UNIT 2: MANUFACTURING PROCESS - EXAM-FOCUS NOTES

1.0 CASTING PROCESSES

1.1 Types of Casting Processes & Applications

Exam Tip: Be ready to compare centrifugal (rotational force for cylinders) vs. continuous (linear withdrawal for slabs/billets). Know material suitability: centrifugal for pipes, continuous for rolling feedstock.

1.2 Pattern Making & Pattern Allowances

Pattern: Replica of casting, used to form mold cavity. Core Box: Used to create sand cores for internal cavities.

Types of Patterns & Uses:

• Solid Pattern: Simple shape, single-piece mold.

• Split Pattern (Two-Piece): Most common; for complex shapes with undercuts.

• Match Plate Pattern: Pattern mounted on a plate with core prints; halves mounted on opposite sides. Used in machine molding.

• Sweep Pattern: Used for rotational symmetric parts (e.g., bells, basins).

• Loose Piece Pattern: Removable pieces for undercuts; increases molding cost.

• Shell Pattern: Thin shell for large, hollow castings.

Pattern Allowances (Extra dimensions on pattern):

Exam Tip: Shrinkage allowance is the most calculation-heavy. Distinguish between solidification shrinkage (liquid→solid) and solid contraction (solid→room temp). For a cubic casting with given volumetric shrinkages, final side = initial side × $$\displaystyle (1 - S_v/100)^{1/3} $$.

1.3 Molding & Core Sands

Properties of Foundry Sands:

• Refractoriness: Resistance to high temperatures.

• Permeability: Ability to allow gases to escape (prevents gas holes).

• Green Strength: Strength when moist (for green sand).

• Dry Strength: Strength after drying/baking.

• Collapsibility: Ability to fall away from casting during cooling (reduces cracking).

• Reusability: Ability to be reclaimed and reused.

Binder Systems:

• Clay (Bentonite) + Water: For green sand (cheap, reusable).

• Clay + Water + Organic Additives: For dry sand (higher strength).

• Resin (e.g., Furan, Phenolic) + Catalyst: For core sand & shell molding (high strength, accuracy).

• Cement: For large, heavy castings.

1.4 Gating System & Risers

Purpose: Gating system (sprue, runner, gate) directs molten metal into cavity. Riser (feeder) acts as a reservoir to compensate for shrinkage.

Design Principles:

• Location: Gate at thickest section, lowest point; riser on top of thickest section.

• Size/Shape: Minimize turbulence (smooth transitions). Riser should solidify last (Chvorinov's Rule).

• Influence on Quality: Poor design → turbulence (inclusions), premature solidification (shrinkage), high scrap.

Riser Design & Chvorinov's Rule:

Solidification time $$\displaystyle t = C \left( \frac{V}{A} \right)^n $$

• $t$ = solidification time

• $V$ = volume of casting/riser

• $A$ = surface area

• $C$ = mold constant (depends on material/mold)

• $n$ = exponent (usually 2)

For a cylindrical riser (height = diameter, h=d):

$$ \frac{V}{A} = \frac{\pi d^2 h / 4}{\pi d h + \pi d^2 / 2} = \frac{\pi d^3 / 4}{\pi d^2 (3/2)} = \frac{d}{6} $$

Modulus (M) = V/A. Riser must have M_riser > M_casting to solidify later.

Exam Tip: Chvorinov's Rule is crucial. For a given volume, the riser with smallest surface area solidifies slowest. For a cylindrical blind riser of constant volume, minimum solidification rate (slowest cooling) occurs when h/d = 1 (a sphere is ideal but impractical). Derivation: For constant V, A is minimized when h = d.

1.5 Solidification, Defects & Remedies

Theory: Volumetric shrinkage (~5-10%) occurs during solidification. Solid contraction (~0.5-1.5%) occurs as solid cools to room temp. Risers feed liquid to compensate for solidification shrinkage.

Common Defects & Remedies:

Aids:

• Chills: Metal/refractory inserts to accelerate local cooling, promote directional solidification.

• Chaplets: Metal supports to hold core in position (become part of casting).

• Core Prints: Projections in pattern to form recess for core positioning.

1.6 Special Casting Processes

Thermit Welding (Thermite Welding):

• Chemical Reaction: $$\displaystyle \text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3 + \text{Heat} $$ (exothermic, ~2500°C).

• Conditions: Preheated mold, thermite mixture (powdered Al + iron oxide), ignition source (magnesium ribbon).

• Applications: Rail welding, repair of large steel castings in field.

