UNIT 4: ADVANCED MANUFACTURING PROCESSES – SHORT NOTES

I. CUTTING TOOL TECHNOLOGY & MACHINING MECHANICS

Tool Nomenclature and Signature (Single Point Cutting Tool)

• Tool Signature (ANSI/ISO): A standardized code defining tool geometry. For a turning tool, the sequence is:

  A-B-C-D-E-F-G-H-J-K-L-M-N-P-Q-R-S-T-U-V-W-X-Y-Z

  • Key Elements:

    • A: Back rake angle

    • B: Side rake angle

    • C: End relief angle

    • D: Side relief angle

    • E: End cutting edge angle

    • F: Side cutting edge angle

    • G: Nose radius

  • Example: -8-8-5-5-6-6-1/8 implies:

    • Back Rake: 8°

    • Side Rake: 8°

    • End Relief: 5°

    • Side Relief: 5°

    • End Cutting Edge: 6°

    • Side Cutting Edge: 6°

    • Nose Radius: 1/8 inch

Orthogonal vs. Oblique Cutting

Cutting Forces and Specific Cutting Energy

• Specific Cutting Energy (U): Energy required to remove a unit volume of material.

$$U = \frac{P_c}{MRR}$$

Where:

*   $$\displaystyle P_c $$ = Cutting Power (W)

*   $MRR$ = Material Removal Rate (m³/s)

*   $U$ has units J/m³.

• Cutting Power (Pc): For orthogonal cutting (neglecting feed force):

$$P_c = F_c \times V_c$$

Where:

*   $$\displaystyle F_c $$ = Main/Tangential Cutting Force (N)

*   $$\displaystyle V_c $$ = Cutting Velocity (m/s)

• Force Components Relation:

  • $$\displaystyle F_c $$ is always the largest.

  • $$\displaystyle F_t $$ (thrust) acts radially into the workpiece.

  • $$\displaystyle F_f $$ (feed) acts axially along the workpiece (in oblique cutting).

[!TIP] Exam Focus: Specific cutting energy problems often give U, Vc, feed (f), depth of cut (d). Remember: $$\displaystyle MRR = f \times d \times V_c $$ (for turning, units must be consistent: m³/s). Then $$\displaystyle F_c = \frac{U \times MRR}{V_c} $$.

Tool Life Equations (Taylor’s Equation)

• Basic Form:

$$V T^n = C$$

*   V = Cutting speed (m/min)

*   T = Tool life (min)

*   n = Exponent (tool material dependent, ~0.1-0.5)

*   C = Constant (for tool-workpiece combination)

• Comparison of Tools: Given two equations:

  • Tool 1: $$\displaystyle V T^{n_1} = C_1 $$

  • Tool 2: $$\displaystyle V T^{n_2} = C_2 $$

  To find speed at which Tool 2 gives higher life than Tool 1, solve for V where $$\displaystyle T_2 > T_1 $$.

  Example from Jun 2022/2025: Carbide: $$\displaystyle V T^{1.6} = 3000 $$; HSS: $$\displaystyle V T^{0.6} = 200 $$.

  Find V where $$\displaystyle T_{carbide} > T_{HSS} $$.

  Solution: $$\displaystyle T_{carb} = (3000/V)^{1/1.6} $$, $$\displaystyle T_{HSS} = (200/V)^{1/0.6} $$. Set $$\displaystyle T_{carb} > T_{HSS} $$, solve for V.

Cutting Fluids (Coolants/Lubricants)

• Types:

  1. Water-based: Emulsions, soluble oils (excellent cooling).

  2. Oils: Mineral oils, animal/vegetable oils (excellent lubrication).

  3. Semi-synthetics & Synthetics: Advanced, multi-purpose.

• Mechanisms of Effectiveness:

  • Cooling: Reduces temperature, prevents thermal damage & softening.

  • Lubrication: Reduces friction, tool wear, and cutting forces.

  • Flushing: Removes chips from cutting zone.

• Uses: Applied via flood, mist, jet, or through tool.

Tool Failure Modes (Four Types)

[!TIP] Exam Tip: Always sketch the wear location for each mode. Flank wear is most common and defines tool life (VB limit).

II. ABRASIVE MACHINING PROCESSES

A. Grinding Processes

Surface Grinding Machine

• Specifications: Wheel diameter & width, work table size, max workpiece weight, spindle speed range, power.

