A countersink is the cheapest tool in the drawer to get wrong. There are five common included angles, they look nearly identical in the tray, and putting the wrong one in a hole does not break anything—it just leaves a screw head that stands proud, or sinks too deep, or sits on a knife edge instead of a cone. This is the reference: what each angle is for, why countersinks chatter when nothing else does, and the two features on a spec sheet that most buyers skip past.
The angle is a fastener spec, not a preference
The number on a countersink—60°, 82°, 90°, 100°, 120°—is the included angle: the full angle across the cone, tip to tip, not the angle from the centerline. A 90° countersink has 45° of flank on each side.
That number is not a style choice. For anything that receives a flat-head screw, it is dictated by the fastener standard the screw was made to, and the two do not mix:
82° — inch flat-head screws. The angle specified for inch-series flat countersunk heads under ASME B18.6.3. If the screw came out of a bin marked 10-32 or 1/4-20, this is the one.
90° — metric flat-head screws. The angle for metric countersunk heads (ISO 7046, DIN 965). It is also the workhorse for non-fastener jobs: chamfering a hole before tapping, breaking an edge, or opening a mouth so a pin starts straight.
100° — aerospace flush fasteners. The standard flush-head angle in aircraft structure. A shallower cone spreads the head over more skin thickness, which is what lets a flush rivet or screw sit in thin sheet without the knife edge that a steeper angle would leave.
60° — deburring and centre work. Not a fastener angle. The steep cone is for knocking the burr off a drilled hole and for the 60° centre form used between centres.
120° — wide, shallow chamfer. Used to break an edge with very little depth, and to open a hole mouth for weld preparation. In thin material it takes off the burr without eating into the wall.
The failure mode is quiet, which is why it is common. Put an 82° screw into a 90° seat and the head contacts on its outer rim only; the joint looks fine and clamps on a thin ring instead of a cone. Put a 90° screw into an 82° seat and the head bottoms on the cone before its top reaches the surface, so it stands proud and no amount of torque brings it flush — it just yields the head.
Body size sets the depth, and it is not the screw size
The other number on the tool is the body size: the largest diameter the cone reaches. It is not the screw diameter and not the hole diameter. It is the widest seat the tool can cut, and it has to be larger than the finished seat you want, because the depth of cut is what sets the seat diameter.
Practically, you drive a countersink to a diameter, not to a depth. The head sinks flush when the cone has opened to the head diameter of the screw, so that is what you measure or what you set a stop to. A body size comfortably above the head diameter gives you room to sneak up on it; a body size barely above it means the tool is buried to its shoulder by the time the seat is right, and the finish suffers.
On a machine, a shank diameter that matches your standard collet matters more than it sounds. Our HTC countersinks run from 1/8" to 1" body size across the range, and in the double-end tools the body and the shank are the same diameter — the tool is the shank.
Why countersinks chatter when drills do not
Chatter is the defining problem of this tool, and it is geometric rather than a matter of technique. A drill cuts with two lips at a constant diameter. A countersink cuts on a cone, so every point along the edge is at a different diameter and therefore a different cutting speed — zero at the tip, maximum at the rim. Add several flutes engaging and disengaging as the cone widens, and the tool has a natural tendency to bounce between teeth. Once it starts, it cuts a lobed, faceted seat: three, five or six flats instead of a circle, and a flat-head screw sitting on that will never pull down evenly.
Two grinds solve it, and both are in the catalogue:
Single flute through a cross hole. One cutting edge, formed by drilling a hole straight through the cone. With one edge there is nothing to bounce between, so the tool stays in contact instead of skipping. The cross hole doubles as chip clearance — the chip goes through the tool rather than packing the flute.
Multi-flute chatterless grind. The teeth are ground so that contact is continuous rather than intermittent. Six-flute chatterless tools give a better finish than a single flute at the same feed, and hold up better in harder material, at the cost of being fussier about rigidity.
Beyond the grind, the usual rules apply and they are worth repeating because they are the opposite of what instinct says: slow the spindle and keep the feed. A countersink that chatters is nearly always turning too fast and being fed too gently. Rigidity is the other half — a countersink on a long extension in a worn holder will chatter whatever its grind.
