Sharpening Forstner bits

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Mar 13, 2010
Saratoga Springs, NY
Recently, there was a thread discussing options for purchasing Forstner bits. The subject of sharpening of Forstner bits was mentioned, but a brief check couldn't find any references on the IAP site on that subject. Hence, this thread was started to capture thoughts on that important issue.

It is my understanding that the steel used in Forstner bits isn't hardened to the same extent as the steel in ordinary twist bits. That means that Forstner's can dull more rapidly that other bits, and a dull Forstner bit requires more effort, and doesn't cut as cleaning as other bits. And because Forstner bits are a bit spendy, developing the ability to sharpen them is important. It turns out that the process is fairly simple and really doesn't take a lot of time.

Forstner bits have three important components - an outer ring, two chipper blades within that ring, and the center spur. All three require sharpening although my experience is that the chipper blades require more frequent attention than the other two components. There are many references on the subject of sharpening on the internet; this discusses my conclusions and methods.

Outer ring:
There are three configurations for the outer ring, and sharpening is slightly different for each.
  • Solid ring - these bits have no teeth on the outer ring. Instead, there is a half-moon-shaped bevel on the leading edge of the ring, and the sharp intersection between that bevel and the ring cuts the fibers in the wood at the perimeter of the hole. This intersection should periodically be sharpened by passing a file over the bevel. It shouldn't take much - just a few strokes with a round or half-round diamond file should suffice.
  • Toothed ring - these bits have a series of triangular teeth around the circumference of the bit - multiple teeth cut the fibers better than one sharp intersection at the leading edge of the outer ring, so larger Forstner bits often have teeth rather than a solid ring. These teeth should periodically be sharpened with a few passes with a triangular file held parallel to the face of each tooth.
  • Wavy ring - some manufacturers use half-round bevels on the inside of the outer ring to create a series of teeth around the perimeter of the bit. These can be sharpened using a half-round file. Go gently - just a few strokes should do the job.

Center spur:

  • The center spur is a pyramidal-spike that creates a pilot hole to keep the bit centered on its axis. A flat file or diamond paddle can be used to hone each of the four faces of the spur, but to avoid decentering the spur, be careful to apply the same number of strokes on each face of the spur.
  • The chippers are the two flat surfaces that extend from the center spur to the outer ring, and that cut the flat bottom in the hole that is characteristic of a Forstner bit. The chippers have to work much harder than the spur or outer ring, and require more frequent sharpening than either the outer ring or the spur.
  • An edge is the intersection between two surfaces, and the edge on the chippers is the intersection between a fairly large flat face (on the 'leading side' as the bit drills into wood), and a bevel ground on the side of the chipper. On most Forstner bits, that bevel is a hollow grind. There are two schools of thought about how the chippers should be sharpened.
  • One approach is to use a flat file or diamond paddle to hone the face of the bevel. Since the bevel starts out as a hollow grind, honing the face means polishing the leading and trailing edges of the hollow. If you take this approach, be careful to hone uniformly across the full width of the bevel. Because this approach only removes microscopic amounts of metal at the leading and trailing edges of the hollow ground bevel, it takes very little time. But it is more tedious to do since its fiddly to keep the hone flat on a small bevel. Also, try to hone both chippers to the same degree so that they remain symmetrical.
  • The other approach is to use a diamond paddle to hone the larger leading face of the chipper. This can also be a bit flddly, especially on small bits, but I find it easier than trying to keep a hone flat against a very narrow hollow-ground bevel. Again, only a few strokes should be required, and it helps to try to use the same number of strokes on each of the two chippers so that they remain symmetrical.

One additional hint - sharpening a Forstner bit is one of those tasks where the hand that is holding the bit being sharpened tends to get in the way of the hand holding the hone doing the sharpening. I find that gripping the bit in a bench vise is helpful. To avoid damaging the shank of the bit, I drilled a 3/8" hole in a scrap of wood that the bits slip into, and then grip the wood in my vise.
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