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.
- 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.
- Selection of DTH Drilling Tools
- General Drilling Method and Application
- DTH Drilling Care & Maintenance – Part One
- DTH Drilling Care & Maintenance – Part Two
- Basic DTH and Top Hammer Drilling Knowledge
- Well Drilling Method
- Rock Properties
- Rock Drillability
- How to Repair Drill Bit
- DTH(Down The Hole) Drilling Application