Advance Machining Processes (ME-702 (A)) - Unit 5 Short Notes

Unit 5: Advanced Machining Processes

(Based on ME-702(A) - Nov 2023 Exam Pattern)

1. Introduction to Advanced Machining Processes

Definition: Non-traditional/advanced machining processes remove material using non-mechanical energy sources (thermal, electrical, chemical, electrochemical, etc.) to machine hard, brittle, or complex-shaped materials that are difficult/impossible with conventional methods.

Necessity:

• Machine hard & brittle materials (ceramics, carbides, glass).

• Achieve complex geometries and intricate shapes.

• Avoid mechanical stresses, tool-workpiece contact, and heat-affected zones (in some processes).

• Enable micro/meso-scale fabrication.

Classification by Energy Source:

[!TIP]

Exam Focus: Be ready to classify any given process (e.g., EDM = electro-thermal, USM = mechanical).

2. Ultrasonic Machining (USM)

2.1 Mechanism of Metal Removal

• Principle: A sonotrode (tool) vibrating at ultrasonic frequency (15–25 kHz) with low amplitude (10–100 µm) impacts abrasive grains (SiC, Al₂O₃) in a slurry against the workpiece.

• Chip Formation:

  1. Impact Mechanism: Abrasive grains driven by vibrating tool indent and fracture the workpiece surface (micro-chipping).

  2. Cavitation: Rapid pressure changes in slurry cause bubble formation & collapse, generating micro-jets that assist in erosion.

• Key Parameters:

  • Tool Amplitude: ↑ amplitude → ↑ MRR (but tool wear ↑).

  • Frequency: Higher frequency → finer surface finish.

  • Abrasive Grit Size: Coarser grit → ↑ MRR but rougher surface.

Material Removal Rate (MRR) Approximation:

$$ \text{MRR} \propto A \cdot f \cdot N \cdot C $$

Where $A$ = amplitude, $f$ = frequency, $N$ = number of abrasive grains, $C$ = constant (material-dependent).

2.2 Applications & Limitations

[!TIP]

Common Pitfall: USM is NOT for ductile metals (poor chip formation). Best for brittle, amorphous, or porous materials.

3. Electrical Discharge Machining (EDM)

3.1 Working Principle

• Spark Erosion: Repeated electrical sparks between tool electrode (cathode) and workpiece (anode) in dielectric fluid (kerosene, deionized water) vaporize & melt material.

• Process Stages:

  1. Dielectric Breakdown: Voltage ↑ → dielectric ionizes → plasma channel forms.

  2. Material Removal: Spark energy (10⁻⁴ to 10⁻² J) melts/vaporizes tiny workpiece volume.

  3. Flushing: Dielectric flushes debris; re-ionization delays next spark.

• Components: Pulse generator, tool electrode, workpiece, dielectric tank, servo-control.

MRR Formula (Approx.):

$$ \text{MRR} \propto \frac{I \cdot T_{on}}{\rho} $$

Where $I$ = peak current, $$\displaystyle T_{on} $$ = pulse-on time, $\rho$ = workpiece density.

3.2 Wire EDM

• Working: Continuous thin wire electrode (Brass, Cu) moves along CNC path; dielectric (deionized water) flushed through nozzle.

• Applications:

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

  • Intricate shapes (dies, molds, medical implants).

  • Narrow kerf (0.02–0.3 mm) with high accuracy (±0.005 mm).

3.3 Errors in EDM

[!TIP]

Exam Key: Wire EDM uses continuous wire & water-based dielectric; Sinker EDM uses shaped electrode & oil-based dielectric.

4. Laser Beam Machining (LBM)

4.1 Effect of Focusing on Performance

• Focusing Lens Role: Converges laser beam to small spot size → high energy density (10⁶–10⁹ W/cm²).

• Impact on Parameters:

  • Spot Size ↓ → Depth of Cut ↑, Kerf Width ↓, Surface Finish ↑ (finer).

  • Depth of Focus: Range where spot size remains small; affects taper in cuts.

  • Beam Quality (M²): M² = 1 (ideal Gaussian); higher M² → poorer focus → reduced precision.

• Key Parameters: Focal length, lens material, beam mode (TEM₀₀).

Energy Density Formula:

$$ E_d = \frac{4P}{\pi d^2} $$

Where $P$ = laser power, $d$ = spot diameter at focal point.

[!TIP]

Focusing Trade-off: Too small spot → shallow depth of cut; too large spot → poor precision. Optimize for material thickness and required aspect ratio.

