Age old problem - Tearout with router

[Carpenter96]

What HP is your router? Even my 40 year old craftsman 1.5 HP router will cut the dado your describing with absolutely no problem.

Regards Bob

[leehljp]

That was cross grain, which affected it. OF course there was probably no way around cross cutting the Oak grain in that situation. Age of the wood and/or dryness/ very low moister content, could certainly affect the cut.

[LCHIEN]

OK, I got my MLCS spiral DOWNCUT bit and tried it tonight.
Here's the same piece of wood in the first pic of the thread with the new downcut bit about 1/4" away.
The bit is a 1/4" solid carbide upcut on the left and a downcut on the right. As you can see there downcut does not have the tearout.

Conclusion, use the downcut or a downshear bit for grooving to markedly reduce or avoid tearout.

Yes the downcut was also a little deeper. .145 deep vs .062 deep.

[leehljp]

That certainly makes a statement of having the "right" tool!

[jussi]

Save the upcut for mortises

[poolhound]

Problem solved! As Jussi said "upcut for mortises!

I had meant to post this pic last weekend when I did this but as usual this week got away from me.

I cut some rabbets in some old plywood for a shop cabinet I am building. This was with a basic straight 2 flute 3/4 bit to a depth of 1/4. Took just one pass and while not 100% perfect edge it only needed a very light swipe with some 220 sandpaper to get rid of the little bit of 'fur' it left.

[LCHIEN]

...

Given that you said this is a 1/4 inch bit and you are taking only a 1/16 and that the router is working hard I am thinking something else may be going on as that doesnt sound right. You should be able to take way deeper cuts than a 1/16 and have the router eat plywood like butter. this is very odd is there any other clues you can give us? how does your setup work with solid stock?

MLCS's advice on spiral carbide bits is that the cross section area is actually quite small and the carbide is brittle. They say:

Important: When the diameter of the bit is less
than the depth of your groove, take the cut in
multiple passes, going deeper each time . For
smaller diameter bits (1/4” or less), we
recommend cutting depths of half the bit diameter
per pass. Do NOT try to take the full depth in
one pass.

So their recommendation is not to take more than 1/8" depth of cut for a 1/4" bit!

As for the last question I get significant tearout for both spiral upcut bits and for straight-fluted carbide tipped bits in both plywood and solid pine regardless of grain direction. While it could be a matter of dull bits, I don't think its that in my case.