Manufacturing Process (ME-305) - Unit 1 Short Notes

UNIT 1: FOUNDRY, WELDING, FORMING & MACHINING PROCESSES

1.0 CASTING PROCESSES

1.1 General Introduction & Types

• Casting: A manufacturing process where molten metal is poured into a mold cavity and allowed to solidify, producing a shape conforming to the cavity.

• Major Types & Comparison:

[!TIP] Exam Focus: Be prepared to compare Die vs. Investment (accuracy vs. cost), Sand vs. Shell (mold properties), and Centrifugal vs. Continuous (product form).

1.2 Pattern Making & Allowances

• Pattern: Replica of the casting, used to form the mold cavity.

  • Types: Solid, Split (two-piece), Match Plate (both halves on one plate), Sweep (for symmetrical revolved parts), Loose Piece (for complex cavities), Cope & Drag (separate top/bottom halves).

  • Materials: Wood (common), Metal (for high production), Plastic/Plaster (for investment).

• Pattern Allowances: Extra dimensions added to the pattern to compensate for post-casting processes.

  • Shrinkage/Contraction Allowance: Largest allowance. Accounts for volumetric shrinkage during solidification and cooling.

    • Calculation: Pattern Dimension = Casting Dimension × (1 + Shrinkage %)

    • Example: For steel (~1.5-2% linear shrinkage), pattern is made larger.

  • Draft Allowance: Taper on vertical surfaces to facilitate pattern removal from mold without damaging cavity.

    • Typical Values: 1° to 3° for external surfaces; 3° to 10° for internal surfaces/core prints.

  • Machining Allowance: Extra material left on surfaces to be machined to final dimensions.

    • Depends on casting size, material, and machining method.

  • Distortion/Camber Allowance: For irregular, long, or thin castings (e.g., "U" shapes). Pattern is bent opposite to expected distortion to counteract it.

  • Shake/Rapping Allowance: Small extra clearance on pattern to allow for rapping/tapping to loosen it from mold. Compensated by reducing draft slightly.

[!TIP] Common Pitfall: Do not confuse Shrinkage Allowance (for solidification & cooling) with Machining Allowance (for final finishing).

1.3 Molding & Core Sands

• Molding Sand Properties: Refractoriness, permeability (gas escape), cohesiveness/strength, plasticity, adhesiveness, thermal stability.

• Common Types:

  • Green Sand: Sand + clay + water. Used for most sand castings. Cheap, but poor surface finish.

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

  • Loam Sand: Sand + clay + water + organic matter (like horse manure). For very large molds.

  • Oil-Bonded Sand: Sand + synthetic oil/resin + catalyst. High strength, good finish, used in shell molding.

• Core Making: Forms internal cavities.

  • Core Sand: Finer grain, stronger than molding sand.

  • Core Binders: Oil-based, resin-based, or inorganic (e.g., sodium silicate).

  • Core Prints: Projections on pattern that form recesses in mold to locate and support the core.

  • Chaplets: Metal supports placed in mold to hold core in position. Must be compatible with casting metal to avoid fusion.

1.4 Gating System & Risering

• Components & Functions:

  • Sprue: Vertical channel connecting pouring basin to runner. Controls metal flow.

  • Runner: Horizontal channel distributing metal to multiple gates.

  • Gate: Constricted opening controlling metal entry into mold cavity. Should be short and rounded to minimize turbulence.

  • Riser (Feeder): Reservoir of molten metal that solidifies after the casting to compensate for shrinkage.

• Design Principles:

  • Gating system should minimize turbulence (to avoid air entrapment and sand erosion) and ensure smooth, directional flow.

  • Riser must have a larger volume-to-surface area ratio (V/A) than the casting to solidify last (Chvorinov's Rule).

  • Riser placed near massive sections, connected by shortest possible neck.

• Chvorinov's Rule (Critical Formula):

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

Where:

*   $t$ = Solidification time

*   $V$ = Volume of casting/riser

*   $A$ = Surface area of casting/riser

*   $B$ = Mold constant (depends on mold material, metal)

*   $n$ = Usually 2 (for most sands)

*   **For a riser to be effective:** $$\displaystyle \left( \frac{V}{A} \right)_{riser} \geq \left( \frac{V}{A} \right)_{casting} $$

• Riser Types: Top (open) riser, Side (blind) riser. Cylindrical shape is common for easy V/A calculation.

