Say I want to cut a hole in a table to fit an irregular but mostly
cylindrical piece. The pattern (the cylinder) is a negative pattern;
it's the shape of the material to be removed.
The only approach I can come up with is to fix some template material
on top of the cylinder, and use a flush cut router bit with a bottom
guide to cut a positive template the size of the cylinder plus the
diameter of the bit.
Then, I use the positive template with a template following bushing to
cut the hole in the table. The hole in the table will be smaller than
the hole in the positive template by the bushing radius - the bit radius.
For example, if I'm using a 3/4" diameter bit (3/8" radius) and a 2 1/4"
bushing, 2 1/4" = 9/4" diameter, 9/8" radius.
3/4" = 6/8" = 9/8" - 3/8"
Is this the best way? Assume that the "cylinder" is not regular, so I
want to trace it and not just measure the diameter and cut a circle of
that size. And, tracing around the cylinder and then carefully
cutting inside the line doesn't work because I'm clumsy and impatient.
I'm really looking for a general procedure for cutting an opening the
same size and shape as an object, using a router. Seems like there
should be a better way to do this, but I can't think of it.