USB Foam Cutter (PVC) - Hot Wire

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Introduction: USB Foam Cutter (PVC) - Hot Wire

Cutting PS foam with a knife can be a pain in the ass. Little balls everywhere and no clean cuts. Using a hot-wire to cut the foam is so much easier and effective.

The theory is simple. You drive a current through a metal wire which will heat up and be able to cut the foam.

For this project I will use a USB cable as a supply source. You can use the hot wire with a power bank or you can use a phone charger in a wall socket.

Step 1: What You Need

What I will use to make this hot wire:

- 2 pieces of 25cm PVC (25mm) tube
- 1 piece of 20cm PVC (25mm) tube
- 2 90° PVC connection pieces (25mm)
- 2 PVC close caps (25mm)
- stainless steal cable
- electric cable
- 2 eye bolts
- 6 washers (max 25mm and fits the eye bolts)
- 1 spring (fits the eye bolts)
- (duct) tape

Step 2: Prepare the Caps

First I prepare the caps for assembly.

1. Drill a hole in the top of the caps to fit the eye bolts

2. Use a washer to fit the eye bolts

3. Use a stripping tool to strip the electric cable. Make a loop in the cable and leave enough cable exposed so it will touch the washers is the next step.

4. Place a washer on the eye bolt inside the cap. Put the loop of electric cable around the bolt on top op the washer. Place a second washer on the bolt and tighten everything with a nut. Be sure the exposed cable touches the washers!

Step 3: Assembly

It is time to assemble the pvc pieces!

I drilled an extra hole in one of the 90° pieces to let the cables out of the PVC. I prefer this place over the middle of the tube to prevent struggles with hand placement.

1. Run the cables coming from the caps through the 25cm tubes.

2. Take one cable and run it through a 90° piece and the 20cm piece.

3. Fit both cables through the second corner piece and inside the drilled hole.

4. Secure everything with the tape (or PVC glue)

Tip: make sure everything is on the same plane when securing everything
Tip 2: the eye bolts should face the same direction

Step 4: Cable

Place the spring on one of the eye bolts. The spring will provide tension in the cable while you go through the PS foam.

I bought a stainless steel cable in my local DIY store. The cable consists of multiple woven strings. I am unraveling the strings until I can't get any further. This gives a very strong small cable. At this point the cable is not straight but bended from the woven structure.

Knot one of the ends of the cable to the eye bolt without spring.

Cut the cable at the desirable length.

Now you take the other end of the cable end put it through the spring. Before making the knot you have to pull the cable so the spring will be under tension. Hold the cable and make a knot so the cable and spring are already under tension.

To straighten the cable you can run it against a screwdriver or a bolt until the bends are away.

Step 5: USB Cable

1. Strip the USB cable. The should be 4 cables inside (red,black, green and white). The green and white are data cables, we don't need these. The red and black cables are our supply.

2. Use a soldering iron and solder to attach the USB cables to the cables of the hot wire.

At this point you can test your hot wire before taping everything together.

3. First cover the exposed soldered cables with tape.

4. Now you can tape them together for extra strength.

Step 6: Finish

Now you can use a phone charger or power bank to use your hot wire in your next project!

Happy building!

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    24 Comments

    how long can you run the cutter on a power bank before the power bank is depleted?

    3 replies

    It depends on your battery bank capacity, battery bank quality, and wire gauge.

    20cm of 36 gauge stainless steel wire has a resistance of about 12 ohms, at 5v it would draw about .42A, a high quality battery bank would last about 24 hours.

    20cm of 24 gauge stainless steel wire has a resistance of about .72 ohms, at 5v it would draw about 6.94A, a high quality battery bank would last about 1.5 hours if it could actually supply that much current. A low quallity battery bank would probably catch fire.

    A couple of hours I think, didn't try it until it runs out. Wouldn't recommend it either, only power it while you cut.

    This would be an awesome use of a momentary switch, (on)-off. Either a push-button style, or maybe a paddle, either one mounted on the handle.

    What foam are you cutting? Forgive me if this is a stupid question but I really don't know.

    2 replies

    That's what I thought but I realize sometimes I'm wrong if you can believe that lol.

    I wonder stainless steel cable has very low resistance and perhaps will burn USB supply. Perhaps we should use nichome wire with a total resistance not less than 5 ohms if the used USB power supply can supply not less than 1 Amp. Maybe Nichrome 60 (Chromel C) Gauge 29 which has a resistance of 5.29 ohms / ft, will do the work. Thanks for this interesting and practical project.

    1 reply

    Actually, it seems that stainless steel and nichrome have comparable resistivity: https://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html. Surprises me too. I would have expected stainless steel to be a pretty bad conductor, but not that bad.

    nice project use full

    Would a nichrome wire provide too much resistance and hurt the battery or charger?

    2 replies

    Too much resistance can't hurt the battery or charger. Only too little can - too little is equivalent to a short circuit.

    However, too much resistance might mean too little heat development to cut effectively.

    The higher the resistance the less current draw.

    How is that going to hurt a battery charger?

    A short can if it is not a current limited.

    Looks like a blackview charger to me :p

    I made something similar to cut the foam insulation for my garage. I found that if the current passed through the spring as you have it, the spring would quickly lose it's tension, due to overheating. My solution was to use an aligator clip lead to pass the current around the second spring I had to put in place. YMMV

    I've been working on a project to insulate the joist bays in my crawlspace using foam board and Every time I work on it I end up covered in foam. Great instructable! This is going to save me a TON of time!

    1 reply

    Not only a time-saver, but with careprovides an extremely close [leak free] fit!

    Love this! Maybe if people can easily do more with EPS...less will end up in the landfill.

    very clever. I would never have guessed that a USB charger could provide enough current. Even still the PVC tubing is Genius

    i am so excited about this instructable! ive wanted one of these for a long time, and this one is within my skill set. thaks for writing this up!