Manufacturing Process (ME-305) - Unit 5 Short Notes

UNIT 5: MANUFACTURING PROCESS

I. CASTING PROCESSES

A. Introduction & Significance

• Definition: Casting is a primary shaping process where molten metal is poured into a mould cavity containing a negative impression of the desired shape, allowed to solidify, and then ejected.

• Significance: Enables production of complex geometries (internal cavities, intricate shapes) that are difficult or costly to make by other methods. It's versatile for a wide range of metals and production volumes.

• Advantages: Design flexibility, near-net shape, large part size capability, material versatility.

• Disadvantages: Potential for defects (porosity, shrinkage), coarse surface finish, dimensional inaccuracy, environmental impact.

B. Types of Casting Processes & Applications

[!TIP] Exam Focus: Be prepared to compare processes (e.g., Die vs. Sand, Centrifugal vs. Continuous) based on principle, cost, accuracy, material, and part geometry.

C. Pattern Making & Pattern Allowances

• Pattern: A replica of the final casting, used to form the mould cavity. Made of wood, metal, plastic.

• Purpose of Allowances: Compensate for dimensional changes during pattern making → moulding → solidification → cooling → machining.

Pattern Dimension Calculation:

$$ \text{Pattern Dimension} = \text{Casting Dimension} \times (1 + \text{Linear Shrinkage Allowance}) + \text{Other Allowances} $$

[!TIP] Common Pitfall: Forgetting to add all applicable allowances (Shrinkage + Draft + Machining) sequentially or misapplying linear vs. volumetric shrinkage.

Types of Patterns:

• Solid Pattern: Simple shape, single piece.

• Split Pattern (Two-Piece): For cavities with undercuts.

• Match Plate Pattern: Pattern mounted on a plate with matching cope/drag patterns.

• Sweep Pattern: For cylindrical/truncated shapes, swept around a central axis.

• Loose Piece Pattern: Removable piece for undercuts.

• Cope and Drag Patterns: Separate patterns for top (cope) and bottom (drag) halves.

D. Moulding Materials & Core Sands

• Foundry Sand Properties:

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

  • Cohesiveness/Strength: Ability to hold mould shape.

  • Refractoriness: Resistance to high temperatures.

  • Plasticity: Ability to be molded.

  • Adhesiveness: Stickiness to pattern.

• Green Sand: Silica sand + clay + water. Used for most sand castings. Advantages: Cheap, reusable. Disadvantages: Moisture causes defects, low strength.

• Dry Sand: Green sand baked/dried. Higher strength, better finish, used for larger castings.

• Core Sands: Require higher strength and refractoriness. Bound with oil, resin (furan, phenolic), or shell process binders. Must be easily removable after casting.

E. Gating System, Runners, and Risers

• Components & Functions:

  • Pouring Basin: Receives molten metal, minimizes turbulence.

  • Sprue: Vertical channel connecting pouring basin to runner.

  • Runner: Horizontal channel distributing metal to gates.

  • Gate: Constricted opening controlling metal flow into cavity.

  • Riser (Feeder): Reservoir of molten metal to compensate for shrinkage.

• Design Principles: Minimize turbulence (smooth transitions, proper sprue base), prevent erosion (low velocity in runners), promote directional solidification (riser last to solidify).

• Riser Design (Chvorinov's Rule):

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

Where: `t` = solidification time, `V` = volume, `A` = surface area, `C` = mold constant, `n` ≈ 2.

**Implication:** For minimum solidification time (riser solidifies slower than casting), **V/A ratio must be larger for riser than casting**.

*   **Height-to-Diameter Ratio (for cylindrical riser):** Optimal `h/d ≈ 1.0` to `1.25` for minimum V/A.

• Impact on Quality:

  • Poor gating → misruns, cold shuts, erosion, inclusions.

  • Inadequate riser → shrinkage cavities.

  • Excessive gating/riser → low yield, high cost.

