Wide Format Hot Wire

A couple of years ago my son was on the Clemson Crew and I

got this wild idea to build him a rowing scull out of foam board and fiberglass. My design called for a double V with a flat channel between to direct water under and a self-bailing aft. It also had a sliding rig as opposed to a sliding seat…both the boat and rig might be another Instructable someday. For this one it is about the tool I needed. The design called for some wide flowing shapes in the foam board. In an early attempt I used my “hobby” sized hot wire which is about 6” wide to try and shape the foam pieces and then glue and stack them. This turned out to be a disaster, for many more reasons than the uneven shapes I ended up with. In my second attempt, I decided to glue and stack my blanks first and then sculpt the form from a large block. My little hot wire was not going to hack it. I needed something more in the range of 30”. I had seen some DIY hot wire plans, but most of them were still too small. Getting the right wire and the power source seemed to be the biggest challenge. So here is what I came up with. Really, really simple to make and it worked perfectly, cutting wide swaths of foam in nice easy slices.

Materials needed

· Three pieces of scrap ¾” plywood or pine, or whatever you have lying around…

o One (1) piece about 3” wide x 20-30” long. (both of these measurements are approximate depending on how big of a cutter you need.)

· Two (2) pieces about 1” wide and about 15-18” long. (again, adjust the measurements to your particular need.)

· 4 each 2” ¼-20 bolts with washers and nuts.

· 1 metal high “G” string from a guitar.

· 3 drywall screws

· 1 medium length Bungee Cord

· 12 Volt car battery charger for a power source.

Assembly:

1. Drill a ¼” hole centered about an inch in from each end of the wide board.

2. Drill ¼” holes in the ends of the narrow boards centered about an inch in.

3. Drill ¼” holes in the middle of the narrow boards.

4. Loosely bolt the two narrow piece center holes to the wider board.

5. Insert bolts into the end of the narrow boards.

6. Wrap the guitar string around the bolt several times and tighten in place with a nut and washer.

7. Decide how wide a cut you need to make, and wrap the guitar string around the other bolt and tighten with a nut and bolt.

8. Next. Partially drive the drywall screws into the opposite ends of the narrow boards and one in the middle of the wide board.

9. Use these to warp and hook the Bungee cord to hold the guitar string taut.

10. You are done, ready to cut some foam!

Cutting:

· Clip the alligator clips of the charger to each bolt at the ends of the guitar string.

· Turn on the charger to 12 volt option.

· The guitar string should heat up within a few seconds.

· As soon as the foam starts to melt, you will need to move the wire through the foam, not too fast but also not too slow in a nice smooth motion. Do some test cuts to get the feel. I was using 2” insulation panels I bought from Home Depot. They are pretty dense but still cut easily. Where I used the foam adhesive to glue the pieces together, the hot wire still cut through but slower, so I had some chatter where the foam was melting before the glue. If you can plan ahead and not glue where you are going to cut or better yet clamp you pieces together and cut before you glue, the cutting will go smoother. I wasn’t that smart.

So the Hot Wire tool worked better than I expected! The boat?

Well it is under my house, never been tested. By the time I got it done, my son had graduated and moved on, so until I find someone with some sculling oars to lend me…