Drill Bits for Aluminum: What the FAA Handbook Specifies

The cobalt question

What the Handbook says. "Cobalt drill bits are excellent for drilling titanium or stainless steel, but do not produce a quality hole in aluminum alloys." It adds how to tell them apart: cobalt bits have "thicker webs and a taper at the end of the drill shank". For aluminum it names HSS with a 135° split point as the standard.

What the listings say. Yardstore's threaded bits are NAS 965 cobalt with a 135° split point, sold in its aircraft sheet metal drill bit category. LAS lists jobber-length cobalt bits with the same point. Yardstore's category also carries HSS bits; we have not yet listed them.

What we can and cannot say. The Handbook's statement is about the type, not about these bits, and no bit here was tested. If your work is mostly aluminum, the Handbook's answer is HSS with a 135° split point; ask the seller for it in the size and length you need. If you also drill stainless or titanium, that is where cobalt earns its place.

Length, shank and size

Short or jobber. HSS bits "come in short shank or standard length, sometimes called jobbers length". Short bits fit tight spaces; Yardstore calls its 1" and 1-1/4" threaded bits short.

Threaded shank for angle drills. "Most right angle drill motors use threaded drill bits". The listed ones are 1/4"-28; check the thread against your drill before buying.

Extensions, with care. Extension bits run 6 to 12 inches. The Handbook's rules: use the shortest that does the job, check it is straight, and support any under 1/4" with a guard of tubing or spring to stop it whipping.

Size by the chart, not by eye. Bits come in number, letter and fractional series. Rivet holes are number sizes: #40, #30, #21 and #11 for 3/32" to 3/16" rivets, on the drill chart. A bent or wobbling bit makes oversize holes; the Handbook says not to use one.

What goes with the bit

Speed. The Handbook gives 200 to 300 surface feet per minute for aluminum and 30 to 50 for mild steel, and the formula rpm = (CS × 4) / D. A 1/8" bit at 300 sfm works out to 9,600 rpm. It also says most drill motors for sheet metal work are rated at 3,000 rpm. The formula sits in its drill press section; the Handbook does not reconcile the two figures, and neither do we.

Lubricant. "Normal drilling of sheet material does not require lubrication"; deeper drilling does. For aluminum, titanium and corrosion-resistant steel the Handbook names a cetyl alcohol based lubricant as the most satisfactory. Yardstore lists Boelube; its listing, as we recorded it, does not state the base.

Drill stops. "A spring drill stop is a wise investment": it stops the bit going through into the structure behind and keeps the chuck off the skin.

Pilot first on larger holes. For 3/16" and up, pilot with a bit about the width of the final bit's chisel edge, or a No. 40 or No. 30, then enlarge. Too large a pilot dulls the final bit's corners and lips.