Failure Pattern: Nominal Geometry Assumption

Pattern Description

The Nominal Geometry Assumption occurs when a design treats idealized geometry as a sufficient proxy for physical reality.

In this pattern, dimensions, alignments, and interfaces are evaluated primarily in their nominal state, while variation introduced by manufacturing processes, assembly sequence, and load is deferred or ignored.

The assumption is not that variation does not exist, but that it will remain small enough to be irrelevant.

This assumption is often implicit. Once locked in, it becomes difficult to surface or challenge.

Where This Pattern Appears

This failure pattern is typically injected at decision time, before:

• tolerance stacks are explicitly analyzed
• manufacturing process capability is known
• assembly sequence is constrained
• validation fixtures or measurement strategies exist

It often enters the system when designs are declared “ready” based on visual completeness rather than behavioral certainty.

Why the Pattern Is Invisible

The Nominal Geometry Assumption persists even in experienced teams because:

• CAD environments default to ideal geometry
• simulations often rely on simplified or averaged inputs
• drawings appear complete and internally consistent
• early prototypes are interpreted as confirmation rather than probes

These factors create a false sense of readiness. Confidence increases, while actual exposure to variation does not.

What This Pattern Breaks Downstream

When this assumption is wrong, failures typically surface as:

• inconsistent fit or alignment
• uneven load distribution
• sensitivity to assembly order
• reduced repeatability across builds
• late-stage negotiation with suppliers over “acceptable” deviation

At this point, correction is no longer a design decision. It becomes a cost, schedule, or quality dispute.

HRF Mapping

Within the Hardware Readiness Framework, this pattern maps to:

• Translation Gap
  Nominal geometry describes intent, not behavior.

• Failure Injection Point
  The decision to freeze geometry without validating variance.

• Readiness vs Confidence
  Visual completeness increases confidence without increasing readiness.

OpenDFM exists to surface this pattern before commitment, not to resolve it after the fact.

Diagnostic Questions

These questions are not checklists. They are probes intended to expose hidden assumptions.

1. What must be true for nominal geometry to be a safe assumption in this system?
2. Which variations are implicitly assumed to be negligible, and why?
3. At what point would this assumption become impossible to reverse?
4. How would failure manifest if this assumption were wrong?
5. Who would bear the cost of discovering that failure?

OpenDFM does not answer these questions. It exists to ensure they are asked before decisions are locked in.