Foam Billets

Just another email I thought I'd post.

I've been getting a handful of emails recently asking how to cut off sections from foam billets - large rectangular foam blocks. Figured this response would be worth posting. 

User in Blue, I'm in Orange.

I'll post more if the reply back from the user has some interesting twists.

-carlo

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I was wondering if you have anything that will cut large dense foam block or if you could suggest what i may need to do so

The blocks are 4 feet by 30 inches and 8 feet long.

They call these billets, right? I've had a few people with that same question in the last few months.

On what dimension are you cutting them? Obviously you'll need to have wire either 30" long, 4' long or 8' long..?

Are they just straight cuts? Or are you going to be shaping and sculpting?

If you're doing the 30-48" side, you can probably get away with building a big Large Scale Cutter found on the Diagrams Page. I think one user even got away with using the Hand Block Cutter, where he mounted the wire in one corner and then just sliced off the end of the billet using the free wire. Sort of like a block cheese slicer ( Link )

I almost exclusively suggest NiChrome Wire for hot wire projects, but you'll get a little better tensile strength and less wire memory with stainless steel leader wire. I've had people go through Terminal Tackle Co ( Link ). The only thing is that I've never actually tested out stainless steel wire for its electrical resistance, and electrical requirements for hot wire foam cutting. From what I've read and gotten feed back on, the electrical requirements are very similar, but just know that you might have to go with a bit beefier power supply to make the wire hot enough. 

Circuit Specialists ( Link ) recently upgraded their website and have started to stock a lot more products. For large scale cutting, you're better of going with an AC transformer - Variac ( Link ) - because you can achieve a higher wattage for less money. I'd probably go with the 130v20a model ( Link ). The top end of 20 amps will allow you to get your wire pretty damn hot, and the 130 volts will give enough electrical pressure to push through the resistance of a really long wire. Whether that wire be nichrome or stainless steel. Just remember, anything over 40 volts is enough to penetrate dry human skin, so be cautious when using these models. And if you are using these in an employer-employee situation, you may have to follow some OSHA guidelines to prevent electrocution. When cutting large sections of foam, especially if your wire is hot (which will create more fumes), be sure to use in a well ventilated area.


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Next email response

If you were cutting these blocks as i described above do you have a wire gage and type you would suggest ?

I would probably go for 26 gauge at the least, but not much above 22 ga, and 24 is probably going to be your best bet. The AWG scale for wire gauge is standard across all wire types, so you can always get some samples of 22, 24, 26 gauge wire at the hardware store and see what you like best. But NiCr is cheap enough to just get a few sizes. Same goes for the leader wire. 

and would you assume that i may have some nichrome wire breakage?

I think it's a possibility, but it really depends on how much pressure you're applying to the wire as it cuts. Which, really becomes a function of how fast you're cutting and the wire temperature.  If it is too cold, and your applying too much force to speed up your cuts, then it will snap. But if it gets TOO hot (red/orange) then the wire will oxidize and become brittle. 

Also as you mention on your web page is 600 deg. F enough heat to cut these big blocks?

This touches on the point before… You're going to have to match speed and temperature. 

Theoretically 600°F would be more than enough to cut the big blocks. But when you're cutting with such a long wire, it really becomes a matter of RE-supplying the heat to the wire, rather than achieving a cutting temperature. If that makes sense..?  As you cut foam, heat transfers from the wire to the substrate and melts the foam to 'cut'. But with such a long wire, the heat will be lost from all portions of the wire and slow the cut down. 

For instance, 24 gauge takes about 3 amps to reach 600°F, but you might have to bump it up to about 5 amps to make sure the entire wire stays at 600°. But if you let the wire rest for a prolonged period of time, the 5 amps will boost your wire temperature up to about 1200°F…   So when you're cutting you want to make sure you have constant pressure throughout the cut, and then turn the transformer down once you've stopped. 

That was sort of my thinking on the 130v20a variac. You'd have a wide range of both voltages and amperages to adjust the temperature while cutting to achieve maximum speed and efficiency. 

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Another reply

I just have one more question... What do you mean by the leader wire?

Stainless Steel Leader Wire - Available through Terminal Tackle Co ( Link ). I don't know if you've ever large lake or deep sea fished, but basically leader wire is metal wire that precedes the fishing line. It gives you more tensile strength over long distances than you would get from normal fishing line, to prevent the line from breaking.

You might have heard of some people using "guitar strings", which is another concept of stainless steel wire. 

I had a user contact me (and send me samples) of Leader Wire to test out and see how it compares to NiChrome Wire. I haven't had time to do that yet, but with Leader Wire theoretically you would get a large amount of tensile strength and prevent line breakage during cutting. Some of the RC Foam Wing creators will exclusively use stainless steel leader wire or guitar wire, because it holds up to large amounts of pressure where NiChrome may fail. 

