So the biggest problem I see with making a router jig is the location
of the router bit. If you want the jig to be variable, at least one of
the two straight pieces has to be able to pivot, but the router bit is
sitting right at the pivot point. If it pivots about any other point,
it will only line up with the edge of the router bit for one particular
angle. Otherwise, the edge of the bit will be offset from the "crotch"
of the V.

One solution is to let one arm pivot an inch or so away from the bit
and just compensate with the placement of your guide nails, but it
might take some practice to figure out how to adjust them.

I think the better way to do it is to fix the arms with bolts which
pass through slots in the base plate . The slots would be arc-shaped
and concentric about the router bit. Personally, I'd cuts arcs that
ran a full 45 degrees so that the arms could be adjusted from straight
apart (180 degrees) down to 90 degrees apart. At 90 degrees seperation
of the arms, you could cut a perfect half-circle. By slightly
oversizing the slots relative to the bolts you could have a little
extra "slop" so that you could line up the arms with the either edge of
the bit (depending on whether you were cutting an inside arc or an
outside one).

If you were cutting a peninsula countertop, you could simply mark the
"corners" of the arc, screw in some temporary wood blocks that line up
with your marks instead of guide nails, mark the furthest point you
want the arc to hit out at the end of the counter, and adjust the angle
of the arms until the router bit reaches just that far. No
trigonometry needed.

Josh


I think you could just screw on a couple of temporary wood blocks to
the countertop