I don't know how big the part in question is but I have found that I use an edge finder. I start as close to one end of radius as possible and zero out the x axis. then I move the edge finder in y axis until it touches part. Lift up the z axis, and move 1/2 the diameter of edge finder toward part. Then zero out the y axis. Then I move a standard distance in x, say .500 and repeat the step for y. Basically you are looking for three points on a curve. When you input the 3 x and y values into a cam, assuming your cam can generate an arc from 3 points, it will give you the radius. I hope this is clear enough.