Worst-Case vs RSS Tolerance Analysis for Modular Equipment Interfaces

An OEM may ask whether a removable module will clear its chassis under every permitted dimensional combination or whether a stable production process will produce an acceptable distribution of clearances. Those questions are related, but they are not identical. Choosing worst-case analysis or root-sum-square analysis changes the evidence needed to answer them.

For medical equipment chassis, the analysis should begin with the functional interface and an approved model. A smaller calculated number is not a reason to choose a statistical method.

Write the functional equation first

Define the quantity being evaluated: clearance, overlap, offset or engagement. Identify the contributors, their signs and their relationship to the drawing references. Include purchased components and finishing effects where relevant; a metalwork-only stack may omit the dominant source of variation.

Keep the model understandable. A simple linear chain can be checked by hand, while rotations, contact changes or flexible parts may require more detailed analysis. Do not force a nonlinear assembly into a convenient one-dimensional equation without reviewing the approximation.

A hypothetical comparison

Assume a linear clearance has a nominal value of 0.50 mm. Three independent dimensional contributors have symmetric limits of ±0.10, ±0.08 and ±0.06 mm, each with sensitivity magnitude one.

The worst-case variation is 0.10 + 0.08 + 0.06 = 0.24 mm. Within this simplified model, the minimum clearance is therefore 0.26 mm when the permitted adverse extremes occur together.

For a separate statistical illustration, assume those limits each represent ±3 standard deviations of stable, centered, independent normal distributions. The combined 3-sigma variation is √(0.10² + 0.08² + 0.06²), approximately 0.141 mm. The resulting illustrative 3-sigma clearance interval is approximately 0.359–0.641 mm.

The latter interval is not a guaranteed minimum and is not valid merely because the drawing has symmetric limits. Real production data must support the assumed distributions, centering and independence. These values are an example, not project results or a DRAmetal tolerance commitment.

Independence is a condition, not a convenience

Two dimensions made in one setup may move together. A common offset, shared tooling or a material change can create correlation or shift a process mean. Ignoring those relationships can understate the spread of the assembly result.

NIST’s discussion of propagation illustrates why independence matters in variance calculations. Measurement uncertainty and manufacturing tolerance analysis are different applications, but neither justifies discarding correlations without examination.

Choose the method from the decision

Adjustment changes the assembly model

Slots, shims and floating hardware may make an interface adjustable, but they also introduce an assembly operation and an acceptance condition. Specify adjustment range, accessible tools, locking method and the final position to be checked. Do not count the full slot length as usable travel without accounting for fastener geometry and edge conditions.

The existing door stack-up guide covers a different application. For modular chassis, include engagement and service removal as well as the assembled position; a module that fits at rest may still lack a clear insertion path.

Send the supplier the assumptions that affect its scope

Provide the released drawings, functional equation, critical contributors and inspection requirements. Explain whether the requested evidence is individual-part conformance, process capability or an assembly fit check. These are different deliverables with different sample and record needs.

DRAmetal can review the drawing-defined metalwork and agreed inspection contribution. The OEM should own the system-level acceptance decision and approve any statistical risk assumptions before they become part of supplier release criteria.