Advance Machining Processes (ME-702 (A)) - Unit 2 Short Notes

Unit 2: Advanced Machining Processes

I. Ultrasonic Machining (USM)

Working Principle:

USM is a non-traditional machining process that uses high-frequency, low-amplitude vibrations (typically 20 kHz) of a tool, coupled with an abrasive slurry, to erode the workpiece material. The tool is not in direct contact with the workpiece.

Mechanism of Metal Removal:

1. Abrasive Impact: The vibrating tool (sonotrode) impacts abrasive grains (e.g., SiC, Al₂O₃) in the slurry against the workpiece surface, causing micro-chipping.

2. Cavitation: The high-frequency vibration creates alternating high and low pressure in the slurry, leading to the formation and collapse of microscopic vapor cavities (cavitation). The collapse generates local shock waves that assist in material removal.

Key Features:

• Tool material: Soft (e.g., titanium, steel) as it does not cut.

• Workpiece: Ideally brittle and hard (ceramics, glass, carbides, semiconductors).

• No heat-affected zone (HAZ).

• Low material removal rate (MRR).

Applications:

• Machining brittle, hard, and fragile materials (glass, quartz, ceramics, germanium).

• Creating complex cavities and non-circular holes in hard materials.

• Finishing operations after traditional machining.

• Semiconductor industry (dicing, scribing).

II. Electrical Discharge Machining (EDM)

Working Principle (Sinker EDM / RAM EDM):

Based on thermal erosion. A pulsed DC spark discharges between a conductive tool electrode (cathode) and a conductive workpiece (anode), submerged in a dielectric fluid (e.g., kerosene, deionized water). The intense heat (10,000–12,000°C) melts and vaporizes a tiny volume of material. The dielectric flushes away the debris and re-ionizes for the next pulse.

Wire EDM (Wire Cut EDM)

1. Working Principle:

• Uses a continuously fed, thin wire electrode (typically brass or copper) as the tool.

• The workpiece is mounted on a CNC-controlled table.

• Sparking occurs along the wire's length, controlled by CNC to follow a programmed path.

• Dielectric (deionized water) is pumped through the wire-workpiece gap for flushing and control.

2. Applications:

• Precision cutting of thick, hard, exotic metals (titanium, Hastelloy, tool steels).

• Tool and die making (molds, punches, dies).

• Prototype production and small batch production.

• Cutting intricate shapes, sharp corners, and very fine features.

3. Stratified Wire (Wire Types & Feeding Mechanisms):

• Wire Types: Brass (standard), Copper (higher conductivity), Zinc-coated brass (reduces breakage, better finish).

• Feeding Mechanism: The wire is fed from a spool through guides. Tension is controlled to maintain straightness and prevent breakage.

• Breakage Issues: Caused by excessive tension, wire fatigue, debris clogging, or power supply instability. Modern machines have automatic wire breakage detection and restart.

Errors in EDM:

1. Tool Wear: Even the tool electrode erodes (though slower). Affects accuracy, especially in deep cavities. > [!TIP] Tool wear is directional; corners wear faster.

2. Taper: The hole or cut profile is slightly tapered (wider at the top) due to lateral sparking and debris flushing inefficiency.

3. Surface Cracks (Recast Layer): Rapid melting and re-solidification creates a thin, hard, brittle, and metallurgically altered layer (recast layer) with micro-cracks. Affects fatigue life.

4. Accuracy Issues: Influenced by dielectric flushing, electrode wear, thermal stresses, and machine rigidity.

III. Laser Beam Machining (LBM)

Working Principle:

A high-intensity, coherent, monochromatic light beam (laser) is focused onto the workpiece. Material removal occurs via:

• Melting & Vaporization (Thermal): For metals (CO₂, Nd:YAG lasers).

• Photochemical Ablation: For polymers/ceramics (excimer lasers, UV).

• The process is non-contact.

Effect of Focusing on Performance:

The focusing lens determines the spot size at the workpiece.

> [!TIP] There is a trade-off: a very small spot gives high precision but has a very shallow depth of focus and is prone to clogging.

IV. Electrochemical Machining (ECM)

Fundamental Principle: Based on Faraday’s Laws of Electrolysis. The workpiece (anode) is dissolved anodically into a conductive electrolyte, while the tool (cathode) is not consumed. No tool-workpiece contact.

Mechanism of Machining Rate:

The theoretical material removal rate (MRR) is governed by:

$$ \text{MRR} = \frac{I \cdot M}{n \cdot F \cdot \rho} $$

Where:

• $I$ = Current (A)

• $M$ = Atomic mass of workpiece material (g/mol)

• $n$ = Valency (number of electrons exchanged)

• $F$ = Faraday’s constant (96,500 C/mol)

• $\rho$ = Density of workpiece (g/cm³)

Practical MRR is lower due to current efficiency (η), typically 90–95%.

$$ \boxed{\text{Actual MRR} = \eta \times \frac{I \cdot M}{n \cdot F \cdot \rho}} $$

Key Parameters: Current (I), Voltage (V), Electrolyte (flow rate, conductivity, pressure), Gap (inter-electrode gap).

