Engineering · Cost
Tolerance vs cost — the ladder.
Eight tolerance planning tiers with candidate processes and example applications. No universal cost ratio follows from tolerance alone; the feature, setup, material, quantity and measurement plan govern the quote.
Interactive Reference
Tolerance Cost Curve Analyzer
These ordinal tiers are not cost indices, measured Digital Machine capability or production forecasts. Inspection descriptions are considerations, not mandatory methods or sampling plans.
Can be run on standard manual or 3-axis mills with minimum setup overhead.
| Tier | Tolerance | Cost vs baseline | Process | Typical features |
|---|---|---|---|---|
| Tier 1 | 0.030″ (0.76 mm) | 1 | Standard milling, single-pass turning, sawing | Bracket envelopes, weldment blanks, non-mating clearances |
| Tier 2 | 0.010″ (0.25 mm) | 2 | Standard CNC, no special tooling | Most general structural features, clearance holes, blank dimensions |
| Tier 3 | 0.005″ (0.13 mm) | 3 | Standard CNC with tighter QC | Mounting bores, dowel-pin holes (clearance), fastener spotfaces |
| Tier 4 | 0.002″ (0.05 mm) | 4 | Precision CNC, in-process probing | Bearing fits, sliding fits, locating features, mating surfaces |
| Tier 5 | 0.001″ (0.025 mm) | 5 | Glass-scale machine, CMM verification | Precision bushing IDs, locating dowel holes, optical mounting interfaces |
| Tier 6 | 0.0005″ (0.013 mm) | 6 | Glass-scale precision mill, careful thermal management | Press-fit interfaces, gauge surfaces, metrology fixtures |
| Tier 7 | 0.0002″ (0.005 mm) | 7 | Wire EDM or grinding for finishing | Bearing races, optical alignment surfaces, gauge masters |
| Tier 8 | 0.0001″ (0.0025 mm) | 8 | Wire EDM, jig grinding, lapping | Tool-and-die work, ultra-precision metrology, quantum/cryo flexures |
Why the cost climbs so fast
It’s not just the cut.
- Tighter tolerance forces tighter machine selection.
A 0.005″ feature runs on any mill. A 0.0005″ feature needs a precision machine with glass scales and thermal stability — different machine, different rate, different schedule.
- Inspection needs a measurement plan.
Measurement method, resolution, uncertainty and sampling depend on the characteristic and quality plan. A CMM is not automatically adequate or required at a particular tolerance.
- Yield needs process data.
Yield depends on the process distribution, measurement system and feature requirements. No scrap-rate percentage can be inferred from tolerance alone; use measured capability and production data.
- Setup time grows with tolerance.
Setup time depends on datum strategy, workholding, thermal stabilization, measurement and trial cuts. No fixed setup duration follows from a tolerance value.
- Process changes — sometimes you have to.
Small internal radii or demanding form requirements may favor EDM, grinding or another finishing process. Material conductivity, access, geometry and demonstrated capability determine the options; a tolerance alone does not mandate one process.
Practical guidance
Three rules for not overspending on tolerance.
- 1. Tolerance only what mates or measures.
Bearing fits, alignment surfaces, sealing surfaces — those need precision. Edges of a bracket, hole locations far from a datum, cosmetic features — those don’t. Apply tight tolerance only where it matters functionally.
- 2. Use ISO 2768 or default-tolerance blocks.
Unnecessary tight callouts can add setup and inspection work. Use a reviewed general-tolerance scheme and explicit requirements on functional features; savings require a feature-specific quote.
- 3. Specify GD&T relative to actual datums.
A 0.001″ positional tolerance referenced to the right datums is much cheaper than 0.001″ on each X/Y/Z dimension. GD&T allows the machinist to optimize stack-up; per-axis tolerance forces them to hit each independently.
Keep exploring
Related tools & references
Achievable tolerances by process and how to call them out.
Worst-case and RSS analysis across a dimension chain.
ISO 286 fit classes for mating holes and shafts.
Find cost-driving callouts before a drawing is released.
See how tolerance and finish choices roll into piece price.
The finish side of the same cost trade-offs.
Want a DFM tolerance review on your drawing?
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