Upcut bits are designed for solid material (hardwoods, MDF) and usually thicker material. If the wood part being milled is too thin, an upcut bit can emerge violently and leave a ragged, torn exit wound. So, most situations call for the best side of the board to be facing down when using an upcut bit.

What is a Downcut bit used for?

Downcut spiral tools are best used for thinner materials which will be pushed down into the machine bed rather than being lifted with an upcut spiral. This can be particularly useful when using vacuum hold down where maintaining the seal between the material and sacrificial bed is crucial.

What is the difference between Upcut and Downcut?

A down-cut bit will leave a clean edge on top but a ragged edge on the bottom; an up-cut bit will accomplish the opposite. ... A down-cut bit will leave a clean top edge, but it sends the chips downward, into the dado where they have no place to go.