Advance Machining Processes (ME-702 (A)) - Unit 1 Short Notes

UNIT 1: Advance Machining Processes

I. Non-Traditional Machining Processes

A. Ultrasonic Machining (USM)

• Mechanism of Metal Removal:

  • A tool (sonotrode) vibrating at ultrasonic frequency (typically 20 kHz) is pressed against the workpiece in the presence of an abrasive slurry (water + fine abrasive grains like SiC, Al₂O₃).

  • The vibrating tool impacts the abrasive grains, which in turn hammer the workpiece surface at high velocity.

  • This causes micro-chipping (brittle fracture) and erosion of the workpiece material. Removal is primarily due to mechanical hammering, not thermal or chemical action.

  • Key Point: USM is ideal for brittle, hard, and fragile materials (ceramics, glass, carbides, semiconductors) as it induces minimal thermal stress and mechanical distortion.

  [!TIP] Exam Focus: Distinguish USM from EDM. USM is mechanical (abrasive impact), EDM is thermal-electrical (spark erosion). USM works on conductive and non-conductive materials.

• Applications:

  • Machining hard and brittle materials: ceramics, glass, quartz, gemstones, carbides, semiconductors (Si, Ge).

  • Creating non-circular holes, slots, and intricate cavities in fragile components.

  • Finishing operations on hardened steel dies and tools.

  • Deburring and micro-machining.

B. Electrical Discharge Machining (EDM)

• Working Principle:

  • Based on thermal erosion by a series of discrete electrical sparks between the tool electrode (cathode) and the workpiece (anode) submerged in a dielectric fluid (kerosene, deionized water).

  • Process: A small gap (spark gap) is maintained. When voltage exceeds the dielectric breakdown, a spark occurs, generating intense heat (10,000–12,000°C) that melts and vaporizes a tiny volume of both electrode and workpiece. The dielectric flushes away debris and re-solidifies the molten material.

  • Material removal occurs without physical contact between tool and workpiece.

• Types of Errors in EDM:

  • Spark Gap Error: The unavoidable gap between tool and workpiece leads to a tapered cut (larger at the top). Compensated by tool design and "finish" passes.

  • Tool Wear: The tool electrode erodes along with the workpiece, causing dimensional inaccuracy and loss of shape fidelity. Measured by Tool Wear Ratio (TWR).

  • Accuracy Issues: Caused by thermal stresses (recast layer, micro-cracks), arc-ing (continuous spark causing deep craters), and dielectric contamination.

  • Corner Wear: Excessive wear at sharp corners of the tool due to higher current density.

  [!TIP] Common Pitfall: EDM can only machine electrically conductive materials. The recast layer (resolidified molten metal) is a defect that may require secondary finishing.

C. Wire Electrical Discharge Machining (WEDM)

• Working Principle:

  • A thin, continuously fed wire electrode (typically brass, copper, or coated wire) serves as the tool.

  • The wire and workpiece are submerged in a dielectric (deionized water). Sparks occur between the wire and workpiece, eroding material along a pre-programmed path.

  • The wire is fed from a spool and guided by precision guides. The dielectric is flushed under pressure to remove debris and cool the wire.

  • Key Feature: The wire is consumable and constantly renewed, allowing for intricate 2D profiles and tapered cuts.

• Applications:

  • Extreme precision cutting of hard, conductive materials: hardened steels, titanium, carbides, tool steels.

  • Manufacturing stamping dies, blanking dies, extrusion dies, and molds with complex shapes and tight tolerances.

  • Cutting prototypes, medical implants, and aerospace components.

  • Producing sharp corners and fine details unachievable by conventional milling.

D. Laser Beam Machining (LBM)

• Effect of Focusing on LBM Performance:

  • The laser beam is focused using a lens to a small spot size at the workpiece surface.

  • Energy Density (Power/Area) is inversely proportional to the spot area. Tighter focus = higher energy density.

