Drilling is the process of creating or enlarging holes by rotary movement of either a circular single or multi-edged tool or of the workpiece.
All drilling processes involve two motions relatively between the tool and workpiece simultaneously: a rotation around and a linear movement along the tool’s centerline axis. Usually, it is the drilling tool rotating and moving forward, but in certain applications, as, for instance, on a lathe, it can be the workpiece rotating and the tool moving forward.
In general, one distinguishes between conventional drilling, where either a new hole is being produced or an existing hole is enlarged; center drilling, where a hole of a particular shape is produced (in most cases, this hole is subsequently used to guide another drilling tool); and trepanning, where an annulus is being cut out of the workpiece material. In conventional drilling, one also differentiates between a hole that goes all the way through the workpiece, as the so-called through hole, and a hole that ends somewhere inside the workpiece, which is referred to as a blind hole.
Drilling exhibits a number of peculiarities, which, as is commonly agreed on, makes it more complex than many other metal cutting operations. One of these peculiarities is that the rotary motion between the cutting tool and workpiece results in the cutting speed not being constant along the cutting edge(s). At the center of the drill, the cutting speed is zero, from where it increases linearly with the radius, reaching its maximum at the drill’s periphery. Moreover, twist drills do not exhibit a constant rake angle along their cutting lips. Toward the drill’s center, the rake angle is even negative, resulting in the cutting edges plowing through the material rather than cutting it, causing the drill’s core to be exposed to considerable compressive forces.
Another reason for the high complexity of drilling is that drills must perform under severe machining conditions, because the cutting zone is located deep within the workpiece. As the cutting zone is not accessible during the drilling operation, the removal of cutting heat is restricted, which causes a strong heat load on the drill. This heat load is further increased by the constant rubbing of the drill’s margins along the borehole wall.
Another problem is that the chips produced at the drill’s tip have to be evacuated through the drilled hole, a certain portion of which is occupied by the drilling tool itself. With growing borehole depth, the chip evacuation becomes increasingly difficult, making it more and more likely for the chips to jam inside the flutes, which can result in a sudden (premature) fracture of the drilling tool.