A. Electrochemical Honing (ECH)

• Process: A hybrid process combining ECM with mechanical honing. A reciprocating, abrasive-coated tool (honing stones) moves slowly while a high DC current flows. The ECM softens the metal surface, and the honing stones remove the softened layer and break down passivation.

• Tool Design: Similar to a honing tool, made of conductive material (copper) with abrasive stones bonded to it.

• Comparison with ECM:

  | Feature | ECM | Electrochemical Honing (ECH) | | ---------------- | ---------------------------- | ----------------------------------- | | Material Removal | Pure electrochemical dissolution | Combined ECM + mild mechanical abrasion | | Surface Finish | Good, but may have passivation issues | Excellent (mirror finish, Rz < 1 µm) | | Tool Wear | Very low | Low (abrasive stones wear slowly) | | Application | Roughing, complex shaping | Finishing of bores, gears, cams |

B. Etch Factor (λ)

• Definition: A measure of the lateral undercutting (anodic dissolution beyond the tool edge) relative to the depth of penetration (machined depth). It indicates the accuracy of the ECM process.

• Calculation:

$$ \lambda = \frac{\text{Depth of Machining (D)}}{\text{Undercut (U)}} $$

Where undercut (U) is the horizontal distance the workpiece has been dissolved beyond the tool edge.

• Influence on Accuracy: A higher etch factor (λ) means less undercut and better dimensional accuracy (closer to tool shape). λ is influenced by electrolyte conductivity, current density, and gap design.

V. Rapid Prototyping (RP)

Definition & Significance:

RP, also known as Additive Manufacturing (AM), is the automated construction of physical 3D models from digital data (CAD). It significantly reduces product development time from months to days, enables design iteration, allows complex geometries impossible with subtractive methods, and facilitates customization and communication.

Classification of RP Processes:

1. Additive (Layered Manufacturing): Build by adding material layer-by-layer (most common: FDM, SLA, SLS).

2. Subtractive: CNC machining of a block (less common for RP).

3. Formative: Selective curing/sintering of a material bed (e.g., SLS, DMLS).

Starting Materials (Three Main Types):

3D Printing Principles (Key Technologies):

• FDM: Heats and extrudes thermoplastic filament through a nozzle, depositing material layer-by-layer.

• SLA: Uses a UV laser to cure and solidify a vat of liquid photopolymer resin.

• SLS: Uses a laser to sinter (fuse) powdered material (polymer, metal) layer-by-layer.

• Material Jetting: Inkjet-like print heads deposit droplets of photopolymer material, cured by UV light.

Application Issues and Challenges:

1. Accuracy & Tolerances: Typically lower than traditional machining (±0.1 to ±0.5 mm). Affected by part geometry, material shrinkage, and layer thickness.

2. Material Properties: Often inferior to bulk materials (anisotropic strength, lower density, different surface finish). Limited material palette compared to subtractive.

3. Support Structures: Required for overhangs in FDM/SLA. Must be designed and removed manually, adding post-processing time and cost.

4. Cost: High for machines and materials. Economical for complex, low-volume parts; not for mass production.

5. Post-Processing: Often needed (support removal, surface finishing, infiltration, sintering for metal).

VI. Microfabrication

A. Basic Types of Microsystem Devices (MEMS - Micro-Electro-Mechanical Systems):

B. LIGA Process

A German acronym for Lithography, Electroplating, and Molding. Used to fabricate high-aspect-ratio (HAR) microstructures (tall, narrow features).

1. Lithography: Create a high-precision, thick resist mask (using X-ray lithography) on a substrate.

2. Electplating: Electrodeposit metal (e.g., Ni, Cu) into the resist mold cavities to form a robust microstructured master.

3. Molding: Use the metal master as a mold to mass-produce plastic or metal replicas via injection molding or hot embossing. Key Advantage: Produces very high aspect ratios (height/width > 100:1) with vertical sidewalls and excellent surface finish.

C. Industrial Applications of Microfabrication:

• Semiconductor: Integrated circuits (IC fabrication), MEMS chips.

• Biomedical: Implantable devices (pacemakers, neural probes), drug delivery systems, diagnostic chips (microfluidics), surgical tools.

• Aerospace & Defense: Micro-propulsion systems, inertial navigation systems (MEMS gyros/accelerometers), miniaturized sensors.

• Automotive: Airbag accelerometers, tire pressure sensors, fuel injectors.

• Consumer Electronics: Inkjet printer heads, projection displays (DLP), microphones, hard disk drive read/write heads.