How to sharpen Auger Bits
With a brace and a sharp auger bit, one has with many different kinds of tasks, better and more precise control over drilling a hole. There are also many woodworkers who know this, but do not often use this tool simply because they don't know how to sharpen auger bits. I wonder how often old, dull, but still good bits are thrown away because someone doesn't know how to sharpen these traditional bits or know exactly what tools are needed. In fact one needs only a special file, and a little practice to discover how simple it is to make old auger bits sharp and useful tools once more.
This special type of file is, usefully, called an Auger Bit File and has single-cut teeth on both ends of the tool. At one end, the wide flat faces have the teeth, and the edges are plain, with no teeth. At the other end, the edges have teeth and the faces are flat with no teeth. This is designed to allow one to work very precisely in one area, without grinding away nearby surfaces as well.
Here you can see how the wide face of the file has teeth, but the narrower edge is plain.
On the other end of the file, the narrow edge has teeth, and the broad faces have none. Normally one uses the wide toothed faces, but with very small bits, or bits with an unusual shape, one uses the narrow edges to do the sharpening.
First you sharpen the main cutter. You hold the tip of the bit firmly against a piece of softwood, and make a couple of passes with the file over the cutting edge. Important: Be careful not to alter the angle of the bevel as originally sharpened into the bit. A second bevel, on plane irons for instance, can be very useful. But with auger bits it is a mistake. Only file the face of the bevel, never the back side! In general, the less material you remove, the better.
On these bits, there is a cutting spur on the opposite edge of the bit from the main cutting edge, which serves to keep the hole clean and round, and allows for the easy removal of the waste wood chips by cleanly cutting the wood fibers around the edges of the hole. This tip must also be regularly sharpened. But always from the inside! As with the main cutting edge, the less material one removes here, the better. The edge to be sharpened is normally slightly convex. Carefully file this surface so as to maintain the convex shape as much as possible, and pay particular attention to remove material as evenly as possible along the edge of the spur. This is much easier to do if the brace end of the bit is firmly held in a vice.
In principle, you are now finished.
But sometimes, in the process of filing the cutting edges, a slight burr forms. This burr can normally be removed by simply starting to bore a hole with the bit. But sometimes this doesn't work, and then one can place the bit flat on a fine grit sharpening stone, and move it with a circular motion (at least 3,000 grit, in the photo a 3,000-grit Naniwa Super Stone is being used).
Important: Keep the side of the bit flat on the stone when doing this. If the brace end is lifted, a slight bevel will be cut on the outside of the edge, and the bit will be permanently ruined! Be especially careful when working with bits shaped so that the side cannot be held firmly and reliably flat against the stone.
The main cutter can also develop a burr during sharpening. Again, normally this can be broken off by simply using the bit. But if not, it can be removed using a very fine stone. One must also be very careful here not to cut a second bevel on the back side of the edge. If one is not absolutely confident about these steps, it is better simply not to try them.