Drill Speed Calculator

Drilling inverts the usual machining calculation: material fixes the cutting speed, diameter is given, and spindle speed is the unknown, so N = Vc x 1000 / (pi x D). The speed being controlled is the peripheral speed at the outer corner of the drill, the fastest-moving and hottest point on the tool. Because the same formula spans every diameter, RPM scales inversely with size: at 30 m/min a 5 mm drill wants roughly 1900 RPM while a 20 mm drill wants about 480. Typical high-speed-steel speeds are 25 to 35 m/min in mild steel, 20 to 30 in cast iron, 10 to 15 in austenitic stainless, 60 to 100 in aluminium, with solid carbide two to four times higher. Feed is per revolution, commonly 0.05 mm/rev for small drills in hard material up to 0.3 mm/rev for large drills in soft, with roughly 0.02 times diameter in millimetres as a first estimate. Twist drilling has a geometric weakness these numbers cannot capture: at the chisel edge across the centre, cutting velocity is zero, so material is extruded rather than cut. That is the source of most drilling thrust and of walking on entry, and the reason for spot drilling or web thinning. Chip evacuation is the other constraint, since chips must spiral back up the flutes against the feed direction; past about three diameters of depth, peck cycles retracting 0.5 to 1.5 diameters prevent packing. The 118 degree point suits mild steel, while a 135 degree split point is self-centring and lowers thrust in harder material.

The modern twist drill is credited to Stephen A. Morse of Massachusetts, who patented the design in 1863 and founded the Morse Twist Drill and Machine Company at New Bedford in 1864. Before this, holes were made with flat spade bits forged and ground by hand, which cut poorly and cleared chips worse. Morse's contribution was the helical flute, giving a consistent rake angle along the cutting lips and a continuous channel for chips to escape; the same firm's self-holding taper shank became the Morse taper still used on drill presses and lathe tailstocks. The drill followed the same materials progression as other cutting tools, moving from carbon steel to high-speed steel after 1900, then cobalt-bearing HSS grades, titanium nitride and titanium aluminium nitride coatings from the 1980s, and solid carbide, whose rigidity permits far higher speeds but demands better runout control. Geometry and dimensions were standardized during the twentieth century through documents such as ISO 235 for parallel-shank drills and ANSI/ASME B94.11M in the United States, fixing diameter series, point angles and tolerances. The most significant recent change is in coolant delivery and control rather than the drill: through-spindle high-pressure coolant and programmed peck cycles allow depth-to-diameter ratios well beyond what a manual drill press could achieve.