Tolerances & Standards

ISO Tolerance Grades (IT Grades)

The ISO 286 system classifies dimensional tolerances into grades IT01 to IT18. Lower numbers are tighter. For CNC-machined parts, common grades are:

Grade	Application
IT5–IT6	Precision fits, bearing journals, critical features (high cost)
IT7–IT8	General engineering fits, good CNC standard
IT9–IT10	General tolerances, typical machined dimensions
IT11–IT14	Coarse tolerances, non-critical features

What CNC Machining Can Achieve

• Standard machining: ±0.05 mm (general features)
• Good machining practice: ±0.02 mm
• Precision / 5-axis high-tolerance: ±0.005 mm on critical features
• Surface flatness / parallelism: from 0.02 mm depending on size

Related Standards

Standard	Scope
ISO 2768	General tolerances for linear and angular dimensions
ISO 286	ISO system of limits and fits
ISO 1302	Indication of surface texture on drawings
ASME Y14.5	Geometric Dimensioning and Tolerancing (GD&T)
ANSI/ASME B46.1	Surface texture measurement

Tip: Only tolerance what you need. A "free" ±0.05 mm costs nothing; tightening a feature to ±0.005 mm can raise cost through extra machining and inspection. Hold-Tech reviews every drawing for over-toleranced features and suggests practical specifications.