F. Chills, Chaplets, and Core Prints

• Chills: Inserts of high thermal conductivity material (graphite, iron) placed in mould to accelerate local cooling.

  • Internal Chills: Placed inside cavity (dissolve or remain as part of casting).

  • External Chills: Placed in mould wall.

• Chaplets: Supports for cores (to prevent sagging/floating). Made of same/alloy metal as casting. Must melt/fuse properly.

• Core Prints: Projections on pattern (in cope/drag) that create recesses in mould to position and support the core accurately.

G. Solidification, Defects & Their Elimination

• Theory: Directional solidification (from extremities towards riser) is essential to feed shrinkage.

• Major Casting Defects:

H. Special Casting Processes (Theoretical & Applied)

• Die Casting - Dimensional Tolerance Management:

  • Die Design: Use of precision-machined, hardened steel dies with proper ejection, cooling channels.

  • Process Optimization: Control of injection pressure, speed, temperature; use of vacuum assistance.

  • Material Selection: Choosing alloys with low shrinkage (e.g., Zamak, A380).

  • Result: Achieves ±0.1 mm tolerances for small parts.

• Investment Casting - Procedure:

  1. Pattern Making: Wax pattern injected into metal die.

  2. Tree Assembly: Patterns attached to a wax sprue (tree).

  3. Shell Coating: Tree dipped in refractory slurry, stuccoed with sand, repeated to build shell.

  4. Dewaxing: Shell heated to melt/remove wax.

  5. Firing: Shell baked at high temperature to remove residues, gain strength.

  6. Pouring: Molten metal poured into preheated shell.

  7. Knockout & Finishing: Shell broken off, parts cut from tree, finished.

• Centrifugal vs. Continuous Casting:

  | Feature | Centrifugal Casting | Continuous Casting | | :--- | :--- | :--- | | Principle | Rotation of mold creates centrifugal force. | Oscillating water-cooled mold, continuous withdrawal. | | Product | Cylindrical (pipes, tubes). | Constant cross-section billets/slabs. | | Grain Structure | Fine, dense, radial. | Fine, equiaxed (if stirred). | | Riser Needed? | No (center feeds). | No (top fed, solidifies from outside). | | Material | Primarily steel, iron, non-ferrous. | Primarily steel, aluminum, copper. | | Production Rate | Medium. | Very High. |

II. WELDING PROCESSES

A. Introduction & Classification

• Fusion Welding: Base metal melted (Arc, Gas, Resistance).

• Solid-State Welding: Joined without melting (Forge, Friction, Explosive).

B. Arc Welding Processes

• Manual Metal Arc Welding (MMAW/SMAW):

  • Consumable electrode with flux coating. Flux decomposes to provide gas shield and slag.

  • Electrode Polarity:

    • DCEN (Electrode +ve): Deep penetration, faster melt rate (for thick sections).

    • DCEP (Electrode -ve): Shallow penetration, faster deposition (for thin sheets/root pass).

    • AC: Balanced penetration, no magnetic deflection issues.

  • Equipment: AC/DC transformers/rectifiers. DC provides stable arc, better control.

• Gas Tungsten Arc Welding (GTAW/TIG):

  • Non-consumable tungsten electrode, inert gas shield (Ar/He).

  • Precise Control: Independent control of heat (current) and filler metal (manual addition). Excellent for thin materials, non-ferrous metals (Al, Mg), critical welds.

  • Shielding Gas: Prevents atmospheric contamination. Argon for most metals, Helium for deeper penetration.

  • Electrode: Thoriated tungsten (DC) for electron emission, pure tungsten (AC for Al).

• Gas Metal Arc Welding (GMAW/MIG):

  • Consumable wire electrode, inert/active gas shield (Ar/CO₂ mix).

  • Control Mechanisms:

    • Voltage-Controlled: Constant voltage (CV) source. Wire feed speed controls current/penetration.

    • Current-Controlled: Constant current (CC) source. Less common.

  • Comparison: Higher deposition rate & speed than TIG, but less precise. Semi-automatic/automatic.

