ISO 2768 General Tolerances in a CNC RFQ: When and How to Specify Them

A mechanical drawing can contain many dimensions that do not need a separate ± value. General tolerances provide a default for dimensions without an individual indication and keep the drawing readable. ISO 2768-1:1989 covers untoleranced linear and angular dimensions in four classes: f, m, c and v. It is intended for parts made by material removal or formed sheet metal and uses nominal-size ranges. A note such as ISO 2768-m does not control every drawing requirement. Position, profile, runout, datum structure, threads, fits, theoretically exact dimensions and features governed by other standards still need the appropriate specification. The RFQ should state the edition and scope.

The class names fine, medium, coarse and very coarse should not be selected simply because one sounds more precise. The class must fit function, process capability, nominal size, stock-making method and control cost. Applying a fine class to the complete drawing may turn non-functional surfaces into costly machining and inspection characteristics. A class that is too broad may fail to protect an assembly. Put individual tolerances on locating faces, fits, hole patterns, seals and safety-related features; reserve the general note for suitable secondary dimensions.

ISO 2768-1 addresses untoleranced linear and angular sizes. It does not establish datums, create a position zone for a hole pattern or replace profile control for a functional 3D surface. Older drawings often show ISO 2768-mK, combining dimensional and general geometrical rules. ISO 2768-2:1989 has been withdrawn, while ISO 22081:2021 provides the current framework for general geometrical and general size specifications. Do not copy an mK note into a new design without reviewing the governing GPS rules and customer contract.

As checked on 14 October 2026, ISO lists ISO 2768-1:1989 as the published and current edition confirmed in 2022, while ISO 2768 Edition 2 dated 2026-10 remains at stage 60.00, under publication. During this transition, writing ISO 2768 without an edition can lead the buyer and supplier to different documents. New drawings should identify the contract edition; old drawings should not be silently upgraded. VietTech will request clarification when the note does not establish the version or scope.

Functional items should not be left to a general note: dowel or bearing bores, locating surfaces, hole patterns, seal thickness, motion clearances, threads, optical faces, post-plating sizes and post-heat-treatment geometry. Basic dimensions, reference dimensions and features governed by a specialist standard also require correct interpretation. For model-based definition, confirm which semantic PMI or document controls. Clear specific requirements allow the process and measurement method to be chosen before quotation.

State the full standard reference, part or edition, class, unit and scope. Attach the 2D PDF and STEP model with the same revision, list functional features with individual tolerances, and define inspection records. If a legacy drawing uses ISO 2768-mK, state whether it remains contractual, is being converted or requires an RFI. Do not ask each supplier to choose the class. When the general note changes, release a new revision and describe the impact. VietTech reviews machining and inspection against the controlled package, and quotation assumptions must be confirmed before order.

• 2D PDF and STEP at the same revision
• Standard, part/edition and tolerance class
• Functional tolerances and GD&T
• Priority rule for 2D, 3D and PO
• Condition after heat treatment or coating
• Reports, quantity and delivery

This article uses the official ISO pages for ISO 2768-1:1989, the ISO 2768 Edition 2 project under publication in October 2026, and ISO 22081:2021. It does not reproduce copyrighted tolerance tables; designers and buyers should use their organization's licensed standards. Status may change after the publication date and should be rechecked before drawing release. The article is RFQ guidance, not a substitute for design authority, contract interpretation or metrology advice.