I'm using a CMT flush trim bit on 3/4" birch ply and am finding that the bit
consistently shaves off the tiniest bit of wood from the surface it supposed
to be "flush" with. Is this a problem with my bit, the plywood or not a
problem at all (i.e. operating as designed)?
Could you be pushing into the wood too hard? The bearing could be
pushing into the plywood edge. Look to see if there is a indentation
where the bearing traveled. This happened to me a couple of times
until I figured out to let the router do the work; esp. on plywood and
Not as uncommon as you may think:
A trimmer that eats its templet. A little bait & switch and also the
reason your cutter is trimming templet at the
I had a bit where the bearing was about 1/64" smaller than the
diameter of the bit, that would be the first thing to check. If
that's the case get out a diamond hone and make them the same size.
Other things which cause this problem is bit defelection, 1/4 shanks
are more prone than 1/2" shank. Make sure that the bering is ridding
as close the surface that you want to trim as possible. And don't
extert a lot of force against or you'll bend the bit a touch just let
the router do the work.
It could be a defective bit. I had this problem with an imported bit
from Grizzly. Ruined a laminate job. They offered to replace the bit
... no thanks, gave me my money back but I still worked a day for
free. Maybe I should write it off as tuition :>) CMT bits I've found
to be of the highest quality so unlikely the bit but not impossible.
Test by running the bearing against a very hard surface, like
laminate. More likely the problem is that you are holding the router
not _exactly_ square with the work or the bearing is crushing the
guide material slightly. When you are ripping the ply make sure the
blade is set exactly 90 deg to the face.
Hope this helps,
Kenneth Burton, author of "Cutting-Edge Table Saw Tips & Tricks," prefers to
use his table saw instead of a router for flush trimming. It's just a tall
fence that attaches to his table saw fence, but the add-on fence doesn't go
all the way down to the table. It sticks up above the table far enough that
the part to be trimmed isn't against the fence. Burton writes that it's
easy to line up and a lot faster than using a router. I think I'll give the
technique a try the next time I need to flush cut.
Mitch, a late reply here; what I haven't seen others mention is how deep to
set the cutter. I like to set the cutting flutes just barely deep enough to
cut through the veneer/laminate. Just because they give you 3/8 or 1/2" of
cutting flute does not mean you have to use it all, and setting it deeper
than necessary make is much easier to tip the router or flex the bit/arbor.
Lastly, I know you shouldn't have to fix a router bit, especially a top of
the line CMT, but in a pinch you can fix that bit by sharpening the flutes
with a diamond hone. It won't take much.
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