{"id":29117,"date":"2026-05-29T18:51:51","date_gmt":"2026-05-29T18:51:51","guid":{"rendered":"https:\/\/leweiprecision.com\/"},"modified":"2026-05-30T19:03:51","modified_gmt":"2026-05-30T19:03:51","slug":"cnc-machining-tolerances-explained","status":"publish","type":"post","link":"https:\/\/leweiprecision.com\/pt\/cnc-machining-tolerances-explained\/","title":{"rendered":"CNC Machining Tolerances Explained: ISO 2768, GD&T, and What to Specify for Your Parts"},"content":{"rendered":"

Tolerance specification is one of the most consequential \u2014 and most misunderstood \u2014 aspects of engineering a CNC machined part. Specify too loosely and you get a part that does not assemble correctly. Specify too tightly and you pay significantly more for precision that provides no functional benefit. Getting tolerances right is a skill that comes from understanding what CNC machines can actually achieve and what different tolerance levels cost in practice.<\/p>\n\n\n\n

This guide covers the ISO 2768 standard that governs general machining tolerances, the GD&T feature controls used for geometric precision, the tolerance values achievable on different feature types, and the specific tolerance decisions that drive cost most significantly.<\/p>\n\n\n\n

ISO 2768 \u2014 The Foundation of General Machining Tolerances<\/strong><\/strong><\/h2>\n\n\n\n

ISO 2768 defines general geometric tolerances for machined parts \u2014 the default tolerances that apply to any dimension on a drawing that does not have a specific tolerance callout. It has four grades for linear dimensions:<\/p>\n\n\n\n

ISO 2768 GRADE<\/strong><\/td>SYMBOL<\/strong><\/td><\/tr>
Fine<\/strong><\/td>f<\/td><\/tr>
M\u00e9dio<\/strong><\/td>m<\/td><\/tr>
Coarse<\/strong><\/td>c<\/td><\/tr>
Very coarse<\/strong><\/td>v<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

ISO 2768-m is the standard default tolerance class used by the majority of M\u00e1quina CNC<\/a> shops for general precision work. When your drawing states ‘General tolerances per ISO 2768-m’ without individual callouts, every unmarked dimension is held to \u00b10.10mm. This is the right default for most functional machined parts. Only call out tighter tolerances where function genuinely requires them.<\/p>\n\n\n\n

Achievable Tolerances by Feature Type<\/strong><\/strong><\/h2>\n\n\n\n
FEATURE TYPE<\/strong><\/td>STANDARD ACHIEVABLE<\/strong><\/td><\/tr>
Milled flat surface<\/a><\/strong><\/td>\u00b10,05mm<\/td><\/tr>
Drilled hole diameter<\/strong><\/td>\u00b10,1mm<\/td><\/tr>
Bored hole (precision)<\/strong><\/td>\u00b10,025mm<\/td><\/tr>
Milled slot width<\/strong><\/td>\u00b10,05mm<\/td><\/tr>
Thread pitch diameter<\/strong><\/td>\u00b10.05mm (class 6H)<\/td><\/tr>
Surface parallelism<\/strong><\/td>0.05mm\/100mm<\/td><\/tr>
Surface flatness<\/strong><\/td>0.05mm<\/td><\/tr>
Positional accuracy (hole pattern)<\/strong><\/td>\u00b10,05mm<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

GD&T Feature Controls \u2014 When to Use Them<\/strong><\/strong><\/h2>\n\n\n\n

GD&T (Geometric Dimensioning and Tolerancing) is the system for specifying geometric precision \u2014 flatness, perpendicularity, parallelism, concentricity, runout, position \u2014 in addition to dimensional tolerances. GD&T controls<\/a> the shape and relationship of features, not just their size.<\/p>\n\n\n\n

Key GD&T controls relevant to CNC machined parts<\/a>:<\/p>\n\n\n\n