When tight matters.
When it doesn't.
Specify wisely.
Tight tolerances cost money. Specify ±0.025mm only where function requires it. Default ±0.1mm for non-critical features. This single DFM principle reduces 30-50% cost on many parts.
Key principles.
Tight = expensive
±0.1 mm default vs ±0.025 mm critical. Each tolerance level adds 30-100% to feature cost.
H7/h6 standard
For bearing bores: H7 hole, h6 shaft. Achievable with reaming and CNC. ±0.025mm class.
Coordinate carefully
Mating features in different parts: coordinate tolerances. Stack-up analysis verifies assembly.
Replace ± stack
GD&T position tolerance (e.g., 0.1 mm) gives 27% larger zone than equivalent ±0.05 stack-up.
Avoid datum chains
Excessive datum chains accumulate tolerance. Reference dimensions to single primary datum where possible.
Review pre-quote
Before quoting, audit drawing for unnecessarily tight tolerances. Common 30-50% cost savings.
FAQ
What's a typical CNC tolerance?
Standard CNC ±0.025-0.05 mm on critical features, ±0.1 mm on general dimensions. Tighter requires specific processes (grinding, lapping).
When is ±0.01 mm justified?
Bearing fits, precision instruments, optical mounts. Most parts don't need this. Cost: 5-10× standard tolerance.
GD&T position vs ± tolerance?
Position tolerance gives circular zone (more area than square). For functional fit verification, more permissive than ±X/±Y while maintaining intent.
Surface roughness affects tolerance?
Yes. Tighter Ra often demanded with tight tolerance. Ra 0.4 µm typical for precision; cheaper Ra 1.6 µm for non-critical.
Temperature stability?
Precision parts need thermal-stable materials (Invar 36) or thermal accommodation. CTE mismatch in dissimilar materials causes tolerance drift.
Inspection cost included?
Tight tolerances require precise inspection — CMM time, calibrated tools. Included in part cost. Tighter tolerance = more inspection time.
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