Manufacturing Process (ME-305) - Unit 3 Short Notes

UNIT 3: MANUFACTURING PROCESS

1. CASTING PROCESSES

1.1 Types of Casting Processes

[!TIP] Exam Focus: Be ready to compare centrifugal vs. continuous (Dec 2023) and die casting tolerances (Jun 2023). Know key applications for each.

1.2 Pattern Making

Pattern Materials: Wood (cheap, easy), Metal (durable, for high volume), Plastic (light, stable), Plaster (for complex shapes).

Pattern Allowances (Added to casting dimensions):

1. Shrinkage Allowance: Compensates for volumetric contraction on cooling.

   • Linear Shrinkage: $$\displaystyle \text{Pattern Dimension} = \text{Casting Dimension} \times (1 + \delta) $$

   • $\delta$ = linear shrinkage % (e.g., steel ~1.5-2.0%).

   • \boxed{\text{Pattern Size} = \text{Final Size} \times (1 + \text{Shrinkage Fraction})}

2. Draft Allowance: Taper on pattern vertical faces for easy removal from mold.

   • $$\displaystyle 1^\circ $$ to $$\displaystyle 3^\circ $$ for external, $$\displaystyle 5^\circ $$ to $$\displaystyle 8^\circ $$ for internal surfaces.

3. Machining Allowance: Extra material left on surfaces for post-casting machining.

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

4. Deformation/Camber Allowance: For long, flat castings prone to sagging. Pattern is cambered (curved) upward.

5. Shake/Rapping Allowance: Extra material on cores/pattern to allow for rapping out without damage. Typically 0.5-1.5 mm per side.

Pattern Types:

• Solid: Single piece, simple shape.

• Split (Two-Part): Most common, along parting line.

• Match Plate: Pattern mounted on plate with core prints; used in machine molding.

• Loose Piece: Removable projections for undercuts.

• Sweep: For large, symmetrical cavities (e.g., turbine housings).

• Skeleton: Frame with attached pattern segments for large savings.

• Shell: Thin shell pattern for investment casting.

[!TIP] Common Pitfall: Forgetting that shrinkage allowance is applied first before other allowances in calculation sequences. Shake allowance is subtracted from pattern dimension.

1.3 Mold Design and Components

Molding Sands:

• Base Sand: Silica (most common), Olivine, Chromite.

• Binders: Clay (green sand), Bentonite, Resins (furan, phenolic), Oils.

• Additives: Coal dust (prevents sand-metal reaction), Cereals (strength).

Gating System:

• Function: Control metal flow, filter dross, minimize turbulence, direct flow.

• Components:

  1. Sprue: Vertical channel from pouring cup to runner.

  2. Runner: Horizontal channel distributing metal to gates.

  3. Gate: Constricted opening into mold cavity. Choke (narrowest point) controls flow rate.

  4. Pouring Cup: Receives molten metal.

• Design: Bottom/side gating, gating ratio (Sprue : Runner : Gate ≈ 1 : 1.5 : 1.2).

Riser (Feeder):

• Function: Reservoir of molten metal to compensate for shrinkage.

• Types: Top (open, blind), Side (exposed, internal).

• Design Principle: Riser must solidify AFTER the casting (Chvorinov's Rule).

• Modulus: $$\displaystyle M = \frac{\text{Volume}}{\text{Surface Area}} $$. Riser modulus $$\displaystyle M_r $$ > Casting modulus $$\displaystyle M_c $$.

• Placement: On thickest section, last to solidify.

Other Components:

• Chills: Metallic inserts (Fe, Cu) to increase cooling rate in specific areas, promote directional solidification.

• Chaplets: Supports for cores (metal spacers).

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

1.4 Solidification and Defects

Solidification Principles:

• Shrinkage: Volumetric contraction during liquid→solid (4-8%) and solid→room temp (0.5-1.5%).

• Directional Solidification: Solidification should progress from thin sections to thick sections, towards riser. Achieved by chills, insulating sleeves, proper gating.

Common Casting Defects:

Quality Control: Visual inspection, dimensional check, NDT (X-ray, ultrasonic), destructive testing (tensile, hardness).

