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Last Updated: , Created: Saturday, January 26th, 2002

Sharpening drill bits

Twist drill bits are actually fairly complicated cutting devices. First there is the cutting angle. Although most bits you buy at the hardware store all have the same cutting angle, specialized bits will have a steeper cutting angle for soft material like wood than for hard material like metal.

As the cutting edge digs into the material, the metal behind it must be lower or it would prevent penetration. Again this relief angle will be less for hard material, because it goes in with less bite per revolution and it needs more support behind the cutting edge. For soft material the relief needs to be more radical to simply get out of the way as the bit moves forward rapidly.

If you look at the graphic of a home made sharpening jig you can best understand what we need to do when sharpening drill bits. The wooden stop is set at the angle that will present the cutting edge correctly to the grinding wheel. The felt pen marked lines are at the angle required at the end of the relief angle. You touch the bit to the stone in such a position as to begin by sharpening that cutting edge and then immediately rotate the bit while swinging to the left to end up at the relief angle by the time you have rotated not quite half way around the bit. Don't spin the bit so far as to hit the other cutting edge. Then you have to do exactly the same thing on the other side, making the bit perfectly balanced with both cutting edges at the same position. You can verify if you have sharpened too little on one side by looking at the tip. It should have a symmetrical pattern jumping from one cutting edge to the other. After a while you will wear a groove into your grinding wheel and will have to move the jig, or replace the wheel.

Now you can see why someone invented special sharpening jigs, even machines, to do all of this. One such machine is the Drill Doctor shown in the photo. You set your bit into the guide with a setting gauge built into the machine. Then you push in and rotate the bit at the same time. A special cam action on the guide creates the relief angle as you rotate. You do each cutting edge separately but the arrangement makes sure that they are ground equally. The Drill Doctor has an inexpensive version that attaches to your drill, or the full motorized stand alone version. Grinding sleeves, rather than wheels, are used that keep their shape right up until they don't sharpen any more and need replacing.

Keywords: Sharpening, Jigs, Drills, Power Tools, Techniques

Article 1686