How to Sharpen a Twist Bit
My father gave me several old twist bits. They’re dull, but otherwise in good shape. How can I sharpen them?
Warren Foley, Reno, NV
You can sharpen a twist drill bit freehand on a grinder. Use the side of the wheel, because it’s easier to judge angles against the large, flat surface. This is a safe exception to the rule against sharpening on the side, as only light pressure is needed.
Rotate and tilt.
Starting with the cutting edge lightly touching the surface, rotate the bit and tilt it downward in one motion until the entire heel has been run over the wheel. The tip will move upward slightly.
On a properly sharpened bit, the area behind the cutting edge, called the heel, spirals down from the cutting lip. Without this clearance the bit won’t cut, so you have to keep the clearance.
Hold the bit with the tip at a slight upward angle to the top of the bench, with the lip you are sharpening horizontal, and the bit at a 60° angle to the side of the wheel. Now, lightly touch the bit against the wheel while you rotate it about one-sixth of a turn, sweeping the tip slightly upward, and decreasing the angle with the side of the wheel to 45°. The trick is to do this smoothly and consistently. It helps to practice against the wheel with the power off.
After taking a pass or two on both sides, check that the two lips are of equal widths and at the same angle. Adjust as needed.
Drawings by Vince Babak