• Construction:

  DiagramSEARCH: "surface grinding machine diagram labeled"

  • Base: Supports entire machine.

  • Column: Houses cross feed mechanism.

  • Table: Reciprocates (horizontal movement).

  • Wheel Head: Mounts grinding wheel, can be lowered for depth.

  • Wheel: Rotates at high speed ( abrasive action).

• Applications: Produce flat surfaces, precision slots, die-sinking.

• Advantages: High accuracy, good surface finish, can hard materials.

• Limitations: High tool cost, heat generation, slow MRR.

Centerless Grinding

• Working Principle: Workpiece supported by a regulating wheel and a grinding wheel, without centers or chucks. Supported on a work rest blade.

  • Through-feed: For long rods. Regulating wheel inclined (~5°) to provide axial feed.

  • In-feed: For discrete parts. Regulating wheel stopped, workpiece fed axially into grind zone.

• Sketch:

  DiagramSEARCH: "centerless grinding principle diagram"

• Applications: Mass production of cylindrical parts (pins, rollers, bearings).

• Advantages: High productivity, no workpiece deformation (no centers), good roundness.

• Limitations: Limited to cylindrical surfaces, setup complex, not for all shapes.

Grinding Wheel Specifications (ISO/ANSI Marking)

• Symbol Sequence: A 36 K 5 V 12

  • A: Abrasive type (Al oxide, SiC, CBN, Diamond).

  • 36: Grain size (higher number = finer grain).

  • K: Grade (hardness) (A=soft, Z=hard).

  • 5: Structure (dense to open, 1-14).

  • V: Bond type (V=vitrified, B=resinoid, S=silicate).

  • 12: Manufacturer's code.

• Parameters:

  • Grain Size: Affects surface finish & MRR.

  • Grade: Controls grain release (hard grade = grains held firmly).

  • Structure: Porosity; open structure for chip clearance/soft materials.

  • Bond: Holds grains; vitrified (porous, strong), resinoid (flexible).

Wheel Dressing vs. Truing

B. Super Finishing Processes

III. GEAR MANUFACTURING TECHNOLOGY

A. Gear Fundamentals

• Elements/Terminology:

  DiagramSEARCH: "gear terminology diagram labeled"

  • Pitch Circle: Imaginary circle where pure rolling occurs.

  • Addendum: Radial height from pitch circle to tooth tip.

  • Dedendum: Radial depth from pitch circle to tooth root.

  • Module (m): $$\displaystyle m = \frac{\text{Pitch Diameter}}{\text{Number of Teeth}} $$ (mm). Fundamental size.

  • Pressure Angle (φ): Angle between line of action and tangent to pitch circle (usually 20°).

  • Base Circle: From which involute profile is generated.

  • Tooth Thickness: Arc thickness on pitch circle.

  • Circular Pitch (p): $$\displaystyle p = \pi m $$.

B. Gear Production vs. Generation

C. Gear Machining Processes

Gear Hobbing

• Working:

  DiagramSEARCH: "gear hobbing machine diagram"

  • Hob: Worm-like cutter with helical teeth, fluted like a tap.

  • Workpiece: Rotated at precise speed ratio to hob ($$\displaystyle \frac{N_w}{N_h} = \frac{Z_w}{Z_h} $$).

  • Relative motion: As hob feeds axially, it generates involute teeth on blank.

• Advantages: High productivity, good accuracy, versatile (wide range of gears).

• Limitations: Cannot cut internal gears, requires special fixture for small gears, crowning difficult.

Gear Shaping

• Principle:

  DiagramSEARCH: "gear shaping machine diagram"

  • Cutter: Pinion-shaped tool (reciprocates vertically).

  • Workpiece: Rotated and fed radially.

  • Relative motion: Cutter and workpiece rotate in synchronization (like meshing gears) to generate tooth profile.

• Advantages: Can cut internal gears, idlers, racks; less expensive tooling than hobs.

• Limitations: Slower than hobbing, accuracy lower, stroke length limits face width.

Gear Shaving

• Principle: Finishing process. A shaving cutter (gear-like) meshes with the finished gear under light pressure and crossed-axis helical motion. Small amounts of metal are sheared off.