[!TIP] Numerical Focus: You will get a problem on calculating riser dimensions using Chvorinov's rule or comparing V/A ratios. For a cylindrical riser of diameter d and height h: $$\displaystyle V = \frac{\pi d^2 h}{4} $$, $$\displaystyle A = \pi d h + \frac{\pi d^2}{2} $$.

1.5 Solidification, Defects & Remedies

• Solidification Zones: Liquid → Mushy (liquid+solid) → Solid.

• Major Defects & Remedies:

1.6 Special Casting Processes & Calculations

• Investment Casting Steps: Wax pattern making → Assembly on tree → Investing (dipping/brush with ceramic slurry) → Dewaxing (steam/autoclave) → Baking → Pouring → Knockout/finishing.

• Die Casting: High-pressure (up to 2000 bar), fast cycle. Tolerances: Controlled by die quality, process parameters, and material shrinkage. Die temperatures and injection speeds are critical.

• Numerical Problems:

  1. Pattern Dimension: $$\displaystyle P_{dim} = C_{dim} \times (1 + \frac{\text{Shrinkage \%}}{100}) $$

  2. Final Casting Dimension after Contraction: If a casting of side L undergoes volumetric solidification shrinkage of S_s% and volumetric solid contraction of S_c%, the final side length L_f is:

$$ L_f = L \times \sqrt[3]{(1 - \frac{S_s}{100}) \times (1 - \frac{S_c}{100})} $$

    *Assumption:* Uniform cooling and isotropic contraction in all directions.

3.  **Riser Design:** Use Chvorinov's rule to find minimum riser size or compare V/A ratios.

2.0 WELDING PROCESSES

2.1 Welding Fundamentals

• Weld: A localized union of two or more pieces of metal.

• Joint Types: Butt, Lap, Corner, Edge, T (Tee).

• Classification:

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

  • Solid-State Welding: No melting (Friction, Ultrasonic, Explosive).

  • Arc Welding: Uses electric arc as heat source (SMAW, GTAW, GMAW).

  • Gas Welding: Uses combustible gas flame (Oxy-acetylene).

  • Resistance Welding: Uses electrical resistance heat (Spot, Seam).

2.2 Arc Welding (Core Focus)

• Manual Metal Arc Welding (SMAW/MMAW):

  • Consumable electrode with flux coating.

  • Flux Functions: Shield arc, stabilize arc, deoxidize/degas, add alloying elements, form slag.

  • Polarity: DC+ (Electrode +ve) → deeper penetration. DC- (Electrode -ve) → faster melt, shallow penetration. AC → medium penetration.

• Inert Gas Welding:

  • TIG (GTAW - Gas Tungsten Arc Welding):

    • Non-consumable tungsten electrode.

    • Inert shielding gas (Ar/He).

    • Filler rod added separately if needed.

    • Advantages: High quality, precise control, clean (no slag), all-positional, wide material range (Al, Mg, Cu, steel).

    • Disadvantages: Slow, requires skill, expensive equipment.

  • MIG (GMAW - Gas Metal Arc Welding):

    • Consumable wire electrode fed continuously.

    • Inert/active shielding gas (Ar, Ar+CO₂).

    • Metal Transfer Modes: Spray (high current, smooth), Globular (medium, droplet), Short-circuiting (low current, spatter).

    • Advantages: High speed, high deposition rate, easy automation, less skill than TIG.

    • Disadvantages: More spatter than TIG, sensitive to wind/gas shielding, equipment complexity.

• TIG vs. MIG Comparison:

• AC vs. DC Welding Machines:

  • DC (Direct Current): Stable arc, easier to start, deeper penetration (DC+), common for SMAW/GTAW/GMAW.

  • AC (Alternating Current): Arc extinguishes/re-ignites each half-cycle, less stable. Used for TIG on aluminum (cleaning action from oxide layer) and for some SMAW electrodes (e.g., E6011).

2.3 Other Welding Methods

• Oxy-Acetylene Welding: Uses flame (~3200°C). Versatile for heating/cutting/welding. Flame types: Carburizing (reducing), Neutral (ideal for welding), Oxidizing.

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

  • Spot Welding: Overlap joint, two electrodes.

  • Seam Welding: Rotating wheel electrodes for continuous seam.

  • Projection Welding: Localized heating on projections.

• Thermit Welding: Exothermic reaction (e.g., Fe₂O₃ + Al → Al₂O₃ + Fe + heat). Molten steel (~2500°C) fills gap. Used for rail welding, large section repairs. No external power source needed.

