This may not be a completely new idea, but it is a good one that works well.

When building foam core wings, it is frequently necessary to bore through the foam after sheeting to allow passage and access to servo wires, pushrods, bellcranks, etc. I am sure the servo wire path is the most needed with todays techniques, as most builders are using one servo per control surface vs. the horrible bellcrank setups of yesteryear.

I constructed a simple foam boring tool to cut my servo wire paths on my most recent build, a (30 year old!) Bridi Lockheed Sirius kit. I bought the kit in 1981 with all intentions of building it that year, but it kept getting moved to the bottom of the pile until this year. Better late than never, I guess. I immediately decided on one servo in each wing instead of the bellcranks called out.

I bought the material for the tool at Home Depot. It was a 5' length of electrical conduit pipe, and has a thin wall, which was important. This stuff is REALLY cheap - $1.30 for the 5' stick. I cut it in half and threw the remainder away. The ID of the tube is 5/8" and the wall is about .040". The only down side of this material is that it has an electrical insulation paint on the inside, which causes the foam to not slide into the bore real easily. I could have got a copper plumbing pipe, but decided not to as it was almost $7 for a 24" piece. The 5/8" bore is plenty big to pass servo connectors through.

In the photos, you can see that I cut rudimentary saw teeth onto the business end of the tool with my Dremel. At first, I bent some of the teeth inward, thinking this would produce a larger kerf, keeping the foam from bunching up inside the tool as I cut. It really did not help, and I ended up straightening the teeth. I also marked a scale on the side of the tool so I could observe the penetration.

I found that I can cut about 4" deep before the tool clogs. Simply withdraw the tool, punch out the impacted foam with a stick, and go back to work. The tool produced absolutely straight, smooth wall tunnels in my foam cores, 14" long in this case.

In one of the photos you will see the arrow shaft taped onto the wing to act as a guide for aiming the tool. This worked really well, and I broke out into the servo boxes within 1/4" of where I was aiming, and in one case (pure luck) within about 1/16". Not bad for 14". The hard part was estimating the vertical distance, and the wing is tapered.

This tool also cuts decent holes in sheets of balsa if I ever need to do it.

Regards,

littlera