Mayday.

                       twist drill & hand tools          Drill bits are cutting tools used to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order to create holes drill bits are usually attached to a drill, which powers them to cut through the workpiece, typically by rotation. The drill will grasp the upper end of a bit called the shank in the chuck. The above is the definition of twist drill, and then introduce the structure of twist drill.The standard twist drill consists of three parts. Clamping part:  the end of a drill bit, used for machine tool coupling and for transferring torque and axial force.According to the diameter of twist drills, they are divided into straight shank (d < 12mm) and taper shank (d > 12m...

     1) drill bit holes size can vary based on rpm, feed, material being drilled, coolant or no coolant, how drill sharpened, etc many many variables. also drill bits tend to have a back taper or smaller dia near shank. this helps with drilling but obviously as drill sharpened it gets to be smaller dia .      2) best drill can be different. some want harder drill which can be brittle and break easy. some want softer but tougher drill which gets dull faster. there are a lot of variations in drill bits. nothing is the best for everything.