Advance Machining Processes (ME-702 (A)) - Unit 4 Short Notes

Unit 4: Advanced Machining Processes

1. Ultrasonic Machining (USM)

Mechanism of Metal Removal

• Principle: Utilizes high-frequency (15–40 kHz) low-amplitude vibrations of a tool, coupled with an abrasive slurry.

• Process:

  1. Tool (sonotrode) vibrates ultrasonically against the workpiece.

  2. Abrasive grains (SiC, Al₂O₃) in slurry impact the workpiece surface.

  3. Micro-chips are removed by brittle fracture and micro-chipping.

  4. Slurry carries away debris.

• Key Point: No significant heat generation; suitable for brittle, hard materials.

• Material Removal Rate (MRR) Formula:

$$ \text{MRR} \propto A \cdot f \cdot V_a $$

where $A$ = amplitude, $f$ = frequency, $$\displaystyle V_a $$ = abrasive flow rate.

Applications

• Machining ceramics, glass, semiconductors (Si, Ge).

• Stamping dies, drilling small holes in hard materials.

• Deburring and surface finishing of intricate shapes.

• Not suitable for ductile metals (poor surface finish).

[!TIP]

Common Pitfall: USM is not a thermal process. No melting occurs—removal is purely mechanical via abrasives.

2. Electrical Discharge Machining (EDM)

Wire Electrical Discharge Machining (Wire EDM)

Working Principle

• Spark Erosion: Rapid, repetitive electrical discharges (sparks) between a thin wire electrode (brass, copper) and the workpiece submerged in dielectric fluid (deionized water).

• Mechanism:

  1. Voltage applied creates an electric field.

  2. Dielectric breaks down → spark jumps across gap (~0.025 mm).

  3. Localized heat (10,000°C) vaporizes/melts tiny material volume.

  4. Flushing removes debris; dielectric re-ionizes.

• Wire Feed: Continuous feed to maintain gap; wire does not touch workpiece.

Applications

• Precision cutting of hard metals (tool steels, carbides).

• Complex 2D profiles (dies, molds, medical implants).

• Small features (slits, notches) with high accuracy (±0.005 mm).

• Stratified wire technique for roughing and finishing in one setup.

Errors in Matching (Accuracy Issues)

Stratified Wire Technique

• Purpose: Improve efficiency by using different wire diameters/parameters for roughing and finishing.

• Method:

  1. Roughing: Thick wire, high energy → fast material removal.

  2. Finishing: Thin wire, low energy → better surface finish and accuracy.

• Benefit: Reduces total machining time while maintaining precision.

[!TIP]

Exam Focus: Wire EDM accuracy depends on wire tension, flushing pressure, and control system. "Matching errors" refer to deviation from programmed path.

3. Laser Beam Machining (LBM)

Effect of Focusing on Performance Parameters

• Focusing Lens: Determines spot size ($$\displaystyle d_f $$) at workpiece.

$$ d_f = \frac{2 \lambda f}{\pi D} $$

where $\lambda$ = wavelength, $f$ = focal length, $D$ = beam diameter.

• Impact:

  | Parameter | Tight Focus (small $$\displaystyle d_f $$) | Loose Focus (large $$\displaystyle d_f $$) | |-----------|--------------------------|---------------------------| | Energy Density | High → deeper penetration, faster cutting | Low → shallow cuts, slower | | Kerf Width | Narrow | Wide | | Accuracy | High (fine features) | Low | | Depth of Focus | Small (critical alignment) | Large (for curved surfaces) |

• Trade-off: Too tight focus may cause excessive heat; too loose reduces precision.

[!TIP]

Key Concept: Energy density ($$\displaystyle J/\text{mm}^2 $$) is critical. For metals, need > $$\displaystyle 10^6 $$ J/cm² to melt/vaporize.

4. Rapid Prototyping (RP) / Additive Manufacturing

Definition & Significance

• Definition: Layer-by-layer fabrication of physical models from 3D CAD data.

• Significance in Product Development:

  • Reduces time-to-market (from months to days).

  • Design validation, fit/function testing.

  • Customization (medical implants, aerospace parts).

  • Complex geometries impossible with subtractive methods.

Starting Materials (Three Types)

3D Printing: Principles & Working

1. CAD Model → sliced into thin layers (0.1–0.3 mm).

2. Material deposition layer-by-layer:

   • FDM: Thermoplastic filament extruded.

   • SLA: UV laser cures photopolymer resin.

   • SLS: Laser sinters powder (polymer/metal).

3. Support structures for overhangs.

4. Post-processing: Cleaning, curing, finishing.

Application Issues & Limitations

[!TIP]

Common Exam Question: Compare additive vs. subtractive. RP adds material; conventional machining removes it.

5. Microfabrication

LIGA Process

• Acronym: Lithography, Imprinting, Galvanoforming, Abformung (German).

• Steps:

  1. Lithography: X-ray lithography creates high-aspect-ratio polymer mold.

  2. Electroplating: Ni or Cu fills mold → microstructure.

  3. Molding: Replicate via injection molding or hot embossing.

• Advantage: Produces tall, precise microstructures (aspect ratio > 100).

• Applications: Micro gears, optical components, microfluidic channels.

Basic Types of Micro System Devices (MEMS)

Industrial Applications

• Automotive: Airbag sensors, tire pressure monitors.

• Biomedical: Drug delivery systems, implants, diagnostics.

• Electronics: Inkjet printer heads, RF switches.

• Optics: Adaptive lenses, optical switches.

• Aerospace: Inertial navigation systems.

[!TIP]

MEMS vs. LIGA: LIGA is a fabrication process; MEMS are the devices made using such processes.

6. Electrochemical Machining (ECM)

Electrochemical Honing

• Hybrid Process: Combines ECM (anodic dissolution) with mechanical honing (abrasive stones).

• Working:

  1. Tool (cathode) reciprocates while electrolyte flows.

  2. ECM removes bulk material; honing stones finish surface.

  3. Advantage: High MRR + fine surface finish (0.2–0.4 µm Ra).

• Applications: Turbine blades, cylinder liners, hardened gears.

Etch Factor

• Definition: Ratio of undercut depth ($U$) to depth of penetration ($D$) in ECM.

$$ \text{Etch Factor} = \frac{U}{D} $$

• Significance: Measures dimensional accuracy. Lower etch factor → less undercut → better precision.

• Dependent on: Current density, electrolyte flow, electrode shape.

Mechanism of Machining Rate

• Faraday’s Law:

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

where $\eta$ = current efficiency, $I$ = current (A), $M$ = atomic weight, $n$ = valence, $F$ = Faraday constant (96,500 C/mol).

• Key Factors:

  • Current Density ($J$): Primary control variable.

  • Electrolyte: Conductivity, flow rate (removes sludge, controls temperature).

  • Gap: Maintains uniform current distribution.

• No Tool Wear: Tool is cathode; only workpiece (anode) dissolves.

[!TIP]

ECM vs. EDM: ECM is non-thermal, no sparks; EDM uses thermal erosion. ECM tool never touches workpiece; EDM wire may contact.