100 Degree Countersinks for Aerospace Fasteners

Why the 6-flute Chatterless design matters

A single-flute or three-flute countersink cuts intermittently: each edge enters, the tool deflects, the edge leaves, and the tool springs back. On a drill press or a hand drill that rebound becomes the faceted, chatter-marked seat every machinist has seen. Resonant vibration is the cause, and a tool with symmetrical cutting edges reinforces it.

Severance Chatterless Countersinks™ use a patented six-flute pattern with unequal spacing. At least two edges are in the cut at any moment, so the load-and-unload cycle behind chatter never starts, and the staggered spacing breaks up the harmonic. Each flute takes a smaller chip, which lowers radial load, spreads wear over six edges, and stops the grab in soft stock. The result is a smooth, burr-free 100° seat in interrupted cuts and difficult materials, at higher feed rates than a conventional countersink allows.

HSS or solid carbide

Both 6-flute series are made in 100°. The HSS tools are premium M2 high speed steel at Rc 63 to 65: tough enough for a hand drill or a drill press with quill play, easy to regrind on standard wheels, and the economical choice for low-carbon steel, nonferrous work, and any material under Rc 45.

The solid carbide tools are sintered by Severance in a tungsten-cobalt grade of 91.7 to 92.2 Rockwell A. Carbide runs at about twice the surface speed, often gives ten or more times the service life in hard or abrasive materials, and pays for itself on a rigid CNC where a tool change costs more than the tool. The catalog rule of thumb is carbide for production runs in cast iron, alloy steel, or glass-reinforced plastics. Carbide is brittle, so keep it off hand-held drills. The HSS vs carbide guide works through cost per hole.

Speeds and feeds

Aircraft skin is mostly aluminum, which wants more speed than steel or the edge rubs and smears. From the speed and feed page, a 1/2″ body HSS countersink runs 1,200 to 1,800 RPM in aluminum and 600 to 1,200 in mild steel; carbide roughly doubles those. On CNC feed 0.002 to 0.005″ per revolution in aluminum. A wax stick or light oil on the tool stops chips welding to the edge; the aluminum countersinking guide has the full table.

Depth control

In flush-riveted skin the difference between flush and proud is a few thousandths, and depth by feel wanders 0.005″ or more between holes. Severance developed its Chatter-Free Stop-Countersinks and the patented micrometer-style stop holder for the aircraft industry: a 100° cutter threads into a Precision Stop Countersink Unit whose micrometer collar sets depth in 0.001″ steps and locks it, on a drill press or a hand power tool. Drill the pilot hole to size first, clamp the part, and set depth on scrap of the same alloy.

Regrinding

Every tool on this page is resharpenable. Severance regrinds its own countersinks in HSS and carbide to the factory 100° geometry, with a standard 1-3 week turnaround and rush service available. Regrinding typically costs 25 to 40 percent of a new tool, and most tools can be reground 2 to 4 times. See the regrind service page for how to ship tools.