What is a countersink bit?
A countersink bit is a cutter that makes a shallow, cone-shaped recess so the head of a countersunk (flat-head) or raised countersunk screw sits flush with or below the surface. It is used in timber, composites, plastics and metal. In Dutch it is called a soevereinboor or verzinkboor.
Types
- Plain countersink. A conical head with one or more cutting edges. Drill the pilot hole first, then countersink.
- Drill and countersink combination. An adjustable countersink collar fitted around a twist or wood drill, so pilot hole and countersink are cut in one pass.
- With depth stop. A non-marking collar that rides on the surface, so every screw head ends up at the same depth.
The angle matters. Metric countersunk screws have a 90 degree head, and so do standard metric countersinks. Imperial (American) flat-head screws use 82 degrees. Mixing them gives a head that either stands proud or sits too deep with poor bearing.
Application
- Drill a pilot hole. In hardwood and near edges, pre-drill for the screw shank so the timber does not split.
- Countersink. Hold the bit square to the work and run at moderate speed until the recess is just wider than the screw head.
- Test on scrap. Check on an offcut of the same material that the head sits flush or slightly below.
- Hide the head if needed. Go deeper and fill with wood filler or a plug cut from matching timber.
- Clear the chips. Withdraw the bit now and then; clogged edges leave a ragged rim.
Common mistake: driving countersunk screws into hardwood or board material without countersinking. The head wedges the material apart, the surface splits or the head ends up proud.
Related terms
- Cordless drill
- Hammer drill
- Dowel
- Dowel hole
- Composite material
- Plastic
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