Brad point bits are designed specifically to cut clean, accurate holes in wood. Like any other cutting tool, they work best when properly sharpened. Although the “business” end of a brad point bit may look a little complicated, it’s easy to sharpen once you know how it works.
COMBINATION BIT Basically, a brad point bit combines the best features of three different drill bits. It’s like a hybrid of a spade bit, a Forstner bit, and a twist bit, see box below.
POINT. Like a spade bit, there's a sharp, tapered point for centering the bit on the workpiece. This prevents the bit from "wandering" off the centerpoint as you start the hole.
LIFTERS. After the point enters the Wood, two cutting edges or "lifters" take over. Like the cutting edges on a Forstner bit, the lifters act like a pair of revolving chisels that shear off and lift out thin wood shavings.
SPURS. In addition to the lifters, some brad point bits also have two knife-edged "spurs". To produce a cleaner hole, these spurs score the wood fibers around the perimeter of the hole.
FLUTES. As the hole is drilled, spiral flutes in the bit pull up and eject the chips from the hole like an auger. The basic idea is the same as a twist bit. Only the flutes are ground at a steeper angle so they eject the chips faster.
SHARPENING
There's really nothing mysterious about sharpening a brad point bit. In fact, you can restore a sharp edge in less time than it takes to drill a hole with a dull bit. All it takes is a few strokes from a file.
CLAMPING JIG. The first step is to provide a way to hold the bit securely in place while you're sharpening it. To do this, I use a simple clamping jig. This is just a scrap block of wood (I used a 2x4)
SHARPEN LIFTERS. With the bit extending about 1" above the block, the next step is to sharpen the litters (cutting edges). The secret is to create a clean, sharp line along the edge. This requires filing the angled "flat" on the end of the bit, see Fig.1.
FILING ANGLE. To determine the correct filing angle, just rest the face of the file on the flat.
Once you’ve "found" the angle, push the file across the flat in a continuous motion. The trick is to file the flat without nicking the brad point or the spur.
To keep from rounding over the lifter, raise the file off the bit at the end of each stroke. Then find the angle again and repeat the filing process until the surface is shiny and flat.
The important thing is not to get carried away. The idea is to hone the cutting edge. Not reshape the bit. I usually keep track of the number of strokes I make on each lifter. This ensures that an equal amount of material is removed off each edge and that both lifters end up doing the same amount of work.
SPURS. If you have a brad point bit with spurs, the next step is to "dress" the inside of the spurs, see Fig.2. Here again, find the angle with the file. Only this time, tilt the file so the face is against the inside of the spur.
Now take one or two strokes until the surface is shiny and flat. A light touch here keeps from damaging the lifters that you’ve just sharpened. Note: Don’t file the outside of the spurs or you’ll reduce the cutting diameter of the bit.
