Wednesday, April 17, 2013

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.



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. 

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. 

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