Drilling means making a cylindrical hole in a part with a drill bit, a rotating pointed tool that sinks into the material and clears the chips through its helical flutes. It is the most widespread machining operation, and often the first step towards a finished hole: you drill, then tap or bore.
Drilling is done on a drilling machine, a machining centre or a lathe. Drill bits are made of high-speed steel or solid carbide, with or without coolant through the tool, which clears the chips from deep holes. A spotting drill starts the hole to stop it wandering. Other tools extend the operation: the step drill for drilling two diameters at once, the counterbore for seating a screw head, the countersink for a conical recess. Beyond a depth of around ten times the diameter, we speak of deep-hole drilling, which requires specific tools and coolant.
A drilled hole is not very precise: its tolerance band is around 0.1 to 0.2 mm for a 10 mm diameter. For a demanding fit (bearing, pin), the hole is finished with a reamer or boring tool. Clearance holes for screws, fixing holes, lubrication holes and fluid passages appear on almost every part: plates, flanges, frames, enclosures. Drilling is part of CNC machining as well as mechanical assembly.
In a quote request. For each hole, state the diameter, the depth, whether it is through or blind, its dimensioned position and its tolerance (for example Ø 8 H7 if a bore is required). For a tapped hole, give the thread designation and the useful thread depth. Also flag cross or angled holes, which complicate set-up and deburring. The guide on machining tolerances explains how to dimension a hole.