• Challenges: Controlling reaction rate, achieving sound weld (porosity), high temperature gradient.

2.0 WELDING PROCESSES

2.1 Welding Methods & Suitability

2.2 Arc Welding Fundamentals

Role of Flux & Electrodes:

• Flux (SMAW): Provides shielding gas, slag (protects weld pool), deoxidizers, alloying elements. Electrode coating contains flux.

• Electrodes: Consumable (SMAW, MIG) melt to fill joint. Non-Consumable (TIG) only provides arc.

Power Sources: AC vs. DC:

Electrode Polarity in DC (Crucial):

• DC Electrode Positive (DCEP): Electrons flow from workpiece to electrode. ~66% heat at electrode → faster melt rate, shallow penetration.

• DC Electrode Negative (DCEN): Electrons flow from electrode to workpiece. ~66% heat at workpiece → deeper penetration, slower electrode melt.

Heat Input Calculation:

$$ \text{Heat Input (J/mm)} = \frac{V \times I \times \eta}{v_s} $$

• $V$ = Arc Voltage (Volts)

• $I$ = Welding Current (Amps)

• $\eta$ = Efficiency (arc ~0.8-0.9, gas ~0.9, SMAW ~0.75-0.85)

• $$\displaystyle v_s $$ = Travel Speed (mm/s)

Exam Tip: Polarity effect is frequently tested. Remember: "Electrode Positive = More heat on Electrode (faster melt, shallow). Electrode Negative = More heat on Work (deeper penetration)." For TIG, DCEN is standard for deep penetration on steel.

2.3 Welding Defects & Remedies

2.4 Special Welding Processes (TIG vs. MIG vs. Traditional)

TIG (GTAW):

• Controls: Manual torch manipulation, separate filler rod feed, foot pedal for current/amperage control.

• Speed: Slow (manual).

• Quality: Highest – clean (no slag/spatter), precise heat input, excellent for thin materials & non-ferrous.

• Significance: Used where weld quality is paramount (aerospace, nuclear, medical).

MIG (GMAW):

• Controls: Semi-automatic (trigger on gun controls wire feed & power). Can be fully automatic.

• Speed: Fast – high deposition rates.

• Quality: Good, but may have spatter; shielding gas critical.

• Significance: High-production welding of mild steel, sheet metal.

vs. SMAW (Traditional):

• SMAW is versatile, portable, but slower, produces slag/spatter, quality operator-dependent.

• TIG/MIG offer superior control/quality but are less portable (require gas) and more equipment-sensitive.

3.0 FORGING PROCESSES

3.1 Theory & Application of Forging

Fundamentals: Mechanical working of metals by compressive forces to shape, improve grain structure, and enhance mechanical properties (forging refines grain, closes porosity, aligns fibers).

Hot Working vs. Cold Working:

Justification for Wire Drawing: Cold working is used to achieve high strength, precise diameter, and excellent surface finish. Strain hardening is desirable.

Open-Die vs. Impression-Die Forging:

3.2 Forging Machines & Operations

Drop Forging (Hammer Forging):

• Mechanism: Hammer (gravity or powered) raised and dropped onto workpiece on anvil. Open-die or impression-die.

• Applications: Large parts, open-die shapes, initial forging of closed-die billets.

• Advantages: Simple, high impact force, good for large deformations.

• Limitations: Impact noise, vibration, less precise than press forging.

Horizontal Forging (Press Forging):

• Mechanism: Workpiece deformed slowly by continuous pressure (hydraulic/mechanical press). Usually impression-die.

• Applications: Complex, precise closed-die forgings (automotive, aerospace).

• Advantages: Complete die fill, better dimensional control, no impact shock, can forge larger sections.

• Limitations: Slower cycle time than drop forging for some parts, higher equipment cost.

Comparison:

• Drop Forging: High speed, impact, good for initial shaping, more flash.

• Press Forging: Slow, controlled pressure, better detail, less flash, finer grain structure.

4.0 PRESS WORKING (Sheet Metal Forming)

4.1 Concept & Importance

Press Working: Sheet metal forming using dies and punches in a press machine. Material is stressed beyond yield strength but within ultimate tensile strength (plastic deformation). Importance: High production rate, excellent surface finish, close tolerances, minimal scrap (for some ops), material property enhancement (strain hardening).

4.2 Press Working Operations

Shearing Operations (Separation):

• Shearing: Cutting straight line (e.g., cutting sheet to size).