1.5 Calculations and Analysis

1. Shrinkage Allowance:

   • Pattern dimension = Casting dimension × (1 + linear shrinkage %).

   • Example (Dec 2023): Linear shrinkage = 1.5%, Casting = 100×80×60 mm.

   • Pattern Length = $$\displaystyle 100 \times (1 + 0.015) = 101.5 $$ mm.

   • Similarly, Width = $$\displaystyle 80 \times 1.015 = 81.2 $$ mm, Height = $$\displaystyle 60 \times 1.015 = 60.9 $$ mm.

2. Riser Design (Chvorinov's Rule):

   • Solidification time $$\displaystyle t = B \left( \frac{V}{A} \right)^n $$, where $B$ = mold constant, $n \approx 2$.

   • Modulus $$\displaystyle M = V/A $$. For safe feeding: $$\displaystyle M_{riser} \geq 1.25 \times M_{casting} $$.

   • Problem (Jun 2023): Cylindrical riser (d=6cm, h=6cm), Casting t=1.36 min. Find riser t.

   • $$\displaystyle V_r = \pi (3)^2 \times 6 = 169.6 $$ cm³, $$\displaystyle A_r = 2\pi(3)(6) + 2\pi(3)^2 = 141.4 $$ cm².

   • $$\displaystyle M_r = 169.6 / 141.4 = 1.2 $$ cm.

   • Given $$\displaystyle M_c $$ from $$\displaystyle t_c = B M_c^2 = 1.36 $$ min. But $$\displaystyle M_r > M_c $$ for riser to solidify later. Without $B$, assume $$\displaystyle M_r = k M_c $$. If riser is "just sufficient", $$\displaystyle t_r = t_c \times (M_r/M_c)^2 $$. But problem likely expects using $$\displaystyle M_r $$ to find $$\displaystyle t_r $$ if $B$ known. [Note: Typically, need B or assume same mold constant. If riser modulus is given, t_r = B M_r^2. Since t_c given, B = t_c / M_c^2. Need M_c from casting dims not given. This problem may be incomplete or expects ratio method.]

   • Optimization: For blind riser of constant volume, minimum solidification time when $$\displaystyle h/d = 1 $$ (spherical shape has min surface area/vol ratio). (Jun 2023 question).

3. Blanking Layout (Dec 2023):

   • Sheet: 300×500 mm. Scrap margin 20% on each side.

   • Usable width = $$\displaystyle 300 \times (1 - 2 \times 0.20) = 300 \times 0.6 = 180 $$ mm.

   • Usable length = $$\displaystyle 500 \times (1 - 2 \times 0.20) = 500 \times 0.6 = 300 $$ mm.

   • If component is rectangular (say a×b), max number = floor(180/a) × floor(300/b). [Note: Component size not given in problem statement, so calculation depends on component dimensions. The question likely expects the usable area calculation first.]

2. WELDING PROCESSES

2.1 Arc Welding

Manual Metal Arc Welding (MMAW/SMAW):

• Consumable electrode with flux coating.

• Flux Functions: Shield arc (gas + slag), deoxidize, alloy, stabilize arc.

• Power Sources:

  • AC: Cheaper, no magnetic deflection, but arc less stable, frequent re-striking.

  • DC: More stable arc, easier to control, deeper penetration (DC+), less spatter.

• Polarity:

  • DC+ (Electrode +): 2/3 heat on electrode → faster melt, shallow penetration.

  • DC- (Electrode -): 2/3 heat on work → deeper penetration, slower melt.

  • AC: Alternating, average penetration between DC+ and DC-.

Heat Input Calculation:

• $$\displaystyle Q = \frac{V \times I \times \eta}{v} $$ (J/mm or J/in)

• $V$ = voltage (V), $I$ = current (A), $\eta$ = efficiency (0.7-0.9 for arc), $v$ = travel speed (mm/s).

• \boxed{Q = \frac{V I \eta}{v}}

• Example (Jun 2023): OCV=62V, I_sc=130A, L=4mm, v=15cm/s=150mm/s, $\eta$=0.85, V=20+1.5L=26V.

