What is the difference between an up cut router bit and a down cut router bit

There are so many options out there for router bits that it may be difficult to decide which one is the right router bit for your application. 

One of the most common questions I frequently get asked is whether to use an up cut or a down cut router bit.

The difference between an up cut router bit and a down cut router bit is the direction of the flutes. 

On an up cut router bit, if you hold the bit by the shank and point it straight down as if you were going to do a plunge cut into the surface of your work piece, you would be able to drill into the work piece by turning the bit in a clockwise rotation.  With a Down Cut bit, you would need to turn it counter-clockwise to be able drill into the surface.

Another way to determine if a bit is an up cut or a down cut bit is to look at the direction the flutes move around the bit toward the tip.  If the flutes twist right around the bit to the tip, then it is an Up Cut Bit.  If the flutes twist left around the back of the bit to the tip then it is a Down Cut Bit.

Now that you know the difference between the two, how do you know which one to use?  An Up Cut bit is very efficient in evacuating chips up and out of the cut.  It will leave a very clean finish at the bottom of the work piece, but will leave a rougher surface on the top of the surface (or the side that the bit enters the work piece).  A down cut bit does just the opposite.  It is best to use a down cut bit for through cuts, as a down cut bit pushes the chips down into the cut.  Down cut bits will leave a very clean cut on the top of the work piece, but may leave a rougher finish on the bottom.

Down cut bits are also great for cutting shallow dados, rabbets, and visible slots because of the clean finish it gives on the edge of the hole or groove.  When cutting dados or grooves with a down cut bit, be sure to slow the feed rate to allow more time for chip removal.

If you need a clean cut on both the top and bottom surface, try a compression bit.  They can be pretty pricey, but Compression bits leave a beautiful smooth finish on both sides of the work piece being cut.