{"id":2834,"date":"2026-06-23T13:50:43","date_gmt":"2026-06-23T05:50:43","guid":{"rendered":"https:\/\/www.cncmaven.com\/?p=2834"},"modified":"2026-06-23T13:50:43","modified_gmt":"2026-06-23T05:50:43","slug":"iso-2768-cnc-machining-tolerances","status":"publish","type":"post","link":"https:\/\/www.cncmaven.com\/de\/blog\/iso-2768-cnc-machining-tolerances\/","title":{"rendered":"What ISO 2768 Means for CNC Machining Tolerances"},"content":{"rendered":"

ISO 2768 is a useful shortcut for CNC machining drawings because it defines general tolerances for dimensions that do not have individual tolerance callouts. Used well, it keeps drawings clean and gives suppliers a shared baseline. Used carelessly, it can hide functional requirements, create quote ambiguity, or make buyers think every feature is controlled more tightly than it really is.<\/p>\n

This guide explains what ISO 2768 means in a CNC sourcing context, when it is enough, and when a drawing still needs specific tolerances or GD&T.<\/p>\n

What ISO 2768 does on a CNC drawing<\/h2>\n

ISO 2768 is commonly used as a title-block or drawing-note standard for general tolerances. In practical terms, it tells the manufacturer how to handle dimensions that do not have their own tolerance. ISO 2768-1 covers general tolerances for linear and angular dimensions. ISO 2768-2 covers certain geometrical tolerances such as straightness, flatness, perpendicularity, symmetry, and circular runout under defined classes.<\/p>\n

The important point is that ISO 2768 does not replace engineering judgment. It gives a default tolerance system. It does not tell the supplier which hole controls assembly, which face seals fluid, which bore receives a bearing, or which surface must stay flat after anodizing, powder coating, passivation, or heat treatment.<\/p>\n

\"CMM
For critical dimensions, a drawing should define the tolerance and inspection method instead of relying only on a general tolerance note.<\/figcaption><\/figure>\n

Common tolerance classes and how buyers should think about them<\/h2>\n

For linear and angular dimensions, ISO 2768 uses classes such as fine, medium, coarse, and very coarse. Many CNC drawings use a medium class for general-purpose machined parts, then add tighter callouts only where function requires them. This keeps cost under control because the supplier does not need to inspect and machine every non-critical edge as if it were a bearing fit.<\/p>\n\n \n
Class mindset<\/th>Typical use<\/th>Buyer caution<\/th><\/tr><\/thead>\n
Fine<\/td>Small precision features where general dimensions need closer control.<\/td>Do not use broadly unless cost and inspection effort are justified.<\/td><\/tr>\n
Medium<\/td>Common default for many CNC machined parts.<\/td>Still add specific tolerances for critical fits, datums, and sealing surfaces.<\/td><\/tr>\n
Coarse<\/td>Large, non-critical, fabricated, or clearance-based features.<\/td>Not suitable for alignment or precision assembly features.<\/td><\/tr>\n
Very coarse<\/td>Forgiving structures or non-critical dimensions.<\/td>Rarely appropriate for precision CNC interfaces.<\/td><\/tr>\n <\/tbody>\n<\/table>\n

When ISO 2768 is enough<\/h2>\n

ISO 2768 is usually enough for non-critical outside profiles, clearance features, general block dimensions, relief cuts, chamfers, visual edges, and features that do not control assembly or performance. It is useful when the 3D model defines shape and the drawing only needs a clean default for everything that is not function-critical.<\/p>\n

For example, a CNC enclosure might use ISO 2768-m for general length, width, and pocket dimensions, while the connector mounting holes, gasket face, and dowel holes receive explicit tolerances. That drawing is easier to quote than one with tight blanket tolerances on every dimension.<\/p>\n

When a specific tolerance or GD&T callout is needed<\/h2>\n

Add a specific tolerance when the feature controls fit, movement, sealing, alignment, interchangeability, or inspection acceptance. Add GD&T when orientation, position, flatness, perpendicularity, or datum relationship matters more than a simple plus\/minus size tolerance. ISO 2768 can support the drawing, but it should not carry the burden of functional design.<\/p>\n\n \n
Feature or requirement<\/th>Use ISO 2768 only?<\/th>Better drawing approach<\/th><\/tr><\/thead>\n
Bearing bore<\/td>No<\/td>Specify bore tolerance, surface finish, roundness if needed, and datum relationship.<\/td><\/tr>\n
Dowel hole pattern<\/td>No<\/td>Use position tolerance relative to datums or clear hole-to-hole tolerances.<\/td><\/tr>\n
Cosmetic chamfer<\/td>Often yes<\/td>Use general tolerance unless the chamfer controls assembly.<\/td><\/tr>\n
Gasket surface<\/td>No<\/td>Define flatness, surface finish, and whether requirements apply after finish.<\/td><\/tr>\n
Overall non-critical length<\/td>Often yes<\/td>General ISO 2768 note is usually sufficient.<\/td><\/tr>\n <\/tbody>\n<\/table>\n

Drawing checklist before sending a CNC RFQ<\/h2>\n