Assemblies

Component Tree

In modern engineering organizations, the assembly model becomes the central digital representation of a product and drives many downstream activities throughout the product lifecycle.

A CAD assembly is a structured collection of:

• Parts (components) : individual pieces like bolts, brackets, housings, gears, etc.
• Relationships (mates/constraints) : how parts are positioned relative to one another.
• Hierarchy (subassemblies) : logical grouping of components into modules.
• Metadata : material, part numbers, suppliers, revisions, configurations, etc.

Note

For example:

• Electric Vehicle
  • Battery Pack Assembly
    • Battery Module
    • Cooling Plate
    • Fasteners
  • Powertrain Assembly
  • Chassis Assembly

The assembly represents not only geometry but also how the product is designed, manufactured, assembled, and maintained.

This hierarchy is essential for:

• Product lifecycle management (PLM)
• Bill of Materials (BOM)
• Configuration management

Constraints

CAD assembly constraints (also called mates, joints, or assembly relationships, depending on the CAD software) define how individual parts are positioned and allowed to move relative to one another within an assembly. They are fundamental to building functional digital prototypes because they capture the design intent, not just the geometry.

Without constraints, the components are just floating bodies in 3D space.

Assembly constraints enable CAD software to:

• Position components accurately.
• Maintain relationships when dimensions change.
• Simulate mechanical motion.
• Detect interferences and collisions.
• Verify manufacturability and assembly sequences.
• Support downstream analyses like stress, kinematics, and tolerance studies.

Note

For example, if you change the diameter of a shaft, bearings constrained concentrically to that shaft automatically update their position, preserving the intended design.

Geometries

Boundary Representation (B-Rep)

Boundary Representation (B-Rep) is arguably the most important geometric representation in mechanical CAD because it forms the foundation for nearly all downstream engineering workflows, from manufacturing to simulation and digital twins.

A B-Rep describes a solid by explicitly defining its boundaries:

• Vertices (points)
• Edges (curves)
• Faces (surfaces)
• Topological relationships between them

Note

For example, a cube can be represented as:

• 6 planar faces
• 12 edges
• 8 vertices
  plus the information describing how these entities connect.

Meshes, Tesselated Mockups

Meshes and mockups focus on how a product looks and is perceived, rather than on the engineering details required to manufacture it. They are often used in product design, marketing, stakeholder reviews, and user testing before the design is finalized.

A CAD visual mockup is a high-quality digital representation of a product created from CAD geometry (or modeled specifically for visualization). Its goal is to communicate the product's appearance, proportions, materials, and interaction with light - not necessarily its manufacturability.

Think of it as the digital equivalent of a physical foam model or appearance prototype.

Model-Based Definition (MBD)

Model-Based Definition (MBD) is a product development methodology in which the 3D CAD model becomes the authoritative source of product definition. Instead of relying on traditional 2D engineering drawings, all manufacturing, inspection, and product lifecycle information is embedded directly within the digital model.

In essence, MBD transforms the CAD model from a geometric representation into a comprehensive engineering document that supports the entire product lifecycle.

What is Model-Based Definition?

Traditionally, product information was split between:

• A 3D CAD model describing geometry.
• 2D drawings containing PMIs : dimensions, tolerances, materials, notes, and manufacturing instructions.

Under MBD, these are unified into a single digital dataset that includes both the geometry and all associated engineering specifications.

This enriched model becomes the primary reference for designers, manufacturers, quality inspectors, suppliers, and maintenance teams.

PMI

CAD PMIs (Product Manufacturing Information) are manufacturing annotations embedded directly in a 3D CAD model. They provide the information needed to manufacture and inspect a part without relying solely on 2D drawings.

Typical PMI includes:

• Dimensions: linear, angular, radial, diametral, ordinate, etc.
• Geometric Dimensioning & Tolerancing (GD&T): position, flatness, perpendicularity, profile, concentricity, and other geometric tolerances.
• Datums: datum features and datum reference frames.
• Surface finish: roughness and texture requirements.
• Welding and manufacturing notes: weld symbols, process instructions, and general notes.
• Material and process information: material specifications, heat treatment, coatings, and finishes.
• Thread and hole callouts: thread specifications, hole sizes, countersinks, counterbores, and fits.

PMI can be represented in two forms:

• Graphical PMI: visible annotations displayed in the CAD model.
• Semantic PMI: machine-readable definitions linked to the model geometry, enabling downstream applications such as CAM, CMM programming, tolerance analysis, and automated inspection.

Design Features

One of the most important concepts in understanding modern CAD systems is the distinction between the geometry of a model and the feature history used to create that geometry.

At first glance, a CAD model appears to be simply a three-dimensional object displayed on the screen. A user sees a bracket, a housing, a shaft, or a mechanical component. However, internally, a parametric CAD system manages much more information than the visible shape.

A CAD model usually contains two interconnected layers:

• A geometric representation that defines the physical shape of the object.
• A feature-based representation that defines the sequence of operations, parameters, and relationships that generated the shape.

Note

• The geometric representation answers the question:
  "What is the final shape of this object?"
  
• The feature tree answers the question:
  "How was this object created, and what engineering decisions define it?"

The difference between these two viewpoints is fundamental because engineering design is rarely about creating a shape only once. Engineers continuously modify dimensions, adjust requirements, explore alternatives, and reuse previous designs. To support these activities, a CAD system must understand not only the final geometry but also the logic behind it.

Machining Features

In mechanical CAD, machining features are intelligent modeling operations that represent the removal, addition, or repetition of material in a way that corresponds closely to real manufacturing processes. Instead of simply creating arbitrary shapes, engineers use feature-based CAD tools to describe why geometry exists and how it is intended to be produced.

The most fundamental machining-related features are:

• Holes : cylindrical or specialized openings created through drilling, boring, reaming, or tapping operations.
• Pockets : recessed regions created by removing material through milling or similar processes.
• Patterns : systems for repeating features according to geometric rules, such as linear spacing or circular arrangements.

These features are central to modern parametric CAD because they preserve design intent, meaning the model remains understandable and editable as requirements change.

Note

For example, a designer does not simply create twelve identical circular cuts. Instead, they define:
"A circular pattern of twelve M8 clearance holes on a 150 mm bolt circle."
This communicates engineering purpose, not just geometry.

2D Drawings

A 2D CAD (Computer-Aided Design) drawing is a precise digital representation of an object, component, building, or system using two-dimensional geometry. Unlike a sketch, a 2D CAD drawing is created to communicate exact dimensions, manufacturing requirements, construction details, or engineering intent.

2D CAD drawings are used to:

• Document designs with precise measurements.
• Communicate design intent between engineers, architects, manufacturers, and contractors.
• Guide fabrication, assembly, or construction.
• Maintain standardized technical documentation.
• Support regulatory approvals and quality inspections.

They remain the industry standard for manufacturing and construction documentation, even when 3D models are available.

Persistent Naming

Persistent naming is one of the most fundamental and technically challenging problems in modern Computer-Aided Design (CAD). It refers to the ability of a CAD system to maintain a stable identity for geometric entities - such as faces, edges, and vertices - even after the underlying model has been modified.

Although users interact with geometry visually, CAD systems operate on a boundary representation (B-rep), where a solid model is defined as a collection of interconnected topological entities. Features such as holes, fillets, chamfers, sketches, dimensions, constraints, assembly mates, finite element meshes, and manufacturing operations frequently reference specific faces or edges of this topology. If those references cannot be preserved when the model changes, downstream operations either fail or become associated with incorrect geometry.

For this reason, persistent naming is essential to any robust parametric CAD system.