Q: I’ve heard of a process for sharpening files that involves dipping them in acid. Do you think that will also work for burs? I have a lot of dull ones, and they’re expensive to replace.
A: No, if the bur is dipped directly in the acid without preparation. But if you first cleaned the bur very well, and
applied something that will act as an acid stop-off agent just to the outer surface of the bur teeth, the acid then would act only on the valleys. That undercuts the worn tooth surface and in use, the previously dull edge can break off at the undercut, giving you a marginally useful cutting edge. In doing this, the size of the bur, and perhaps the accuracy of its shape, is shot to hell, but you can still use it to cut metal. This method, when used to “sharpen” dull files, works best on those with coarser teeth. Similarly, the above method works more easily with coarse tooth burs. I tried it once using a simple Magic Marker (well, actually it was some sort of marker that looked exactly like a Sharpie marker, but came from Radio shack, sold as an etch stop off pen. It did work. Not all that well, and perhaps not worth the time and trouble, but it did, at least somewhat, sharpen the bur.
Frankly, it’s easier to take one of the very thin separating disks, true up the edge, and use it as a fine grinding wheel to literally sharpen the bur by re-grinding it freehand. This won’t work with the carbon or vanadium steel burs like the Busch burrs. They’re too fine a cut, and too cheap to be worth the bother. But with some of the larger, costlier, high speed steel burs, which have a much coarser tooth structure than the Busch burs, it’s quite possible to return a bur from uselessly dull to useful. Use magnification to grind the valleys between teeth, maintaining the angle. Grind the back of the tooth, not the front leading edge. In doing this, you thin out the worn, flat dulled surface of the tooth. Take it to the point where there’s just the faintest hint of the shiny surface that told you it was dull. Don’t try to fully grind it sharp. Done like this you end up with the same dimensions and shape that the dull bur had, which might be more accurate than the random inaccuracy you’ll get if you take the bur to fully sharp teeth. The bur will be sharp enough to be useful again—slower than new, but usable. Small diamond grinding bits do the job well also, if they’ve got a clean enough cutting edge to get into the grooves.
Best, though, is not to waste your time sharpening your own burs except in an emergency. Instead, put the worn (high speed, not carbon steel) burs aside until you’ve got enough to make it worthwhile, and then send them to one of the several suppliers that offer sharpening services. What you get back are almost as good and sharp as new, except they’re now slightly undersize. Actually, this is a rather useful thing to have. Bur sizes midway between standard sizes can be pretty useful in some situations; the cost for resharpening is much less than the cost for new high speed steel burs, so it’s well worth doing.
by Peter W. Rowe M.F.A., G.G.