  • Impact on Performance:

    1. Material Removal Rate (MRR): Increases with higher energy density (up to a point).

    2. Cut Quality: Smaller spot size yields narrower kerf width and better precision.

    3. Depth of Focus: A very tight focus has a shallow depth of field, requiring precise positioning for deep cuts.

    4. Threshold Effect: For some materials, a minimum energy density (power/area) is required to initiate machining.

  [!TIP] Formula: Energy Density, $$\displaystyle E_d = \frac{P}{A} = \frac{P}{\pi r^2} $$, where $P$ = laser power, $r$ = spot radius.

E. Electrochemical Processes

• Electrochemical Honing (ECH):

  • Concept: A hybrid process combining electrochemical machining (ECM) with mechanical honing.

  • Working: A metallic abrasive stone (honing tool) is rotated and reciprocated inside a workpiece hole (e.g., engine cylinder). A low voltage DC current flows between the conductive workpiece (anode) and the tool (cathode) through an electrolyte (e.g., NaCl solution).

  • Mechanism: Material is removed anodically (dissolved) from the workpiece peaks. The abrasive stone simultaneously mechanically rubs the surface, removing the passivated layer and ensuring uniform dissolution. Results in excellent surface finish and dimensional accuracy.

• Etch Factor:

  • Definition: The ratio of the depth of cut ($D$) to the lateral undercut ($U$) in an electrochemical or chemical etching process.

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

*   **Significance:** It is a **measure of process selectivity and accuracy**. A **higher etch factor** (D >> U) indicates **minimal lateral etching** and better **anisotropy** (directional control), which is desirable for precise micromachining and pattern transfer. Low etch factor leads to poor resolution and feature distortion.

F. General Concepts

• Mechanism of Machining Rate in Non-Traditional Processes:

  • The material removal rate (MRR) depends on the energy density delivered to the material and its material removal mechanism.

  • General Form: MRR $\propto$ (Energy Source Parameter) × (Process Efficiency) / (Energy Required to Remove Unit Volume of Material).

  • Key Influencing Factors:

    1. Energy Source Parameters: Power/Intensity (laser, beam), voltage/current (EDM, ECM), amplitude (USM).

    2. Process Parameters: Feed rate, tool geometry, gap distance, pulse parameters (for EDM).

    3. Workpiece Properties: Thermal/electrical conductivity, melting/boiling point, hardness, fracture toughness (for USM).

    4. Medium Properties: Dielectric strength (EDM), viscosity (USM slurry), conductivity (ECM electrolyte).

II. Rapid Prototyping (RP)

A. Fundamentals

• Definition: Rapid Prototyping is a group of additive manufacturing techniques used to quickly fabricate scale models of physical parts or assemblies directly from 3D CAD data, typically by building layer upon layer.

• Significance in Product Development:

  • Reduces Time-to-Market: Accelerates design verification and iteration.

  • Improves Communication: Provides tangible models for design review, marketing, and client approval.

  • Enables Design Optimization: Allows testing of form, fit, and function early.

  • Facilitates Customization: Ideal for one-off or low-volume production of complex geometries.

  • Reduces Tooling Costs: Eliminates need for expensive molds/dies for prototypes.

B. RP Materials and Processes

• Three Types of Starting Materials:

  1. Polymers (Most Common): Thermoplastics (ABS, PLA, Nylon) or photopolymers (SLA). Fed as filament, powder, or liquid resin.

  2. Metals: Metal powders (stainless steel, titanium, Inconel) or wire fed into a high-energy source (laser/electron beam) for full melting (SLM, DMLS) or partial melting (EBM).

  3. Ceramics: Ceramic powders bound with a polymer binder (binder jetting) or processed via stereolithography with ceramic-filled resins, followed by sintering.

• Stratified Wire Process (Fused Deposition Modeling - FDM):

  • Working Principle:

    1. A thermoplastic filament is unwound from a spool and fed into a heated extrusion head.

    2. The head melts the filament and extrudes it through a small nozzle.

    3. The nozzle moves in the X-Y plane, depositing a thin bead of molten plastic to form one layer of the part, following the CAD slice data.

