For a 12-inch cast iron pulley rim, the recommended turning speed is

• A. 20 RPM
• B. 200 RPM
• C. 2 RPM
• D. 120 RPM

Answer: (A)

Keeping the cutting edge moving at a safe, appropriate surface speed for cast iron is what guides turning speed. The relationship between surface speed (SFM), RPM, and diameter is SFM = (RPM × π × diameter) / 12, so RPM = (SFM × 12) / (π × diameter).

For cast iron, a typical roughing or finishing surface speed is around 60 SFM. With a 12-inch diameter, that gives RPM ≈ (60 × 12) / (π × 12) ≈ 19 RPM, which rounds to about 20 RPM. So 20 RPM keeps the surface speed near the recommended range for a 12-inch cast iron pulley rim.

The other speeds would push the surface speed far outside that safe range: 200 RPM would yield about 628 SFM—too fast and risks overheating and tool wear; 2 RPM would be about 6 SFM—too slow for practical cutting; 120 RPM would be about 378 SFM—also too fast for this size.