Introduction
Digital Mock-Up (DMU) in CATIA is the practice of using the 3D assembly model as the primary medium for design validation — checking that parts fit together, that there are no interferences, that maintenance access is achievable, and that assembly sequences are feasible — before any physical prototype is built. For organizations that have made the transition from physical to digital validation, DMU has become one of the highest-value activities in the design process. For engineers who are new to the practice, understanding what DMU can and cannot do, and how to do it well, is the starting point.
What DMU Covers
CATIA DMU encompasses several analysis types that collectively replace or reduce the need for physical fit checks:
- Interference analysis (clash checking): Identifying volume intersections between parts that physically cannot occupy the same space. This is the most basic and most commonly used DMU function.
- Clearance analysis: Identifying pairs of parts where the distance between them falls below a defined threshold. This catches near-misses that would cause problems in service even without hard interference — thermal expansion, assembly variation, vibration contact.
- Sectioning: Cutting through the assembly with planes to inspect internal configurations that are not visible from outside. Useful for verifying thread engagement depths, internal channel clearances, and hidden interface conditions.
- Kinematics simulation: Simulating the motion of mechanisms to verify that moving parts do not contact adjacent structure throughout their range of travel. Links, cams, and slider assemblies should all be validated kinematically before prototype build.
- Disassembly simulation: Verifying that individual components can be removed for maintenance without removing or damaging other components. This is frequently neglected until a service engineer reports that a part is inaccessible in the field.
Setting Up a Clash Analysis in CATIA
The quality of a clash analysis is only as good as the setup. Key decisions:
- Scope selection: Define which parts will be checked against each other. Checking every part against every other part in a large assembly can generate thousands of results, most of them irrelevant. A more practical approach is to define checks between specific groups — for example, all moving parts against all fixed structure, or all pipes against all brackets.
- Clearance values: For clearance analysis, the minimum acceptable clearance value should reflect the functional requirement. A clearance check for thermal expansion should use the calculated expansion differential at maximum operating temperature, not an arbitrary value.
- Contact versus penetration: CATIA distinguishes between contact (parts touching at a surface) and clash (parts overlapping in volume). For tight-fit designs, contact results can be intentional. Configure the analysis to separate these results so that intentional contacts do not obscure real problems.
Interpreting and Acting on DMU Results
A clash analysis report from a real assembly will almost always contain many results. The engineering work is in triage:
- Intentional contacts: Press fits, sliding interfaces, and mating surfaces will appear as contacts. Identify these, document them as accepted, and filter them from the active problem list.
- Assembly-state dependencies: Some apparent clashes only exist in a specific assembly configuration that is not the operating condition. A cable routed through a channel may show interference in a specific assembly position that is only used during assembly, not during operation.
- Real design problems: Unintentional interferences and clearances below thermal or vibration requirements are genuine problems that require design action. Each should be assigned to a responsible engineer with a resolution deadline.
A well-managed DMU process maintains a living issues list: all identified problems, their status (open, in resolution, closed), and the responsible engineer. This list feeds directly into design review risk sections.
DMU for Maintainability and Assembly Sequence
DMU analysis is often used exclusively for interference checking, missing its value for maintainability and assembly feasibility. A few questions DMU can answer that physical prototypes used to be required for:
- Can component X be removed if all adjacent components remain installed? Simulate the removal path in the assembly and check for clearance throughout.
- Is there sufficient tool access for the torque values specified? Check clearance not just to the fastener, but to the tool envelope required to reach it.
- Does the assembly sequence work? Can sub-assembly A be installed before sub-assembly B, and is the interface accessible in that sequence?
Summary Table
| DMU Function | What It Detects | When to Use |
|---|---|---|
| Clash / interference check | Volume intersections between parts | Every major design iteration |
| Clearance analysis | Near-contact conditions below threshold | When thermal expansion or vibration is a concern |
| Kinematics simulation | Contact during range of motion | Any mechanism with moving parts |
| Disassembly simulation | Inaccessible or stuck components | Before finalizing design for production |
| Sectioning | Hidden internal conditions | When internal geometry cannot be inspected visually |
FAQ
Q: How accurate does the CAD geometry need to be for DMU results to be meaningful?
DMU results are only as reliable as the geometry they are based on. Parts modeled to nominal dimensions without tolerance representation will show only nominal conditions. For critical clearance analysis, consider modeling worst-case conditions explicitly, or supplementing DMU analysis with tolerance analysis that accounts for stack-up. DMU is not a substitute for tolerance analysis — it is a complement to it.
Q: Can CATIA DMU validate flexible components like hoses and cables?
Flexible component routing in CATIA requires specific modeling approaches — hoses and cables are typically modeled as flexible members with defined routing paths. The resulting models can be included in clash analysis, but the validity depends on whether the routing path has been modeled accurately for all relevant assembly and operating conditions. Flexible component DMU is more involved than rigid body DMU but is achievable with the appropriate modeling technique.
Q: How often should a DMU analysis be run during a design project?
At minimum, before each formal design review. In active design phases, a clash check should be run after any significant geometric change, particularly changes that affect spatial relationships between parts. The cost of running a clash analysis is low enough that it should never be the bottleneck — if it is, the assembly structure or analysis setup needs optimization.
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