I am making small (.67" diameter) scale model wheels. I use blanks cut from aluminum bar stock. I prepare the blank on the lathe, face and cut to length. The blank is clamped to the table, and the spindle is centered over the blank using a needle point. The X&Y dro's are set to zero. The z height is set with the end mill and is zeroed. I run the part, and the final product is off center by about .04. what am I doing wrong?