CAD Lab (ME-605) - Unit 3 Short Notes

UNIT 3: ADVANCED 3D MODELING, ASSEMBLIES & SIMULATION

3.1 Advanced 3D Modeling Techniques

3.1.1 Parametric vs. Direct (History-based vs. History-free) Modeling

• Parametric (History-based): Model is defined by a sequence of features (sketches, extrudes, cuts) stored in a design tree. Changes propagate from parent to child features. Design intent is captured.

  • Pros: Easy to edit, family of parts, driven by dimensions/relations.

  • Cons: Can be fragile; complex edits may fail if feature order/logic is broken.

• Direct (History-free): Model is a single, editable "dumb solid." Geometry is manipulated directly (push/pull) without a feature history.

  • Pros: Flexible, fast for concepting, robust for imported geometry.

  • Cons: Difficult to make intelligent, parametric changes; no automatic update propagation.

• Key Difference: Parametric is rule-driven (change parameter -> update model). Direct is geometry-driven (change geometry directly).

[!TIP] Exam Focus: Be prepared to identify which paradigm is best for a given task (e.g., family of brackets = Parametric; quick modification of a scanned mesh = Direct).

3.1.2 Advanced Feature-Based Modeling

• Loft: Creates a smooth transition between two or more profile sketches.

  • Guide Curves: Additional 3D curves that control the loft's shape along its path. Essential for complex, non-uniform transitions.

• Sweep: Profiles a 2D sketch along a 3D path.

  • With Guide Curves: Path is supplemented by curves that control orientation/profile shape.

• Complex Blends: Fills between multiple faces/edges with varying cross-sections (e.g., Boundary Blend, Blend between 3+ profiles).

3.1.3 Multi-Body Parts & Boolean Operations

• Multi-Body Part: A single part file containing multiple, separate solid bodies.

  • Used for complex castings, machined parts with internal cavities, or as a precursor to assembly.

• Boolean Operations (Combine/Subtract/Intersect):

  • Combine (Union): Merges two or more bodies into one.

  • Subtract (Difference): Removes volume of one body from another.

  • Intersect: Keeps only the common volume shared by bodies.

  • \boxed{\text{Result Body} = f(\text{Body}_1, \text{Body}_2, \text{Operation})}

3.1.4 Sheet Metal Design

• Core Features: Flange (base bend), Bend (adds angle), Rip (relieves material), Jog.

• Key Parameters: Bend Radius, Bend Allowance/Bend Deduction (for flat pattern accuracy), K-Factor.

• Flat Pattern: Unbent 2D representation of the sheet metal part, critical for manufacturing.

  • \boxed{\text{Flat Length} = \text{Total Bend Length} + \text{Sum of (Bend Deduction)}}

3.1.5 Mold & Die Design Fundamentals

• Core/Cavity Separation: Splitting the mold into two halves (core for internal features, cavity for external).

• Parting Line: The 3D curve where the two mold halves meet. Must be continuous and on a "draft" surface.

• Draft Analysis: Visual check for adequate draft angle (typically 1°-3°) on faces perpendicular to the parting direction to allow ejection.

  • \boxed{\text{Draft Angle} = \arctan\left(\frac{\text{Offset Distance}}{\text{Part Height}}\right)}

3.2 Assembly Modeling

3.2.1 Bottom-Up vs. Top-Down Design

3.2.2 Component Placement & Mates/Constraints

Mates define geometric relationships between components, restricting Degrees of Freedom (DOF).

[!TIP] Common Pitfall: Over-constraining (redundant mates) causes solver errors. A fully constrained rigid body in 3D space needs exactly 6 constraints.

3.2.3 Assembly Structure

• Sub-assemblies: Logical grouping of components (e.g., Pump-Assembly, Motor-Assembly). Improves performance and organization.

• Flexible Components: Components (like springs, cables) that can change length/shape within the assembly (e.g., Make Flexible command).

