There are several characteristics to look for when buying router bits; each of them is shown below and on page 25. As it cuts through wood, a bit should only contact the workpiece with its cutting edges; the body should never touch the wood. As shown in the photo at left, you can check for this feature by measuring the cutting circle of the bit— the distance between the cutting edges— and the wing diameter. In a properly made bit, the cutting circle diameter exceeds the body diameter; the difference is known as side clearance.
A bit should also slice through wood with the edge of the cutter rather than the face. Two features make this possi-
blc. The first is the hook angle, which is the angle formed by the intersection between the cutting edge and the spinning axis of the bit. The second is bit shear, as illustrated on page 25. On bits with shear, the cutting edge is tilted vertically with respect to the shank. Bits with shear and a hook angle of about 20° will produce a smooth cut with little tearout and are less likely to cause kickback.
Bits with anti-kickback, or chip – limiting, designs are becoming increasingly common. The cutters on these bits protrude from the bit body by only one – half as much as on a standard bit. By taking a shallower bite, the bits place less strain on the router motor. In addition, the bodies of these bits are virtually solid, with only a small gap between each cutting edge and the bit body. This reduces the risk of kickback.
Some straight bits are manufactured with a spiral design. Upcut spiral bits remove waste faster because they expel wood chips upward. Downcut spiral bits are slower, but they provide a cleaner cut.