Assembly Design and Digital Mockup - Practical Guide

CATIA · Assembly & DMU

Overview

Once a single part is done, assembly follows. This page bundles how to build the Product structure and Constraints in Assembly Design, manage the BOM, and then verify interference, kinematics, and assembly paths in DMU (Digital Mockup).

The focus is on Top-down Skeleton design, the criteria for choosing between Flexible and Rigid Sub-Assemblies, and which real-world situations each of the 16 Kinematics Joints is actually used for.

1. Product Structure Basics

Product (top level) — the .CATProduct file
Component — an Instance inside a Product. The same Part used multiple times has a distinct Instance Name each time but a single Reference.
Part (.CATPart) — the lowest-level element, carrying the actual geometry
Sub-Product — a Product inside a Product (nestable)

The core concept is the Reference + Instance structure. For example, placing ten of the same bolt yields one Part file + ten Instances. Edit the Part and all ten update simultaneously.

2. Inserting Components

Existing Component — Insert → Existing Component, or the icon. Pick a file.
New Component — creates an empty Product
New Part — creates a new Part. In the origin-alignment dialog, "Yes" = align the Part Origin to the Product Origin; "No" = use the Compass location.
Existing Component With Positioning — insert an existing file with snap guidance
Replace Component — swap a component (revision update)

Note Failing to pick "Yes" when inserting a New Part leaves the origin misaligned, which tends to tangle the assembly later. In my experience it is among the most common root causes of assembly defects.

3. Assembly Constraints

Coincidence — axis / face / point coincident (the most common)
Contact — two faces touching (direction is automatic)
Offset — specified distance
Angle — specified angle (includes Parallelism and Perpendicularity)
Fix — lock in place
Fix Together — lock as a group
Quick Constraint — automatically recommends a type fitting the selected elements
Change Constraint — change the type
Reuse Pattern — auto-place parts against a Pattern

Tip Always select the part that should move (child) first, then the part that should stay fixed (parent). Reverse the order and the side that should stay put tries to move, producing a Conflict.

Note Green Constraint icons are OK, yellow means update needed, and red means Broken. Revive a red Constraint by double-clicking it and using Replace or reselecting the reference.

4. Flexible vs. Rigid Sub-Assembly

Item	Rigid (default)	Flexible
Nature	The whole Sub-Assembly is a single rigid body	Parts inside the Sub-Assembly can be independently constrained from the top level
Instances	Multiple instances of the same Sub-Assembly share the same pose	Pose can vary per instance
Performance	Light	More load

How to toggle: right-click the Sub-Product → Flexible / Rigid Sub-Assembly.

Use Case: If you model a six-DOF robot arm as a single .CATProduct and drop several copies into a top-level line, switch them to Flexible so each robot can hold a different pose. This is a prerequisite for assembly-line simulation.

Note Flexible mode carries a performance cost. Avoid overuse in large assemblies, and apply it only to the Sub-Products that truly need it.

5. BOM Management

Path: Analyze → Bill of Material. The default columns are Part Number, Quantity, Revision, Nomenclature, Description. Nomenclature and Description are edited from the Part's right-click → Properties → Product tab.

Customize the format: Analyze → Bill of Material → Define Format
Export: txt / xls / html

Tip A production-grade BOM pipeline typically uses Knowledgeware Parameters to auto-extract PartNumber / Material / Mass and export to Excel. Automating the weekly BOM via a Macro eliminates manual-entry errors.

→ See also: /cad-tips/catia/macro-3dx-troubleshoot.md for Macro / BOM automation details.

6. Assembly Features

There are also features at the Assembly level rather than the Part level.

Split — cut a Part inside the Product with a Surface
Hole — a Hole that passes through multiple Parts (drilled in the assembled state)
Pocket / Add / Remove / Rib / Slot — assembly-level versions

Note Assembly Features are convenient, but they actually edit the Part files themselves. In a PDM environment this complicates revision and ownership management, so it is safest to use them sparingly.

