|Router Fence Alignment
generally like to cut dadoes with my table saw and dado blade. But some pieces
are just too big to do this safely. So with especially wide or long panels, I
often use a hand-held router and a long straightedge.
The trick is clamping the straightedge in just
the right place. Layout lines will show you where the dado needs to be cut. But
you also have to allow for the router base, which means measuring the distance
from the edge of the base to the cutting edge of the bit. This measurement can
then be transferred to the workpiece. However, it's easy to see that some
errors could creep into this process. And with a large piece of plywood, any
error can be costly.
So for accurate dadoes, I make a simple dado
alignment gauge. It's just a piece of scrap with a dado cut across it. You can
use the gauge to align the straightedge fence parallel to your layout lines, as
you can see in the drawing above.
To make the gauge, clamp a piece of
scrap down to the bench and clamp a higher fence at one end, see drawing below.
(I used a 2x4 block.) Now mount the bit in the router and run the router base
against the high fence to rout a shallow dado across the piece of scrap. (The
depth of the bit isn't critical at this point.)
At this point, you're
ready to use the gauge. Simply turn it over on the panel so the dado aligns
with the layout lines, as you can see in the drawing above. Then butt the
straightedge fence against the end of the gauge and clamp it down. Repeat this
process at the opposite end of the straightedge, and you'll be ready to rout
the dado. The result should be a dado that matches the layout lines
One more thing. Since a router base can be mounted off center
in relation to the bit, always keep the router facing the same direction that
it was when you routed the dado in the alignment gauge.