• Purpose: Improve surface finish, reduce noise, correct minor errors in heat-treated gears.

D. Gear Finishing Methods (Four)

1. Shaving: As above. For heat-treated gears.

2. Honing: Similar to cylinder honing. Uses honing wheels with abrasive sticks. For large, coarse-pitch gears.

3. Lapping: For highest precision (master gears, bearings). Uses lap and abrasive.

4. Burnishing: Cold working with hardened rollers/polygons. Improves surface finish & fatigue strength, no metal removal.

E. Special Cutters - DP Cutters

• DP Cutter: Double-Path or Double-Phase cutter for involute gear cutting (used in gear shaping).

• Characteristics:

  • Has two distinct rake faces on each tooth.

  • First phase (roughing): High rake, high clearance → heavy chip, fast removal.

  • Second phase (finishing): Low/negative rake, low clearance → light cut, good finish.

  • Single tool performs both roughing & finishing in one stroke → high efficiency, good accuracy.

IV. UNCONVENTIONAL MACHINING PROCESSES

Need for Unconventional Machining

• To machine: Very hard, brittle, complex-shaped, heat-sensitive, or exotic materials.

• To achieve: High precision, complex geometries, no tool-workpiece contact (avoid mechanical stresses, heat).

• To overcome: Limitations of conventional machining (tool wear, accessibility, thermal damage).

Individual Processes

1. Electrical Discharge Machining (EDM) / Spark Erosion

• Working Principle:

  DiagramSEARCH: "EDM wire cut and die sinking diagram"

  • Tool (electrode) & workpiece submerged in dielectric (oil, kerosene).

  • Voltage applied → dielectric breaks down → spark jumps across small gap (µs).

  • Spark generates intense localized heat (~10,000°C) → metal vaporizes/melts → removed.

  • Dielectric flushes away debris, re-ionizes.

• Advantages: No mechanical force, complex shapes, hard materials, good surface finish.

• Limitations: Slow MRR, electrode wear, only conductive materials, thermal affected zone (HAZ).

• Applications: Die-sinking (molds), wire-cut (2D/3D profiles), small holes, surgical tools.

2. Electrochemical Machining (ECM)

• Working Principle:

  DiagramCANVAS: "ECM logical diagram: Power supply (DC) positive to tool (anode), negative to workpiece (cathode). Electrolyte (e.g., NaCl) pumped at high pressure through gap. Tool shaped as inverse of desired cavity. Metal dissolves from workpiece (anodic dissolution) according to Faraday's law. No tool wear."

  • Anodic Dissolution: $$\displaystyle M \rightarrow M^{n+} + ne^- $$

  • Faraday's Law: $$\displaystyle W = \frac{I \cdot t \cdot M}{n \cdot F} $$

• Procedure: Tool (cathode) shaped as final cavity. Workpiece (anode) fed slowly. Electrolyte flows at high pressure (10-30 m/s) to remove sludge & heat.

• Advantages: No tool wear, no thermal stress, fast for hard materials, smooth surface.

• Limitations: Only conductive materials, electrolyte handling, dimensional control difficult, not for small features.

• Applications: Turbine blades, die cavities, deep holes, deburring.

3. Electron Beam Machining (EBM)

• Characteristics:

  • Uses focused beam of high-velocity electrons in vacuum (~10⁻⁵ torr).

  • Electrons kinetic energy → heat → vaporization/melting.

  • No physical contact, very small feature size (µm), high aspect ratio.

  • Material removal by vaporization (not melting).

• Advantages: Extremely precise, no tool wear, can machine any material (conductive/insulator), no mechanical stress.

• Limitations: Very expensive, vacuum system, slow for large areas, safety (X-rays), conductive materials preferred.

• Applications: Micro-holes, thin slots, aerospace, medical devices.

4. Abrasive Jet Machining (AJM) / Abrasive Water Jet Machining (AWJM)

• AJM Principle:

  DiagramSEARCH: "abrasive jet machining diagram nozzle"

  • High-pressure air/gas carries fine abrasive (Al₂O₃, SiC) through nozzle.

  • Abrasive impacts workpiece at ~150 m/s → erosion by micro-cutting.

• AWJM: Uses water as carrier (high pressure ~2000-4000 bar). Can cut without abrasive for soft materials.

• Abrasive Selection Factors:

  • Hardness: Must be harder than workpiece.

