Counter-sinking seems to be all the rage, but where’s the love for counter-bores? You can find no end to counter-sinking bits sold online, yet for some reason the options for counter-bores is severely limited.
A counter-sink is a conical hole that lets a tapered screw head sink below the surface so it’s flush with the surface or slightly recessed. A counter-bore is a cylindrical hole with a flat bottom. It also allows the head of a bolt or screw to be flush with the surface or recessed.
When I need to, I cut counter-bores in two steps, using a Forstner bit for the counter-bore, then following up with a twist bit for the screw hole (usually it ends up being the other way around). It’d be nice to do it in one operation, though. Timberline sells a series of moderately priced bits for the job: They have seven different-sized bits for size 4 to 16 wood screws. The three smaller sizes have a 5/16″ shank, and the four larger sizes have a 3/8″ shank. The drill bit passes through the shank and is held in place by two set screws. This allows you to adjust the depth of the hole before the counter-bore starts cutting. Two carbide tipped wings cut the flat-bottomed counter-bore.
You can purchase an individual bit for around $20, which seems expensive since you can usually pick up a set of counter-sinking bits for the same price.