90 Degree Countersinks for Metric Fasteners

Why the 6-flute Chatterless design matters

Resonant vibration is the cause of chatter in rotating cutting tools. Every tool, machine and workpiece system has natural frequencies at which that vibration occurs, and a tool with symmetrical cutting edges tends to multiply those frequencies and reinforce the vibration. On a drill press or a hand drill the result is the faceted, chatter-marked seat every machinist has seen.

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 90° seat in interrupted cuts and difficult materials, at higher feed rates than a conventional countersink allows.

HSS or carbide

Both 6-flute series are made in 90°. 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 carbide tools (individually tipped teeth over 1-1/2″ diameter) 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

From the speed and feed page, a 1/2″ body HSS countersink runs about 530 to 760 RPM in mild steel, 1,150 to 1,910 in aluminum, and 230 to 610 in free-machining stainless (110 to 380 in work-hardening grades); carbide runs about 610 to 1,300 in mild steel and 2,290 to 3,820 in aluminum. On CNC feed 0.001 to 0.002″ per tooth, which is 0.006 to 0.012″ per revolution for a six-flute countersink. For other sizes, RPM = SFM × 3.82 ÷ diameter, using the catalog surface speed for your material and tool.

Depth control

Depth by feel on a drill press wanders 0.005″ or more between holes, and a metric flat head sits flush only when the seat depth is right. A Stop-Countersink cutter in a Precision Stop Countersink Unit stops every hole at the same depth: the micrometer collar sets depth in 0.001″ steps and locks it, on a drill press or a hand power tool. The units take only the threaded stop-countersink cutters; on CNC the CNC-K series holds length tightly so a preset repeats.

Regrinding

Every tool on this page is resharpenable. Severance regrinds its own countersinks in HSS and carbide to the factory 90° 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.