Many woodworkers buy and use regular twist drills that are actually designed to drill metal. They use them not only to drill wood, but also to drill holes in a variety of other materials. These twist drills do become dull, and their cutting edges (called a lip) can become chipped; when the latter happens, they don't even cut woods very well, and certainly not metal. It is also not uncommon to break off a drill bit, in which case, you can either attempt to grind a new point or throw the bit away.
With a little practice, you can grind or sharpen bits so that they cut wood reasonably well. Perhaps, they will even do a fair job of drilling metal. Metal workers demand closer tolerances to their hole diameters than is usually necessary for woodworking and general utility work.
There are generally three essential requirements in twist drill sharpening:
1. Equal drill-point angles, which are usually 59 degrees each for a total of 118 degrees
2. Cutting lips of equal length
3. Correct clearance behind the cutting lips, which is approximately 8 to 12 degrees
A handyman's adjustable protractor can be used to check both the drill-point angle and the lengths of the cutting edge lips. The appropriate clearance behind the cutting lip can be determined by visual inspection. The resulting clearance angle is not critical as long as it is approximate 8 to 12 degrees.
Clearance must be provided all along the cutting lip. Little or no clearance prevents the cutting edge from producing a chip, and the drill will just not drill. The cutting edge will be held off the work. Too much clearance weakens the cutting edge because too much metal behind the edge is removed.
Grind all twist drills without overheating them. Keep their points cool enough so that you can touch them with your bare fingers. Making very light passes over the wheel can do this.
It is easy to freehand-sharpen a twist drill. Even if you do not sharpen the twist perfectly, it will still cut wood, although the hole may be slightly oversize. Begin by holding the bit on your forefinger, with its cutting lip horizontal and the axis of the drill at an angle of about 59 degrees. The actual grinding process actually involves three distinct motions of the shank while the bit is held lightly against the wheel. The three motions are:
1 .To the left
2. In a clockwise rotation
The three steps involved in sharpening a twist drill. Note: The operator moves the shank of the bit left and downward while simultaneously rotating it.