First load the sound track.
http://www.stevenson-engineers.co.uk...dambusters.wav
or
http://www.stevenson-engineers.co.uk...dambusters.mp3
Now read the history.
http://en.wikipedia.org/wiki/The_Dam_Busters_(film)
Now watch the pictures.
Two cross haired lasers [ no it's not a breed of dog ] mounted either side the router head at 45 degrees. My finger is over one so only the other shows.
The camera is picking the laser line up a lot fatter than it actually is, in real life it's quite fine.
Both lasers shining together, head is too low at this point.
Raise head.
Both lasers converged, bombs away. That is bang on under the centre of the tool and Z hight is exactly 40mm above the work.
Set tool hight, zero axis and open bomb bay doors.
In real life I can get between 5 thou and 10 thou of where I want to be, may not be accurate enough for some in metal but for the router guys who work from the start point then everything from that point is accurate, mostly outside shapes are discarded.
Won't suit everyone but it's very quick.
[Edit] the tracks in the chipboard as the outline of Phenolic brush rings out of large DC motors. It can make a complex ring with curved slots in about 2 minutes.
John S.