I done a quick google and found the following article. Excellent article and I
am sure this is your problem and this should help solve it! Happy woodworking
to you!
The
combination of a square chisel preceded by a round drill requires a specific gap
between the two for proper operation. Because chips are evacuated through a slot
in the chisel housing, there must be sufficient space between the drill bit and
the tip of the chisel for the chips to be carried up to the slot by the
specially designed flutes on the drill.
Most
manufacturers give a range of bit to chisel clearances, sometimes indicating the
range of settings are meant to accommodate different woods. Unfortunately, they
seldom describe what setting is for what type of wood.
The
logical interpretation is that the greater bit to chisel clearance is for softer
woods that produce larger chips and shavings. Hardwoods, which normally generate
much finer chips when drilled, can use the smaller end of the tolerance range.
While researching this story I cut mortises in every species of wood I could
find and this bit to chisel concept seems to hold up.
In
reality, most woodworkers use a bit to chisel setting in the middle of the range
for all of their work. This one-setting-fits-all approach usually works, but
remember this setting in case you encounter a problem with chip extraction in
the future as it might be the cure.
While
manufacturers sometimes provide instructions on how to attain this bit to chisel
setting, they are often more complicated than need be. Enterprising woodworkers
discovered that American coinage provided an alternative that simplifies the set
up task.
Locate
the recommended bit to chisel clearance in the instructions that came with your
mortiser, and then find a coin with a thickness that most closely matches that
number.
Insert
the chisel into its bushing and raise it up to the bushing face trapping the
appropriate coin between the bushing and chisel face (see photos) before
tightening the setscrew to secure the chisel. Insert the drill bit through the
chisel and into the chuck, holding the tip of the bit fully up against the end
of the chisel. Tighten the chuck to secure the bit. Loosen the chisel set screw,
remove the coin and slide the chisel up, butting it against the bushing. Make
sure the chip port in the chisel faces to the right or left, not front or back,
before tightening the setscrew to secure the chisel housing.
You
should now have the proper bit to chisel clearance for your
machine.
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