• Punching: Punch cuts hole in sheet, slug (scrap) falls through.

• Blanking: Punch cuts part from sheet, slug is part, sheet is scrap.

• Piercing: Punch cuts hole in sheet, slug is scrap, sheet is part.

• Trimming: Removing excess material (flash) from around a formed part.

• Notching: Cutting small shapes from edge.

• Lancing: Shearing and bending a tab in the sheet (no reduction in thickness).

• Slitting: Continuous shearing to cut wide sheet into narrower strips.

Drawing Operations (Forming without thinning too much):

• Shallow Drawing: Depth < diameter. Minimal thinning.

• Deep Drawing: Depth > diameter. Defects: Wrinkling (compressive buckling in flange), tearing (excessive tensile stress in wall).

  • Remedies: Proper blank holder force, correct die radius, lubrication, multi-stage drawing.

Bending: Forming straight line bend (V, U, channel). Springback (elastic recovery) is key issue. Embossing: Creating shallow, raised (or recessed) design without significant thinning. Perfecting: Two operations in one stroke (e.g., punching and bending).

4.3 Press Machines & Auxiliaries

Hydraulic Presses vs. Pneumatic Presses:

Tool Dies:

• Components: Punch (male), Die (female), Stripper plate, Guide pillars/bushings, Blank holder.

• Types: Simple Die: One operation per stroke. Compound Die: Multiple operations in one station (e.g., blanking & piercing). Progressive Die: Series of stations, part completes progressively.

Auxiliary Equipment:

• Stock Feeders: Automatically feed sheet/coil into press (roll feed, NC feeder).

• Scrap Cutters: Cut scrap into manageable pieces.

• Safety Devices: Light curtains, guards, two-hand controls, interlocks.

4.4 Process Selection & Calculations

Selection Factors: Part geometry, material, thickness, production volume, required tolerance/finish, cost.

Blanking/Piercing Layout (Strip Utilization):

• Goal: Maximize number of parts per strip/coil, minimize scrap.

• Considerations: Pitch (part width + clearance), side margin (scrap), strip width.

• Calculation: Number of parts = $$\displaystyle \left\lfloor \frac{\text{Strip Width}}{\text{Pitch}} \right\rfloor $$ (for single row). For multiple rows, optimize arrangement (rectangular, staggered).

Force, Pressure & Power:

• Cutting Force (Approx): $$\displaystyle F = \tau \cdot t \cdot L $$

  • $\tau$ = Shear strength of material (MPa)

  • $t$ = Sheet thickness (mm)

  • $L$ = Total cutting length (mm)

• Press Capacity: Must exceed cutting force + stripping force + blank holding force.

• Power: $$\displaystyle P = \frac{F \cdot v}{60} $$ (Watts) or $$\displaystyle P = \frac{F \cdot s \cdot N}{60,000} $$ (kW)

  • $v$ = Slide speed (mm/s)

  • $s$ = stroke (mm), $N$ = strokes per minute.

Exam Tip: Blanking layout problems are common. Remember: Blanking = part is the punch-out; Piercing = part is the sheet with hole. Scrap margin is added outside the part dimensions for blanking.

5.0 ROLLING PROCESSES

5.1 Machines & General Process Description

Rolling Mill Types:

• Two-High: Two rolls, reversible or non-reversible. Basic.

• Three-High: Three rolls, one driven, two rotating. Work passes up/down. For heavy plates/sections.

• Four-High: Two small working rolls + two large backup rolls. Reduces roll deflection → better flatness for sheets/strips.

• Cluster (Z-High): Multiple backup rolls. For very wide plates/sections.

• Tandem Mill: Series of stands (2-18). High speed, continuous rolling for strips.

• Continuous Mill: Coil-to-coil, with looping towers.

Process Sequence: Slab/Bloom → Roughing Mill → Intermediate Mill → Finishing Mill → Coil/Plate.

5.2 Hot Rolling vs. Cold Rolling

Impact on Properties: Cold rolling increases yield strength and hardness significantly (work hardening). Hot rolling produces a more ductile, isotropic material.

5.3 Rolling Products

• Structural Sections (I-beams, H-beams, rails): Rolled on section mills (breaks, stands). Used in construction, bridges.

• Plates: Thick (>3mm), wide, flat. Rolled on plate mills (four-high, reversing). Used in ship hulls, pressure vessels.

• Sheets: Thin (0.4-3mm), large area. Rolled on sheet mills (four-high, tandem). Used in automotive bodies, appliances.

