CNC Machining IT tolerance grade: A Complete Classification Guide

Precision is the cornerstone of CNC machining. Whether you're producing aerospace components or agricultural equipment, selecting the right accuracy grade directly affects part performance, cost, and lead time. This guide breaks down the three major classification systems engineers rely on.

IT Tolerance Grade System (ISO Standard)

The internationally recognized IT tolerance system spans 20 grades from IT01 to IT18. The lower the number, the tighter the tolerance. Each tier corresponds to specific manufacturing processes and industries. The following are the commonly used tolerances in CNC Machining.

Tip: IT7 is the industry's most common  precision— achievable by precision turning or milling without specialized equipment, and suitable for most mechanical fits and assemblies.

Machine Tool Performance Grades (GB/T 25372-2010)

China's national standard classifies machining centers into three performance grades based on positioning and repeat positioning accuracy.

Ultra-precision centers push positioning accuracy to 0.002–0.004 mm, while 5-axis multi-axis machines enable complex surface machining at a stable IT7 level.

Choosing the Right Precision Grade

The optimal accuracy grade balances functional requirements, assembly tolerances, and production cost. Aerospace fasteners demand IT5-grade high-precision machines; automotive gearbox gears fit within IT6–IT7; general structural brackets need nothing beyond IT9.

Excessive pursuit of precision can increase costs and delivery lead times. On the other hand, insufficient precision can render the parts unusable. Therefore, achieving the most suitable precision is crucial. Feel free to contact LVMA, and we will analyze your part design to help you determine the most appropriate precision.