  • $$\displaystyle Q = \frac{26 \times 130 \times 0.85}{150} = 19.1 $$ J/mm.

2.2 Gas Welding

TIG vs MIG (Dec 2023):

• Control: TIG offers superior heat input control (foot pedal), MIG control via voltage/current settings.

• Speed: MIG faster due to continuous wire feed.

• Quality: TIG produces cleaner, higher-quality welds (no spatter), MIG may have spatter.

• Material: TIG for thin/non-ferrous, MIG for thicker steels.

2.3 Other Welding Methods

• Thermit Welding: Exothermic reaction (Al + Fe₂O₃ → Al₂O₃ + Fe + heat). Used for rail welding, large sections. Requires precise preheat, no external power.

• Resistance Welding: Heat from electrical resistance at joint (spot, seam, projection). Fast, localized, no filler.

2.4 Welding Defects and Remedies

[!TIP] Exam Focus: Porosity control (Dec 2023) and hydrogen-induced cracks are critical. For gas welding, ensure proper gas flow and electrode manipulation.

2.5 Welding Parameters and Quality

• Current (I): Increases penetration and deposition rate.

• Voltage (V): Affects arc length and bead width. Higher V → wider, flatter bead.

• Travel Speed (v): Inversely affects heat input. Too slow → burn-through; too fast → lack of fusion.

• Heat Input (Q): Governs HAZ size, distortion, microstructure. $$\displaystyle Q \propto \frac{V I}{v} $$.

• Quality Assessment: Visual, destructive (tensile, bend), NDT (X-ray, ultrasonic, dye penetrant).

3. FORGING PROCESSES

3.1 Types of Forging

• Open-Die Forging: Deformation between flat/contoured dies. Hand (small) / Machine (large). Operations: Cogging (length reduction), Fullering (cross-section reduction), Upsetting (thickness increase).

• Impression-Die (Closed-Die) Forging: Metal flows to fill die cavity. Flash forms in gutter. Drop Forging: Hammer impact. Press Forging: Slow, continuous pressure (better for complex shapes).

• Horizontal Forging (Upsetting/Heading): Workpiece held horizontally, die moves vertically. High production for fasteners (bolts, screws).

• Other: Swaging (reducing diameter), Hubbing (embossing).

3.2 Forging Machines and Equipment

• Drop Hammers: Gravity (energy = mgh) or Power-assisted (steam/air). High impact, rapid.

• Horizontal Forging Machines: Workpiece clamped, heading die moves. High speed, automated.

• Hydraulic Presses: Slow, high force, uniform deformation, good for large parts.

• Mechanical Presses: Faster than hydraulic, limited stroke, cam-driven.

• Forging Dies: Made from high-speed steel or carbide. Must have draft, radius, clearances. Flash design critical.

3.3 Hot Working vs Cold Working

3.4 Forging Defects and Remedies

3.5 Forging Operations and Applications

• Sequence: Cogging → Fullering → Final impression (with blocker die for complex shapes).

• Applications:

  • Open-die: Large rings, shafts, discs.

  • Closed-die: Automotive components (crankshafts, connecting rods), tools.

  • Heading: Fasteners, bolts, screws.

4. PRESS WORKING (SHEET METAL FORMING)

4.1 Press Machines

• Mechanical Presses: Crank, eccentric, knuckle joint. High speed (100-1000 spm), moderate force, short stroke.

• Hydraulic Presses: Fluid pressure (piston). Low speed, high force, long stroke, adjustable.

• Pneumatic Presses: Air pressure. Lower force, faster than hydraulic, used for light operations.

• Selection: Speed (mechanical), force (hydraulic), flexibility (hydraulic), cost (mechanical).

4.2 Press Working Operations

• Shearing: Cutting sheet by shear stress (no chip). Blanking: Cutting out the part (part is product). Punching: Cutting out waste (hole is product). Piercing: Punching hole in sheet.

• Trimming: Removing excess flash from forgings/trimming edges.

• Deep Drawing: Forming cup-shaped part by drawing sheet into die. Single-stage (one draw), Multi-stage (multiple draws with anneals).

