In the work of the car, the tip of the blade and the center of the workpiece have any impact on the workpiece roughness.
The tip of the knife is higher than the center of the workpiece, and the finish will be better because of the continuous contact with the workpiece. It is below the center of the workpiece, because only the cutting edge cuts the workpiece, the smooth finish is not good.
And if the workpiece is very small in diameter, the tip of the knife should not be lower than the center of the workpiece, or it will be easily broken.
A large diameter piece of work is a little bit shorter and it doesn't matter. When you have a very thin piece of diameter, you want to be as flat as possible