So it might be worth testing out, especially if your nichrome ends up breaking a lot. 

Battery Power

This message was submitted through the online contacts form. Great project and questions regarding using batteries in heating elements or even just foam cutters...

User in Blue, my response in Orange


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I am creating a heating element with 22gage nichrome wire. Using 3.7 volt 3800 mAh Battery, trying to reach a stable temp of 500F degrees.
Using the Jacobs Calculator it states with 3.3amps and 3.7volts a 12.75 inch straight wire would reach the 500F temp. As i understand when coiling the wire you cut the amps needed in half to reach the desired temperature. 

Does this mean that the same 12.75 inch length of wire coiled will still reach the 500F temp and only draw from the battery 1.65 amps and make the battery last longer?

or

Does the battery still push 3.8 amps through the wire and damage the power supply?

Do I need a longer or shorter piece of nichrome?

It looks like your estimates are correct. 3.7volts applied to a 12.75" length of 22 gauge nichrome wire will yield 3.3 amps, a theoretical temperature of 500°F

Here is a small section of the website dedicated to fixed voltage / batteries - Link - There might be some information there worth reading.  But I have a few things to say.

I've tested NiChrome wire in the coil arrangement on a heater project I was working on for a customer. And I've never found that "half amperage" fact to be true. It still seems like it requires the fully calculated value for amperage to achieve the desired temperature. I suppose I understand where that statement is coming from. In a coil arrangement, you are trapping and saving more heat into the empty space of the coil and less is lost to the environment. But honestly, I've only ever seen the calculated amperage to truly being effective.

The other thing is, you may not actually be able to get 3.3 amps out of that battery and expect to achieve longevity from the battery.  Theoretically if you pulled 3.3 amps from a 3.8Ah (3800mAh) battery, it would run for 69 minutes. But as a rule of thumb you should never pull more current (amps) than 1/4 the rated Ah (or mAh).  Since it's a 3800mAh battery or 3.8Ah, 1/4 would put you around 0.95 amps, or damn near 1 amp. 

The problem is two fold...

One, not every battery can physically establish a current that high. The internal salt bridge and internal resistance of the battery actually limits the total output amperage. 

Two, if you force a battery to output more than it can comfortably handle, the battery will heat up. Now, it might not explode, but with increased battery heat, comes increased battery resistance. The higher battery resistance, the lower the amperage output, and eventually the battery will die quicker. So rather than 69 minutes, you may only get 20 minutes per battery charge (just a guess). 

So the 1/4 rule is really more so that you can MAXIMIZE both the output current as well as the longevity. If you go above the 1/4 rule, in most cases you will be able to achieve the desired current, but you may not achieve the desired battery life / time. 

I kinda rambled a bit, but does all that make sense?

In any event, when people are asking about these custom projects I always tell them that calculations are one thing.. but in-field application is another. Try to do as much on paper as you can, but eventually you're just going to have to build your heater and try it out. Some times the real life applications works better than it does on paper, which is a win-win.  

So don't be too afraid of going forward with your current project as-is, but if it doesn't work out like you plan, my explanation may be the reason why. 

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Just a great question for those interested in batteries, hope it helps some of you out!

Laptop Power Supply

Hello again, second posting for the day.... continuing my email purge.

Hopefully someone will find this useful

I'll be in Orange, the user will be in Blue. This posting starts just after the received email. 

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Two years ago I built a foam cutter out of a laptop power supply that is still in use today at my old school.

That leads me to my next question.  I am attempting to build a more permanent and robust version and I have come across some new materials that I would be grateful if you could offer me some advice.

I have an older power supply from a computer which tells me it can put out several different supplies depending on the connectors I choose.  Before I give you that info, I also want to try to use copper wire from an old IDE cable which I believe to be 30ga.and will run about 12 inches.

No no no, don't use copper. Copper wire has very little resistance and will short circuit your power supply. You're going to want to stick with NiChrome Wire or Stainless Steel wire (fishing leader wire). Some people will use guitar strings, but I almost exclusively suggest NiChrome Wire. 

Jacobs Online ( Link ) has so many different options for nichrome wire and he is VERY reasonable on prices. I usually suggest 24 gauge for straight wire cutters, and 20 gauge for rigid wire (bendable) cutters.

Anything smaller than 26 gauge won't really be able to withstand the force of cutting foam. Anything larger than 18 gauge is too big and unnecessary.

The power supply says I can do 5v @ 15A, 12v @ 12A, 5vfp @ 2A, and 3.3v @ 10A.  I was thinking about using the 12v @12A because that is pretty similar to what I was running off the laptop power which was 18v at about 3A with NiCr 40ga. at about 10 inches.