• Bending: Forming angle/channel. Springback is elastic recovery.

• Notching: Cutting notch on edge.

• Lancing: Shearing and bending a tab (no reduction in area).

• Embossing: Shallow drawing for stiffening/decoration.

• Perfecting: Pressing both sides of sheet simultaneously for flatness.

• Slitting: Longitudinal cutting of sheet/coil into strips.

4.3 Tooling and Dies

• Components: Punch (male), Die (female), Stripper (removes part), Guide pillars/bushings (alignment).

• Materials: Tool steel (high carbon, high chromium), carbide for high wear.

• Tolerances: ±0.05 to ±0.1 mm for precision dies.

• Auxiliary: Stock feeders (automatic feeding), scrap cutters.

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

4.4 Process Parameters and Forces

• Shearing Force: $$\displaystyle F = \tau \times t \times L $$

  • $\tau$ = shear strength (MPa), $t$ = thickness (mm), $L$ = cut length (mm).

  • For blanking/punching, use perimeter of part.

• Clearance ($c$): Gap between punch and die. Typically 5-10% of thickness for sheet metal.

  • Too small → high force, rapid wear.

  • Too large → poor edge quality, burrs.

• Drawing Force: $$\displaystyle F_d = \pi d t \sigma_u \left( \frac{d}{D} - 0.7 \right) $$ (approx)

  • $d$ = punch diameter, $D$ = blank diameter, $$\displaystyle \sigma_u $$ = UTS.

• Power: $$\displaystyle P = F \times v $$ (where $v$ = slide speed).

4.5 Calculations and Layout

• Blanking Layout: Maximize yield from sheet/coil.

  • Stripping: Components separated by cutting all around.

  • Nesting: Arrange shapes to minimize scrap. Consider scrap margin (edge distance).

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

    • Usable width = $$\displaystyle 300 \times (1 - 0.4) = 180 $$ mm (since 20% left + 20% right = 40% total).

    • Usable length = $$\displaystyle 500 \times 0.6 = 300 $$ mm.

    • Max components = floor(180/component width) × floor(300/component length).

• Yield: $$\displaystyle \text{Yield (\%)} = \frac{\text{Area of all parts}}{\text{Total sheet area}} \times 100 $$.

4.6 Defects in Press Working

5. ROLLING PROCESSES

5.1 Rolling Fundamentals

• Hot Rolling: Above recrystallization temp. Large reductions, coarse surface, scale. Used for ingots → slabs/billets/plates.

• Cold Rolling: Below recrystallization temp. Small reductions, smooth surface, strain hardening, accurate dimensions. Used for sheets/foils/finishing.

• Effect on Properties: Cold rolling increases strength/hardness, decreases ductility. Hot rolling refines grain, improves ductility.

Rolling Mill Types:

• Two-High: Most basic, reversible/non-reversible.

• Three-High: Three rolls, work passes up/down. For blooms/slabs.

• Four-High: Two small working rolls, two large backup rolls (support). For sheets/plates (reduce roll deflection).

• Cluster (Z-High): Multiple backup rolls. Very large forces.

• Tandem: Series of stands (2-18), continuous rolling. High production.

5.2 Rolling Products

• Structural Sections (I-beams, channels): Rolled in section mills with shaped grooves. Requires multiple passes.

• Plates: Thick (>3mm), hot rolled on reversing mills or Steckel mills.

• Sheets: Thin (<3mm), hot or cold rolled on four-high/tandem mills. Cold rolling gives better finish/straightness.

• Defects: Camber (roll bending), crown (thicker center), edge cracking (low ductility, high reduction).

5.3 Rolling Operations and Control

• Roll Pass Design: Sequence of reductions to achieve final shape without defects. Consider draft (thickness reduction per pass), spread (width increase).

• Control: Automatic gauge control (AWC), rolling force/torque monitoring.

• Roll Deflection: Causes non-uniform thickness. Compensated by crowning (parabolic roll shape) or hydraulic backup rolls.

6. MACHINE TOOLS

6.1 Lathe Machine

Main Components:

• Bed: Base, guides for carriage.

