Spiral Down or Up?

[Dale Murray]

I have a maple bench top 90"x30"x2-1/4" and need to form a 1/2" tongue around the perimeter to accept the skirts for my bench build. Currently I am leaning toward using a 1/2" 3-flute spiral down-cut bit unless unless there are compelling reasons to do otherwise. Are there?

This would be used in a handheld router, not a router table.

As an aside, I have a dedicated mortiser so future mortising use of the but is not a consideration.

[glenn bradley]

So you will cut down from the large surface on both sides to leave the tongue, yes? I am fine with that bit. Multiple passes of course. I try to take no more that 1/4" square of surface area per pass but, other do 3/8" x 3/8" and a rule of tumb. In your case I would cut the full 1/2" wide going 1/8" or so deep per pass ending with a very light pass; repeat on other side.

[Dale Murray]

That is exactly what I was thinking. Cool.

Personally, I've never known anyone with a spiral down or up bit, so I needed a bit of guidance. Thanks.

[Bruce Page]

I use down spiral bits on my cnc router often. There is also what's known as a compression bit that have both up & down spirals ground on them, great for reducing/eliminating edge tare out when panel routing.

[pat warner]

Way too much deflection on a skinny bit like that, solid carbide or not.
Especially on full extension. Moreover, there are hidden risks with that much cutter
engagement, (especially for a guy without priors on spirals).
Would use a carbide clad steel straight bit with at least a 5/8" cutting diameter.
(There is very little grinding into the shank on cutters = or >5/8 CD.)

[Mel Fulks]

The spiral probably leaves better surface ,but that is not an issue making a tongue. I still say fastest router bit is single flute straight .

[Gerry Grzadzinski]

Way too much deflection on a skinny bit like that, solid carbide or not.


In a handheld router, he's limited to a 1/2" shank. The carbide spiral will be more rigid than a carbide tipped bit.

And he's only cutting 7/8" deep on each side.

[Dale Murray]

Actually, I am only cutting down 3/4" on each side leaving a tongue 3/4" tall and a 1/2" deep. I would think a straight cutting bit would be must more grabby as it rotates than a spiral bit - never used a spiral so I dont know.

Solid Carbide 1/2" shank, 1/2" bit, three flute.