Center-Drill Large Holes


Ever need to match drill a hole for an AN3 bolt? If you use a #12 bit, there’s always a possibility the bit will start slightly off center and enlarge the parent hole in the process. The result is an egg-shaped hole through both parts.

The solution is to buy three sections of straight brass tubing. You can find them in the aviation department at your local hardware store. The smallest is 1/8 inch in diameter, and the largest is 3/16 inch in diameter. The three pieces should slide inside each other.

Brass tubing keeps the #41 pilot drill bit centered in the #12 guide hole.

Cut off a piece of each tube. You want each one progressively longer than the next-larger tube, so they are easy to separate. Cut them long enough to go through the part, but shorter than the drill bit. About 1 inch in length works well.

Apply some Boelube to each tube and slide the three pieces together. Insert into the AN3 bolt hole. Apply some Boelube to the #41 bit and guide the bit into the hole nice and straight. Any off-axis position can be felt. Hold the tube and drill the hole. The result is a centered pilot hole.

Now simply remove the inner tube and drill the next size up (#31). Remove the middle tube and drill (#21). Finally, finish it off with a #12 reamer for a perfectly aligned hole and no damage to the parent hole.

Other size tubes can also be stacked to center-drill larger holes.

Drill bits and brass tubing used to center the hole.


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