The 5v 15a terminal can be used if you want to have a rigid wire cutter using 18 or 20 gauge. With large gauge wire you want low voltage and high amperage.

For your standard table top cutter or straight wire hand held cutter go with the 12v terminal as long as your length is over 10-12". If your cutter is less than 6" go with the 5v terminal. 

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The user ended up using a different power supply, and here is his response.

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I used an old wall wart transformer that was 15v @ 500mA and I'm using a 9" - 38 gauge nichrome.  And, so far, it's holding up quite well.  I then wanted to try slightly higher production (thin foam slices) and so I strung up a total of 4 wires, and now after 5 passes, the transformer is heating up a little bit.  If I wanted to to more wires, that would equal less resistance, right?  So, that means I would need a higher amperage transformer, correct?  Do you think I should just hook up my 350w power supply and see what happens?

I'm pretty much only doing thin foam slicing for now.  However, because of the amount of donated foam I'm getting, I was thing of building a jigsaw like cutter for the art teacher in my building.  Or, if I could run two cutters off of one power supply, that would even be better yet.

Let me know your thoughts.  I'm not afraid to get a little crazy.  Already popped the breaker a couple times when I tried to wire up a switch and pilot light.  Still can't seem to get the pilot to stay lit while the cutter is on...

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I used an old wall wart transformer that was 15v @ 500mA and I'm using a 9" - 38 gauge nichrome.  And, so far, it's holding up quite well.  I then wanted to try slightly higher production (thin foam slices) and so I strung up a total of 4 wires, and now after 5 passes, the transformer is heating up a little bit.  If I wanted to to more wires, that would equal less resistance, right? 

Sort of… It depends on how you have your circuit arranged. Check out the Resistance Page ( Link ), but more specifically the Circuit Theory Page ( Link ). With multiple wire foam cutters, you have to determine whether you're attaching your wires in series or parallel. The circuit theory page discusses the effects series/parallel arrangement has on resistance, voltage and amperage. 

Long story short, yea, if you're wires are in parallel it dramatically reduces the resistance of your overall circuit. That will in turn draw an incredible amount of amperage from your power supply UNLESS you find out a way to reduce voltage. You can always put your cutter wires in parallel, but then toss a resistor in series to reduce the current flow. 

If they're placed in series, it basically just mimics one long wire and one additive resistance. SO you'll have to boost voltage to achieve a cutting temperature. 

So, that means I would need a higher amperage transformer, correct?  Do you think I should just hook up my 350w power supply and see what happens?

The 0.5amp (500mA) max output of your 15v power supply really won't be able to handle much of a load, so definitely, use your 350w power supply. Again, you may only have to use the 5v terminal if in parallel, but the 12v terminal if in series. 

I'm pretty much only doing thin foam slicing for now.  However, because of the amount of donated foam I'm getting, I was thing of building a jigsaw like cutter for the art teacher in my building.  Or, if I could run two cutters off of one power supply, that would even be better yet.

Definitely. Just think of each cutter as their own resistance circuit. So two FULL cutters placed in series or two FULL cutters placed in parallel. The same electrical properties apply as if you were using two wires in series/parallel.

Let me know your thoughts.  I'm not afraid to get a little crazy.  Already popped the breaker a couple times when I tried to wire up a switch and pilot light.  Still can't seem to get the pilot to stay lit while the cutter is on…

Lol, thats weird… Check out my DIY power supply diagrams ( Link ). Towards the middle of the page, you can see how you're supposed to wire a switch & pilot light with your power supply/cutter. The idea is that the switch controls the electrical flow to both your power supply and the pilot light with one throw. So you just have to split the hot wire after it leaves the switch. But that's all I got for you on that… I can't imagine why you're popping a fuse unless you've accidentally crossed a hot and neutral return - that would cause the electrical flow to short circuit to your breaker box every time you throw the switch. 

60" Cutter

G'afternoon!

Cleaning out my saved email folder, hoping some of you can find this information useful. Read through how the conversation progressed between me and a user.  Their text will be in Blue, I'll be in Orange.

- Carlo

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Nice website. I am a foam fabricator and I am in need to build another wire cutting table/bow. I need a transformer to heat a 60" long wire. I contacted Sean at Mastech power supply, and he asked me how much current I need. I do not know. Can you help?

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If you'd like to talk by phone, I'll be available tomorrow any time :)

What type of wire are you using for your cutter? Do you plan on using NiChrome wire, or are you going with stainless steel?

I know a lot of the fabricators that have large CNC machines will use Rene Wire, or stainless steel. Nichrome wire is the preferred wire, because it has known electrical properties (resistance per foot, heat to amperage ratio, etc etc). But there is a bit of sag that occurs with nichrome wire, that is really difficult to eliminate at lengths greater than 2-3 feet.  So if you're going with a 5' cutter, it might be in your best interest to go with Rene wire. I know the hobby RC plane wing creators swear by either Rene or Stainless steel leader fishing wire. 