The double-end tool, and why it is not just a spare
A double-end countersink carries a cutting end on each end of the body. When the first end loses its edge you turn the tool around and keep going. It is the same purchase, and the second edge is free in the sense that it costs nothing extra to have it there.
The reason this matters more on countersinks than on most tools is that countersinks dull in a specific, annoying way. The tip does almost no work — it is at zero surface speed — while the outer rim does nearly all of it. Wear lands on the part of the edge that defines your seat diameter, so the tool goes out of tolerance long before it feels blunt. Being able to flip it means you notice the problem once per tool rather than replacing it.
We list 120 double-end countersinks, in HSS and solid carbide, at all five angles, in single-flute and six-flute chatterless grinds, from 1/8" to 1/2" body size.
HSS or carbide
Both are stocked, and the choice is less about hardness than about how the tool is held and driven.
High-speed steel takes a keener edge, tolerates a flexing setup, and survives being nudged. In a drill press, a hand-held tool, or anywhere the work is not clamped like a vice, HSS is the right answer — carbide in a setup that moves will chip, and a chipped carbide countersink is scrap.
Solid carbide earns its price in a rigid machine at production rates, and in the materials that eat high-speed steel: hardened stock, stainless that work-hardens under a rubbing edge, abrasive composites. It holds the seat diameter far longer, which on a part with fifty countersunk holes is the whole argument.
Of the countersinks we stock, 513 are HSS and 301 are solid carbide — roughly the split you would expect in a shop that does both jobbing and production work.
Micro stop: when every seat has to match
On a part with a row of flush fasteners, "sneak up on the diameter" is not a method — it is fifty chances to be slightly different. A micro stop countersink solves it mechanically. The cutter screws into a micro stop cage, the cage rides on the surface of the work, and the depth is set by a calibrated adjustment rather than by the operator's judgement. Every seat comes out the same depth regardless of who is holding the tool.
The cutter also runs a pilot — a pin that drops into the drilled hole and locates the cone concentric with it. Many are sold by pilot range rather than a single pilot size, so one cutter covers a span of hole sizes and you change the pilot, not the tool. This is standard practice in aircraft work, which is also why micro stop cutters are usually 100°.
We list 126 micro stop countersinks, identified in the catalogue by body diameter, pilot range and mounting thread — commonly 1/4-28 UNF.
Sets, and when a set is the honest answer
If you countersink occasionally and across whatever sizes walk in the door, a boxed set beats ordering cutters loose: one box covers the common seat sizes at one angle, and you stop discovering at 4pm that the size you need is the one you do not have. We list 33 countersink sets, some sold as a fixed list of body sizes and some as a range.
If you countersink the same hole a thousand times, buy the single size in carbide and a spare. The set is insurance against variety, not against wear.
A short checklist
- Match the angle to the fastener standard, not to what is in the drawer. Inch flat head, 82°. Metric, 90°. Aircraft flush, 100°. Deburring or centres, 60°. Light edge break or weld prep, 120°.
- Pick a body size above the finished seat diameter, with room to approach it.
- Cut to a diameter, not to a depth — unless you have a micro stop cage, in which case set the cage and stop thinking about it.
- If it chatters, slow down and keep feeding, and check the tool is not hanging out further than it needs to.
- For a run of holes that must match, use a micro stop or a stop collar. Judgement does not repeat; a stop does.
- HSS for flexing setups, carbide for rigid ones and for hard or work-hardening material.
What we stock
FM Carbide lists 849 HTC countersinks, all made in Michigan. Across the range: 214 at 82°, 215 at 90°, 184 at 60°, 136 at 100° and 100 at 120°; 463 in chatterless grinds, 126 micro stop cutters, 120 double-end tools and 33 sets. Every listing carries body size, shank diameter, overall length, included angle, flute count and material on the page, so you can choose without opening a catalogue PDF.
Browse HTC countersinks → See the sets
Keep learning
- Counterbore Holes: Metric or Imperial Dimensions — the flat-bottomed cousin, for socket-head screws.
- Best Uses for a Solid Carbide Center Drill — where the 60° form comes from, and why a centre drill is not a countersink.
- The Most Common Uses for Carbide — when the extra cost pays for itself.
Not sure which angle or size your job needs? Send us the fastener callout and the material and we will point you at the right tool — info@fmcarbide.com.
FM Carbide helps you machine better!
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