What a countersink is
A countersink is a cone-shaped recess at the top of a hole that lets a flat-head screw, bolt or rivet sit flush with, or slightly below, the surface. The cone angle must match the screw head, or the head will bear only on its edge and may loosen or crack the part.
The formula
For a countersink of surface diameter D over a hole of diameter d, with an included angle α:
Depth = (D − d) ÷ 2 ÷ tan(α ÷ 2)
The depth is measured from the surface down to where the cone meets the hole wall. The full depth of the cone, to its imaginary point, is D ÷ 2 ÷ tan(α ÷ 2), which is what you set on a stop collar if you measure from the tip of the countersink bit.
Example
A #10 flat-head screw has a head about 0.375 in across, in a 0.19 in clearance hole, with an 82° countersink:
Depth = (0.375 − 0.19) ÷ 2 ÷ tan 41° ≈ 0.0925 ÷ 0.869 ≈ 0.106 in
Which angle?
| Angle | Typical use |
|---|---|
| 82° | inch flat-head machine and wood screws (US standard) |
| 90° | metric countersunk screws (ISO) |
| 100° | aircraft rivets and some aerospace screws |
| 120° | sheet metal rivets and some plumbing |
| 60° | center drill holes for lathe centers |
Using a 90° bit for an 82° screw, or the reverse, leaves the head touching on only one edge. In wood the difference is often hidden, but in metal it matters.
Tips
- Set the stop about 0.005 in (0.1 mm) deeper than the calculated value so that the head ends just below flush; you can sand or file a slightly proud head, but you cannot fill a hole that is too deep.
- In thin sheet, where the countersink would go right through the material, use dimpling instead of countersinking.
- Run single-flute or zero-flute countersinks at low speed with cutting fluid in metal to avoid chatter marks.
- Test on scrap first, then check with the actual screw.