I'm making a bunch of mortises in oak and burnt out a straight bit in
the process. Picked up a Freud 1/4" spiral bit that's supposed to
clear material more easily and be good for such applications??
However, I am finding that I'm getting a lot of burn, easily. I slowed
down the router speed and took very small passes, and this improved the
situation somewhat, but I still have to proceed slowly. Is this to be
expected with these bits? Am I doing something wrong?
You want the bit to move at a pace that will keep it from heating up the
wood and burning it. This often requires shallow passes but not necessarily
a slower spin speed with a bit that small. Secondly a 1/4" bit will heat up
much more quickly than a larger bit. You might also consider using a single
flute carbide bit . You should be making relative shallow mortises with a
bit that small.
I think burn is a fact of life when cutting mortises with a router. I never
worried about it because its going to be inside the joint anyway. Its an
obvious question, but I assume you got a spiral bit that spirals to pull the
shavings up. How small are your cuts? I usually cut about 1/16" at each
pass. Actually I just set the depth near the final depth I want to cut and
then move back and forth with light pressure on the top of the router. It
cuts lightly in each direction when I do that. I've never used a 1/4" bit
for mortises. I was doing larger stuff with a half inch bit. I used a
straight bit and had no problem, but my router has built-in dust collection
that is very effective for this application.
They make up cut and down cut spiral bits. Up cut will clear a blind mortise
of chips, down cut will move the chips in to the hole. Each has it's own
uses. A downcut will work well trimming veneer on the top of the board you
are cutting, an upcut works well when you are routing on the back of a piece
and the finished surface is on the other side. I think the burning is from
the too large a feed. I would rough out the hole with something else and
finish with the spiral bit.
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