CNC Tolerances Reference Table

ISO 2768-1 sets general linear tolerances for machined parts by size range and class (fine f, medium m, coarse c). This table gives the full standard range from 0.5mm up to 4000mm, plus the tolerance a capable shop can actually hold for common materials. Apply fine (f) for machined metals and medium (m) as the general default.

How ISO 2768-1 general tolerances work

General tolerances apply to every linear dimension on the drawing that does not carry its own tolerance. The standard sorts dimensions into size ranges (0.5 to 3mm up to 2000 to 4000mm) and into four classes of precision: fine (f), medium (m), coarse (c), and very coarse (v). The table lists f, m, and c; very coarse is left out because it rarely applies to machined work.

Bands widen with size; class declared once

Each band widens as the nominal size grows, because larger features pick up more variation from machine travel, thermal growth, and setup error. The class is declared once in the drawing title block, for example as ISO 2768-mK, and then governs every untoleranced dimension at once.

Choosing a tolerance class

Picking the class by function

Pick the class by how the feature functions, not by a blanket precision target. Fine (f), such as ±0.05mm on a 0.5 to 3mm feature, suits mating and locating features on machined metals where fit matters. Medium (m), such as ±0.10mm on the same feature, is the general default and covers most non-critical dimensions. Coarse (c) fits non-critical features where looser bands save cost. Tightening every dimension to fine roughly doubles inspection and cycle effort, so reserve fine for the features that affect fit, seal, or function and leave the rest at medium.

A worked example shows the trade. A 50mm shaft bore on a bracket falls in the 30 to 120mm row, where fine is ±0.15mm, medium ±0.30mm, and coarse ±0.80mm. If the bore only locates a bushing that is itself toleranced, medium is plenty. If the bore directly seats a bearing, call out fine, or write the specific value (for example ±0.02mm) on the feature and let medium govern the rest of the part. The cost difference between medium and fine on that one feature is small; applying fine to every dimension on the part is where cost climbs.

What a shop can actually hold

Achievable precision by material

The ISO class is a default, not a ceiling. A capable shop can hold tighter than the general band on the right setup, and what it can achieve depends strongly on the material. Aluminum 6061, carbon steel 1018 and 1045, and free-machining brass C360 reach about ±0.025mm with good practice because they machine cleanly. Stainless 304 and titanium Ti-6Al-4V run about ±0.05mm because stainless work-hardens and titanium builds heat at the tool. Engineering plastics such as PEEK and Delrin also land near ±0.05mm, with some swelling risk. These material-specific numbers are the practical precision a feature can reach; they are separate from the general class the title block declares.

Feature type also sets the achievable band. A bored hole reams to ±0.025mm more readily than a deep pocket, a thin wall, or a long threaded bore, which all flex or deflect under cut. Geometry matters as much as material: a rigid, well-supported feature holds tighter than a delicate one in the same metal.

Putting tolerances on the drawing

State the general-tolerance class in the title block, for example ISO 2768-mK, where the letter pair names the linear and geometrical classes. Then tolerance only the features that need a specific value, and let the general class govern the rest.

Geometrical control and the rest of the drawing

For geometrical control such as flatness, perpendicularity, or runout, call out ISO 2768-2 classes (H, K, L) or ASME Y14.5 GD&T directly on the feature, because ISO 2768-1 covers size and angle, not form or orientation. Also list the material, process, and surface finish on the drawing so a shop can quote and plan against the right class and achievable precision.

Limitations

The table values are the ISO 2768-1 standard classes, not a capability promise for a specific machine or shop. Real achievable precision moves with machine class, fixture rigidity, part geometry, lot size, and shop practice, and a capable shop can often beat the general band on suited features. Always confirm the class and the material-specific achievable tolerance with the shop and against the governing standard (ISO 2768-1 and ISO 2768-2) before locking a specification.