One Hole, Many Bits

How many bits does it take to drill one hole? Well, if it’s an important hole, it takes as many as it takes—and that can be quite a few! Let’s start from the end, and work backward ….

This is the rear spar attach bolt on our F1 Rocket project—but it is the same kind of attachment you’ll find on most RVs, at least the short-wing RVs. This has to be drilled “in assembly” once you have set the wing’s incidence and sweep. The bolt (in this case) is AN5, or nominally 5/16 inch. That means that you need an edge distance of 5/8 inch to make sure that under ultimate load, you don’t pull it out. The box drawn in blue ink shows where the center of the hole must fall—and there’s not a lot of margin. You can see in this “finished” (not yet safetied) photo that the center of the hole is as far outboard as allowed (to give clearance from the fuselage for the drill motor, and right in the middle of the top and bottom lines).

There is the hole—a nice, smooth, and square (to the surface) hole, reamed with a 5/16-inch reamer to give a nice close fit for the bolt. But we didn’t start out with a 5/16-inch bit—that wouldn’t have been prudent—the hole would have ended up triangular, and probably not square or in the right spot. We worked our way up from a tiny pilot hole.

And this is what it takes to do the job—along with time and patience! No rushing allowed—we spent a day checking incidence and sweep several times, then walking away. Then we let it sit overnight, while our mind went over the process, measured once more, clamped the pieces together, then began drilling—first with a #40, then a #30, then a #20, #10, 1/4 inch … and then through the lettered drill set until we were just shy of 5/16 inch (that was an “N” bit). Then we chucked up the reamer and finished the hole. You’ll also see our steel drill block that we used for the initial holes to make sure we’re drilling “square” to the workpiece. This is an important hole, and you only have one shot at it—so take your time and get it right!