• Component Patterns: Linear Pattern, Circular Pattern, Pattern along Path. Drive patterns from a seed component.

3.2.4 Interference Detection & Clearance Analysis

• Interference Detection: Checks for unwanted overlap between solid bodies.

  • Result: Highlights interfering volume. Critical for ensuring parts don't occupy the same space.

• Clearance Analysis: Checks if a minimum gap exists between components (e.g., for thermal expansion, tolerance).

  • \boxed{\text{Clearance} = \text{Distance between surfaces} - (\text{Tol}_1 + \text{Tol}_2)}

3.2.5 Exploded Views & Animation

• Exploded View: Manually or automatically offset components along a vector to show assembly sequence.

• Animation: Uses motion drivers (motors, springs) or manual keyframes to animate assembly/disassembly.

  • Output: Video file (.avi, .mp4) for documentation/presentations.

3.3 Motion & Mechanism Analysis

3.3.1 Degrees of Freedom (DOF) in Assemblies

• A free rigid body in 3D space has 6 DOF (3 translational, 3 rotational).

• Mates reduce DOF. A fully constrained assembly has 0 DOF (unless driven by a motor/spring).

• Grübler's Equation (for planar mechanisms): \boxed{DOF = 3(n-1) - 2j_1 - j_2} where n = links (including ground), j_1 = 1-DOF joints (pin, slider), j_2 = 2-DOF joints (cam).

3.3.2 Creating Basic Mechanisms

• Slider-Crank: Concentric mates on pin joints, Coincident on slider faces, Distance or Angle on crank arm.

• Linkages: Use Revolute (pin) or Cylindrical (pin+slider) mates to allow rotation.

• Key: Ensure at least one component has 1+ DOF to allow motion.

3.3.3 Simple Motion Study

• Motor: Imparts constant/variable velocity/rotation to a component (e.g., motor on a shaft).

• Spring: Applies a force proportional to displacement between two points/components.

• Damper: Applies a force proportional to velocity (resists motion).

• Gravity: Applies uniform gravitational acceleration.

3.3.4 Results Extraction

• Run simulation over a time span.

• Results Plots: Displacement, Velocity, Acceleration vs. Time for selected component/point.

• Trace Path: Plots the 3D path traveled by a point during motion.

3.4 Design Validation & Simulation (FEA)

3.4.1 Simulation Workflow

1. Pre-processing: Define study type, apply fixtures (constraints), loads (forces/pressures), select material, create mesh.

2. Solving: Software computes nodal displacements/stresses (solves [K]\{u\} = \{F\}).

3. Post-processing: Visualize displacement, stress, strain, factor of safety (FOS). Generate reports.

3.4.2 Static Stress Analysis

• Fixtures (Constraints): Immobilize geometry (e.g., Fixed Geometry, Roller/Slider, Symmetry).

• Loads: Force (point/face), Pressure (normal to face), Gravity, Bearing Load, Torque.

• Assumption: Loads are static (non-accelerating), small deformations.

3.4.3 Mesh Generation

• Types:

  • Solid Mesh (Tetrahedral/Hexahedral): For volumetric stress. Hexahedral (brick) is more accurate but harder to generate.

  • Shell Mesh: For thin-walled parts (e.g., sheet metal). Uses mid-surface.

• Mesh Quality Metrics: Aspect Ratio (ideally ~1), Jacobian (0 to 1, >0.6 good), Skewness.

• Mesh Control: Refine mesh in high-stress areas (fillets, holes, load application points) using Mesh Control (element size, curvature-based).

3.4.4 Interpreting Results

• von Mises Stress (\sigma_{vm}): Scalar stress value from tensor. Used for ductile materials. Failure predicted when \sigma_{vm} \geq \sigma_{yield}.

  \boxed{\sigma_{vm} = \sqrt{\frac{(\sigma_1-\sigma_2)^2 + (\sigma_2-\sigma_3)^2 + (\sigma_3-\sigma_1)^2}{2}}

• Displacement: Total vector displacement (mm/in). Check for excessive deflection.

