Cutting Speed & Feed Rate
Converts between spindle speed and cutting speed for turning, milling and drilling, and gives the feed rate. Give a diameter and a spindle speed to get the cutting speed and the feed rate, or give the target cutting speed for the tool and work material to get the spindle speed to set. Machinists and manufacturing engineers use it to set up jobs and to write CNC programs.
Equations
V = π·D·N fr = f·N N = V/(π·D)
V is the cutting speed, the surface speed at which the cutting edge passes over the work. D is the diameter being cut: the workpiece in turning, the cutter in milling or drilling. N is the spindle speed in revolutions per minute, f the feed per revolution and fr the feed rate, the speed of the tool along its path [1]. With D in mm and V in m/min, N = 1000·V/(π·D).
Recommended cutting speeds come from tool makers’ catalogs and handbooks for each pairing of tool and work material [1]. The page turns that speed into a spindle setting; it doesn’t choose the speed for you.
Limits and common mistakes
- In turning, the cutting speed falls as the diameter shrinks. Use the diameter at the cut. When facing toward the center, use constant-surface-speed mode on a CNC lathe, with a maximum spindle speed.
- In milling, feeds are given per tooth. The feed per revolution is the feed per tooth times the number of teeth.
- Speeds in tables are often in ft/min (SFM). Pick ft/min in the unit list rather than converting by hand.
- Round the spindle speed to one the machine can run, and check it against the machine’s and the chuck’s maximum.
Worked example
A 60 mm steel bar is to be turned with a carbide insert rated for a cutting speed of 180 m/min at a feed of 0.25 mm/rev. Find the spindle speed, then the cutting speed and feed rate at the nearest available speed of 950 rpm.
- Choose Solve For Spindle speed, given target cutting speed.
- Enter Diameter (D) 60 mm.
- Enter Target Cutting Speed (Vc) 180 m/min and press Solve. The Solution shows: Required Spindle Speed (N): 954.93 rpm
- Choose Solve For Cutting speed & feed, given spindle speed.
- Enter Spindle Speed (N) 950 rpm and Feed per Revolution (f) 0.25 mm, and press Solve. The Solution shows:
Cutting Speed (V): 179.071 m/min
Feed Rate: 237.5 mm/min
Check: 1000 × 180/(π × 60) = 954.9 rpm, and 0.25 mm × 950 rpm = 237.5 mm/min. Running at 950 rpm is 0.5 % under the target speed, well within the range a catalog speed allows.
