After cutting several hundred feet of hard rock the rate has slowed down. What is the probably cause?

• A. Formation became more porous.
• B. Operator fatigue.
• C. Bit wear or dulling, reducing cutting efficiency.
• D. Increased pump pressure.

Answer: (C)

In air rotary drilling, the rate at which you advance depends largely on the cutting efficiency of the bit. When drilling hard rock, the cutting teeth do the work of shearing the rock, so as the bit wears and the edges become dull, each revolution cuts less effectively. That means less rock is removed per pass, and the penetration rate slows down even if other conditions stay the same. Bit wear or dulling directly reduces cutting efficiency, making it the most likely cause after several hundred feet of hard rock. Other options don’t fit as well: porosity changes don’t directly impede the cutting action in this context, increased pump pressure isn’t a factor in a dry air rotary setup, and fatigue could contribute but isn’t as consistently tied to a sustained slowdown in hard rock as worn cutting edges are.