  • Shape: Angular for cutting, round for polishing.

  • Size: Finer for smoother finish, coarser for MRR.

  • Toughness: Resists fracture on impact.

  • Cost & Availability.

• Applications: AJM: Glass, ceramics, silicon, delicate parts. AWJM: Cutting thick metals, stone, composites, food.

5. Ultrasonic Machining (USM)

• Working Principle:

  DiagramSEARCH: "ultrasonic machining diagram tool horn slurry"

  • Tool (sonotrode) vibrates at ultrasonic frequency (~20 kHz) with small amplitude (~20 µm).

  • Slurry (abrasive + water) flows between tool & workpiece.

  • Impact & micro-chipping: Abrasive grains hammer workpiece due to tool vibration.

• Important Elements: Transducer (piezoelectric/magnetostrictive), Horn/Amplifier, Tool, Abrasive Slurry, Workpiece Fixture.

• Advantages: No heat, brittle materials, complex shapes.

• Limitations: Slow, tool wear, only hard/brittle materials.

• Applications: Ceramics, glass, carbide, silicon, quartz.

6. Plasma Arc Machining (PAM)

• Construction & Working:

  DiagramSEARCH: "plasma arc machining diagram torch"

  • Plasma Torch: Tungsten electrode, orifice, shielding gas (N₂, Ar, H₂).

  • Process: Gas ionized by arc → plasma jet (~20,000°C) → melts & vaporizes metal → high-velocity jet blows away molten metal.

  • Types: Transferred (arc between electrode & workpiece) for cutting; Non-transferred for spraying.

• Applications: Cutting thick conductive metals (steel, Al, Cu), fast cutting, shipbuilding.

7. Laser Beam Machining (LBM)

• Working: High-power, coherent laser beam (CO₂, Nd:YAG, fiber) focused on workpiece.

  • Absorption → heat → melting/vaporization/ablation.

  • Assist gas (O₂, N₂, Ar) may be used to blow debris or enhance cutting.

• Advantages: Non-contact, high speed, precise, can cut any material, narrow kerf.

• Limitations: High cost, low efficiency, reflective materials problematic, thermal HAZ.

• Applications: Cutting, drilling, welding, marking, micro-machining.

Comparative Studies

EDM vs. EBM

Plasma Arc Cutting vs. Wire Cut EDM

V. NUMERICAL CONTROL & COMPUTER NUMERICAL CONTROL

A. NC Machine Tools

Constructional Features and Functions

Coordinate Systems (NC Machines)

• Cartesian (Rectangular): Most common. X, Y, Z linear axes (right-hand rule). Rotational: A, B, C.

• Polar: Radius (R) and angle (θ). Used in turning, some drilling.

• Spherical: Radius (R) and two angles (θ, φ). Rare.

• Cylindrical: Z (axial), R (radial), θ (angular). Used in cylindrical grinders.

• Machine Zero (Home): Fixed reference point on each axis.

• Workpiece Zero (Program Zero): User-defined origin on workpiece.

Types of Control Systems

1. Point-to-Point (PTP): Controls only final position of each axis independently. Path not controlled. (e.g., drilling, punching).

2. Straight Line: Controls path to be a straight line between points. One axis may be in motion at a time.

3. Contouring (Continuous Path): All axes move simultaneously under control to generate any curve (circle, parabola). Requires interpolation. (e.g., milling, turning complex shapes).

B. CNC & Adaptive Control

NC vs. CNC

Adaptive Control (AC) of NC Machines

• Concept: Closed-loop control that automatically adjusts cutting parameters (speed, feed, depth) in real-time based on sensor feedback (force, torque, vibration, temperature) to optimize performance.

• Goals: Maximize MRR, prevent tool breakage, maintain constant force/temperature, extend tool life.

• Types:

  • AC for Constraint: Adjusts to stay within machine/tool limits (e.g., max force).

  • AC for Optimization: Adjusts to achieve optimal objective (e.g., max profit, min cost).

C. NC Part Programming

Manual Programming (G & M Codes)

• G-Codes (Preparatory): Define motion and mode.

  • G00 - Rapid traverse (PTP).

  • G01 - Linear interpolation (contouring).

  • G02/G03 - Circular interpolation (CW/CCW).

  • G20/G21 - Inch/mm.

