Tolerances protect function. They also add cost. The job of tolerance allocation is to assign the allowed variation for every dimension so the assembly works, manufacturing can build it, and the product isn't unreasonably expensive.
This article (7–9 min read) shows practical allocation strategies, how to use process capability (Cp/Cpk), and a worked example that balances cost and risk.
Customer complaints if critical function is missed
Allocation is where design meets manufacturing: the optimal solution minimizes cost while meeting requirements and preserving manufacturability.
Practical tip: Always start allocation with the function — identify the
minimum tolerances that preserve product performance, then relax where
possible.
Manufacturing cost vs. tolerance typically follows a nonlinear curve: cost is fairly flat for loose tolerances, then rises slowly, and climbs sharply as tolerances tighten toward the process limits.
Key points:
Small improvements near the process capability limit are very expensive.
Some processes have asymmetric cost behavior (e.g., grinding vs. turning).
Volume matters: a $0.50 per-piece cost increase at 100k units is significant.
Simple conceptual formula (illustrative, not universal):
Cost(T)≈a+Tnb
where T is tolerance (width), n > 0 controls steepness, and a,b tune baseline cost and sensitivity.
Use cost curves from vendor quotes or your shop’s historical data whenever possible.
Split the required tolerance equally among contributing dimensions.
Pros: simple, quick.
Cons: ignores sensitivity — may tighten unimportant features and leave critical ones loose.
Weighted (sensitivity-based) allocation:
Weight each link by its contribution to the critical dimension (sensitivity) and cost impact.
Pros: minimizes cost for given performance.
Cons: requires a sensitivity analysis and cost model.
How to compute a simple weighted allocation:
For each contributor i, calculate sensitivity s_i = ∂(critical)/∂(dimension_i) (often +1 or −1 for simple chains).
Estimate cost function slope c_i(T) or marginal cost for reducing tolerance.
Allocate tolerances such that marginal cost per unit improvement is equal across contributors:
dTidCosti=λ⋅si(KKT-like balance)
This balances where money buys the most functional improvement.
Rule of thumb: if a feature has low sensitivity (small s_i) and high cost to
tighten, leave it loose and tighten high-s_i features instead.
If a 100 mm aluminum spacer is used over a −20°C → 80°C range (ΔT=100°C):
ΔL=23×10−6×100mm×100∘C=0.23mm
This 0.23 mm is a significant fraction of a 0.5 mm tolerance budget and must be included in the allocation (either as an environmental tolerance or by specifying material/assembly conditions).
Design actions:
Use matched materials (similar α) when stack is temperature-sensitive.
Allocate a portion of budget to thermal variation.
Control assembly temperature or specify functional conditions.
Always request capability studies (σ, Cp/Cpk) for the actual feature/process.
For long-term production, negotiate price vs. tolerance: vendors can often offer tiered pricing by tolerance band.
Consider single-source tooling for high-volume parts — fixture/tooling investments reduce per-piece cost for tighter tolerances.
If a vendor cannot provide capability data, assume conservative capability and
validate with prototypes — underestimating process variability is costly.
Tolerance allocation is a trade-off: cost vs. risk. Use sensitivity, cost curves, and process capability to make data-driven decisions. Weighted allocation guided by marginal-cost balance typically outperforms equal splits. Always validate allocations with vendors and prototypes.
