Large Multicutter Design

Another fairly common question, large multi cutter design

I'm planning to build a prototype for a large foam block multi-wire slicer.

Scenario:

1. I will need 5-8 wires

2. The length of the NiChr are going to be 8-9 feet in length

3. I will be cutting different densities of foam, 1lb to 3lb

4. I will be using 220V power

What do yo recommend in this situation?

With a cutter that large, there are going to be a few difficulties.

1. Power Supply

2. Wire Tension

1. Power Supply

The size and power of your power supply will solely depend upon the wire you choose and the overall resistance of your circuit. Over those lengths, I'm not sure that NiChrome wire is going to be your best option. You may want to consider stainless steel wire.

The biggest factor with resistance and necessary amperage to heat your wire, is how you lay out your design. If you go with stainless steel wire, chances are you're going to want to set up your wires in circuit series, rather than parallel. But if you use NiChrome wire, you'll probably want to use a combination of series and parallel to achieve the best combination of voltage and amperage requirements. 

This is discussed in further detail on the website, and should help explain that recommendation a bit more.

http://hotwirefoamcutterinfo.com/_MultiWireCalc.html

Don't even both trying to find a DC power supply. For something like this an AC Step Down Transformer is going to be your cheapest option for overall output power. If you're using 220v power, I'd assume you're not in the US? If so, I do not have a ton of international options for power supplies. But feel free to scan through my power supply section for some US suppliers that can ship to you.

http://hotwirefoamcutterinfo.com/___BenchTop.html

2. Wire Tension

For lengths over 24" I typically recommend using 24 gauge NiChrome wire. But if you're looking at 8-9 feet, you're going to need something quite strong that will respond to both the pressure of cutting as well as the linear applied wire tension. I still think stainless steel wire is going to be your best option, but I do not have an idea of wire size. Good news is stainless steel wire is cheap, so buy a few rolls and see what gauge wire works best for your design. Keeping in mind that wire gauge and length have a bearing on circuit resistance, and require amperage to heat. You're going to want to find the smallest gauge wire, that can hold up to cutting pressure. Probably 24-18 gauge.

Next, I'd recommend designing your cutter so that at one end you have a rigid extension spring and at the other end some type of winding device. Even if that winding device is just an eye bolt you turn to increase wire tension. Keep in mind that as a wire is heated it will expand, and with expansion comes a saggy wire. If you're cutting a grid out of large foam billets, the last thing you want is wavy lines. I suppose it all depends on your application, but you're going to want wire tension. I have actually seen a design were someone used a FULLY threaded pipe, from end to end. And all they had to do was twist the entire pipe, and it increased resistance on all the wires. It was basically a huge, large diameter machine screw at that point. Simplistic design, just requires you to know someone who can thread pipe. 

I’m thinking of using the Transformer from MastechPowerSupply.

Volteq 5KW Transformer Variac 5000VA 0-250V 220V Input, with maximum current output of 20amps (@250V)

or do think I can get away with….

Volteq 2KW Transformer Variac 2000VA 0-250V 220V Input, with maximum current output of 8amps (@250V)

Not just because I'm American.. but I'd have to say "bigger is better." When it comes to non hobby large scale cutters, especially unique set ups, you do NOT want to be 100% ready to go and have an inadequate power supply.  With the 5kW unit, you have the same voltage regulation on the dial but have 2.5 times the power. Using the 5kW model, you'll have the best power supply, just in case you need to wire in series on the fly...

Just remember… Greater than 45 dry volts will penetrate human skin. If you're moving 5kW across something like that… there is a pretty high risk of shock. Just be incredibly careful. 

I’m also thinking of having them wired is Parallel setup, just so I can hook them up on two shafts, and have adjustability on the thickness of the slices, just by moving the Springs up and down along the shafts.

I love the idea, and would love to see it in action. Send some pictures when you get it set up. 

Just remember your electrical properties when placing a circuit in parallel. An easy way to think of it is like this….

Placing four wire in series, is like making one wire four times as long

Placing four wires in parallel is like making one wire four times as thick

When you have a long wire or wires placed in series, the resistance dramatically increases, the voltage required to heat dramatically increases, but the amperage pretty much stays the same. 

When you have a larger gauge wire or have wires placed in parallel, the resistance dramatically decreases, the required voltage decreases but the necessary amperage to heat the wires dramatically increases. 

I hope that wasn't confusing. 

Long story short… depending on how this pans out, you may end up needing a combination of series and parallel to find the right combination of circuit resistance, voltage and amperage. You may even consider having switches to "activate" a cutting line. You'll want something rated at 20amp max, which is usually a house circuit switch.

