woodnthings

The upcut will get the chips out of the narrow groove, but may cause a little tearout. Try the painters tape trick to reduce tearout.
The downcut bit should work on the table mounted router since the groove is facing down, not up. Such a narrow groove/dado is prone to clogging no matter what, that's why I asked about the TS. But it just occured to me you could sandwich two circ saw blades together with a cardboard or other spacer to get 1/4" groove with an exact fit. No problem with the chips, but might have some tearout, depending on the blades. You could still use the same jig for either circ saw or router if it's a simple fence or clamp bar. bill

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autoBrad

Yeah, I don't think it matters how it's mounted. An upcut bit, is more likely to create surface tearout.. a downcut is more likely to not.. Table mounted or handheld in my mind it shouldn't matter, a bit still works the same as it would regardless.

Also if you have a shop vac or some sort of dust collection on the router, i would think the downcut wouldn't rreally clog if it can suck the stuff out of the way.

rrich

As Brad said...

An Up cut spiral bit brings the chips TOWARD the router motor.

A Down cut spiral bit brings the chips AWAY FROM the router motor.

The Up cut is for faster operation and is normally used when the dado is more of a mortise application. i.e. The part going into the dado has shoulders. The up cut has a tendency to produce some chip out.

The down cut has a tendency to fill the dado with chips however makes a cut with very little chip out. When the down cut is used for a deep dado or mortise operation must be done in steps. The down cut does not hog out material as well as an up cut.

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Rich (Tilting right)
Huntington Beach, California
Remember that when we have the "BIG ONE" everything east of the Rockies falls into the ocean.