Hotwiring Foam

Hot Wiring Tips
My hot wiring experience went very well, thanks in large part to John Slade'sassistance. We discovered some tips and tricks that I'd like to pass along toother builders. Our cores looked almost laser-cut when they were done, usingthe techniques described below.

Saw Construction
I've seen a number of methods of constructing a hot-wire saw. Ours cost less than$8, and was very fast and simple to assemble:

1. Cut 3/4" electrical conduit to 18" (small saw) or 24" (large saw) lengths.
2. Cut a 2x4 to 46" (small saw) or 64" (large saw). Bigger is not better. 64" will handle the biggest cuts you need to make, and you can stand close to the templates for better control.
3. Use an 11/16" spade bit to drill holes in each end of the 2x4 at a 15-degree outward angle.
4. Pound the conduit into the holes with a hammer. It'll be tight, but this friction gives good tension for the wire.
5. Drill small holes in the ends of the conduit for the hot wire.
6. Install the wire and wrap it around itself to hold it in place. I bought 40' of 26ga nichrome wire on eBay for $5.
7. Tighten by turning the conduit with a pair of Channel-Lock pliers.
8. Install conduit couplers at the base of the saw to provide an attach point for the cord. Alternatively, drill the conduit and install a self-tapping screw.
9. Use a 12' extension cord with its end cut off for the power cord.

Constructing the saw took only a few minutes. We used conduit joints to hook up the electrical wires - see picture 2 above. A pair of Channel-Lock pliers made an easy tightener - you just turn one arm a bit, until the wire "sings".

Cutting Guides
The plans include a set of straight edges, but John and I discovered two much moreuseful options. The first is the corner of a small sheet of melamine/formica. Youcan get this in 12x24 pieces cheaply, and the corners are already perfect 90-degreeangles. Each piece gives you four triangles that can act as squares. If your tableis flat, you can very quickly set up a perfectly vertical cut when joining blocks.

For longer, or angled cuts, we used two drywall squares. We drilled out the rivets,and separated the two pieces. We then drilled holes about every 6" for nails. Thesemade great short and long straight edges, and we've since found dozens of uses forboth lengths in other chapters of the project.

It helps to make a few cutting tools/guides. Here you can see a pair of triangles cut out of the corners of a piece of melamine, with a factory-cut 90-degree corner. Verify your corners - not all sheets are exactly 90 degrees! With a pair like this you can make perpendicular cuts in just a few moments - set the block flat on your bench, and the triangles will automatically give you a vertical cut! In the background of the second picture you can see two aluminum straight edges, made by cutting the cross piece off a drywall square. The inch markers are handy both for measuring and as hotwire talking points.

Lessons Learned
We learned a number of things that make life easier while cutting:

1. Don't leave the saw on between cuts. This fatigues the wire more quickly.
2. Tighten the wire for every cut, AFTER turning the saw on.
3. Replace the wire occasionally because it will fatigue. Two replacements over the course of cutting all of the cores would be a good guideline for nichrome.
4. Split a mixing stick about an inch down, and once the saw is hot, run it down the wire. This cleans the wire much more effectively and quickly than sandpaper, with no damage to the wire. It is very important to have a clean wire - the quality of the cuts is much higher.
5. If you come off the template and get a bump, don't spline-sand it off. This takes more time than it's worth. Hit it quickly with the hot-wire saw to define the shape, then a bit of sanding will take off the residue left behind. It's more accurate, and much faster.

Half a popscicle stick, carefully split for about an inch, makes a great wire cleaner. With the wire hot, slide the split end over the wire, and run it back and forth a few times. All of the melted foam will get scraped off by the stick, without harming the wire as sandpaper would.

Hot-Wiring Micro
Yes, you can! Wicks had a fire sale on foam blocks that I jumped on. They weren't thehighest quality, and they weren't the large size - they were 7x14x41. We had topiece them together to make the wings, and in doing so ended up with a micro jointbefore we had made all of the cuts (in the aileron torque tube hole area). Thinkthis is a problem for a hot-wire saw? Think again. Our cuts were perfect - circular,and went right through. You can't do this if the micro is fully cured; ours was onlya day old. I'm not recommending this, just saying if you happen to have a cut youforgot to make, and now have to make it through micro, don't panic - give it a shot,and you may be pleasantly surprised.

Stopping Midway
Finally, a well-known trick is to put popscicle sticks across the spar capdepressions and cut those in a second pass. The idea here is that the wire willbe lagging in the center of the cut, and even a two second pause may not beenough to let it catch up. Cutting a curved trough is not good, so a secondpass ensures they will be perfect.

We took this a step further. We started at the leading edge, cut to the centerof the popsicle stick, paused two seconds, then lifted the saw out. We then cleanedand tightened the saw, and cut the other side by starting at the trailing edge. Theidea here is that the start of the cut is always the best part - you have thetightest possible wire, no lag, and the wire is hot so it makes its best cut.

It's not uncommon to see cuts where the leading edge looks great, but there isa curve in the center of the trailing edge that needs to be spline-sanded out dueto the wire lag. By making the cut in two passes, from the edges to the center, youdon't see this problem. We used the same technique in the spar cap trough, and theylooked perfect. This technique also gives you a chance to rest your armshalfway through the cut, shift your feet to a new position, etc.