The Delta Mortiser instruction manual says the clearance between the
chisel and bit should be between 1/16" and 3/16" --- depending on the
type of wood. In the past, with a drillpress mortising attachment, I've
always set the clearance at about 1/16". From your experience, which
species of wood require the larger clearance?
3/16"? I guess I'd ask Delta. I won't pretend to know more than they
do about mortising chgisels and bits, but it sounds like a misprint to
me. I usually set mine around 1/32" - jst enough to keep the bit from
The bit in hardwood will make fine shavings. In soft wood you
get the grain to tear rather than be shaved. If you leave a bigger
gap between the chisel and the bit you're apt to have the bit deflect
in the cut - and leave room for chips to jamb between it and the chisel.
Here's The Forty Cent Method for bit to chisel gap.
And if you tune up the chisel and bit - sharpen and polish - you'll get
less wood tearing and clogging things up.
You MUST tune up the chisel and bit. They are NOT sharp from the factory.
Lee Valley sell a tool for doing the inside of the chisel. Hone the sides
on stones as you would any cutting tool. A small file will do the side of
have the bit deflect in the cut - and leave room for chips to jamb between it
and the chisel. ...
Thanks. I was planning to machine some steel shims to the right
thickness, but the Forty Cent Method sounds a lot cheaper.
I had a conical stone on a shaft, but it seems to be on a long
vacation. I'll get the Lee Valley set-up -- and file the inside corners
Thanks again, Charlie.
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