ISO 2768 on CNC Drawings: Where General Tolerances Stop

ISO 2768 for CNC machining drawings is a way to assign general tolerances to dimensions that do not have individual limits. It can make a drawing easier to read, but it does not replace functional fits, GD&T, material definition, surface finish, or a supplier’s project-specific review. The useful question is not whether ISO 2768 appears in the title block; it is whether the selected class matches the non-critical dimensions and leaves critical features explicitly controlled.

What ISO 2768 Can-and Cannot-Do

General tolerance review

It provides a general framework for unspecified linear, angular, and geometric dimensions.
It does not automatically define bearing fits, hole patterns, sealing faces, threads, or assembly-critical relationships.
The drawing must state the applicable class and units clearly.
Critical dimensions should override the general note with their own requirement.

Understand the scope

Tolerance scope review

A general tolerance note applies only where the drawing does not provide a more specific requirement. It should be read together with the drawing’s units, decimal conventions, title-block notes, material callout, finish, and any geometric controls. A 3D model alone cannot reveal those defaults reliably.

The selected ISO 2768 designation also has a defined scope. Do not assume that one general note controls every geometric characteristic or every feature in the same way. If a feature affects fit, sealing, motion, alignment, or safety, identify the requirement directly.

Select a practical class

CNC tolerance class comparison

A finer class may reduce variation on ordinary dimensions, but it can also add machining, inspection, and quotation risk. A coarser class may be suitable for clearance geometry or nonfunctional outside features. The class should follow the product function and process capability, not a blanket preference for “precision.”

Feature role	General-note approach	Better drawing action
Non-critical edge	General tolerance may be adequate	Confirm it cannot affect assembly or safety
Bearing or locating feature	Do not rely on the default alone	Specify size, fit, datum, form, and surface condition
Cosmetic profile	General note may not describe appearance	Add profile, edge, and visual acceptance criteria

Separate exceptions from defaults

The drawing should make exceptions easy to find. Use individual limits for holes, shafts, slots, threads, sealing surfaces, critical thicknesses, and mating faces. Use GD&T when the risk concerns position, orientation, form, or profile rather than size alone. This prevents a supplier and inspector from applying a general note to the wrong feature.

For a wider explanation of how functional requirements should be separated from general dimensions, see the site’s CNC machining tolerances guide.

Protect critical features

Critical feature inspection

ISO 2768 does not tell a supplier how a coated bore should be measured, which datum controls a hole pattern, or whether a flat face must remain suitable for sealing. Those are project requirements. Include final surface state, material condition, edge treatment, inspection timing, and any functional test when they affect acceptance.

Highlight features that locate, seal, rotate, slide, or carry load.
Define the final condition after heat treatment, plating, anodizing, or polishing.
Link position and orientation controls to a usable datum reference frame.
State whether the supplier should provide a dimensional, CMM, or functional report.

Prepare the supplier review

Supplier drawing review

Before an RFQ, check the title block, revision, units, general tolerance note, material grade, finish, critical callouts, and inspection requirements as one package. Ask the supplier to flag a general tolerance that is tighter than the function requires or a feature whose requirement cannot be reproduced by the proposed inspection method. Process context is available on the site’s CNC machining capabilities page, but the controlled drawing remains the acceptance reference.

ISO 2768 FAQs

Does ISO 2768 apply to every CNC dimension?

It applies only within the scope stated by the drawing and only where no more specific requirement overrides it. Critical features should have their own limits or geometric controls.

Can ISO 2768 replace GD&T?

No. A general dimensional tolerance does not fully describe position, orientation, form, or profile relationships that may control assembly.

Should a bearing bore use only a general tolerance?

Usually not. Define the fit, size limits, axis relationship, surface condition, datum, and final inspection state according to the bearing and housing function.

What is the risk of choosing a very fine class?

It may increase machining and inspection effort for features that do not benefit from the tighter default. Apply precision where the product needs it.