How to calculate drill speeds and feeds
Getting drill speed and feed right is the difference between clean holes and long tool life, versus burnt material and broken bits. The two numbers that matter are how fast the drill spins (RPM) and how fast it advances into the work (feed rate).
Spindle speed (RPM)
RPM comes from the material's recommended cutting speed and the drill diameter. Bigger drills spin slower for the same surface speed:
Cutting speed depends on the material and drill type. As a guide for HSS drills: mild steel ≈25, stainless ≈12, aluminium ≈80, brass ≈45 m/min. Cobalt drills run around 1.5× faster, carbide several times faster.
Feed rate
Feed rate is how fast the drill moves down, set by the RPM and the feed per revolution:
Feed per revolution scales with drill size — roughly 0.04 mm/rev at 3 mm up to 0.32 mm/rev at 20 mm. Too high a feed overloads and breaks the drill; too low causes rubbing, heat and work-hardening, especially in stainless.
Drilling time
Once you know the feed rate, drilling time is simply the hole depth divided by the feed rate. Deep holes need peck drilling — pecking and retracting to clear chips — which adds roughly 10–15% to the time.
Tap drill size
To cut an internal thread you first drill a hole slightly smaller than the thread, then run a tap through it. For metric coarse threads the tap drill is approximately the thread diameter minus the pitch — an M6 × 1.0 thread uses a 5.0 mm drill. The clearance drill is slightly larger than the thread, for bolts to pass straight through.
Frequently asked questions
How do I calculate drill RPM?
RPM = (cutting speed × 1000) ÷ (π × diameter), with cutting speed in m/min and diameter in mm. Cutting speed depends on material and drill type.
How do I calculate drill feed rate?
Feed rate (mm/min) = RPM × feed per revolution. Feed per rev runs roughly 0.04 mm/rev at 3mm up to 0.32 mm/rev at 20mm.
What cutting speed should I use for drilling steel?
For HSS: mild steel ≈25, stainless ≈12, aluminium ≈80, brass ≈45 m/min. Cobalt ≈1.5× faster, carbide several times faster.
What is a tap drill size?
The hole you drill before tapping a thread — roughly thread diameter minus pitch. An M6×1.0 thread uses a 5.0mm tap drill.