Rotary Drilling Method

March 4, 2018

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.

Hardware

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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