Up Spiral Bit or Down Spiral Bit?

up to pull the shavings UP from the mortise towards the router (and since it’s upside doen, it means = down to the floor)

-- When in doubt - There is no doubt - Go the safer route.

upcut…. both for handheld and router table… not gonna make it any more confusing than that

Up, to clear the chips towards the bit shank from the fully housed hole being cut. There might be a small amount of tearout around the hole, but nothing that will compromise the joint or show after assembly.

Down spirals are used when you want the cleanest edge closest to the bit shank, especially in veneered stock and plywood, but have somewhere for the chips to go. For instance, edge routing or a through mortise.

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Okay, I know this is the exact opposite of what everyone just said, but I use the down spiral bit for mortises, to get the clean edge as noted above. Some of the shavings do tend to build up in the hole sometime, but no worse than a straight bit, at least in my experience. Of course, if you weren’t using a table, just the router, you would need the up spiral or your mortise would really fill up with shavings. Either one gives a cleaner cut than any straight bit I’ve tried.

-- Dale Manning, Carthage, NY

For the reasons stated, use an upcut bit for mortises to eject the chips more efficiently when the mortise is being cut.

The only time you need to use a doowncut bit is if you need a very cleanly cut opening such as on a through mortise or when inlaying one piece into another.

On the standard mortise, the mortise is covered by the piece with the tenon so using the upcut bit trades off the efficiency of chip ejection and ease of cutting for possible tearout at the surface (which still almost will never happen with an upcut bit)

-- Mark, Webster New York, Visit my website at http://thecraftsmanspath.com