How does down pressure vary due to formation changes in air rotary drilling?

• A. Down pressure decreases in harder formations
• B. Down pressure increases in harder formations; decreases in softer formations
• C. Down pressure remains constant
• D. Down pressure is not related to formation hardness

Answer: (B)

The weight on the bit should match the rock’s resistance. When you hit harder formations, the bit meets more resistance, so you need more down pressure to push the bit into the formation and keep it cutting effectively. In softer formations, the rock breaks away more easily, so less down pressure is needed to maintain contact and prevent excessive bit wear or wandering. So, down pressure increases with harder formations and decreases with softer formations. This approach helps maintain a steady rate of penetration and efficient drilling. The idea that down pressure stays the same or isn’t related to formation hardness doesn’t fit how cutting work responds to rock strength.