2.4 Welding Consumables & Equipment

• Electrode (SMAW): Core (filler metal) + Flux coating. Specified by AWS (e.g., E7018).

• Flux: Provides shielding gas, slag, deoxidation.

• Power Sources:

  • Constant Current (CC): Current stable, voltage varies (for SMAW, GTAW).

  • Constant Voltage (CV): Voltage stable, current varies (for GMAW, FCAW).

2.5 Welding Defects & Quality

• Common Defects & Remedies:

• Quality Factors: Heat input (affects HAZ), welding parameters (V, I, speed), joint design, material properties.

2.6 Welding Calculations

• Heat Input (J/mm or kJ/mm): Energy supplied per unit length of weld.

$$ \boxed{Q = \frac{V \times I \times \eta}{v}} $$

Where:

*   $V$ = Arc voltage (Volts)

*   $I$ = Welding current (Amperes)

*   $\eta$ = Efficiency (arc efficiency, typically 0.6-0.9 for arc welding)

*   $v$ = Travel speed (mm/s)

• Weld Bead Area from Heat Balance: If electrode melting efficiency $$\displaystyle \eta_m $$ and heat required to melt electrode $$\displaystyle H_m $$ (J/mm³) are known:

$$ \text{Cross-sectional area of weld bead } (A_w) = \frac{Q \times \eta_m}{H_m} $$

[!TIP] Numerical Focus: Heat input problems are very frequent. Always check units (convert cm/min to mm/s). Remember efficiency factors.

3.0 BULK DEFORMATION (FORGING & ROLLING)

3.1 Forging

• Principle: Plastic deformation of metal under compressive force to shape.

• Types:

  • Open-Die Forging (Smithing): Deformation between flat/contoured dies. Material flows freely. Used for large, simple shapes (shafts, discs). Can be hand or power hammer.

  • Impression-Die (Closed-Die) Forging: Metal fully confined in die cavity. Produces near-net shape. High initial die cost, high production. Includes drop forging (hammer) and upsetting (horizontal forging machine).

• Forging Machines:

  • Drop Hammer (Forge): Gravity/steam/air hammer. Rapid impact.

  • Horizontal Forging Machine (Upsetter): Workpiece held, heading tool moves horizontally. Ideal for making heads on bolts/screws.

  • Presses (Hydraulic/Pneumatic): Slow, uniform pressure. Better for complex shapes, less shock.

• Defects: Hot tears (cracks from stress), laps/folds (improper metal flow), scale pits (surface oxidation).

3.2 Rolling

• Principle: Metal passed through rotating rolls to reduce thickness/length.

• Rolling Mills:

  • Two-High: Simple, reversible/non-reversible.

  • Three-High: Three rolls, one passes metal through two. For large reductions.

  • Four-High: Two small working rolls + two large backup rolls (for stiffness). For plates/sheets.

  • Cluster: More than four rolls. For very wide plates.

  • Tandem: Series of stands (2-18). For high production (hot/cold strip).

• Products:

  • Structural Sections: I-beams, rails (hot rolled, thick).

  • Plates: > 3 mm thick (hot/cold rolled).

  • Sheets: 0.4 mm to 3 mm (often cold rolled for finish).

  • Foil: < 0.2 mm (final passes with pack rolling).

• Hot vs. Cold Rolling:

4.0 SHEET METAL WORKING (PRESS WORKING)

4.1 Introduction & Presses

• Press Working: Sheet metal forming using press and dies. Includes shearing & forming.

• Press Types:

  • Mechanical: Crank, eccentric, knuckle. Fast, intermittent, for stamping/blanking.

  • Hydraulic: Constant pressure throughout stroke. Slower, for deep drawing, forming.

  • Pneumatic: Similar to hydraulic but with air. Lower force, faster.

4.2 Press Operations

• Shearing Operations (Cutting):

  • Shearing: Cutting sheet with straight blade (guillotine).

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

  • Blanking: Punch cuts out part (blank), sheet is waste.

  • Piercing: Punching hole in already formed part.

  • Slitting: Continuous shearing to reduce sheet width.

  • Trimming: Removing excess from edge of drawn/formed part.

• Bending Operations:

  • Bottoming: Punch forces sheet to bottom of die, high springback.

  • Air Bending: Sheet touches punch & die at three points, common.

  • Coining: Extreme pressure, full contact, no springback, high precision.

• Drawing Operations (Forming):

  • Deep Drawing: Depth > diameter. Requires blankholder to prevent wrinkling.