• Submerged Arc Welding (SAW): Arc submerged under granular flux. High deposition, deep penetration, no spatter/fume. Used for long straight seams (pipes, tanks).

[!TIP] Exam Focus: TIG vs. MIG comparison is frequent. TIG = precision, quality, manual filler; MIG = speed, automation, higher deposition.

C. Other Welding Methods

• Oxy-Acetylene: Fuel gas (acetylene) + oxygen flame. Flame Types: Carburizing (reducing), Neutral (stoichiometric), Oxidizing. Used for welding, cutting, brazing.

• Resistance Welding: Heat from electrical resistance at joint interface.

  • Spot Welding: Overlapping sheets, electrodes apply pressure/current.

  • Seam Welding: Rotating wheel electrodes for continuous weld.

  • Projection Welding: Localized heating at projections.

• Thermit Welding: Exothermic reaction (Al + Fe₂O₃ → Al₂O₃ + Fe + heat). Used for in-situ repair of heavy sections (rail, shaft). Requires preheating, precise mix, skilled operation.

D. Welding Defects, Causes & Remedies

E. Welding Calculations & Parameters

• Heat Input (Energy per unit length):

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

Where: `V` = Voltage (V), `I` = Current (A), `S` = Travel speed (mm/s), `η` = Efficiency (arc ~0.8-0.9, gas ~0.5-0.7).

• Weld Bead Geometry: Influenced by Current (↑ → ↑ penetration/deposition), Voltage (↑ → ↑ width, ↓ penetration), Travel Speed (↑ → ↓ size, ↑ cooling rate).

III. FORGING PROCESSES

A. Theory & Application of Forging

• Definition: Plastic deformation of metal under compressive forces to achieve desired shape.

• Significance: Improves mechanical properties (grain flow aligned with stress, increased strength, toughness) vs. casting.

• Hot Working (Recrystallization Temp >): Lower forces, larger deformations possible. Disadvantages: Oxidation, poor surface finish, dimensional inaccuracy.

• Cold Working (Room Temp): Higher strength (strain hardening), better surface finish, dimensional accuracy. Disadvantages: Higher forces, limited deformation, residual stresses.

• Justification for Wire Drawing: Cold working is suitable. Produces high-strength, smooth, precise-diameter wire.

B. Types of Forging Operations

• Open-Die (Smith Forging): Deformation between flat/简单 dies. Operations: Drawing out (reduce cross-section, increase length), Upsetting (increase cross-section, reduce length), Punching, Bending. Used for large, simple shapes (shafts, disks).

• Impression-Die (Closed-Die): Deformation in die cavity resembling final shape. Sequence: Edging (rough shaping), Blocking (pre-forming), Finishing. Produces net/near-net shape with flash. Used for high-volume, complex parts (crankshafts, gears).

• Drop Forging: Hammer blows (open or closed die).

• Press Forging: Slow, continuous pressure (hydraulic press). Better control, less shock.

• Upset Forging: Specialized for increasing cross-section (e.g., bolt heads).

• Swaging: Reducing diameter by radial blows.

C. Forging Machines & Equipment

• Drop Forging Hammers: Gravity/steam/air driven. Advantages: High impact energy, fast. Disadvantages: Vibration, noise, limited energy control.

• Horizontal Forging Machines (Upsetters): Workpiece held, moving header applies force. Advantages: High production for long parts with localized upsetting (bolts, fasteners).

• Hydraulic Presses: High force, slow, controllable, no vibration. Used for large, complex forgings.

• Mechanical Presses: Faster, lower force, cam-driven. Used for precision closed-die forging.

D. Forging Defects & Remedies

IV. PRESS WORKING (Sheet Metal Forming)

A. Concept & Importance

• Definition: Sheet/blank formed into shape by shearing (cutting) or deforming (forming) using dies and punches in a press.

• Importance: High-volume, precision, economical production of sheet metal components (automotive, appliances, enclosures).

