In the former article, the Down the Hole and Top Hammer Drilling method was introduced. Today we will explain more on rotary drilling method

Rotary Drilling Fundamentals

  • Rotary drilling is a versatile method for blast hole drilling.
  • It is the dominant method for holes larger than 203 mm (8”) diameter.
  • In smaller holes, it is most common in softer rock formations.
  • The drilling principle is to use a high pull down force (weight-on-bit), rotate the drill bit, and blow the rock cuttings to the surface with compressed air.
  • Hardrock drills typically use the Rotary method to drill blast holes from 200 mm (7 7/8”) to 311 mm (12 ¼”) diameter.






  • The critical drill features are pull down force, rotation power, and compressor capacity.
  • For optimum hole cleaning, the outside diameter of the drill pipe must be balanced with the hole size and air compressor.
  • All actual drilling is done by the drill bit and choosing the right bit has significant impact on productivity and total cost.
  • There is a variety of rotary drill bit cutting structures to select from for different rock formations.
  • Visit Hardrockdrills for the newest technology and learn more about proper bit selection.

Drilling Parameters

  • The pull down force must be sufficient to fracture the rock.
  • Excessive force causes bit wear and vibration damage to the drill.
  • The optimum pull down depends on rock strength and bit size. For example, in a typical coal mine with a rock strength of 10 000 psi (69 MPa), the optimum pull down is about 67 kN (15 000 lb ).
  • The compressor output should be matched with the drill pipe and bit to give an up-hole air velocity of 30-40 m/s (6000-8000 ft/min).
  • An over sized compressor causes excessive steel wear and fuel consumption.
  • Theoretically, once enough force has been applied to overcome the strength of the rock, increased rotation speed should result in higher penetration rates.
  • In reality, vibrations and bit bearing life limit the maximum rotation.

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