April 3, 2012

Hand-Sharpening Small Drill Bits

Hold that dull bit up to the light, then head for the grinder. Your eyes can detect problems as well as any gauge.

Sharpening drill bits by hand is a task commonly made harder than necessary. The accepted practice is to use a small gauge or a protractor to match the flutes in a perfect 59-degree (included) angle. The flutes are ground one at a time and then matched by eye to the gauge. Some gauges have length lines scribed along the gauging surface, to be used in measuring the length of each flute. These gauges are just clumsy enough to use that you are often tempted to toss the offending drill bit in the trash and get a new one.

A problem is that the gauges leave too much room for human error, something that can be compounded on each flute. There are three places you can go wrong: on flute angle, flute length, and clearance angle. Making one flute 2 degrees flat, and the other flute 2 degrees sharp results in a wide mismatch even though each mistake, when taken by itself, is small.

There is a better way. By throwing a shadow on the bit and looking closely at it in silhouette, you can quickly see the difference in the flutes and correct them at the bench grinder. The process is simple. Go to the grinder with a dull bit, put on those safety glasses and carefully touch up each flute, trying to follow the factory angles as closely as possible.

Hand-Sharpening Small Drill Bits

Courtesy, American Gunsmith

Above: Using a small gauge or protractor to match the drill bit’s flutes to a perfect 59-degree angle, grinding them, then comparing them again is the accepted way of sharpening bits. This process is just clumsy enough to tempt you to throw the dull bit out.

Now, hold the bit vertically in one hand and cup the other hand over the bit to put a shadow on the flutes. As long as you are facing the light—it doesn’t matter whether it’s a window, an open door or an overhead fluorescent—you should be able to see the flutes in silhouette. This is the same image you’d get on an optical comparator. Any discrepancies in the flutes are quickly noticed, and you can return to the grinding wheel to correct them.

For a drill bit to cut efficiently and on size, both flutes have to contact the work. This means the flutes need to be the same, in length and angle. Forget about that magical 59 degrees. Don’t give in to the temptation of reaching for that protractor to check flute angles; just follow the factory edge as closely as possible. Remember, you are better off going flatter than 59 degrees if you are going to go one way or the other. The important thing is to make the two flutes look alike. Length and angle both will be glaringly apparent once you look at the bit in silhouette.

Hand-Sharpening Small Drill Bits

Tools & Techniques, Chapter 18 Hand-Sharpening Small Drill Bits

Above: You can check flute sharpness easily by holding them up to the light. The silhouette created will identify the differences that need to be corrected. An optical comparator would do the same thing.

Once you are satisfied with the flute angles, use the edge of the grinding wheel to thin the web of the bit. This takes a bit of practice, but there’s really nothing to it. Make sure the cutting side of the flute is close to the side of the wheel on your right and use a tiny bit of pressure to let the edge of the wheel bite into the center of the drill bit’s web. Rotate the bit 180 degrees and do the same on the other side.

When you return to the work and begin to drill, you can quickly see how well you did. If the drill produces only one shaving, that tells you that only one flute is cutting and it is time to try again. If the drill produces two shavings, note their size and shape. Hopefully, they will be similar, indicating that the drill is close to perfect.

The final test is to measure the hole. A properly sharpened drill bit will cut a hole that is within a few thousandths of nominal size. The easiest way to measure small drilled holes is to use other drill bits as go/no go gauges. Once you have a drill bit cutting on size, you can pat yourself on the back.