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

Ok Littlera, this is a great idea. How much effort was needed to turn the pipe through the foam wing? Was this done by hand, or drill motor? I liked your use of an arrow shaft to help direct the coring tool to the servo location.

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Richard Scherer

Yes, this is a simple hand tool, and is turned by hand. You twist it as you might a screw driver or wrench. I kept turning in one direction, about 1/4 turn at a time. It probably would work as well twisting it back and forth, but I did not try it that way. Just apply gentle pressure into the hole as you turn, letting the tool progress at its own rate. After about 4 inches, you feel resistance as the tool plugs. Withdraw, knock out the impacted foam, reinsert into the hole and go again. It took only 3 or 4 minutes to drill the 13.25 inches I needed in my case.

Be diligent on the alignment for the first 6 inches or so. After that, the didrection is pretty much "set" and "steering" the bore becomes harder. Not that different than the rules for drilling with a hand drill.

I suppose you could rig the tool up to use a drill, but it doesn't make much sense to do so. You would probably tend to drill too fast, and could overheat the foam, causing melting and a big mess.

Regards,

Randy L.

Great idea and the pictures are woth a thousand words.

Bruce

Ouch!!!!! Fantastic idea!

Something else to try. I use an old piece of aluminum wing joiner tube for bigger stuff. Trick with it is not to put teeth on it. Sharpen the inside bore with a drill bit.
Makes a nearly razor sharp edge and will slice through foam like the proverbial hot knife through butter.

For smaller holes, a piece of CF tubing, same sharpening of the inside bore, but use a razor knife instead.


On the smaller tube I can do it by hand, chuck it in a hand drill, or I've even used a sharpened CF tube in a drill press to install the rods in foam birds for strength.
Use the drill on slowest speed, and walk the bird into the rod, get the length needed installed and trim off the excess.

I can do a 24 inch long 3/8" CF tube in about 5 seconds. Even got good enough at it I can run a 1/32" CF rod in 1/16" foam for strength in micro birds.

Oh one other thing with the sharpened aluminum tubes. Heat em up a bit and they make a perfect circle in covering. No ragged edges, covering stays tight, and no corners to tear.

Put your pipe in before you foam it but put oil all over the pipe and after the foam sets it should pull out easy.