CNC speeds and feeds formulas, with a worked example

Convert cutting speed into spindle speed

For metric inputs, spindle speed in revolutions per minute is cutting speed in metres per minute multiplied by 1,000, then divided by π times the effective cutting diameter in millimetres.

Using a 10 mm cutter at 100 m/min gives approximately 3,183 RPM: 100 × 1,000 ÷ (π × 10). A smaller cutter must rotate faster to maintain the same surface speed.

Convert chip load into table feed

Table feed equals chip load per effective tooth multiplied by RPM and the number of effective cutting teeth. At 0.05 mm per tooth, 3,183 RPM and four effective teeth, the calculated feed is about 637 mm/min.

Use effective teeth rather than blindly entering the flute count. Engagement and tool geometry can mean not every flute contributes in the same way for a particular operation.

Treat the result as a starting point

The formulas keep the variables consistent, but they cannot see machine rigidity, runout, workholding, tool overhang, radial engagement, coolant, chip evacuation or the exact material condition.

Start with cutting data from the tool manufacturer and remain within machine and spindle limits. Reduce or adapt the setting when the real setup is less rigid than the reference conditions.

• Check maximum spindle speed and available power
• Confirm units before transferring values
• Listen for chatter and inspect chip formation
• Record proven settings with the material and operation

Estimate cutting time separately

Ideal cutting time is cutting length divided by table feed. Production time also includes acceleration, rapid moves, tool changes, probing, repositioning and operator work. Keep the feed calculation and the job quotation model separate so hidden time is not accidentally ignored.