Q:

Some of my Forstner bits are getting a little dull. How do I sharpen them?

Del Fussel, Parker, CO

A:

Sharpening a forstner bit isn’t difficult. All you need is a small file and a slip stone. It takes only a few strokes to get a sharp edge, so don’t overdo it. It is also critical that you never hone or file the outside of the rim, which would lessen the bit’s diameter, or the beveled side of the chippers, which could change their cutting angle.

Start by thoroughly cleaning the bit to remove any built-up pitch or other debris. Sharpen the inside of the rim, then the chippers, and the centering spur last.

Hone the rim first

Use a slip stone with a rounded edge and sharpen the inside of the rim only.

File the back of teeth

Some Forstner bits have sawlike teeth. A few strokes with a small triangular file will sharpen them.

Hone the front side of the chippers

Make sure the chipper and rim meet at a crisp 90º corner. A rounded corner won’t shear the grain cleanly.

 Right on point

Lightly file the centering spur to a sharp point, but keep the point centered. Johnson uses an index card to protect the chippers from the file.

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