Fisher Innovation FI-FoamBurner (2010)

The FI-FoamBurner is a lightweight easy to use hot wire foam cutter. Built from easy to get and assemble parts, its can be made in a afternoon and used for a lifetime. Parts are easy to replace and the power supply system can variate the temperature to cut the desired type of foam.

Overview

During my initial testing of my FI-AUAV project I needed a lightweight airframe that I could fly to test autopilot code. Knowing that 70%+ flights would end in a crash due to runtime code issues, I needed a method to reproduce airframes easily and with little cost. I decided to use high density foam for the airframe which allow for easy customization for experimentation.

Foam had a couple of drawbacks, one being it's not easy to cut thick slices of dense foam with a knife or saw, and when you do use those methods, it’s very messy. Professionals use hot wires to cut foam fast and precise, so I figured the easiest way to cut foam at home was to emulate the professionals.

The first step was getting a spool of nichrome wire. Nichrome wire is a electrically resistive material made up of nickel and chrome used in heating elements & hair dryers. At first I was unsure how much electricity to apply to the wire to achieve the proper temperature to cut foam. So I decided to get my temperature sensor from my reflow solder plate project (See FI-ATCSRP Project page), I attached the glass braided wire to the nichrome wire and attached the nichrome wire to a variable DC power supply.

After a hour or so of testing with the variable DC power supply, I decided to switch to a AC power supply. Curious to see the raw power of 120VAC 60Hz wall plug power, I connected a 12 inch piece of wire only to watch the wire almost evaporate in less then a second. After some math and some final tests, I decided to use a variable AC door bell transformer. The transformer I found would output 120VAC 60Hz electricity to either 8VAC, 16VAC or 24VAC.

One interesting fact I found with the nichrome wire was that the heat generated by the wire is only done between the points of VCC and GND. Thus you can use a larger piece of wire then needed and not worry about insulating the heat from the frame that holds the wire. With that new found fact in mind I started working on the frame to hold the wire.

In the beginning I had a couple goals in mind for this project. The most important to me was that the tool in its final state would need to be durable and simple. Durable so that I could use it for a long time and simple so that when there was an issue, it could be fixed fast and cheap.

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Email: fisher.matt at gmail.com

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