B. Press Machine Types

• Hydraulic Presses: Fluid pressure drives ram. Advantages: Full force throughout stroke, adjustable speed, overload protection. Applications: Deep drawing, large parts.

• Pneumatic Presses: Compressed air. Faster, lower force, less precise.

• Mechanical Presses: Crank, knuckle, eccentric mechanisms. Advantages: High speed, precise timing. Applications: Blanking, punching, shallow drawing.

C. Shearing Operations (Cutting)

• Process: Material fails in shear along line of cut by punch penetrating into die.

• Clearance (c): Gap between punch and die edge. Critical!

  • Effect: c too small → high force, rapid tool wear, poor cut. c too large → rough edge, large burr, large deformation.

  • Typical: c = 5-10% of material thickness for steel.

• Operations:

  • Shearing: Cutting sheet along a straight line.

  • Punching: Cutting out a scrap piece, desired part remains in sheet.

  • Blanking: Cutting out the desired part, scrap is remainder.

  • Piercing: Punching a hole in sheet (part remains).

  • Trimming: Removing excess/flash from a formed part.

  • Notching: Cutting a shape from edge.

  • Lancing: Partially cutting and bending a tab (no scrap separation).

  • Perforating: Multiple holes punched simultaneously.

D. Forming Operations

• Bending: Strain material beyond yield point to form angle. Types: V-bending, edge bending, channel bending. Defect: Springback (elastic recovery).

• Deep Drawing: Forming a cup/box from a flat blank by drawing it into a die with a punch. Defects:

  • Wrinkling: Buckling in flange due to compressive stress. Remedy: Increase blank holder force.

  • Tearing: Tensile stress in wall exceeds UTS. Remedy: Increase die radius, use proper blank shape, lubricate.

  • Earing: Waviness at top of cylindrical cup due to anisotropy. Remedy: Use isotropic material, proper blank orientation.

• Embossing: Shallow drawing to create raised/relief pattern.

• Spinning: Forming axially symmetric part by rotating blank and applying tool pressure.

E. Tooling, Dies & Auxiliary Equipment

• Die Components: Punch (male), Die (female), Stripper (ejects part), Guide Pillars/Bushings (alignment).

• Die Types:

  • Simple Die: One operation per stroke.

  • Compound Die: Multiple operations in one station (e.g., blanking & piercing).

  • Progressive Die: Series of stations, part progresses through each stroke.

  • Transfer Die: Part transferred between stations by separate mechanism.

• Auxiliary Equipment: Stock Feeder (feeds sheet/coil), Scrap Cutter (cuts scrap), Safety Devices (light curtains, guards).

F. Process Parameters & Calculations

• Shearing Force (Approximate):

$$ F = \tau \times A_s = \tau \times (t \times L) $$

Where: `τ` = shear strength (~0.6-0.8×UTS), `t` = thickness, `L` = cut length.

• Blanking Layout & Scrap Calculation:

  • Problem (Dec 2023): Sheet 300mm × 500mm, 20% scrap margin on each side.

  • Effective Area for Components: Width = 300 - 2*(0.2×300) = 180mm, Length = 500 - 2*(0.2×500) = 300mm.

  • Components per sheet: Assume rectangular component size a×b. Max number = floor(180/a) × floor(300/b).

  • Key: "Scrap margin on each side" means total width/length reduced by 40% (20% left + 20% right).

G. Defects in Press Working & Remedies

V. ROLLING PROCESSES

A. General Description

• Principle: Metal passed through rotating rolls to reduce cross-sectional area/length.

• Rolling Mill Types:

  • Two-High: Two rolls, reversible/non-reversible. Simple, for primary rolling.

  • Three-High: Three rolls, continuous pass. For blooms/slabs.

  • Four-High: Two small working rolls + two large backup rolls. For sheets/strips (backup rolls prevent work roll deflection).

  • Cluster (Sendzimir): Multiple backup rolls for very thin strips.

  • Tandem Mill: Series of stands (2-18), each does partial reduction. High speed, high accuracy for sheets/coils.

