They look a little like the striped pole in front of a high-tech barber shop, but spiral bits do more than take a little off the top. Use them wherever you'd use a straight bit -- and get cleaner cuts.
Spiral-fluted router bits leave the edges of your cuts virtually fray-free because, as they turn, the two corkscrew-shaped cutting edges stay in contact with your workpiece longer than the vertical cutting edges of a straight bit. This results in a shearing action instead of the rapid chop-chop-chop-chop of the traditional double-fluted straight bit.
Unlike most router bits that have a carbide cutter brazed to a steel bit body, spiral bits are solid carbide. Carbide, however, is harder than steel, but also more brittle, so you must work with more care than with non-carbide bits. Don't force the work, and avoid sudden plunges or starts.
Let's take a look at the three kinds of spiral bits, and how to choose the right bit for the task at hand.