ISO 286 Guide
A practical guide to ISO 286 notation, hole basis systems, shaft basis systems, IT grades, formulas, and GD&T relationships.
What is ISO 286
A code system for limits and fits that defines tolerance grades and tolerance zone positions for linear sizes.
Hole basis system
The H hole lower deviation is zero, so standard tooling can hold the hole at nominal while shaft zones set the fit behavior.
Shaft basis system
The h shaft upper deviation is zero, useful when stock shafting or standard journals control the assembly basis.
IT grades
International tolerance grades describe the tolerance width. Lower grade numbers represent tighter production capability.
Fundamental deviations
Letters A–ZC (holes) and a–zc (shafts) define the position of the tolerance zone relative to the zero line (nominal size).
GD&T relation
Limits and fits size the mating features while GD&T controls form, orientation, location, and runout of those features.
Fit types
Clearance: hole always larger than shaft. Interference: shaft always larger than hole. Transition: overlap where either can result.
Rule #1 connection
ISO size tolerances interact with ASME Y14.5 Rule #1 — at MMC the feature envelope must be perfect in form.