Up-cut or down-cut for saddle slot?

[Jack Lord]

StewMac sells a spiral down-cut 3/32" bit for routing saddle slots.  The powdered shavings really pack into the bottom of the slot, and I clean them out after each pass.  An up-cut would clear the debris.  I know the advantage of the down-cut is a cleaner edge around the slot, but that can always be cleaned up with a little sandpaper at the end.

[Russell Vance]

Yep, had this discussion with a colleague the other day as well, we use an overhead router which is quite hefty and it does not appear to make much difference in the finish regardless of up cut or downcut. But, I used a sliding jig and a small laminate trimmer before we got the OHR and I noted the upcut spiral gave a better finish of the slot with less chatter due to less pressure and flex on the bit and jig through the cut process.Obviously, multiple passes with the either of the bits helps with the accuracy and finish as opposed to digging in deep and flexing the tool. R.

[Pierre-Antoine Roiron]

I clearly see a difference using a downcut bit : I get a much cleaner cut (I make one or two big passes and then one finish passe). But I believe it's because I have an exhausted Dremel with a certain amount of play in the mandrel.
I use regular or up-cutting spiral bits with my heavier Festool (great router!) and get with sharp bits a very clean cut too.

[Howard Klepper]

I use upcut spiral end mills with good results. In theory the downcut bit is for rabbeting or trimming rather than for slotting because it packs the swarf down in a slot with no good place for it to go. In practice, either make work for a saddle slot. But there is very little chance of splintering the woods we use for bridges with a shearing upcut.