• Factor of Safety (FOS): \boxed{FOS = \frac{\text{Allowable Stress (e.g., Yield)}}{\text{Maximum von Mises Stress}}}. FOS > 1.5-2.0 typically required.

3.4.5 Design Studies & Optimization

• Design Study: Automatically runs multiple simulations while varying parameters (dimensions, material, thickness).

• Goal: Find parameter set that minimizes/maximizes an objective (e.g., minimize mass while keeping \sigma_{vm} < 100 MPa).

• Result: Response graph showing relationship between input parameter and output result (stress, displacement).

3.5 CFD & Thermal Analysis (Introductory)

3.5.1 Basic CFD Workflow

1. Define Fluid Domain: The volume the fluid occupies (internal flow) or the volume around the object (external flow).

2. Assign Fluid: Select fluid properties (air, water, custom: density \rho, viscosity \mu).

3. Boundary Conditions:

   • Inlets: Velocity, Pressure, Mass Flow Rate.

   • Outlets: Static Pressure (often 0 gauge), Outflow.

   • Walls: No-slip (velocity=0), symmetry, moving walls.

4. Solve & Post-process: Velocity vectors, pressure contours, streamlines.

3.5.2 Thermal Analysis (Steady-State)

• Heat Transfer Types: Conduction (through solid), Convection (to fluid), Radiation (often ignored in basic).

• Thermal Loads: Heat Power (W), Temperature (fixed), Heat Flux (W/m²).

• Convection Boundary: Define Convection Coefficient (h) and Ambient Temperature (T_∞).

  \boxed{q_{conv} = h \cdot A \cdot (T_{surface} - T_{\infty})}

• Results: Temperature distribution plot, heat flux vectors.

3.5.3 Interpreting Plots

• Streamlines: Trace of fluid particles. Shows flow direction, separation, recirculation.

• Contour Plots: Color map of scalar values (Pressure, Temperature, Velocity Magnitude).

• Vector Plots: Arrows showing direction/magnitude of vector quantities (Velocity).

3.6 Data Management & Industry Practices

3.6.1 File Formats for Interoperability

3.6.2 Drawing Creation from 3D Models

• Automated View Generation: Model View (standard 3-view), Projected View, Auxiliary View (from inclined face), Section View (full, half, offset), `Detail View** (zoomed callout).

• Best Practice: Views are associative to the 3D model. Changes in model update drawing.

3.6.3 Bill of Materials (BOM) Generation

• Tabular BOM: List of components in an assembly, typically with columns: Item No., Part Number, Description, Quantity, Material.

• Generated Automatically from assembly structure. Can be placed on drawing sheet.

• Structured BOM: Shows parent-child relationships (sub-assemblies).

3.6.4 Introduction to PLM & CAD Data Management

• PLM (Product Lifecycle Management): System to manage all product data (CAD files, BOM, revisions, approvals) from concept to disposal.

• Key Concepts: Check-in/Check-out (prevents overwrites), Revision Control (A, B, C...), Workflow/Approval Process, Where-Used (find all assemblies using a part).

• Goal: Single source of truth, collaboration, change management.

3.6.5 Design Intent & Best Practices

• Design Intent: How the model is built to behave predictably when edited (e.g., "this hole always stays centered on this face").

• Best Practices for Robust Models:

  1. Sketch Fully: Define all geometry with relations (horizontal, vertical, concentric, equal) and dimensions.

  2. Use Origin/Base Planes: Anchor sketches to stable references.

  3. Avoid In-Context References unless necessary (top-down). They can cause circular references.

  4. Name Features/Sketches: Instead of Extrude1, use Base_Block, Holes_Counterbore.

  5. Keep Design Tree Organized: Use folders, suppress unused features.

  6. Model Symmetrically: Use Mirror features/patterns instead of drawing symmetric geometry.