Countersink Angle: 82° or 90° for Flat-Head Screws?
Countersink angle has to match the screw head: 82 degrees for inch flat heads, 90 for metric. What a mismatch does and how to call it out on a drawing.
The parts come back, you drive the flat-head screws, and every one of them sits a few thou proud. Usually the screws are fine. The countersink angle doesn't match the head, and 82° and 90° look the same from across the bench.
Should an 82° Flat-Head Screw Get an 82° or 90° Countersink?
An 82° countersink. Match the cone to the head: inch flat heads are 82°, metric flat heads are 90°.
That was the answer on an r/Machinists thread asking exactly this. The top reply said it "should match the screw head angle." Others boiled it down to 90 for metric and 82 for SAE, and one pointed out that 90° tools also get used for plain deburring.
What a wrong countersink angle does
Put an 82° head in a 90° cone and the hole narrows faster than the head. The head bears near the shank and its top edge stands proud. Flip it, a 90° head in an 82° cone, and only the outer rim of the head touches. Either way you get a thin ring of contact instead of a full cone. That ring carries all the clamp load, it can dent under torque, and the joint loosens as it does.
Cutting deeper doesn't fix it. It moves the contact ring; it doesn't widen it.
Calling it out on the drawing
Fictiv's guide to countersink holes says a countersink needs three numbers: the countersink diameter, the angle, and the pilot hole diameter. It also notes the angle differs between ANSI and ISO hardware and is matched by the fastener.
A few habits that save a phone call:
Name the hardware next to the callout, like "for #10 flat head" or "for M5 ISO flat head". The machinist picks the tool from that.
Watch for a third angle. Aircraft flush hardware is commonly 100°, so check the screw's data sheet if it came from an aerospace supplier.
If the head doesn't need a cone at all, a socket head in a counterbore may be simpler. See spot face vs counterbore. And if you only want a light edge break on a hole, say that instead, since it isn't a seat. Back-side holes are their own problem: deburring M5 holes from one side.
Countersinks at DigiForge
Our CNC work is "3-, 4- and 5-axis milling and turning in metals and plastics." The quote engine costs detected features such as holes, threads and fillets individually, and every quote is reviewed by an engineer before production. Countersinks aren't named on that feature list, so put the angle and diameter on the drawing.
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