If given the choice, try to stay between 20-24 gauge wire. Although, 20ga will be QUITE stiff, so go with 22-24 gauge. Anything smaller than 26 gauge will have too high of a resistance at 60" and require a high voltage output from the power supply. Anything larger than 18-20 gauge will require a much higher amperage output.  Rather than getting TOO much into the details (which are also available on the website), basically you need voltage to overcome your circuit resistance, and amperage to heat the wire. The rule of thumb with ALL wire (regardless of stainless, rene, nichrome) :: The larger the wire, the lower the resistance, the lower the voltage, but the higher the amperage. The smaller the wire, the higher the resistance, the higher the voltage, the lower the amperage. 

Here are some links

NiChrome Wire - Use Jacobs-online.biz

Rene Wire - I recently found this link to SkyKing ( Link )

Stainless Leader Wire - A user contacted me and sent me some samples from Terminal Tackle ( Link ) They have a great selection of stainless steel leader wire. 

As for current, it will depend on the wire. But if I were to buy ONE power supply that will work for almost every application, I'd go with either a 30v10a or 30v20a power supply. Depending on your budget, I'd spend the little bit more and get the 20a power supply. 

Here are two from Sean

30v10a - Link

30v20a - Link

If you know what type of wire you're going to use, let me know and I should be able to estimate you a bit closer on the your required power supply. But in all honesty, I'd say the 30v20a unit will be more than enough for any hobby project. 

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Thanks for the response. I am using Rene .014 " wire.

Spool says Rene 41. My current transformer is a power stat 116ct 10 amp 50/60~ kV a 1.4 whatever all that means. It works pretty good and would like a second unit like it. I emailed Sean the same info so I will let you know what he says. I am trying to figure out an easy way to slice 30" x 48" x 96" billets into 12" thick and thinner sheets. My current table slicer table is only 30" x 72".

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Thanks for the response. I am using Rene .014 " wire.

Spool says Rene 41.

Okay, I figured as much. Rene 41 is used much more in industrial applications and actually has a lot of beneficial properties over nichrome wire. I haven't had the time to personally test the wire, but from what I've been able to find it does quite well. 

The only thing I haven't been able to find is a resistance per foot (ohms/foot) of Rene 41. That helps gauge power supply requirements. But if you have an electrical multimeter, you can always test the resistance of your current wire and do a little math.

My current transformer is a power stat 116ct 10 amp 50/60~ kV a 1.4 whatever all that means.

Not sure what the 116ct is though, maybe a model number.

the 50/60 should be 50/60Hz, which is the AC frequency of US electricity.  10 amp is the maximum safe working output amperage.   If i were going to guess, it's probably a 1.4kVA power supply which is a measurement of maximum output wattage (kiloVolts * Amperage)

If i were to guess, it looks like you have an AC step up transformer. Is it a Variac? A large turn dial on top, with output nodes on the front?

Anyway, because this power supply is a 10a power supply, you can probably get away with another Variac or either of those two power supplies i linked in my previous email. 

It works pretty good and would like a second unit like it. I emailed Sean the same info so I will let you know what he says.

Yea, sean will definitely have a better idea of what he has in stock that will work for you. But I'm sure those two DC supplies i sent you will work, but also any Variac

Here is a 2kVA Variac that would work well - ( LInk ), or if you wanted to go a bit bigger a 3kVA Variac - ( Link )

I am trying to figure out an easy way to slice 30" x 48" x 96" billets into 12" thick and thinner sheets. My current table slicer table is only 30" x 72".

If you're looking to cut them in 96" long sheets the best would be to build what I call a "horizontal cutter ( Link ). Obviously what I have diagramed on the website won't work for you, but it should give you an idea of what you can set up at your own shop. I've seen some units where people just have two plumbing pipes mounted on a table with a flange and then make notch cuts in the pipe every 6" to wrap the wire around. 

If you're looking to cut the face of the billets off, that might be a bit trickier. Using plywood, you could build a square opening that is 32" x 50", and mount the rene wire at one top corner. Then the loose end of rene wire is free and mounted on a wooden block you would hold with one hand. You pass 12" through the wooden opening, then hand cut using the wood as your guide/fence to make sure the cut is straight and flush. You'd basically be making a large adult version of a play dough factory…. Advance the foam, cut it, advance, cut it, etc

Anyway, I don't know if that helped you with the design at all?

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Thanks Carlo, I think I will get the 2K model. You have been a great help.

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No problem! Glad to help!

Yea that 2kVa model is only a little bit bigger than the one you currently stock, and allows 13amp maximum output. So you'll be able to get the wire hotter and cut faster…

Take it easy, and send me some pictures when you have your cutters made!