The majority of the time, I’ll be using one wire, but I want to make sure that I can do 5-7 wires without any issues.

With the Transformers I have listed above, what do you think there limits are as far as the number of 8-foot wires they can handle?  We’ve been using two sizes of wire, 0.010” and 0.014”, in 8-foot lengths for many of our cuts, usually lettering and shapes, so we need smaller wire gauge.  Do you think that Stainless Steel is better to use that NiChrome?… with the wire gauges we are using, 30 AWG and 27 AWG, which one would be best for heat and length? Right now we have it automated to cut at around 45in/min on 1 lb density foam.

Well… There are a few ways to answer. 

For lengths greater than 2-3' I typically tell people to shy away from NiChrome. Its a good wire, don't get me wrong. But often the resistance is too high at those lengths. Where as, for a small cutter <2' I exclusively recommend NiCHrome wire and NOT stainless steel because NiCr's resistance is higher and makes power supply selection easier. So both are good options, it just depends on your application. 

BUT… 

Jacobs-online.biz JUST started to carry 27 gauge Rene wire…. I think you might like this… Rene wire is considered a super alloy, and virtually indestructible. It is an "up and comer" in the foam cutting world, and a lot of guys use it to cut large (8') foam wings. You might like it a lot, I know Gary Jacobs really likes it. Personally, I am looking into using it for my combination foam cutter as a good straight wire cutter. 

27 gauge Rene wire has similar electrical properties to 28 gauge NiChrome wire, so you can use that reference for calculations. As you would imagine, high wire resistance will be counteracted when placed in parallel. 

Just for kicks, lets see what we can do with a little math. 

A single 8' length of rene (34ohms) would take about 150 volts to heat to 1800° and only draw about 4.5 amps from your power supply.  If you put six 8' lengths of rene wire in parallel (5.667 ohms), my guess would be it's similar to 20 gauge based on length and resistance. Here's the difference… you'd  require about 80 volts to heat to 1800° but drawing close to 15 amps from the power supply. 

Very small cutter

This question came through YouTube, in response to my combination foam cutter


I am just curious, why aren't these cutters made to be a little smaller and more manageable...something like a little wire that is shaped like a U where the loop is like 1/32" so you can carve intricate stuff, or a cutter that is several inches long, shaped like an F with the wire going between the short lines of the F, where the wire itself is only 3" or so long??  For me, working at the end of that huge thing would be impossible;/


You can make cutters as small as you want, this is just an all-in-one prototype that I came up with. It is a work in progress, and I am moving forward with commercial fabrication. But more on that later.

Long story short, when it comes to a hot wire foam cutter, you're electrical requirements are a function of wire diameter/thickness and wire length. With a bendable wire, you cannot use smaller than 20 gauge wire. That size wire requires about 4-6 amps to become hot enough to cut foam. However, a large gauge wire has little resistance, and at 1/32" length, the maximum applied voltage would be incredibly low. In fact, you could not apply more than 0.007 volts to the circuit before overcoming 6 amps. There are virtually no commercially available power supplies that can micro-control voltage that low.

The solution would be to add a resistor to your circuit, to raise the overall circuit resistance. The best case scenario would be finding a variable voltage power supply, that can out put at least 10 amps. You'd have to find a 1 ohm / 50 watt resistor, and place it in-line with your 1/32" 20ga cutter. Your optimal cutting settings would be ~6 volts, drawing 6 amps from your power supply (36 watts), and not overloading the wattage of your resistor (50 watt max).

A very small cutter is definitely an option with the right tools and electrical supplies. But small resistors that can handle that level of heat dissipation (wattage) are hard to come by. In most circumstances you have to buy them in bulk. That said… I THINK I have 1ohm 50watt resistors in my shop.. I'd just have to look.


Wow......man how can something so simple be so complicated:)  Hey does cutting foam with a hot wire give off any fumes?

Thanks for your response, that was very thoughtful of you

I know!! It is both simple and complex at the same time!

Yes, cutting foam will emit fumes depending on the temperature of your wire. It is a bit of a trade off… A hot wire lets you cut faster but will emit more fumes but a cold wire will cut slower with less fumes. When you have a cold wire, you have to apply more pressure to cut the foam which will increase your chances of breaking the wire. A hot wire usually won't touch the foam because the radiant heat will melt the foam at a distance. Although, the hot wire will cut away more foam (a larger kerf) so sometimes your fine work is lost. Like I said… it's all a series of trade offs. It's a fine balance between temperature, cutting speed, fumes emitted and cutting kerf. 

I hope that doesn't overcomplicate!!

