Spiral Downcut Bit Creating Tearout

Just cut a job using the Nomad Tools #251 Spiral Downcut Bit. I was getting a massive amount of tearout along the top edges (and bottom edges - thru-cut). I don’t recall ever getting tearout like this from a downcut bit. I think that the speeds and feeds were fine. I didn’t hear a lot of chatter and the edge quality looks fine for the most part. Is this just a case of a dull bit? The bit looks ok though. Thoughts?

What material were you cutting?

A spiral down bit cutting through the other side will cause tear out especially if you have a spoil board that is really worn and not supporting the bottom of the project. A down cut bit turns into an up cut on the bottom side it will push the material down and you will have tear out if not supported.

Inspect your bit for dull or small chipped areas on the cutting surface. It should feel sharp on the leading edges. if you have been using it on plywood the glues used to hold the plys together will dull bits.


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Thanks Anthony.

Material is White Ash Wood - nice hardwood with a pretty tight grain.
Good point about the tearout on the underside. Yes, that should be expected some with the downcut bit. Main thing was the surprising amount of tearout on the topside. Would a dull bit do this? I’m starting to suspect maybe the feeds and speeds. :face_with_monocle: I was experimenting with more aggressive settings, but I don’t see why it would make such tearout on the topside.

Your bit could be dull or it may be the direction of the grain. Is the tear out the same all around or in sections that change with the grain.

How about trying a slow and shallow first pass about .020 kind of like people do on chop saws to score a line. Then do the rest of your cut normally. You could try this on a scrap of the same material and see how it works.


White ash, while tight grained is also a bit… ‘stringy?’ and likes to chip out. Splitting ash firewood is fun when it’s cold. It almost explodes.
One thing that might really help is to climb cut instead of conventional cutting.
To do this on a profile in CC, Offset the profile by the diameter of the cutter & cut the offset vector.
It will conventional cut the offset, resulting in a climb cut on the actual profile.
If it’s to be an outside profile, cut the offset vector using ‘inside’.

CC could really benefit from a “Climb Cut” option. Personally, I would make that the default.


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