How to sharpen Forstner bits

Forstner bits, especially the big ones can get expensive. Running them a bit too fast or boring too many holes can dull them fast. Sharpening then can be relatively easy.

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  • Expensive bits are worth saving.

    Are your bits losing their edge? Rather than giving them the heave-ho, try the ol' re-hone.

  • We dulled our bits first.

    You'd never bore through sandpaper as we did to quickly dull the bits for this slideshow, but some woods, such as cedar, contain silicates that have a similar, albeit slower, dulling effect.

  • Choose your file, found for rims.

    For bits with a cutting rim, chuck a round file in your drill press. At the drill's lowest speed, lightly hone the interior of the rim taking care to match the angle of the interior of the rim to the file as you turn the bit through a few quick strokes.

  • Choose your file, triangle for teeth.

    If your Forstner bits have serrated teeth on the cutting rim, skip the round file and instead clamp the bit in a vise and hone the back of each tooth with a triangular file. Don't overdo it; a few strokes per tooth will suffice.

  • File the flats.

    Next, with the bit clamped in a vise, hone the leading edge of the chippers with a fine mini-hone, ensuring a flat plane and sharp cutting edge. Pay close attention to the intersection where the chipper meets the cutting rim. A crisp corner makes clean cuts.

  • Keep the center spur centered.

    Finally, use the triangular file to hone the center spur. Use your thumb as a guide to protect the chippers and rim. Don't overdo it here: Use the same number of light strokes on each face to keep the point of the spur centered.

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