Workshop Calculation: Cutting Speed, Feed, RPM and Machining Time
Cutting speed, feed, rpm and machining time
This domain tests the relationship between cutting speed, workpiece diameter, spindle speed, feed rate and the time to machine a length. It applies to turning, drilling, milling and similar operations.
Core rules
- Cutting speed v = πDN/1000, where D is diameter in mm, N is rpm and v is m/min.
- Spindle speed N = 1000v/(πD).
- Feed is usually expressed in mm/rev; feed rate in mm/min = f × N.
- Machining time for a single pass: t = L/(fN), where L is the total travel including approach and overrun.
- For drilling, add an approach allowance; a common estimate is about 0.3D for a 118° drill point, but use the allowance stated in the question.
Worked example
A 50 mm diameter bar is turned at v = 30 m/min. N = 1000 × 30/(π × 50) = 191 rpm approximately. Feed f = 0.2 mm/rev, so feed rate = 0.2 × 191 = 38.2 mm/min. Cut length = 120 mm, plus 5 mm approach = 125 mm. Time = 125/38.2 = 3.27 min. The reasoning is to find rpm from the diameter and cutting speed, then divide total tool travel by the feed rate.
Traps
- Using radius instead of diameter in v = πDN/1000.
- Forgetting the 1000 factor when D is in mm and v is in m/min.
- Mixing feed per revolution with feed per minute.
- Omitting approach and overrun from the tool travel.
- Rounding rpm heavily before calculating time.