Hotwire Bow - Parts list and build instructions

This article will walk the reader through the process of assembling a simple hotwire foam cutter similar to the ones used during Joshua Orchard's "Hotwire Foam Cutting 101 and 102" class at Flite Fest 2016.   

Required Parts

This simple hotwire bow was designed to be easily constructed from inexpensive materials with no soldering required.  The bow and power supply together cost less than $100 and the design can be easily modified to fit a wide range of sizes.   

Attached is a full parts shopping list including Home Depot part numbers.  You can find similar parts at any large home improvement store.  

Parts Shopping List - Excel

Parts laid out

Tools

The following tools will be required to assemble your hotwire bow:

• Saw
• Ruler/measuring tape
• Drill (Drill press preferred, hand drill OK)
• Drill bits (11/64th, 1/4, 1/2)
• Plyers
• Wire Cutters
• Scissors/knife
• Screwdriver (philips)

Assembly 

• Assembly begins by cutting your 48" board into 2 12" sections and 1 24" section

• On the 12" sections measure 1" from the end and mark the center of the thin side (see Picture).  Using a drill put a hole through the arm, being careful to keep the hole as straight as possible.  A drill press helps with this but it can be done freehand as well.  For the size of the suggested hardware I used an 11/64th" bit.

• Using the #8 -32 x2" Machine Screws and the matching nuts secure the machine screws to the arms.  The extended portion of the screw will go on the OUTSIDE of the arm.

• Using a file, saw, or razorblade carefully score a channel into the end of each arm closest to the machine screw.  This will serve as a guide for the wire and keep it from shifting while cutting.

• On each arm, on the side opposite the machine screw, measure a line 3" from the end.

• On the main cross bar (24" long section) mark a spot 3/4" in from each end on the centerline of the piece.  If you're using different sized lumber you will need to adjust this as needed.
• Using the 3" line on each of the arms as a reference, line up the arms with the cross bar section.  The cross bar should line up with 3" above the bar and the longer section below.
• Using a 1/4" drill bit drill a hole straight through each arm, going all the way through the cross bar.  A drill press helps to keep these aligned but is not necessary.

• Using a larger drill bit drill a hole in the upper portion of each arm (away from the machine screw and well above the pivot point, See above).  I used a 1/2" bit.

• Assemble the arms to the cross bar with the 1/4" bolts, 1/4" washers, and 1/4" wing nuts.

• On each arm add 2 #8 washers and a #8-32 wing nut to the machine screw.  This will serve as the attachment point for the cutting wire and will make it easy to replace a worn out or broken wire.

• Using your paracord, tie the cord off and wrap it between the holes in the upper arms several times.  This should be relatively loose as tension will be added later.

• Cut a section of nichrome wire longer than the desired bow length.  For this bow I'm using about 36" of wire.
• Loosen one of the wing nuts on the lower arm and wrap the wire several times between the two washers.
• Repeat this on the other arm, ensuring that the wire runs through the grove you cut on the bottom of the arm.
• Cut a 7-8" section of dowel and insert it between the paracord lines.  Twisting slowly you will feel tension come onto the bow.  

It's Electric!

The frame for your bow is now complete and it is time to move on to running the power wires.  If you're building with the suggested parts no soldering is required.  If you want a stronger finished product you can solder up each end of the connection, but it is not necessary.

• Determine the length of wires you will need.  This will depend on your workspace, the location of your power supply, and your personal preference.  My recommendation is to give yourself a few extra feet... You can always make it shorter if the longer cables get in the way.
• Cut your wires to the desired length and strip off about 1/4" at each end.
• The suggested banana plugs and alligator clips attach to the stripped wire easily with a set screw.  It is so easy I neglected to take pictures while assembling them!
• I opted to attach the wires to the bow using small zip ties.  Small holes were drilled into the frame and the wires were zip-tied in place
• The alligator clips will clamp onto the wing nuts on the arms when you're ready to use the bow, and the banana plugs will connect to your power supply.
  

  

  Done!

Using the Bow

• Before using the bow you should refer to the instructions for your particular power supply.  If you're using the charger suggested in the parts list you'll power up the charger and select the appropriate mode for powering a hotwire cutter.  Other power supplies will be configured differently but the banana plugs used are a standard fit for most options.
• You may need to adjust the volts/amps to the bow to achieve a smooth cut without overheating the wire.  A few test cuts should be made at various power levels to determine the best combination.
• ALWAYS have adequate ventilation and use caution when using a hotwire bow.  Safe use and cutting techniques will be covered in Joshua Orchard's "Hotwire Foam Cutting 101 & 102" classes.  I plan to update this article with a link to these presentations as soon as one is available.