9Mm Utility Knife,Retractable Utility Knife,Box Cutter Knife,Retractable Cutter Blade Yucheng Weisite Measuring Tools Co., Ltd , https://www.wsttool.com When observing a hole drilled with a gun drill, it is sometimes found that there are annular grooves (annular cuts) in the hole, sometimes periodically on the entire circumference, or on a part of the circumference, and sometimes along the length of the hole, if a hole is required. When the surface roughness is good, this is not allowed. This phenomenon mostly occurs on steel with good soft toughness.