7. Part-Positioning Helpers

Manipulation — translate along individual axes (DOF control)
Snap — temporarily align two elements
Smart Move — Snap + automatic Constraint creation
Explode — exploded 3D view

Drop the red square of the Compass onto a Part and drag it to move only that Part while preserving its degrees of freedom. Indispensable for preliminary placement before applying Constraints. → See also: /cad-tips/catia/shortcuts-ui.md for Compass basics.

8. Scenes and Overload

Scene — saves a "deformed state" of the assembly. Explosion or movement states stored as Scenes can be reused in drawings.
Overload — override a Part's color, material, or Hide/Show at the assembly level. The original Part is left untouched.

9. Skeleton-Based Top-down Design

The key technique for keeping large assemblies safe.

Place a Skeleton.CATPart in the top-level Product to host reference lines, planes, and axes
Publish the Skeleton's elements
Downstream Parts reference only the Published elements
When a reference dimension changes, edit only the Skeleton and every downstream part follows

Tip Publication is not merely a Named Reference — it is a safety net for Links. Even if the file name changes or the internal structure is reshuffled, as long as the Publication name stays the same, the link does not break.

9.1 Assembly-Design Principles in Brief

Go top-down with a Skeleton part
Use Publication for safe references
Apply Constraints only to the actual degrees of freedom (avoid redundancy)
Group into Sub-Products once you pass ~100 parts
Use automatic Mass / CoG in Product Properties (essential for body-weight control)

10. DMU Navigator — Lightweight Review of Large Assemblies

Walks through the assembly with minimal memory footprint, backed by CGR representations.

Assembly exploration / Hide and Show / Isolate / Overload
Sectioning
Measure

Editing is disabled in this mode, but even projects with tens of thousands of parts open smoothly.

11. DMU Space Analysis — Interference Checks

Clash Detection — three levels: Contact / Clearance / Clash
Interference (Clash) modes — Between All Components / Inside One Selection / Selection Against Selection
Distance & Band Analysis
Sectioning — sliding X / Y / Z section planes
Measure Between / Item / Inertia

Tip Publishing the Clash Detection result generates an HTML report. You can hand it off to a design-review meeting as-is.

12. DMU Kinematics — Mechanism Simulation

12.1 The 16 Joints

Joint	Description	DOF
Revolute	Rotation (hinge)	1
Prismatic	Linear translation	1
Cylindrical	Rotation + translation	2
Planar	Motion on a plane	3
Rigid	Fixed	0
Spherical	Ball joint	3
Universal	Universal joint	2
Point-Surface	Point-on-face contact	3
Point-Curve	Point-on-curve	5
Roll-Curve	Rolling	1
Slide-Curve	Sliding	1
Screw	Thread	1 (coupled)
Gear	Gear coupling	–
Rack	Rack	1
Cable	Cable	–
Constant Velocity	Constant-velocity coupling	1

12.2 Commands and Playback

Convert Assembly Constraints — automatically converts existing Assembly Constraints to Joints. Quick-start option.
Simulation with Commands — drive each Command value with a slider
Simulation with Laws — specify a Law (time function) via Formula for periodic motion
Replay / Trace / Swept Volume — playback, path trace, swept volume

12.3 Interference Checks During Simulation

On — inactive
Off — only highlight when interference occurs
Stop — stop immediately on interference

Tip For robot-cell layouts and automotive-assembly process validation, Stop mode is effectively mandatory. Missing even a single interference balloons real-world cost.

13. DMU Fitting Simulation — Assembly / Disassembly Paths

Track — path of part motion
Recorder — records manual motion as a track
Player — playback
Shuttle — a temporary DMU object (outside the Product)
Smooth — smooths the track

Use Case: Used for serviceability assessments — for example, whether the alternator can be replaced in the engine bay (Serviceability). Lets you weigh after-sales serviceability already during design.

Field Checklist

When inserting New Part, confirm origin alignment with "Yes"
Select Constraints in child → parent order
Use Flexible Sub-Assembly only when strictly necessary
Drive Top-down with Skeleton + Publication
Automate the BOM with Knowledgeware + Macro
Coordinate PDM impact before using Assembly Features
Run DMU Clash Detection on a regular cadence
Catch interference with Stop mode during Kinematics simulation
Pre-verify serviceability with Fitting Simulation
Open large assemblies through DMU Navigator