What cutting speed and feed are, and how to calculate drill RPM with the formula.
Cutting speed
Cutting speed (V) is the speed at which the cutting edge passes over the material, given in metres per minute (m/min). It depends on the job material, tool material, coolant and finish required.
| Material (HSS drill) | Cutting speed (approx.) |
|---|---|
| Mild steel | 25 – 30 m/min |
| Cast iron | 20 – 25 m/min |
| Brass | 40 – 60 m/min |
| Aluminium | 60 – 90 m/min |
| Tool steel | 10 – 15 m/min |
Spindle speed (RPM)
V = (π × D × N) ÷ 1000
N = (1000 × V) ÷ (π × D)
N = RPM, V = cutting speed in m/min, D = drill diameter in mm
N = (1000 × V) ÷ (π × D)
N = RPM, V = cutting speed in m/min, D = drill diameter in mm
Example: Find the RPM for a 10 mm drill in mild steel at a cutting speed of 25 m/min.
N = (1000 × 25) ÷ (3.14 × 10) = 25000 ÷ 31.4 ≈ 796 RPM. Select the nearest lower speed on the machine.
Remember Bigger drill → lower RPM. Smaller drill → higher RPM (for the same material).
Feed
Feed is the distance the drill moves into the job in one revolution, in mm/rev. Smaller drills get less feed (e.g. 0.05–0.1 mm/rev); larger drills more (0.2–0.3 mm/rev). Too much feed breaks the drill; too little causes rubbing and work hardening.
Quick Revision
- Cutting speed in m/min; feed in mm/rev.
- N = 1000 V ÷ πD.
- 10 mm drill, MS, 25 m/min → ~796 RPM.
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