  • G40/G41/G42 - Cutter compensation (cancel, left, right).

  • G90/G91 - Absolute/Incremental programming.

• M-Codes (Miscellaneous): Define machine functions.

  • M00 - Program stop.

  • M03/M04 - Spindle on (CW/CCW).

  • M05 - Spindle stop.

  • M06 - Tool change.

  • M08/M09 - Coolant on/off.

  • M30 - End of program.

Computer-Assisted Programming (APT-like)

• Motion Control Statements (Five Types):

  1. Point-to-Point (PTP): GOTO/ x, y, z

  2. Linear (L): GOLFT/ x, y, z (Go Linear To)

  3. Circular (C): CIRCLE/ x, y, z, radius, direction

  4. Parabolic/Curve: CURVE/ ...

  5. Pattern/Repetitive: MIRROR, ROTATE, LOOP.

• Geometry Statements: Define points, lines, circles, planes.

• Postprocessor: Converts APT-like statements to specific machine G/M codes.

Part Programming for Machining Centers (Milling) – Example

Geometry:

DiagramCANVAS: "Simple 2D contour: A rectangle 50x30 mm with a central hole Ø20 mm. Coordinate system origin at bottom-left corner of rectangle."

Sample Program (Absolute, G54):

N10 G21 G90 G54 (Metric, Abs, Work Coord)

N20 S1200 M03 (Spindle 1200 RPM CW)

N30 G00 X0 Y0 Z50 (Rapid to safe Z)

N40 G00 X10 Y10 (Rapid to start point)

N50 G01 Z-5 F100 (Plunge to depth)

N60 G01 X40 Y10 F200 (Cut along Y=10)

N70 G01 X40 Y40 (Cut along X=40)

N80 G01 X10 Y40 (Cut along Y=40)

N90 G01 X10 Y10 (Cut along X=10 - close rectangle)

N100 G00 Z50 (Retract)

N110 G00 X25 Y25 (Rapid to hole center)

N120 G01 Z-10 F80 (Plunge)

N130 G01 X35 (Cut Ø20 hole - simple arc or use G02/G03)

... (complete circle)

N140 G00 Z50 M05 (Retract, stop spindle)

N150 M30 (End)

VI. PLASTICS PROCESSING

A. Plastics Fundamentals

• Thermoplastics: Soften on heating, harden on cooling. Reversible (can be re-melted). e.g., PE, PP, PS, PVC, Nylon.

• Thermosets: Undergo irreversible chemical change (curing) during first heating. Once set, cannot be remelted. e.g., Phenolics, Epoxy, Polyester, Melamine.

• Additives:

  • Plasticizers: Increase flexibility, reduce hardness/brittleness (e.g., in PVC).

  • Fillers: Reduce cost, improve strength/stiffness, reduce shrinkage (e.g., CaCO₃, wood flour).

  • Stabilizers: Prevent degradation (thermal, UV, oxidation). e.g., Antioxidants, UV absorbers.

B. Welding of Plastics

• Process: Joining plastics by localized heating along joint line, then applying pressure to fuse.

• Methods: Heated tool, ultrasonic, friction, hot gas, induction.

• Advantages: Can join complex shapes, no fasteners, lightweight, good aesthetics.

• Limitations: Limited to thermoplastics (some thermosets), joint strength < base material, surface preparation critical, not for all plastics.

C. Molding Processes

Injection Molding

• Process: Plastic granules melted in barrel → injected under high pressure into closed mold → cools → opens → eject part.

• Mold Types:

  • Cold Runner: Runner (channel) solidifies with part → waste.

  • Hot Runner: Runner kept molten → no waste, better control.

• Benefits: High production rate, excellent surface finish, complex shapes, automatic.

• Limitations: High mold cost, not for very large parts, flash possible.

Blow Molding

• Process for Bottles:

  DiagramSEARCH: "extrusion blow molding process diagram"

  1. Extrusion: Parison (tube) extruded.

  2. Mold Closes: Parison captured in mold cavity.

  3. Blow Air: Air injected → parison expands to mold shape.

  4. Cool & Eject.

• Advantages: Low cost for hollow parts, fast, good for thin walls.

• Defects: Flash (excess material), sink marks, thin spots, stress cracking, poor wall thickness uniformity.

