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.
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