Drill Speed RPM
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RPM is derived from the recommended surface (cutting) speed for the material being drilled and the drill bit's diameter: RPM equals surface speed divided by (pi times diameter), with a unit conversion factor depending on whether you're working in inches or millimetres.

For a fixed cutting speed at the edge of the bit, a larger diameter bit covers more distance per rotation, so it needs fewer RPM to maintain the same edge speed. This is why large bits run much slower than small bits in the same material.

Harder materials generate more heat and tool wear per unit of cutting, so they call for lower cutting speeds (and thus lower RPM at a given diameter) than soft materials like aluminum or wood, which tolerate much faster cutting without overheating the bit.

Excess speed generates heat that can rapidly dull or burn the cutting edges, especially in steel, and can cause the bit to walk or chatter rather than cut cleanly. In softer materials like plastic, too much speed can melt the material around the hole instead.

Yes, high-speed steel (HSS) bits, cobalt bits, and carbide bits tolerate different cutting speeds in the same workpiece material, with carbide generally allowing significantly higher speeds than HSS because it retains hardness at higher temperatures.