Thanks but yes you have misunderstood. It is the center of the tools position I am after when it is at the end of the straight bit. It is where the tool path for the straight bit crosses the tool path for the circular bit. Or put it another way it is the difference in the angle between the center and the bit where the straight joins the curve (which I know as that was part of defining the shape) and the angle between the tool center and the center of the hole. These angles are not the same as I first thought.