Engineering snapshot · Nº 07
What Tolerance Actually Costs
There's no fixed multiplier, but every zero you add narrows the process list and grows setup and inspection effort. Spend tolerance where the function is.
Digital MachineQuick-Turn CNCTolerance tier → typical route
Bar = how much narrower than T1
- T10.030″0.76 mmBaseline
Typical route · Standard milling, single-pass turning, sawing
- T20.010″0.25 mm3× narrower
Typical route · Standard CNC, no special tooling
- T30.005″0.13 mm6× narrower
Typical route · Standard CNC with tighter QC
- T40.002″0.05 mm15× narrower
Typical route · Precision CNC, in-process probing
- T50.001″0.025 mm30× narrower
Typical route · Glass-scale machine, CMM verification
- T60.0005″0.013 mm60× narrower
Typical route · Glass-scale precision mill, careful thermal management
- T70.0002″0.005 mm150× narrower
Typical route · Wire EDM or grinding for finishing
- T80.0001″0.0025 mm300× narrower
Typical route · Wire EDM, jig grinding, lapping
Inspection scales with tolerance
- 0.030″Caliper or depth-gauge check at the machine
- 0.002″Micrometers, bore gauges, in-process probing
- 0.0001″CMM in a temperature-stable room, documented results
Considerations, not mandatory methods: the measurement plan depends on the feature.
Where the money goes
- 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: a different rate and schedule.
- Setup and stabilization
Datum strategy, workholding, warm-up and trial cuts all grow as the band narrows.
- A measurement plan
Method, resolution and uncertainty depend on the characteristic. A CMM isn't automatically adequate, or required.
- Process changes
Small inside radii or demanding form may favor wire EDM or grinding. Geometry and material decide, not the number alone.
- Scrap risk
Risk climbs as the band narrows. The real rate comes from process data, not from the tolerance.
Tolerance the function, not the title block: a reviewed general-tolerance block (ISO 2768 or your own) covers the rest.
Tiers are planning bands, not price or capability predictions. The feature, setup, material, quantity and measurement plan govern the quote.