• Headstock: Holds workpiece, contains spindle & motor.

• Tailstock: Supports other end, holds drill/center.

• Carriage: Moves along bed, holds tool post.

• Slide & Cross-slide: Tool movement (longitudinal, transverse).

• Tool Post: Holds cutting tool.

Basic Operations:

• Turning: Cylindrical surface (straight, tapered).

• Facing: End surface perpendicular to axis.

• Threading: Helical grooves (single/multi-start).

• Drilling: Hole in center (using tailstock drill).

• Boring: Enlarging/trueing hole.

• Knurling: Creating patterned surface.

Types: Engine lathe (general), Turret lathe (multiple tools, no manual indexing), Capstan lathe (similar), CNC lathe.

6.2 Shaper and Planer Machines

• Shaper: Single-point tool, reciprocating motion. Cutting stroke (forward), return stroke (idle). Workpiece clamped on table. Types: Horizontal, Vertical, Universal.

• Planer: Large workpiece, tool stationary, workpiece reciprocates. For very large jobs (bed, frames).

• Working Principle: Quick return mechanism (slotted crank, Whitworth). Cutting time only during forward stroke.

• Applications: Shaper - small/medium parts, keyways, slots. Planer - large flat surfaces, multiple small parts on table.

6.3 Milling Machine

• Types: Horizontal (spindle horizontal), Vertical (spindle vertical), Universal (both), CNC.

• Operations:

  • Face Milling: Cutter axis perpendicular to surface.

  • Peripheral (Side) Milling: Cutter teeth on periphery.

  • Slotting: Making slots (end mill).

  • Indexing: Dividing periphery (using index plate).

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

• Arbors: Hold cutters on horizontal mill (stub, shell).

6.4 Drilling Machine

• Types: Bench (small), Pillar (medium), Radial (large, arm adjustable), Upright (heavy), CNC.

• Operations: Drilling, Reaming (finishing), Tapping (threads), Boring (enlarging).

• Drilling Time Calculation:

  • $$\displaystyle T = \frac{L}{f \times N} $$ (minutes), where:

    • $L$ = total travel = hole depth + approach + overrun (mm)

    • $f$ = feed (mm/rev)

    • $N$ = speed (rpm)

  • Approach/Overrun: Typically = drill radius (for through hole) or 0.5-1 times diameter.

  • Example (Jun 2023): d=20mm, t=40mm, N=400 rpm, f=0.1 mm/rev, approach+overrun = radius = 10mm.

    • $$\displaystyle L = 40 + 10 = 50 $$ mm.

    • $$\displaystyle T = \frac{50}{0.1 \times 400} = 1.25 $$ minutes.

• Common Drills: Twist drill (most common), core drill (for existing holes), step drill (multiple diameters).

6.5 Grinding Machine

• Types:

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

  • Cylindrical Grinder: External/internal cylindrical surfaces.

  • Centerless Grinder: No centers, high production for cylinders.

  • Tool & Cutter Grinder: Sharpening cutters, complex shapes.

• Abrasives: Al₂O₃ (steel), SiC (cast iron, non-ferrous), CBN (hard steels), Diamond (carbides, ceramics).

• Bonds: Vitrified (strong), Resinoid (flexible), Rubber (flexible).

• Advantages: High accuracy (±0.002 mm), fine finish (Ra 0.1-0.8 μm), can grind hardened materials.

• Applications: Finishing, sharpening, precision forms.

6.6 General Machine Tool Considerations

• Safety: Guards, emergency stop, proper clamping, safe speeds/feeds.

• Tool Holding: Collets, chucks (3/4-jaw), arbors, tool holders.

• Work Holding: V-blocks, angle plates, fixtures, vises, magnetic tables.

• Maintenance: Lubrication, alignment checks, tool sharpness, cleaning.

[!TIP] Exam Focus: Drilling time calculation (Jun 2023) and lathe operations (Jun 2025) are frequent. Know difference between shaper and planer (Dec 2024). For grinding, emphasize abrasive selection based on material.

END OF UNIT 3 NOTES