Spindle speed and feed

How it is calculated

Spindle speed: n = 1000 × Vc / (π × D), where Vc is cutting speed in m/min and D is the drill or workpiece diameter in mm. For carbide drills the table speed is multiplied by 0.8, because a drill works in harsher conditions than a turning insert.

Drill feed in structural steel: up to 6 mm - 0.08 mm/rev, up to 12 mm - 0.15, up to 20 mm - 0.22, above 20 mm - 0.3 mm/rev. Multiply by 0.7 for stainless and by 1.4 for aluminium. Turning feed: 0.15 mm/rev for HSS and 0.25 mm/rev for carbide. Feed rate = n × f.

Example: a 10 mm HSS drill in steel - 1000 × 25 / (3.14 × 10) ≈ 796 rpm, feed 0.15 mm/rev, feed rate about 119 mm/min.

Cutting speed by material

Practical tips and common mistakes

• Benchtop drill presses usually top out at about 2500 rpm. A 6 mm drill in aluminium wants roughly 3200 rpm, so you cannot reach the optimum - use the top step and do not drop the feed to nothing.
• Do not drill stainless gently: a light feed and excess speed work-harden the metal and the drill stops cutting. Keep a steady feed without dwelling.
• Cast iron produces powdery chips and is cut dry; coolant mixed with cast iron dust turns into an abrasive paste.
• The numbers are a starting point. If the chips turn blue or the drill squeals, drop one speed step.
• Before tapping, size the hole with the tap drill calculator.

When these formulas are not enough

For deep holes (over 5 diameters), hardened steels, titanium and interrupted cuts on lathes, take cutting data from the tool maker's catalogue - there it is tied to a specific carbide grade and coating.