20" Cutter

I need is kind a large cutter which has the dimension more than 20", basically 0.3m or 0.45m would be the best. So would it be possible for you to make one and how much would it be

Check out this link, these cutters are really easy to make and all parts should be available at your local hardware store. 

( http://hotwirefoamcutterinfo.com/_LargeHandDesign.html )

That is going to be your best bet at a practical design for any cutter over 12"/30cm.

I would also suggest that you use 24 gauge nichrome wire, available through Jacobs online

( http://www.jacobs-online.biz/nichrome_wire.htm )

You will need between 10-12 volts, and 3.5-4.5 amps. Here are a few options

- Option 1 - 30volt 5amp

- Option 2 - 30volt 5amp

- Option 3 - 30volt 5amp

If you're still interested in me making you a hot wire foam cutter, we can discuss cost of shipping and production. 

Beefy 16g Cutter with a battery?

I have a quick question for you, how long can I run 16 AWG nichrome wire for continuously off of a battery? The Nichrome wire is 8" each piece run in series. 

I was using a 18v drill battery and a 12v motorcycle battery and had horrible results. Both setups melted the wire I was using that was connected to the power source. Whenever I managed to power the nichrome wire I wasn't able to get a long run time before the battery would die, maximum run time was about 1 minute for the motorcycle battery and approximately 30 seconds with the 18v battery. 

Thanks for your response in advance.

I sure have been getting a ton of battery questions lately… weird!

Thank you for your praise of the site, and enjoying its content. It has grown quite vast over the last few years, and some of the information is hard to find. I'm not sure if you found this page, but I have a WHOLE page dedicated to battery power - Link ( http://hotwirefoamcutterinfo.com/__FixedVoltageCalc.html )

There are a few problems with battery powered circuits…

Problem 1 - Fixed Voltage

First and foremost, they are a fixed voltage power supply. Obviously, meaning that they will always output their rated voltage - i.e. 12v, 18v, etc. What you've done with the 12v & 18v batteries is short circuited them both. It's probably good that the wire melted, if they'd have run for a longer duration of time, they'd probably have heated up and exploded.

16 gauge nichrome, at 8 inches, is approximately 0.173 ohms of resistance. Ohms law states that Amperage or Current is equal to Voltage divided by Resistance… A=V/R

A = 12v / 0.173o A = 18v / 0.173o

A = 69.3 amps A = 104.0 amps

That is some SERIOUS current pulled from those batteries. They'll die extremely quickly and/or heat up extremely quickly which is incredibly unsafe. 

Problem 2 - Available Amperage

Not every battery is designed to output amperages necessary to cut nichrome wire. There are internal resistances which restrict output amperage from a batter. Also, as a battery heats up, its internal resistance goes up, which also decreases output amperage. Ask any construction contractor who tries to use battery powered tools left in the sun on a HOT summer day. Their battery life is noticeably reduced. 

As a rough gauge, we say that you should not draw more than 1/10th of the Amp-hour (Ah) Rating of your battery. You'll notice the Ah or mAh listed somewhere on your battery.

So if you have a 20 amp-hour battery, it really should not output more than 2 amps for maximum efficiency.

Solution

Unfortunately there probably isn't an effective battery option for 16 gauge nichrome wire. It's liner foot resistance is far too low, and its current-to-heat demands are far too high. 

If you plan on using your 8" cutter to cut foam, I would recommend no less than 8-10 amps of current, which is about 1.4-1.8 amps. 

Jacobs-online.biz has a bunch of really effective and inexpensive AC step-down transformers that you can use to power your beefy 16g 8" cutter. Check them out, you'll be happy you did. 

I hope that answers your questions, but dive into the battery page for a lengthy explanation, and don't be afraid to ask more questions if you have them!

Common Battery Question

q about hotwire =) can i use a 18650 battery to run this thing instead of plugging in the wall….maybe add a momentary switch??? thanks in advance…

I'm not entirely sure what a 18650 battery is, I'm assuming it's a 3.7v Lithium Ion battery? In that case, the short answer is no, it won't really work.

I have a whole section dedicated to fixed voltage power supplies and batteries. Check out this link.

The problem with batteries is that they are a fixed voltage power source, so you can only regulate your wire temperature by adding a resistor or altering wire length. The second, is that not every battery can withstand or is designed to output currents/amps necessary to heat nichrome wire. With most NiCr wires requiring ~3-4 amps to cut effectively, it is typically not possible with most batteries.

I do have a few battery sources listed on my website as well. Linked here.  Again, the problem is fixed voltage. You'll only be able to moderate wire temperature by changing the wire length. That may or may not work for you and your design!

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

____

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.


_____
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!