공차가 쌓이면 조립이 안 된다 — Tolerance Stackup 실전 계산

"Every part was within tolerance, but the assembly doesn't fit." If you've heard this, tolerance stackup was ignored. When individual tolerances accumulate in the same direction, worst-case total error equals the sum of all individual errors.

1. Worst-Case Method

The most conservative approach — assumes all tolerances accumulate in the worst direction simultaneously.

• Total stackup = Σ|individual tolerances|
• Example: 5 parts in series, each ±0.05mm → total stackup = ±0.25mm
• Pros: 100% assembly guarantee
• Cons: Tolerances become very tight, driving up machining costs. Probability of all tolerances being worst-case simultaneously is extremely low.

2. RSS Method (Root Sum Square / Statistical)

Assumes each tolerance follows a normal distribution.

• Total stackup = √(t₁² + t₂² + t₃² + … + tₙ²)
• Example: Same 5 parts × ±0.05mm → √(5 × 0.05²) = ±0.112mm (≈45% of worst-case)
• Pros: Realistic tolerances, lower machining cost
• Cons: Statistically ~0.27% non-conformance (3σ). Use worst-case for safety/medical applications.

3. Design Strategies to Reduce Stackup

• Reduce part count: Integrating 5 parts into 3 directly reduces stackup contributors.
• Share datums: All parts referencing the same datum minimizes accumulated error.
• Adjustable elements: Place shims or adjustment screws at final assembly to compensate accumulated error.
• Tight tolerances only where functional: Don't specify ±0.05 everywhere — apply tight tolerances only to dimensions that directly affect function.