Tolerance basics.
± vs GD&T.
Specifying right.
Tolerance specification reference. ± tolerance vs GD&T. Tight vs loose. ISO 2768. Achievable per process.
Key principles.
Simple
Plus-minus from nominal. Easy to specify. Less flexible.
Functional
Position, profile, runout. Functional fit verification.
Default
Default tolerance class. -m (medium) common. -f (fine) tighter.
Cpk > 1.33
Process capability. Cpk > 1.33 capable. Production verifies.
Springback
Aluminum springs back. Steel stable. Plastic creeps.
Worst case vs RSS
Worst case (additive) vs root-sum-square (statistical).
FAQ
When use GD&T?
Functional fit critical. Bearing bores, mating features. ASME Y14.5 standard.
Default tolerance?
ISO 2768-m (medium) covers most. Tighter only on critical features.
How tight is too tight?
±0.005 mm requires grinding/lapping. ±0.025 mm CNC capable. Below 0.005 specialty.
Tolerance stack-up?
Multiple parts mating. Worst case = sum. RSS = √(sum of squares). RSS more realistic at production.
Cost vs tolerance?
5-10× cost premium per tighter class. Don't over-specify. Default loose, specify tight per function.
Specifying default?
Note on drawing: "All tolerances per ISO 2768-m unless noted." Explicit tolerances override.
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