Downcut spiral bits push the cuttings into the cavity or away from the table on a tablemounted external/internal routing. I don't own any. Maybe someone else can shed some light on the value of owning them.
Down spiral has the "edge" on the top surface (router side) coming out nicer, less splinters (think thin veneer). I've used it only as a through cutter, when hand held it makes it easier to see what is happen as the chips are taken away from the router.
They also make a compression up and down spiral bit that cuts down from router and up from the bottom side...... I don't have one of these.........
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