Compression Molding

• Process:

  DiagramSEARCH: "compression molding process diagram"

  1. Charge: Pre-measured polymer (powder/granule) placed in open, heated mold cavity.

  2. Close & Apply Pressure: Mold closes, heat & pressure applied → material flows & cures (for thermosets).

  3. Cool & Open: Cool (thermoplastics) or cure (thermosets), open mold, eject part.

• Applications: Thermoset composites (Bakelite), PTFE, large flat parts.

Transfer Molding vs. Extrusion Molding

Calendaring Process

• Process:

  DiagramSEARCH: "calendaring process diagram rolls"

  • Plastic compound passed sequentially through 2 or more heated, rotating rolls (calender).

  • Rolls have decreasing gap → sheet thickness reduced, surface smoothed, embossed.

  • Final sheet pulled off, cooled, wound.

• Applications: PVC sheets, films, coated fabrics, rubber sheets.

D. Other Processes

• Film Blowing:

  DiagramSEARCH: "film blowing process diagram bubble"

  • Extruded tube (bubble) inflated with air → drawn up → cooled → flattened → wound.

  • Produces thin, flexible films (polyethylene).

• Thermoforming:

  • Process: Plastic sheet heated → softened → formed over mold by vacuum, pressure, or mechanical means → cooled.

  • Types: Vacuum forming, pressure forming, twin-sheet forming.

  • Applications: Packaging (blisters), trays, automotive interior panels.

VII. EXTRUSION PROCESSES

A. Fundamentals

• Working Principle: Material (billet) forced by ram pressure through a die with desired cross-section → emerges as continuous profile.

• Applications: Windows (Al extrusions), pipes, rods, structural shapes, wire sheathing.

• Advantages: Continuous, complex cross-sections, good surface finish, can combine materials (co-extrusion).

• Disadvantages: High initial cost, limited to constant cross-section, residual stresses.

B. Classification & Types

Hot Extrusion vs. Cold Extrusion

Tube Extrusion

• Process:

  DiagramSEARCH: "tube extrusion process diagram mandrel"

  • With Mandrel (Fixed/Spider): Billet pierced by stationary mandrel → material flows around mandrel legs → hollow tube.

  • With Bridge/Spider: Mandrel supported by legs (bridges) → weld lines in tube wall.

  • Without Mandrel (Tube Sinking): Solid billet extruded → later pierced/expanded.

• Applications: Pipes, tubes, hollow structural sections.

Metal Flow in Extrusion (Types & Importance)

• Types:

  1. Uniform Flow: Ideal, all elements move parallel to axis.

  2. Inlet (Dead Metal) Zone: Material near container walls stagnant.

  3. Extrudate (Butt) End: Material at billet end flows slower → "butt end" defect.

  4. Center-Burst (Chevron) Cracking: Tensile stress center → cracks.

  5. Surface Cracking (Fins): Due to friction, uneven flow.

• Importance: Understanding flow helps design billet geometry, die shape, lubrication, temperature control to avoid defects and ensure product quality.

VIII. OTHER CONVENTIONAL MACHINING PROCESSES

A. Broaching

• Cutting Action of a Broach:

  DiagramSEARCH: "broach tool diagram teeth pitch"

  • Broach: Long, multi-tooth tool with progressive tooth rise (roughing → finishing).

  • Action: Each tooth takes a small, incremental cut as broach is pushed/pulled through/over workpiece.

  • Stroke: One pass completes the operation (like shaping with multiple tools).

• Continuous Surface Broaching Machine:

  • Construction: Workpiece mounted on fixture that reciprocates. Broach is stationary, mounted vertically/horizontally. Workpiece moves up & down past broach teeth.

  • Useful Component Surfaces: Internal surfaces (keyways, splines, square holes, gear teeth) and external surfaces (flat, contoured) on medium to large production parts. Ideal for blind holes (keyways).

B. Sawing Machines

IX. ADVANCED FORMING & SHEET METAL ANALYSIS

A. Yield Criteria

• Von Mises (Distortion Energy Theory): Yielding begins when distortion energy per unit volume reaches critical value.

$$\sigma_{vm} = \sqrt{\frac{1}{2}\left[(\sigma_1-\sigma_2)^2 + (\sigma_2-\sigma_3)^2 + (\sigma_3-\sigma_1)^2\right]}$$

Yields when $$\displaystyle \sigma_{vm} = \sigma_y $$ (yield stress in tension).

