Garage Engineering Episode 3: Pocket Rib Drilling, Punching Holes

Source: 2022 Q3 Beartracks, Mark Johnson
I was recently having a tough time drilling my Patrol’s pocket ribs to the rear spar, and came up with a method that works really well for me, with better ergonomics, no air tools or hoses, and superior results. Hopefully this is helpful to you too, with pocket ribs or other similar situations.
My Patrol wings are framed vertically in the garage, side by side, leading edges toward the ceiling. The rear spars are 2 feet off the floor with pocket rib attach angles installed on the spars. The angles are predrilled to #40 on the rib side. I fabricated and primed the pocket ribs a while ago, but held off on drilling them until ready to install on assembly.
My original plan was to match drill the rib through the angle. This was not going well. Even with an angle drill or long flexible bit, clearance was an issue and the holes in the angle were often ovalized, needing to be drilled out to #30. (Starting with a small pilot hole would have been smarter).
Reaching up to drill while lying on the floor was awkward and not helping. I reconsidered my approach and the idea came to punch the holes instead of drilling. The hole punch (sometimes called a “Roper Whitney punch”, after one of the manufacturers of them) is a great old tool that I don’t need too often but when I do it’s priceless and outperforms a drill. I first marked the hole centers by hand twisting a #40 bit through the angle a few turns. Then to make the divot more convincing I tapped it lightly on the bench with a pin. Fitted with a 3/32 die, the hole punch easily settled in the divot and knocked out a precise, round hole. With a minimal drilling out to a #40 and deburring, she was ready to install. As you can see in the picture with the rib clecoed, a #40 drill bit slides right through the adjacent holes. The alignment was spot on, and the angles are fine with no damage to the original holes. The picture below shows all the tools I used, and the angle drill I ended up not using at all. I’m still debating how and when to rivet these. I welcome any hints!


Drilling Holes – General Introduction

It’s best to pre-drill all holes slightly undersize at first, then drill the hole to full size. This tends to make more round holes, as the final sized bit is not removing as much metal. For thin sheets of aluminum, you want a fast air drill. For steel, you need a slower turning drill, with lots of torque, so an electric drill is better. A drill press facilitates bit angular alignment, so use a drill press whenever possible. When drilling holes larger than 3/16” or so in thin sheets, consider using a step drill. These are more likely to drill a round hole than a traditional twist drill, and if you aren’t using the largest size step, you can use the same bit to debur the back of the hole.