    4. After a layer is completed, the build platform lowers (or the head rises), and the next layer is deposited on top, fusing with the previous layer as it cools and solidifies.

    5. This layer-by-layer process repeats until the 3D part is complete.

  • Support Material: Often required for overhangs; a second, dissolvable or breakaway material is deposited simultaneously.

C. RP Implementation

• Application Issues in Rapid Prototyping:

  • Accuracy & Tolerance: Limited by layer thickness, machine calibration, and material shrinkage/warpage. Typically ±0.1–0.5 mm.

  • Surface Finish: "Stair-stepping" effect on curved surfaces due to layered construction. Post-processing (sanding, chemical smoothing) often needed.

  • Material Limitations: Properties (strength, temperature resistance) of prototype materials often differ significantly from final production materials.

  • Build Size & Speed: Constrained by machine envelope; large parts require segmentation. Process can be slow for high-resolution parts.

  • Cost: High for machines and materials, especially for metal RP.

• Principles and Working of 3D Printing (Additive Manufacturing):

  • Core Principle: Additive fabrication—constructing a physical object from a 3D digital model by successively adding material in thin layers.

  • General Workflow:

    1. CAD Modeling: Create 3D model.

    2. STL Conversion: Model is sliced into thin 2D cross-sectional layers (STL file format).

    3. Machine Preparation: Orient part on virtual build platform, generate support structures if needed, load material.

    4. Layer-by-Layer Build: Machine executes the slice data, depositing or solidifying material for each layer.

    5. Post-Processing: Remove part from platform, clean, remove supports, finish surface, cure (if needed).

  • Common Technologies: FDM (thermoplastic extrusion), SLA (photopolymer UV curing), SLS (laser sintering of powder), SLM/DMLS (full melting of metal powder).

III. Microfabrication

A. Micro System Devices

• Basic Types:

  • MEMS (Micro-Electro-Mechanical Systems): Integration of mechanical elements (levers, gears, membranes) with microelectronics on a single silicon chip. Examples: accelerometers, pressure sensors, inkjet nozzles.

  • Micro-Sensors: Devices that convert a physical/chemical parameter (temperature, pressure, chemical concentration) into an electrical signal at a microscale. (e.g., MEMS gyroscope).

  • Micro-Actuators: Devices that convert an electrical signal into micro-scale motion or force. (e.g., micro-mirrors for projectors, micropumps, micro-valves).

B. LIGA Process

• LIGA Microfabrication Process:

  • Acronym: German for Lithographie, Galvanoformung, Abformung (Lithography, Electroplating, Molding).

  • Steps:

    1. Lithography: A thick layer of X-ray sensitive photoresist (PMMA) is exposed to synchrotron X-ray radiation through a precise mask. The exposed resist is developed, creating a high-aspect-ratio mold.

    2. Electplating (Galvanoformung): The resist mold is filled by electroplating with a metal (Ni, Cu, Au) to form a robust, high-aspect-ratio microstructure.

    3. Molding (Abformung): The metal microstructure serves as a mold insert. It is used in injection molding or hot embossing to mass-produce plastic replicas or to create secondary metal molds.

  • Features & Advantages:

    • Extremely High Aspect Ratios: Height-to-width ratios > 100:1.

    • Vertical, Smooth Sidewalls: Due to parallel X-ray exposure.

    • Metallic Microstructures: Enables fabrication of precise metal parts.

    • Suitable for Mass Production: Via molding step.

C. Applications

• Industrial Applications of Microfabrication:

  • Medical: Microneedles, drug delivery systems, implantable sensors, microfluidic diagnostic chips (Lab-on-a-Chip).

  • Optics & Photonics: Micro-optics (lenses, gratings), optical switches, waveguide components.

  • Automotive & Aerospace: Pressure sensors, accelerometers (airbag deployment), fuel injection nozzles.

  • Electronics: RF MEMS switches, connectors, components for mobile devices.

  • Energy: Micro-turbines, fuel cells, solar cell components.

  • Biotechnology: Cell culture platforms, DNA analysis chips.