically recommend stepping it up to 24 gauge NiCr wire. You'll get better tension and less sag using 24 over 26.

If you're truly going to work with 5 feet of cutting wire, you're going to need to have a power supply with a bit of a higher voltage output. Five feet of 24 gauge will need at least 30-40 volts if you want to get up to about 5 amps. Now… five amps is just a relative value. Most foam will cut at about 2.5 amps (when using 24 gauge). But at such a long wire, you're probably going to have to go with a hotter wire to make sure your cuts are nice and fast. A wire of that length will definitely lose some heat to the environment and take a while to replenish heat when consumed by cutting the substrate.

You probably won't be able to find a cheap DC power supply that can output 40v5a. But some of the AC step down transformers are really nice, they're called "Variacs"

Circuit Specialists have a few that they stock ( http://www.circuitspecialists.com/variacs ). Honestly, if you don't mind spending the 100 bucks, I'd go with the 20amp model ( http://goo.gl/9o5ho ). That thing is a BEAST. And because it can go well beyond 5 amps, you'll be able to fiddle with your cutter until you get a nice and fast cut. 

But be cautious, any voltage setting above 45 volts has the ability to penetrate dry human skin. They do have an in-line fuse which will help prevent against short circuits and electrocution. But just be really careful that no one touches any of the metal components on your cutter. Also, be careful when you set the cutter down that it doesn't touch a metal bench or a puddle on the ground, or that kinda thing. I know what some people have done is spray the metal brackets/bolts with GrillPaint. Its a high temperature and nonconductive paint, available at most hardware stores. Just a thought. 

One other thing you might consider is using stainless steel leader wire, used in deep sea fishing. A lot of the model RC wing builders will use stainless steel wire because they can put it under higher tension. Now, I haven't tested it myself for the electrical requirements, but apparently it is similar to nichrome wire.

Check these guys out - http://www.terminaltackleco.com/prod_detail_list/144

For leader wire gauge, I think the 80-100lb test is typically used, but maybe thinner. The stuff is pretty cheap, so you can experiment too. 

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If you guys have any questions, get a hold of me!

-carlo

AC v DC

Another question emailed by a user, with my reply...

I found your video and info on NiChrome wire interesting.  I am working on a much longer run of NiChrome wire to burn through some tough fabric.  Are you available to discuss AC vs DC voltages?

No problem at all.

Long story short, when it comes to a resistance wire application (nichrome wire) it really doesn't matter. The fact of the matter is, NiChrome wire only requires an applied electrical pressure (voltage) and a current running through the circuit. Whether its a constant flow from direct current, or an alternating flow from alternating current it doesn't make any difference.

But as far as what your specific project requires, it really depends on the length of wire and the diameter you'll require - as you may be aware, both will impact the electrical requirements. 

AC power supplies are usually cheaper and have a higher voltage capability. Whereas DC power supplies are sometimes easier to come by.  I always warn users to be cautious of any power supply that outputs more than 45 volts, which is the threshold for penetrating dry human skin. Not many circuits require voltages this high, so why run the risk with a high output AC transformer? But both are perfectly acceptable options.

Jacob's Online has some good solid state step down AC transformers that are beefy and can handle quite the load. So if you intend on making your own supply, check out what he has stocked. I've personally made a power supply from his units, and it works great!

Otherwise check out the Benchtop Power Supply Section of the website which lists a few vendors that have great customer service and a variety of power supplies. I've referenced MastechPowerSupplies.com a few times, they have a great section on AC transformers. 

Hope that helps!

-carlo

Power Supplies & Wire

Here is another emailed question from a user regarding power supplies and type of wire. 

Hello, 

I was looking at your webpage regarding hot wire foam cutters. I tried making a cutter but it seems that I may have bought the wrong wire. I have a toy train transformer that cuts of in about 15 seconds after it heats the wire to cut. I bought fishing wire (stainless steel)... I am very frustrated with this and really want to make this work. I'm not interested in cutting foam but as I am in cutting PVC boards. 

Hello!

I would recommend spending a little time and cruise through my website, it has all the information you desire. But I think the Introduction Page may answer alot of your questions.

There are a few things, stainless steel wire does not have published heating/electrical requirements so a lot of it's use is experimentation. NiChrome wire is preferred because we know how much electricity it takes to heat different lengths and gauges. That information is located here. 

It is unlikely that the train power supply will be effective for long term use. It wasn't designed to output the wattage (amperage (x) voltage) required to heat nichrome wire (or stainless steel wire) for extended periods of time or even short periods of time. Check out the power supply calculations page which will help explain why, and also provide you with ways of estimating your power supply requirements. 

The Introduction Page does have some general power supply information, but on the Materials Page there is a link to recommended power supplies for general small scale use. You may end up spending a bit to get a decent power supply, but it is worth alleviating the headache! On the Materials Page you'll also find a link to purchase NiChrome Wire, but I have always had superior customer service and recommend Jacobs-Online for nichrome wire. 

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I know there wasn't a lot of content in this FAQ, but I just wanted to post so you can see the types of questions we receive. Also, I wanted to post the links to the website in case you too have similar questions. I always recommend people start out by reading through the introduction page for abridged notes to foam cutters. 

-Carlo

Adding Resistance

Here is another email from a user that has a perfect question regarding power supply voltage... check it out!! This is the exact question (in blue) and my reply (in black)

Question ::

I want to make a hot wire cutter to cut rabbets in the edges of rigid styrofoam insulation. I have calculated my voltage and current requirements with the calculator on this site.

To heat my 1" 18ga Nichrome wire to 300F/600C, I need ~6.5A at 0.25V.

My variable power supply won't go that low, and because this is a one-time project, I don't want to spend $30-40 on batteries and another $30-40 on an appropriate charger.

I'm looking for suggestions for a voltage divider that would allow me to use an existing [variable voltage] 12VDC 10A power supply to power the cutter.

No problem at all, and great research! I always love when people like you utilize the website and have almost all your own answers!

You're close :) All you have to do is increase the resistance of your circuit to account for the lowest power supply voltage. 

Just play with ohms law to see how much you need to add, but here is the link to the main page for resistance modification

http://hotwirefoamcutterinfo.com/Hot_Wire_Foam_Cutter_Info/_ResistanceCalc.html

You'll be adding resistance in series with your nichrome wire circuit. 

Here is a quick series of calculations you can do, figuring the lowest voltage your power supply can output. Lets just use 2 volts (just guessing) as the lowest voltage your supply can provide. 

Resistance = Voltage / Current

Resistance = 2/6.5

Necessary Circuit Resistance = 0.307 ohms

Resistance of 1 inch 18 gauge wire = 0.035ohms

Unaccounted Resistance = 0.307 - 0.035ohms

Unaccounted Resistance = 0.272 ohms

So basically, all you have to do is add about 0.3 ohms and you can use a power supply that can only go as low as 2 volts (theoretical).

A REALLY cheap way to make a resistor is like this

http://www.youtube.com/watch?v=P92V6jBYCA0

I've never tried to pull more than a few amps through that wire, but theoretically it could handle quite a lot. I get a little wordy in the video, so I apologize, i haven't updated that video yet. But you should get the idea. I had also considered using a frayed telephone cable (which has 4x small cables in it) but you might melt that small of a cable. Anyway, a little playing around might save you some money.

Otherwise check out Digi-Key, this link goes straight to their resistor search page

http://search.digikey.com/us/en/cat/resistors/through-hole-resistors/66690

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On hand, I have 1ohm5Watt resistors. So placing a handful in series or combined series / parallel will provide a lot of circuits with the resistance and wattage they'd require.

I hope that wasn't too confusing!!

-carlo