My question is this :is there a trick to making a matching female/male template - or does the kerf (whether it be saw blade or router bit) always make this impossible to do in one step?
No trick - just math.
When using a template to cut a hole (i.e. the female part), the offset is equal to half the difference
between the guide bushing OD and the bit diameter. The resulting hole width (or diameter) will be twice this amount smaller than the template dimension since the offset goes all the way around. You need to make your template bigger than the desired hole by twice this offset.
When using a template to cut a filler (i.e. the male part), the offset is equal to half the sum
of the the guide bushing OD and the bit diameter. This resulting filler width (or diameter) will be twice this amount smaller than the template since the offset goes all the way around. You need to make your template bigger than the desired hole by twice this offset.
If the template and the bit is the same for both of the above, you have to use a smaller guide bushing when cutting the filler. If you do the math, the guide bushing for the filler piece needs to be twice the bit diameter smaller than the guide bushing for the hole.
-Hole Guide bushing = 1" OD
-Bit diameter = 1/4"
- The filler guide bushing must be 1"-(2 x 1/4") = 1"-(1/2") = 1/2".
A 1/4" diameter bit should fit inside a 1/2" diameter guide bushing so this solution should work. Make sure you consider the wall thickness of the guide bushing when selecting sizes. If you change the above example to use a 3/8" bit (i.e. change the 1/4" to 3/8" in the above example and do the math), the math still works but you can't fit a 3/8" bit thru a 1/4" bushing (this is actually a nonsense solution but it illustrates the point).
You could also change both the bit and the bushing to get the desired results but it's a bit more complicated. Here's the equation you have to solve: D1-d1=D2+d2 Where:
D1=Bushing OD for hole cut
d1=Bit dia for hole cut
D2=Bushing OD for filler cut
d2=Bit diameter for filler cut
One last thing to consider. If using a bit that has a larger shank diameter than cutting diameter in a plunge router, make sure the shank will clear the bushing ID at the desired plunge depth. You may need to use a smaller shank bit, or larger bushing to get what you need.