B. Hot Rolling vs. Cold Rolling

C. Rolling Products

• Structural Sections (I-beams, channels, angles): Rolled in section mills (specialized roll sequences). Used for construction, frames.

• Plates: Thick (>3mm), flat. Rolled on plate mills (4-high, reversing). Used for pressure vessels, shipbuilding.

• Sheets & Strips: Thin (<3mm). Sheets (discrete), Strips (coiled). Rolled on sheet mills → tandem mills. Used for automotive bodies, appliances, packaging.

[!TIP] Key Difference: Hot rolling for shape/size reduction; Cold rolling for final dimensions, surface, properties.

VI. MACHINING PROCESSES & MACHINE TOOLS

A. Lathe Machine

• Fundamental Components:

  • Bed: Base, guides for carriage.

  • Headstock: Holds workpiece, provides rotation (spindle, chuck, gear train).

  • Tailstock: Supports other end, holds tools (drill, center).

  • Carriage: Moves parallel to axis (saddle, cross-slide, compound rest, tool post).

• Basic Operations:

  • Turning: Generate cylindrical surface (straight, tapered).

  • Facing: Generate flat surface perpendicular to axis.

  • Thread Cutting: Single-point tool, precise tool-work motion synchronization.

  • Drilling/Boring: Drill held in tailstock; boring tool in tool post for internal diameters.

  • Knurling: Produce serrated pattern for grip.

• Types: Engine Lathe (general), Centre Lathe (between centers), Capstan/Turret Lathe (multiple tools, no manual feed), Automatic Lathe (fully automated, cam/CNC).

B. Shaper & Planer Machines

• Shaper: Single-point tool on a ram moves horizontally in reciprocating motion; workpiece fed vertically/rotated. Applications: Small/medium parts, internal surfaces, non-flat surfaces.

• Planer: Workpiece on table moves under multiple stationary tools. Applications: Very large, heavy work (machine bases, long beds).

• Comparison: Shaper = small work, single tool, simple. Planer = large work, multiple tools, heavy.

C. Milling Machine

• Principle: Rotating multi-point cutter (milling cutter), feed motion by workpiece/cutter.

• Types:

  • Horizontal Milling: Cutter on horizontal spindle. Plain milling (wide), Face milling (end).

  • Vertical Milling: Cutter on vertical spindle. End milling (slotting, contouring).

  • Universal Milling: Table can swivel.

• Operations: Slotting, Keyway cutting, Gear cutting, Contouring.

D. Grinding Machines

• Principle: Abrasive machining using a rotating grinding wheel (multi-point, randomly oriented grains). For high precision, fine surface finish.

• Types & Applications:

  • Cylindrical Grinder: External/internal cylindrical surfaces.

  • Surface Grinder: Flat surfaces (workpiece on magnetic chuck, wheel reciprocates).

  • Centreless Grinder: Through-feed, high volume for cylindrical parts (no centers).

  • Tool & Cutter Grinder: Sharpening/reshaping cutting tools.

E. Drilling Machine

• Principle: Rotating drill (twist drill) with axial feed.

• Types: Pillar (bench/floor), Radial (arm moves for large work), Multi-spindle (multiple holes simultaneously).

• Common Drills: Twist drill (general), Center drill (starting hole), Spot drill (countersink).

• Drilling Time Calculation:

$$ T = \frac{L + A + O}{f \times N} $$

Where: `T` = time (min), `L` = hole depth, `A` = approach distance, `O` = overrun/return, `f` = feed rate (mm/rev), `N` = rpm.

> **Note:** `A+O` often taken as **radius of drill** for simplicity.

F. Introduction to Machining & Role

• Role: Secondary/Finishing processes to achieve final dimensions, tolerances, and surface finishes after primary (casting/forging) or forming (press/rolling) processes.

• Capability: Achieve µm-level tolerances and nanometer-level surface finishes (grinding).

END OF UNIT 5 NOTES