In hard rock drilling with roller cone bits, which statement correctly describes the speed and feed relationship?

• A. Low RPM requires higher down pressure (feed) for effective penetration
• B. High RPM with too much feed always improves drilling efficiency
• C. Low RPM with high feed reduces bit wear
• D. RPM and feed are independent of rock type

Answer: (C)

In hard rock drilling with roller-cone bits, how fast the bit spins and how much weight you push on it must work together to keep cutting action efficient while keeping wear in check. The cutters wear primarily from how aggressively they engage rock and how much heat is generated. Running at very high rotational speed tends to drive rockbreaking events too quickly, increasing heat, bearing wear, and tooth abrasion, especially if the feed isn’t adjusted accordingly. Lowering the RPM while increasing the feed keeps the teeth in positive cutting contact rather than letting them just skim or rub, so rock is fractured effectively per revolution but with less heat buildup and bearing stress. This combination tends to reduce overall bit wear while maintaining penetration in hard rock.

That’s why the idea of operating with lower speed and higher feed is favored for minimizing wear on roller-cone bits in hard rock.