  • Shallow Drawing: Depth < diameter.

  • Redrawing: Multiple draws to increase depth.

  • Defects: Wrinkling (compressive buckling → increase blankholder force), Tearing (tensile stress → increase die radius, lubrication).

4.3 Tooling & Auxiliaries

• Dies: Simple Die: One operation per stroke. Compound Die: Two+ operations in one station (e.g., blanking & piercing). Progressive Die: Series of stations, part progresses through each.

• Auxiliaries: Stock Feeders: Automatically feed strip/coil. Scrap Cutters: Cut scrap between parts. Lubrication Systems: Reduce friction, wear.

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

4.4 Process Parameters & Calculations

• Blanking/Shearing Force: $$\displaystyle F = \tau \times A_s $$

  • $\tau$ = Shear strength of material

  • $$\displaystyle A_s $$ = Total shear length (perimeter of part for blanking)

• Strip Layout (Blanking): Minimize scrap. Consider pitch (center-to-center spacing) and stripping force.

• Drilling Time Calculation (from Unit 5.4):

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

Where:

*   $T$ = Time (min)

*   $L$ = Thickness of workpiece (mm)

*   $A$ = Approach distance (mm)

*   $O$ = Overrun/retract distance (mm)

*   $f$ = Feed rate (mm/rev)

*   $N$ = Spindle speed (rpm)

5.0 MACHINING PROCESSES & MACHINE TOOLS

5.1 Lathe Machine

• Basic Components:

  • Bed: Base, guides for carriage.

  • Headstock: Holds & rotates workpiece (spindle, chuck, gears).

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

  • Carriage: Moves along bed. Contains cross-slide (perpendicular), compound rest (angular), tool post.

• Primary Operations:

  • Turning: Remove material from rotating workpiece (external/internal).

  • Facing: Cut end surface perpendicular to axis.

  • Parting: Cut off deep groove to separate part.

  • Drilling/Boring: Enlarge/true hole.

  • Knurling: Roll patterned surface for grip.

  • Threading: Cut internal/external threads (using leadscrew).

5.2 Shaper & Planer Machines

• Shaper: Single-point tool reciprocates horizontally; workpiece fed vertically. Used for flat surfaces, internal profiles, non-production.

• Planer: Workpiece reciprocates under stationary tool. For very large, heavy workpieces (e.g., machine bases).

• Comparison: Shaper - tool moves, small/medium parts. Planer - workpiece moves, very large parts.

5.3 Milling Machine

• Types: Horizontal: Spindle horizontal, arbor-mounted cutters. Vertical: Spindle vertical, end mills. Universal: Table can swivel.

• Operations:

  • Plain Milling: Width of workpiece < cutter width.

  • Face Milling: Cutter diameter > workpiece width.

  • Straddle Milling: Two cutters on common arbor, machine two parallel faces.

  • Gang Milling: Multiple cutters on arbor for simultaneous operations.

  • Indexing: Rotating workpiece to equal angles (simple: direct, compound: for large divisions).

• Milling Cutters: End mill, face mill, slab mill, fly cutter, form cutter.

5.4 Drilling Machine

• Types: Bench/Pillar: Small, vertical spindle. Radial: Arm can swing, for large workpieces. Upright: Heavy-duty, box column.

• Drill Bits: Twist Drill: Most common (flutes, land, point angle 118°). Center Drill: For starting hole. Countersink: For flat-head screws. Spot Drill: For precise starting.

• Drilling Time: $$\displaystyle T = \frac{L + A + O}{f \times N} $$ (See 4.4).

5.5 Grinding Machine

• Principle: Material removal by abrasive grains (bonded in wheel).

• Types:

  • Surface Grinder: Workpiece on magnetic chuck, wheel moves horizontally/vertically. For flat surfaces.

  • Cylindrical Grinder: Workpiece rotates, wheel traverses. For external/internal cylindrical surfaces.

  • Centerless Grinder: Workpiece supported between two wheels (regulating & grinding), no centers. High production.

  • Tool & Cutter Grinder: For sharpening/reshaping cutting tools.

• Advantages: High accuracy (µm), excellent surface finish, can machine hardened materials, brittle materials.

5.6 Introduction to Modern Machining

• CNC (Computer Numerical Control): Machines (lathe, mill, grinder) controlled by coded instructions (G/M codes). Allows complex, repeatable, high-precision parts with minimal operator intervention. Integration of multiple operations (machining centers).