5. Rapid Prototyping (RP) / Additive Manufacturing

5.1 Definition & Significance

• Definition: Layer-by-layer fabrication of physical models directly from 3D CAD data (STL file).

• Significance:

  • Reduced lead time (weeks → days).

  • Design iteration without tooling cost.

  • Customization (medical implants, aerospace parts).

  • Complex geometries (lattice structures, internal channels).

5.2 Starting Materials in RP

5.3 Layered Manufacturing Principle

1. CAD Model → STL file (triangulated surface).

2. Slicing: Software slices model into thin layers (0.025–0.5 mm).

3. Sequential Deposition: Each layer built from bottom up via:

   • Material extrusion (FDM),

   • Vat photopolymerization (SLA),

   • Powder bed fusion (SLS).

4. Post-processing: Support removal, curing, finishing.

"Stratified Wire" Interpretation: Likely refers to Wire Arc Additive Manufacturing (WAAM) where metal wire is melted layer-by-layer via arc welding for large-scale metal parts.

5.4 Application Issues in RP

5.5 3D Printing – Principles & Working

[!TIP]

Exam Distinction:

• FDM: Filament, visible layers, support needed.

• SLA: Liquid resin, finest detail, support needed.

• SLS: Powder, no support, strong parts, rough surface.

6. Microfabrication

6.1 Basic Types of Micro System Devices

• MEMS: Micro-sensors (pressure, accelerometer), actuators, resonators (e.g., quartz crystals).

• Microfluidic Devices: Lab-on-a-chip (DNA analysis, drug delivery).

• Optical Microsystems: Microlenses, optical switches.

• Biomedical Implants: Cochlear implants, stents, drug delivery pumps.

6.2 LIGA Process

• Acronym: Lithographie (X-ray lithography), Galvanoformung (electroplating), Abformung (molding).

• Steps:

  1. X-ray Lithography: Thick photoresist (PMMA) exposed to synchrotron X-rays through mask → 3D microstructures.

  2. Electroplating: Ni or other metal electroplated into resist cavities → high-aspect-ratio metal structures.

  3. Molding: Metal structure used as mold to replicate plastics/ceramics via injection molding.

• Advantages:

  • High aspect ratio (height/width > 50:1).

  • Precise vertical sidewalls (roughness < 50 nm).

  • Batch fabrication → low cost per unit.

6.3 Industrial Applications

• Electronics: Microchips, connectors, RF switches.

• Medical: Microneedles, implantable sensors, micro-pumps.

• Optics: Microlens arrays, diffraction gratings.

• Automotive: Pressure sensors, fuel injectors.

7. Electrochemical Honing (ECH)

• Definition: Hybrid process combining ECM (anodic dissolution) with abrasive honing for precision finishing of hard materials.

• Process:

  1. Workpiece (anode) and tool (cathode) with abrasive-coated tool.

  2. Electrolyte flows under pressure.

  3. Simultaneous action: Electrochemical dissolution removes bulk material; abrasive honing refines surface.

• Applications:

  • Finishing of hardened steels, superalloys.

  • Internal surfaces (gears, cylinders).

  • Achieves Ra < 0.1 µm with minimal residual stress.

8. Key Parameters & Concepts

8.1 Etch Factor

• Definition: Ratio of undercut ($U$) to depth of cut ($D$) in ECM/etching:

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

• Significance:

  • Higher etch factor → more undercut → poor dimensional accuracy.

  • Controlled by current density distribution, electrode shape, electrolyte flow.

  • Aim for low etch factor (close to 0) for vertical walls.

8.2 Mechanism of Machining Rate

General Principle: Material removal depends on energy concentration and interaction time.

Universal Trend:

$$ \text{MRR} \propto \frac{\text{Energy Input}}{\text{Interaction Time}} $$

But over-input causes excessive wear (EDM tool), recast layer (LBM), or stray corrosion (ECM).

[!TIP]

Exam Strategy: For "mechanism of machining rate," pick one process (e.g., EDM) and explain:

1. How energy (spark) removes material.

2. How parameters (current, pulse time) affect MRR.

3. Trade-offs (MRR vs. surface finish).

Final Note: This unit is application-heavy. Focus on comparisons (e.g., EDM vs. ECM), parameter effects, and process selection criteria for given materials/shapes. Diagrams for EDM spark gap, USM setup, LBM focusing, and RP layer buildup are frequently asked—practice neat sketches with labels.