JNTU B.Tech IV ROCK FRAGMENTATION ENGINEERING Previous Question Paper (October/November - 2020 - R16)

Rotary-percussive drilling combines two distinct mechanical actions to achieve rock fragmentation: high-frequency percussive impacts and continuous rotary motion. The process relies on the interaction between the drill bit (usually a button bit) and the rock surface under high axial thrust.

The mechanics of breakage involve three primary stages:

1. Percussive Impact: The piston strikes the drill string, transmitting a stress wave to the bit. This impact generates a high-intensity compressive stress pulse that exceeds the compressive strength of the rock, causing local crushing and the formation of a crater beneath the bit buttons.
2. Rotary Indexing: Between successive blows, the drill string rotates by a specific angle. This ensures that the next impact occurs on a fresh, undamaged portion of the rock face, preventing the bit from striking the same location repeatedly and maximizing the volume of rock removed per blow.
3. Shearing and Crushing: The combination of the percussive force and the torque applied through rotation causes the rock between individual craters to shear off. The continuous rotation also helps in sweeping the crushed rock particles (cuttings) away from the bottom of the hole towards the flushing medium.

The efficiency of this process is governed by the energy transfer from the piston to the rock, the frequency of blows, the rotation speed, and the flushing efficiency. A sketch would illustrate the bit face with tungsten carbide buttons, the direction of the axial impact force, the rotational force (torque), and the resulting crater formation pattern on the rock surface, with arrows indicating the flushing path for debris removal.

The general theory of rock cutting describes the mechanics of how mechanical tools, such as disc cutters or picks, interact with rock to induce fracture and fragmentation. The process is governed by the interaction between the tool's geometry, the rock's physical properties, and the state of stress applied.

The mechanism of rock cutting typically involves three distinct phases:

1. Crushing Phase: As the tool makes contact with the rock surface, high compressive stresses develop directly beneath the tool tip. This creates a zone of pulverized rock known as the crushed zone or the 'crushing bulb'.
2. Crack Initiation and Propagation: As the penetration increases, tensile stresses develop at the boundaries of the crushed zone. When these stresses exceed the tensile strength of the rock, radial cracks initiate and propagate into the intact rock mass.
3. Chip Formation: As the tool advances, the cracks propagate towards the free surface or intersect with cracks from adjacent cutting paths. This leads to the detachment of rock fragments, commonly referred to as chips. The efficiency of this process is defined by the Specific Energy, which is the work done per unit volume of rock excavated.

The efficiency of rock cutting is influenced by several key factors:

• Rock Properties: Compressive strength, tensile strength, hardness, and abrasivity significantly dictate the force required for cutting.
• Tool Geometry: Parameters such as the tip angle, rake angle, and clearance angle determine the distribution of forces and the ease of chip formation.
• Operational Parameters: The depth of cut, spacing between cutters, and the speed of the tool influence the cutting force and the size of the chips produced.

Optimal rock cutting occurs when the spacing between cutters is proportional to the depth of cut, allowing for the interaction of stress fields between adjacent cuts to promote maximum chip volume with minimum energy consumption. If the spacing is too large, the rock between cuts remains intact, leading to inefficient 'grooving' rather than effective fragmentation.