Mortising by Machine

Q & A: Mortising By Machine

Q:

I’m having trouble with my
mortising machine. I cut mortises
by slightly overlapping each hole,
but I’ve broken a few bits and my
chisels have overheated and turned
blue. Is there a problem with my
method?

A:

Yes, there’s a better method than
overlapping holes. When you overlap,
you’re essentially cutting a hole with
three sides. It’s like trying to drill
overlapping holes with a twist bit.
The bit wants to follow the path of
least resistance. It walks into the
neighboring hole, bending as it goes.
When a mortising bit cuts an overlapping
hole, it bends, too, and rubs
against the side of the chisel. Both get
hot. The chisel turns blue as the
temper is drawn out, and
the bit eventually snaps.

The solution is to drill
four-sided and two-sided
holes. Drill the two ends
of the mortise first. Because
the bit meets equal resistance
on all four sides, it
goes straight down.

Now, instead of overlapping,
leave a space about half the width of
the bit, and drill another hole. Keep
skipping half a hole like this until you get to
the other
end of the mortise.

Next, drill two-sided
holes in the half spaces. There’s
equal resistance on opposing sides of
the bit, so the bit travels straight down.