• Strips: Very thin (<0.4mm), coiled. Rolled on strip mills (tandem, continuous). Used in tinplate, galvanized sheets, foils.

6.0 MACHINING PROCESSES (Machine Tools)

6.1 Lathe Machine

Fundamental Components:

• Bed: Base, supports all parts, guides carriage.

• Headstock: Houses spindle, motor, gear train. Holds workpiece.

• Tailstock: Supports other end of workpiece, holds drills/centers.

• Carriage: Mounts tool post, moves parallel/perpendicular to axis (cross-slide).

• Apron: Contains feed mechanisms (levers, gears).

• Tool Post: Holds cutting tool.

Basic Operations:

• Turning: Remove material from rotating workpiece (cylindrical surface).

• Facing: Cut flat surface perpendicular to axis (using cross-slide).

• Drilling: Drill held in tailstock, workpiece rotates.

• Boring: Enlarge existing hole (tool mounted on carriage).

• Threading: Use threading tool with lead screw engagement.

• Knurling: Roll pattern onto surface (no chip removal).

• Parting/Chopping: Cut off work from bar (deep, narrow cut).

6.2 Shaper & Planer Machines

Shaper:

• Working Principle: Single-point cutting tool moves reciprocating over stationary workpiece. Return stroke idle (rapid return).

• Operations: Horizontal/vertical surfaces, grooves, keyways.

• Workpiece: Small/medium, clamped on table.

Planer:

• Working Principle: Workpiece moves reciprocating under stationary cutting tool(s). Multiple tools can cut on return stroke.

• Operations: Very large, heavy workpieces (machine bases, long beds).

• Workpiece: Large, heavy, clamped on table.

Comparison (Shaper vs. Planer):

6.3 Milling Machine

Introduction: Multi-point rotating cutter (milling cutter) feeds past stationary (or moving) workpiece. Working Principle: Rotary motion of cutter + linear feed of table/workpiece. Types:

• Horizontal Milling: Cutter on horizontal arbor. Good for heavy cuts, slots, gears.

• Vertical Milling: Cutter on vertical spindle. More versatile (drilling, boring, end milling). Basic Operations: Plain milling, face milling, end milling, slotting, keyway cutting, gear cutting.

6.4 Grinding Machine

Introduction: Uses abrasive wheel (multi-point, very hard) for precision machining and surface finishing. Working Principle: High-speed rotating abrasive wheel removes small chips (micro-cutting). Main Applications:

• Achieve high dimensional accuracy (±0.001 mm).

• Obtain excellent surface finish (Ra 0.1 μm).

• Harden surfaces (grind hardened parts).

• Machine hard materials (carbides, ceramics). Types:

• Surface Grinder: Flat surfaces (horizontal/vertical spindle).

• Cylindrical Grinder: External/internal cylindrical surfaces.

• Tool & Cutter Grinder: Grind cutting tools (drills, mills).

6.5 Drilling Machine

Basic Functionality: Rotating drill (twist drill) feeds into stationary workpiece to create a hole. Working Principle: Rotational motion + axial feed. Common Types of Drills:

1. Twist Drill: Most common, helical flutes for chip removal.

2. Core Drill: Used to enlarge holes, has 3-4 flutes, no center point.

3. Center Drill: Short, rigid, for starting hole (countersink).

4. Spot Drill: Short, for starting hole to prevent walking.

Drilling Time Calculation:

$$ T = \frac{L}{f \times N} + \text{Approach/Overrun} $$

• $T$ = Drilling time (min)

• $L$ = Total drill travel = Thickness of workpiece + Approach + Overrun (mm)

• $f$ = Feed rate (mm/rev)

• $N$ = Spindle speed (rpm)

Approach/Overrun: Typically 0.3D to 0.5D (D = drill diameter). Given as equal to radius in problem.

Exam Tip: Drilling time problems are frequent. Total travel L = Thickness + Approach + Overrun. Approach/Overrun ensures full hole depth and clean break-through. Always convert units consistently (mm, min).

6.6 General Machining Concepts

Elastic vs. Plastic Deformation:

• Elastic Deformation: Temporary, reversible. Stress < Yield Strength ($$\displaystyle \sigma < \sigma_y $$). Material returns to original shape when load removed (e.g., spring).

• Plastic Deformation: Permanent, irreversible. Stress > Yield Strength ($$\displaystyle \sigma > \sigma_y $$). Material retains new shape (e.g., forging, rolling, machining chip formation). Machining relies on plastic deformation to form chips.