There are several reasons this could be happening: Check to make sure that your drill is not bent; even the smallest bend will produce oversized holes. Next check to make sure that your tool holder is holding the drill tight. If you are still having problems next check to make sure that your drill is split correctly, an over split drill can cause oversized holes. Next make sure that you have correct relief on the drill, if not then make small adjustments to your relief setting until you get properly sized holes. If none of these solutions are working then re-grind your drill making sure that you use even pressure when sharpening both sides of the drill because even a slight difference in the material takeoff could cause oversized holes.