Spiral router bits - up cut vs down cut

[Bill Wilson]

I make small chests and use a PC 4212 dovetail jig to make box joints in the ends.  I've been using a Freud 1/2" straight bit to cut the joints and by & large it works OK.  I get a little tear out in some places.  I'm using red oak and have always presumed that because it tends to be a little more splintery than some other woods, that was the main contributing factor.  However, I've been wondering if a spiral bit might offer a little cleaner cut.  If I were to get one, would I be better served with the up cut or down cut version?  I'm using a hand held router and I don't see chip evacuation as being a big problem, so my first instinct is to try the down cut, but wanted to see what experienced users thought.


TIA

[Cooler]

If it is a blind groove the down cut should eliminate tear out.  If you are routing through the board, the down cut spiral will cause tear out on the bottom of the board.

That said, I have not used either down or up cut bits.  My bits are all straight cut.

[cputnam]

FWIW - Leigh sells downcut bits for their dovetail jig

[handi]

The shear angle on either will cut the wood more smoothly. The curved edge cuts more like a knife and less like a chisel as a straight bit does.

In this case, cutting along the same axis as the end grain, up or down shear will make little if any difference. 

I typically use down shears when cutting grooves and Rabbets, the down shear cuts toward the center mass leaving a clean top edge. 

I only use up shear bits for deep cuts where chip removal is difficult. 

With CNCs I typically use compression bits, also known as Up/Down shear. The tip is an up shear, but only about 3/16” long, the rest is down shear. This bit can cut dados 1/4” or deeper, but can also, when cutting all the way through, cut both faces toward the center leaving clean edges on both, like with melamine.

[Bill Wilson]

Thanks guys.  I'll give the down cut a try.

[fixtureman]

Thanks guys.  I'll give the down cut a try.

I would try a compression bit also known as an up cut down cut bit it should give you the best cut if used properly