In the spindle speed calculation for a 1/2 inch HSS drill bit in mild steel on a manual milling machine, what value of surface speed (SFM) is used?

• A. 50 ft/min
• B. 100 ft/min
• C. 150 ft/min
• D. 200 ft/min

Answer: (B)

Surface speed is the speed at which the edge of the drill meets the material, and it’s the main parameter used to set spindle RPM for a given tool and workpiece. For a half-inch high-speed steel drill bit drilling mild steel, a typical, safe cutting speed is about 100 ft/min. Using that value keeps the drill from overheating and maintains good tool life in steel.

If you plug into the standard formula for spindle speed N = (SFM × 12) / (π × D), with SFM = 100 and D = 0.5 in, you get N ≈ (100 × 12) / (3.1416 × 0.5) ≈ 760 rpm, which is a practical speed range on a manual milling machine for this setup.

Choosing a much lower SFM (like 50) would slow cutting and increase rubbing; a much higher SFM (like 150–200) can overheat an HSS drill in mild steel. Hence, 100 ft/min is the appropriate value.