This article is part of the CNC Tolerances & Quality Control Guide: GD&T, CMM, Cpk, PPAP on CNX Precision.
ISO 2768 tolerances give engineers a fast way to define acceptable variation without covering a drawing in individual notes. When the title block calls out ISO 2768, every dimension that lacks its own tolerance automatically falls under a general tolerance class. This keeps drawings clean, speeds up quoting, and gives CNC machinists a clear acceptance limit for every feature. The standard splits into linear and angular classes on one side and geometrical classes on the other. This guide explains what each class means, which defaults shops rely on, when to pick fine over medium or coarse, and how the choice affects cost and lead time.
What the ISO 2768 Classes Mean
ISO 2768 has two parts. Part 1 covers linear and angular dimensions and defines four classes: f (fine), m (medium), c (coarse), and v (very coarse). Machined parts almost always use f, m, or c. Each class sets a permitted deviation that grows as the nominal size grows. A length between 6 mm and 30 mm, for example, may vary by ±0.1 mm under class f but ±0.2 mm under class m. The same dimension at class c would allow ±0.5 mm. One letter in the title block changes the acceptance window for every open dimension on the page.
Part 2 covers geometrical tolerances and defines classes H, K, and L. These limit flatness, straightness, perpendicularity, symmetry, and related form errors for features that carry no individual geometrical callout. The two parts are normally combined into a single note. ISO 2768-mK pairs medium linear tolerances with class K geometrical tolerances, while mH pairs medium linear tolerances with the tighter H geometrical class. If a drawing shows only ISO 2768-m, only Part 1 applies, and geometrical errors fall back to whatever individual callouts exist. The combined note tells the shop both halves of the story in four characters.
Typical Defaults on CNC Drawings
Most CNC shops default to ISO 2768-mK or plain ISO 2768-m for machined parts. Medium class matches what standard milling and turning hold comfortably in a normal production setup, so it adds little or no cost. Fine class f appears on jigs, fixtures, and parts with many mating features. Coarse class c shows up on weldments, flame-cut plates, and parts that get machined only on critical faces. Angular dimensions follow the same class logic, with the permitted deviation expressed over the length of the shorter side of the angle.
ISO 2768 tolerances govern only dimensions that carry no individual tolerance. If a bore is called out at 10 H7, the general class does not touch it, and a position or flatness frame overrides the Part 2 default for that feature. Individual callouts always win. Placement matters too. The note belongs in the title block or in the general notes where it cannot be missed. A drawing that states no class at all forces the shop to ask questions before quoting, which delays the one thing every buyer wants first: the price.
When you send RFQs to CNX Precision, state which ISO 2768 tolerances apply, either in the title block or in a drawing note. If you are unsure, mK is a safe default for machined parts. Our engineers review every drawing before production and flag any feature that needs a tighter individual tolerance, so nothing important hides behind the general class.
When to Choose Class f, m, or c
Start with class m for the body of the part and reserve tighter control for the features that truly need it. An individual ±0.02 mm note on one critical width costs far less than tightening ISO 2768 tolerances across the entire drawing. The general class should describe what the whole part can live with, not the tightest thing on it. This approach keeps inspection quick and keeps the price where it belongs.
Choose class f when many untoleranced dimensions interact in an assembly, when mating edges must align, or when the part functions as a reference for other parts. Class f still suits normal machining, but it demands closer attention to setup, tool wear, and in-process checks. Choose class c for features that never mate with anything: overall stock sizes, non-critical widths, and dimensions that exist only to remove material. Fine tolerances across a whole drawing raise cost without improving function.
Geometrical pairings follow the same logic. mK covers most parts, while mH adds tighter flatness and symmetry control where plates must sit flush or slots must stay centered. If a feature needs coaxiality, position, or runout control, add a GD&T callout instead of tightening the general class. General tolerances are a floor, not a design tool.
How General Tolerance Classes Affect Cost and Capability
Tighter general classes raise price in three ways. They slow machining, because operators check dimensions more often and may take lighter finishing cuts. They increase inspection time, because more dimensions move into the formal report instead of sitting as reference values. They also raise scrap risk. A part that would pass at ±0.2 mm can fail at ±0.1 mm even though it functions perfectly in the assembly.
Capability matters as much as cost. A three-axis mill holding ±0.1 mm on aluminum all day is routine work. Holding that same band on a thin-walled housing, a slender shaft, or a material that moves with temperature is not. Part geometry and material often decide which class is realistic, which is why ISO 2768 tolerances work best as defaults while critical features get their own callouts. The shop can then concentrate process control where it counts instead of inspecting every dimension to a fine limit.
Common Misunderstandings
The most common mistake is assuming ISO 2768 tolerances cover everything on the drawing. They do not. Surface roughness belongs to ISO 1302 and related standards, threads follow their own tolerance system, and any dimension with an individual tolerance or a geometric frame ignores the general class entirely.
A second confusion involves the class letters. Lowercase f, m, and c refer to Part 1 linear and angular tolerances, while uppercase H, K, and L refer to Part 2 geometrical tolerances. Notes like mH and mK are combinations, not typos, and they mean different things. A third issue is scope. The standard applies to the machined dimensions of the finished part, not to raw bar or plate as delivered. Finally, some buyers treat the general class as a substitute for GD&T on critical features. Datum-based controls such as position and runout communicate far more than a general tolerance ever can.
ISO 2768 Tolerances: Frequently Asked Questions
Does ISO 2768 override a directly toleranced dimension?
No. Individual tolerances and geometric callouts always take priority over the general class. The standard applies only to dimensions and features that carry no specific requirement of their own. If you need tight control somewhere, put the requirement directly on that feature.
What is the difference between ISO 2768-m and ISO 2768-mK?
ISO 2768-m sets only linear and angular general tolerances at the medium class. ISO 2768-mK adds the Part 2 geometrical classes at K, which controls flatness, perpendicularity, symmetry, and related errors. If your part has flatness or symmetry concerns, specify the combined note.
Can ISO 2768 replace GD&T on critical features?
No. General tolerances act as a background safety net, not as functional control. Bores that must align, faces that must sit square, and shafts that must rotate true need explicit GD&T callouts with datums. Use the general class for everything else.
For related information, see our guide to cnc machining tolerances, and iso 9001 machining.