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new bevel cut method: hot wire

utnuc

New member
#1
I'm getting ready to start my third FT plane build, and I have to say the most difficult part of the process is getting the bevel cuts right. Bixler makes it look so easy to get those perfect freehand 45 degree bevel cuts with a sharp razor blade. My experience was that I'd dangerously hack away at the foam, ruining the part I'd just spent 30 minutes cutting out.

So I reverted to the sanding method. It's pretty effective, but has some definite cons
  • leaves a very ratty paper edge that tends to peel back from the foam
  • it's easy to sand all the way through the foam, requiring tape reinforcement on the back side
  • it's really messy, leaving foam powder everywhere
  • it takes a long time to get right, if you leave a section too thick the entire control surface will have a limited range
There is a better way. I searched the forums and couldn't find anything about hot wire bevel cuts, so I thought I'd build a simple handheld foam cutter and give it a try. There are a million tutorials on youtube on how to build a foam cutter, but it's just a simple circuit with a piece of steel wire (usually nichrome) and a 12v power supply that heats up and slices through foam like butter. Most everyone here already has a power supply and some high current batteries, so really you just need something to hold the wire taught. The nice thing is that if you can find the sweet spot of the temperature range, the wire will cut foam but not paper. Here's a large format cutter that will show you the basics. For this one all material here can be picked up at the hardware store:


Since I didn't need a large cutter, I decided to build a small handheld version. Again, just find some youtube tutorials on how to build one. I used an old plastic clamp like this one, a couple of nuts and bolts with washers, a single strand of steel wire from some cable I had laying around, some 14g speaker wire and a xt-60 connector. The key to a straight cutting wire is to keep tension on it, since it expands as it heats up. With my 12v/12a power supply, the wire will turn red hot in seconds. Nice thing about this setup is that I can use a lipo battery too. Here's a picture of the somewhat janky appearing handheld foam cutter.

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So to make a 45 degree bevel cut using the hot wire:
  • Eyeball 5mm away from the hinge, place a ruler there parallel to the hinge and run your knife just barely through the paper
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  • Remove the thin strip of paper
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  • Turn on the foam cutter
  • Holding the hot wire so it lines up with the paper on either side of the bevel cut, slowly drag it through

After practicing with it for 5 minutes I'm getting perfect bevel cuts. One note, when you first put the wire to foam it'll be hot enough to burn paper, so it's useful to have a scrap piece to touch it to before you start your cut. Once it's going through the foam you can keep enough pressure on it to touch both pieces of paper without it burning either one. Here's the final result:
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Arcfyre

Well-known member
#2
This is pretty cool. You got me thinking now if I want to try making that foam cutter to cut out airplane parts. I wonder if it would be faster or cleaner than cutting them by hand with an exactly knife.
 

Bricks

Well-known member
#3
This is pretty cool. You got me thinking now if I want to try making that foam cutter to cut out airplane parts. I wonder if it would be faster or cleaner than cutting them by hand with an exactly knife.
utnuc mentioned it will not cut the paper so I believe it would not work to cut parts out.
 

Matagami Designs

Well-known member
#5
Would something like this work on a LiPo battery and a pot? I think something like that could be a handy tool. Normally I just use an exacto but I definitely dont get the best cut every time.
 

Merv

Well-known member
#6
Great idea, I think a utility knife blade is still faster, easier and with less overhead.

I have found that using a utility knife holder give me far more control over the blade. The secret, besides a sharp blade, as you hold the knife, let one finger go under the foam. You will be able to feel the change far more accurately than you can see it. Control the depth of cut by twisting the blade. Don't try to get perfect with the blade, just get close. Then just a few strokes with a sanding block will remove any imperfections. I use 60 grit for fast removal and 120 grit to finish.
 

utnuc

New member
#7
Great idea, I think a utility knife blade is still faster, easier and with less overhead.

I have found that using a utility knife holder give me far more control over the blade. The secret, besides a sharp blade, as you hold the knife, let one finger go under the foam. You will be able to feel the change far more accurately than you can see it. Control the depth of cut by twisting the blade. Don't try to get perfect with the blade, just get close. Then just a few strokes with a sanding block will remove any imperfections. I use 60 grit for fast removal and 120 grit to finish.
Nice tips. From this novice's perspective, the utility knife blade method is most definitely slower and more difficult to get right. The 30 minutes of building the hot wire cutter was well worth it IMHO.
 

utnuc

New member
#9
You have a great system, that is what this forum is all about. The free exchange of ideas. Thanks for sharing
Thanks Merv, I wasn't at all offended and didn't mean to offend. My novice opinion is worth as much as... a novice opinion. 🙂
 

utnuc

New member
#11
Would something like this work on a LiPo battery and a pot? I think something like that could be a handy tool. Normally I just use an exacto but I definitely dont get the best cut every time.
@Matagami Designs I did some testing with a Tattu 3s 75C LiPo hooked up directly, thinking that since the voltage was the same that it would be similar temps. Big mistake. The steel wire immediately turned red hot and melted. My guess is that it delivered 40-50 amps immediately. Definitely need a pot to regulate it. Just hope I didn't damage the lipo... Anyways, I replaced the wire with a new strand and it's working well using my regulated power supply at 12A.
 
#12
@Matagami Designs I did some testing with a Tattu 3s 75C LiPo hooked up directly, thinking that since the voltage was the same that it would be similar temps. Big mistake. The steel wire immediately turned red hot and melted. My guess is that it delivered 40-50 amps immediately. Definitely need a pot to regulate it. Just hope I didn't damage the lipo... Anyways, I replaced the wire with a new strand and it's working well using my regulated power supply at 12A.
I doubt you would have damaged the LiPo. I think all that happened was you made a short circuit that burned like a fuse from pulling to many amps as you mentioned. I'm guessing if you were to only use a battery you would need a lower C rating but i'm not sure how low it would have to be. Is your power supply actually giving 12 amps for this? I'm assuming its lower.

Another way of doing these bevels i have seen is by just using an iron on the edge held at a 45° angle. If you keep it moving it folds the paper down into the seam. I just need to figure out where i put mine.
 
#14

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HilldaFlyer

Well-known member
#15
Thanks for sharing this technique. I've been doing it for years but haven't shared it... so thanks! It is by far the fastest, most reliable methods do get a bevel in the foam.

Here is my rig...
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Details of the power supply can be found in the article: HOT WIRE FOAM SHEET CUTTING. For those not wanting to build their own power supply, I note that most LiPo battery chargers now days have a hot wire setting.