Machining Calculations (cutting speed, feed, rpm, machining time)
Machining calculations relate cutting speed, spindle speed, feed and length to time and output. Standard formulas use diameter in mm and speed in m/min.
Formulas
- Cutting speed: V = πDN/1000 (m/min), so N = 1000V ÷ (πD).
- Feed f is in mm/rev; feed rate = f × N mm/min.
- Turning time: T = L ÷ (f × N), where L includes approach and overtravel.
- Drilling time: T = (L + 0.3D) ÷ (f × N) for through holes, where 0.3D approximates approach.
- Milling time: T = L ÷ table feed in mm/min.
- Material removal rate for turning is approximately V × f × d with consistent units.
Worked example
Drill a 20 mm through hole in mild steel, 40 mm deep, at V = 25 m/min and f = 0.2 mm/rev. N = 1000 × 25 ÷ (π × 20) = 397.9 rpm; use 400 rpm. L = 40 + 0.3 × 20 = 46 mm. T = 46 ÷ (0.2 × 400) = 46 ÷ 80 = 0.575 min = 34.5 s. At 800 rpm the time halves to 17.25 s, but tool life and surface finish may suffer.
Traps
- Using radius in πDN/1000.
- Forgetting approach and overtravel.
- Mixing feed per revolution with feed per minute.
- Using V in mm/min without converting to m/min.
- Assuming time falls proportionally without checking power and tool life.