Recently, my neighbor Paul asked me if I had a good way to drill nuts for safety wiring. The landing gear on his F1 Rocket project is fixed with large hex nuts and he wanted to safety wire them.
DIY drilling of bolts for safety wire is made simple by many aftermarket jigs. Since the hole is usually drilled perpendicular to the face (re: hex bolt) or square to the tangent (re: socket head cap screw), drilling is no sweat. But drilling nuts for safety wire is another story. You can’t drill them square to the face (well, I guess you could, but … ). Nuts have to be drilled diagonally from flat-to-flat. This is a challenge because whenever you’re drilling other than square, twist drills tend to deflect. Even when drilling square, smaller diameter twist drills tend to “walk” off-axis and deflect. That’s why center-punching or spot drilling is considered essential for precise layout.
One way to drill a nut for safety wire is to start by pecking, or dimpling, with the face square. Then, progressively angling the nut and re-pecking the same spot. Eventually the dimple will get angled enough to drill the desired hole without deflecting. I watched a video on YouTube of a guy doing this and it worked, but he was drilling a much larger nut and using a much larger drill bit than anyone would use for aircraft safety wire.
Even so, we gave that method a try and found that the 0.050-inch-diameter twist drill was much too flexible for anything other than dead square. I suppose with enough patience, we could’ve got the “peck and rotate” system to work. But it seemed like a lot of fiddling with no guarantee of a good outcome.
Plan B was to use a miniature end mill (0.098 inch [2.5 mm] diameter with 6 mm shank, available on Amazon) and my milling machine to plunge cut a shallow pocket (aka spot face) and then drill the through-hole with the 0.050 twist drill.
The setup was simple: With the entry and exit points marked across the corner of the hex, the nut was positioned in the mill vise and the cutter lined up. Once the pocket was milled, the end mill was swapped for the 0.050 drill bit. A shot of WD-40 was used as cutting fluid.
There you have it. Questions or comments are always welcome.
(Note: Advisory Circular 43.13-1B, chapter 7, sections 3 and 4, cover the types of and uses for aircraft bolts and nuts but offer no guidance for DIY drilling for safety wire.)
Another reason I need a milling machine! Probably could do this with a drill press, but I want a milling machine! One of these days. Thanks, Bob. Always enjoy your content.
You can buy a milling machine for a few thousand dollars, or if you just need to drill a few nuts, you could buy the $40 drill jig from Pegasus Auto to accomplish the same task. I would love to have a milling machine, or better yet a friend with a milling machine, but don’t have the money to invest in an expensive new tool.
Thanks John. The Pegasus drill jig part number is 3750-001 and “allows use on virtually
any hex-head fastener with up to a 1” hex…” (per catalog page 120).
It is definitely worth looking into if you need to drill more than one or two nuts for safety wire.
In any case, the hex nut on Paul’s F1 Rocket landing gear is larger than 1″, so we still would have had to find an alternative solution.
As a machinist myself, never underestimate the power of the machinist’s favorite brand of beer to get such government jobs accomplished after hours.