• Tresca (Maximum Shear Stress Theory): Yielding begins when maximum shear stress reaches critical value.

$$\tau_{max} = \frac{\sigma_{max} - \sigma_{min}}{2}$$

Yields when $$\displaystyle \tau_{max} = \frac{\sigma_y}{2} $$.

• Relation: For pure shear, Von Mises predicts $$\displaystyle \tau_y = \frac{\sigma_y}{\sqrt{3}} \approx 0.577\sigma_y $$; Tresca predicts $$\displaystyle \tau_y = 0.5\sigma_y $$. Von Mises is more accurate for ductile metals.

B. Forging Analysis – Pressure Distribution (Rectangular Block)

• Derivation Goal: For a rectangular block (width w, height h, length b) under sliding friction ($\mu$), with constant shear strength $K$, show:

$$\frac{p}{2K} = e^{-\frac{2\mu x}{h}} \left[ 1 - e^{-\frac{2\mu b}{h}} \right]$$

where `p` is pressure at distance `x` from centerline.

• Concept: Consider force equilibrium on an elemental slice. Friction on vertical sides opposes flow. Pressure varies exponentially from center to surface.

• Importance: Predicts required forging pressure, helps design die geometry, assess die filling.

C. Sheet Metal Dies

D. Punching Operations

• Punch/Die Sizes (Clearance):

  • Clearance (c): Gap between punch & die. Usually 5-10% of material thickness (t).

  • For Hole Punching (Punch smaller than die):

    • Punch Diameter (Dp): $$\displaystyle D_p = D_{hole} - 2c $$

    • Die Diameter (Dd): $$\displaystyle D_d = D_{hole} + 2c $$

  • For Blanking (Die smaller than punch):

    • Die Diameter (Dd): $$\displaystyle D_d = D_{blank} - 2c $$

    • Punch Diameter (Dp): $$\displaystyle D_p = D_{blank} + 2c $$

• Shear Angle (ϕ): Angling the punch/die to reduce cutting force & improve life.

  • Force Reduction: $$\displaystyle F_{sheared} = F_{straight} \times \frac{1}{\sin \phi} $$

  • Example (Jun 2023): To reduce force to fit press capacity, calculate required shear angle.

    • Given: Hole Ø100 mm, t=5.6 mm, τ=550 MPa, clearance 10%, cutting completes at 40% penetration.

    • Straight Shear Force: $$\displaystyle F = \pi \times D \times t \times \tau $$

    • With Shear Angle: $$\displaystyle F_{actual} = \frac{F}{\sin \phi} $$

    • Solve for $\phi$ such that $$\displaystyle F_{actual} \leq $$ press capacity (30 tonnes = 300 kN).

X. WELDING TECHNOLOGY (METALS)

Friction Welding (FW)

• Working Principle:

  DiagramSEARCH: "friction welding diagram rotating stationary"

  • Solid-state welding (no melting).

  • One workpiece rotated, other stationary.

  • Axial force applied → friction heats interface → material softens → rotation stopped → forge pressure applied → bond formed.

• Types: Rotary (most common), linear friction, orbital friction.

• Applications: Welding dissimilar metals (Al-Steel, Ti-Steel), tubes, rods, aerospace, automotive. Excellent for non-weldable combinations.

Weldability

• Definition: Ability of a material to be welded under given conditions without defects and to perform satisfactorily in service.

• Factors Affecting Weldability:

  1. Material Composition: Carbon content (steel), alloying elements (Cr, Mo reduce weldability).

  2. Thermal Properties: Thermal conductivity, expansion coefficient (affect HAZ, residual stress).

  3. Phase Transformations: Hardening tendency (martensite in steels → cracking).

  4. Impurities: S, P (hot cracking), H (cold cracking, hydrogen embrittlement).

  5. Physical Properties: Melting point, viscosity of molten metal.

  6. Welding Process & Parameters: Heat input, preheat, post-weld heat treatment (PWHT).

XI. NON-DESTRUCTIVE TESTING (NDT)

[!TIP] Exam Tip: Know the key difference: UT/RT for internal, MT/PT/ET for surface/near-surface. MT only for ferromagnetic, PT for non-porous, ET for conductive.