Milwaukee’s® 21 PC. Thunderbolt® Black Oxide Set offers superior durability and long operational life with its strong oxide coating and Thunderbolt web core. These black oxide drill bits feature parabolic flutes that drive debris and chips away from the bit to reduce heat from friction. A 135 split-point tip bits into the drilling surface on contact to keep the bit from walking so you get the exact hole you need with no damage to your work piece. These black oxide kits range in size from 1/16-in. to 1/2-in. in 1/64-in. increments to make a hole in virtually any size your job requires. The 3-Flat Secure-Grip™ shank keeps bits secure in the chuck, which results in straighter holes and reduced damage to the drill. The bit shanks fit 3/8-in. chucks.