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.
Counter-Bores [Timberline]
Street Pricing [Google Products]
Via Amazon [What’s This?]
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2 Responses to A Proper Counter-Bore
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{ Where can I purchase a set of the Dowel Rounding Planes or just the 1/2 inch one? I volunteer at an 1800's period national park... } – Jun 06, 9:49 PM
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{ These are miss-sized the 1/2 inch is actually 7/8 inch and calls for a 1/2 inch pilot hole } – Jun 02, 9:57 AM
I’m one of those who makes counterbores in the same two steps as you, first with a forstner bit for the counterbore itself, then a drill for the screw hank itself. The Forstner bit leaves a nice center for the followup drill. Bits like these can be nice for production settings, but it looks like the sizes themselves look a bit limited for me, especially since I often use larger sizes for timber framing and other construction work.
Here is a collare we often use for counterboring in timbers:
http://www.amazon.com/Makita-714114-4-Inch-Countersink-Diameter/dp/B0002H4CIW
In the machine shop – piloted counterbores are sized to cur specific sized recesses for cap screws etc. – work in metals rather than wood