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.
Downcut. As the name implies, the cutting action of this bit is downward, or away from the router base. That shearing motion imparts a clean edge on rabbets, dadoes, grooves, shallow mortises, and plunge cuts in both sheet goods and solid stock.
When cutting grooves or dadoes deeper than the diameter of the bit, don't try to take the full depth at once. Instead, make several progressively deeper passes. A downcut bit tends to pack the wasted material down into a deep cut, rather than ejecting it, and shallow cuts reduce the problem.
Upcut. This bit wasn't designed to leave a clean edge like a downcut bit, but rather to remove the chips created in a deep plunge cut. That makes it ideal for plowing out a deep mortise in solid stock. Tear-out caused by the upward shearing will be hidden by the tenoned workpiece.
You also can use an upcut bit in your router table for any edge treatment that you perform with the workpiece face up, such as jointing solid or highly figured stock. (Remember that in a router table, the upcut bit is now cutting down.)
Upcut/downcut or compression bit. The unique geometry of this bit cuts from the top down and the bottom up at the same time, and it's ideal for cleaning up the edges of hardwood plywood or melamine-coated particleboard (MCP). For such easily chipped materials, WOOD® magazine shop manager Chuck Hedlund first cuts the pieces oversized on the tablesaw, leaving an extra 1⁄16 " on all sides. He then loads up a compression bit in the router table, setting the center of the bit's cutting flutes to about the middle of the workpiece's thickness. Finally, he offsets the outfeed fence 1⁄16 " and joints away the chipped edges.