JNTU B.Tech IV UNCONVENTIONAL MACHINING PROCESSES Previous Question Paper (December - 2020 - R16)

Unconventional machining processes are classified based on the primary form of energy used to remove material from the workpiece. This classification is essential for understanding the mechanism of material removal, which typically involves thermal, mechanical, electrochemical, or chemical energy.

The classification is as follows:

• Mechanical Energy Processes: These processes utilize high-velocity abrasive particles or mechanical impact to erode material. Examples include Abrasive Jet Machining (AJM), Water Jet Machining (WJM), Abrasive Water Jet Machining (AWJM), and Ultrasonic Machining (USM).
• Thermal Energy Processes: These processes rely on the conversion of electrical or light energy into thermal energy, causing material removal through melting and evaporation. Key examples are Electrical Discharge Machining (EDM), Wire Electrical Discharge Machining (WEDM), Laser Beam Machining (LBM), Electron Beam Machining (EBM), and Plasma Arc Machining (PAM).
• Electrochemical Energy Processes: These processes utilize the principle of electrolysis to dissolve material at the atomic level. The primary example is Electrochemical Machining (ECM), along with its variants like Electrochemical Grinding (ECG) and Electrochemical Deburring (ECDB).
• Chemical Energy Processes: These processes involve the controlled dissolution of material using chemical reagents or etchants. The main example is Chemical Machining (CHM), which includes chemical milling and chemical blanking.
• Hybrid Processes: These combine two or more of the above energy sources to enhance machining performance, such as Electrochemical Discharge Machining (ECDM) or Abrasive Electrical Discharge Machining (AEDM).

The choice of process depends on the material properties, such as hardness and electrical conductivity, as well as the required surface finish, accuracy, and geometry of the component.

Ultrasonic Machining (USM) is a mechanical material removal process where material is removed by the abrasive action of hard grit particles suspended in a slurry, driven by a tool vibrating at ultrasonic frequencies (typically 20 kHz). The process relies on three primary material removal mechanisms:

1. Hammering action: The tool vibrates at high frequency, forcing abrasive particles to strike the workpiece surface with high impact energy, causing brittle fracture and micro-chipping.
2. Impact of free-flying abrasives: Abrasive particles gain kinetic energy from the tool face and strike the workpiece, causing localized erosion.
3. Cavitation effect: The rapid movement of the tool creates pressure fluctuations in the slurry, leading to the formation and collapse of cavitation bubbles, which assists in flushing debris and cleaning the machining zone.

The Material Removal Rate (MRR) in USM is influenced by several critical factors:

• Amplitude and Frequency of vibration: Higher amplitude and frequency increase the kinetic energy of the abrasive particles, directly increasing MRR.
• Abrasive grain size: Larger grit sizes increase the impact force but may result in a poorer surface finish.
• Abrasive slurry concentration: An optimal concentration is required; too low reduces the number of active particles, while too high causes particle crowding and cushioning.
• Tool material: Materials with high wear resistance (e.g., stainless steel or carbides) are preferred to maintain tool geometry and consistent MRR.
• Workpiece hardness: Materials with higher brittleness (e.g., glass, ceramics) exhibit higher MRR due to the ease of crack propagation and brittle fracture compared to ductile materials.
• Static load: A specific static force must be applied to ensure continuous contact between the tool, abrasives, and workpiece; insufficient force leads to low impact, while excessive force prevents proper vibration.