Professionals and Amateurs Together

Up vs Down Spiral Router Bits

Question

What situations do you use up spiral vs. down spiral router bits in?—George Adams

Answer

Bill Taylor replies: Rather than talk about specific situations I would rather you think about the cut in this manner. Are the fibers supported or unsupported where the cutting edge exits the wood? Unsupported you get tearing like when an up cut bit is shearing the fibers at the wood surface while a down cut has support for the surface fibers.

Of course sharpness is also key. The trade off is speed of cut versus a clean cut when specifically talking router bits. A down cut tends to pack cut material at the bottom of the hole and is slower. Personally I slow down the speed of a down cut bit for less heat generation as well.

Bob Couch replies: The spiral direction refers to the direction of the material from the cut. Spiral down is preferred when you want a clean top surface because it cuts and draws the waste downward. The exception to this is when it is a stopped cut (not all the way through) because the waste will build up in the cut.

Elliot Savitzky replies: Up-cut bits are very efficient at evacuating chips from the hole or slot it is cutting. The problem with up-cut bits is that the same action of the flutes that brings the chips up can also lift or fray the wood fibers around the edges of the cut. Up-cut bits are frequently used for cutting mortises because of their depth. Since the edges of a mortise are eventually hidden by the shoulders of the tenon, this chipping or fraying will be hidden after assembly.
Down-cut bits leave a very clean, crisp edge around the hole or groove it cuts. While chips still are evacuated from the hole, a down-cut bit is far less efficient. This chip ejection deficiency of down-cut bits is minimized by the relatively shallow cuts. Down-cut spiral bits are a good choice for cutting dados or shallow grooves, especially when the edges will be visible when the project is assembled. Down-cut bits require a slightly slower feed rate.

Richard Oedel replies: These are for two different uses. The downcut forces the chips down into the cut. In a blind cut (not all the way through the material) the downcut bit forces the chips into the bottom of the cut, forcing the bit to swirl around in the chips. The result may be a better surface at the top, but much more chattering. If you are going the entire way through the material, the chips will come out the bottom, leaving a nice surface on the top. The upcut bit brings the chips up and out of the hole, but leaves a less-perfect cut on the top surface of the wood. Your choice. I almost always use upcut bits, although I have one or two downcut bits in case I really need them.

Steve Colello replies: All my joinery work uses slip tenons. I have a horizontal boring machine (slot mortiser) and I use up-cut spiral router bits because they evacuate the chips well. The up-cut bits are made to remove the chips upward. However, when I rout a groove for stringing, I use a down cut spiral bit. The stringing groove is usually shallow and the down cut spiral bit will prevent chipping on the surface. This is especially important because most of my stringing work involves thin delicate veneers.

Bruce Wedlock replies: Up spiral bits leave a clean bottom groove, while down spiral bits leave a clean, sharp edge at the groove surface. The down bits have a tendency to “screw themselves” into the stock, deepening the groove. In a router table I have had the bit surprisingly cut through the stock surface! That could cut your hand that is pushing the stock. Use a push stick for safety.

Tags: Router