Tolerances are one of the most important decisions in a CNC machining project. They define how much a finished part may vary from the nominal CAD or drawing dimension while still performing correctly. Tolerance choices influence function, inspection effort, lead time, scrap risk, and machining cost. A drawing that is too loose may not protect the assembly. A drawing that is too tight may force unnecessary precision, longer cycle times, more inspection, and higher cost.

For that reason, engineers rarely specify every feature one by one. Instead, most technical drawings use general tolerance standards for ordinary, unmarked dimensions and then add special callouts only where the function of the part requires tighter control. In CNC machining and other subtractive manufacturing processes, ISO 2768 and ISO 286 are two of the most common systems used to organize this work.

This guide explains how those standards are used, what kinds of features they cover, and how to think about them when preparing CNC drawings for production. For related engineering context, see DEBAOLONG’s CNC machining tolerance standards guide, CNC machining tolerances explained, and tolerance engineering handbook.

Why General Tolerance Standards Matter

Designers and engineers use tolerance notes to control angles, sizes, shapes, holes, shafts, radii, chamfers, and other manufactured features. Without a standard system, every non-critical feature would need its own tolerance note. That creates extra drawing work, makes drawings harder to read, and can lead to inconsistent manufacturing expectations.

General tolerance standards solve this problem. They provide a shared manufacturing language between design, machining, inspection, and procurement teams. When a drawing states a general standard, unmarked features can be interpreted consistently without adding a separate tolerance to every line, hole, radius, or angle.

The key point is that general tolerances are not a substitute for engineering judgment. They are a baseline. If a feature controls sealing, bearing fit, sliding motion, alignment, press fit, cosmetic location, or another functional requirement, that feature may still need an explicit tolerance or GD&T callout.

Common Standards Used for CNC Machining Tolerances

Different manufacturing processes often use different tolerance standards. For subtractive manufacturing processes such as CNC milling and turning, ISO 2768 and ISO 286 are widely used. The source article identifies five selectable tolerance levels within those systems:

• ISO 2768-f fine class
• ISO 2768-m medium class
• ISO 286 IT6
• ISO 286 IT7
• ISO 286 IT8

The main difference is application. ISO 2768 covers general tolerances for linear and angular dimensions when no individual tolerance is shown on the drawing. ISO 286 covers fit tolerances for cylindrical features and two parallel opposite surfaces, such as holes and shafts. If a part includes both ordinary unmarked dimensions and functional mating features, both standards can appear in the same drawing package.

ISO 2768: General Tolerances for Unmarked Drawing Dimensions

ISO 2768 and related geometrical tolerance standards are commonly used for machined and subtractively manufactured parts. The standard is especially useful when a drawing contains many ordinary dimensions that do not require a unique special tolerance.

ISO 2768 applies only where the drawing does not already specify a separate tolerance. If a feature has a special tolerance note, the special note controls that feature instead of the general tolerance block.

Typical ISO 2768 applications include:

• linear dimensions, including external dimensions, internal dimensions, diameters, radii, distances, and chamfer heights
• angular dimensions
• linear and angular dimensions on machined assemblies where no special tolerance is otherwise stated

ISO 2768 Linear Dimensions

The table below preserves the source article’s useful ISO 2768 linear-dimension ranges for fine and medium classes. These values help designers understand how tolerance bands change as nominal size increases.

ISO 2768 Radii and Chamfer Heights

Radii and chamfer heights are often used for edge breaks, corner relief, assembly clearance, deburring, and safer handling. When they are not function-critical, ISO 2768 can provide a general tolerance range.

ISO 2768 Angular Dimensions

Angular tolerance requirements depend on the length of the relevant feature. The source article gives the following fine and medium class angular ranges.

ISO 286: Fit Tolerances for Holes, Shafts, and Parallel Surfaces

ISO 286 is used for linear size tolerances related to fit conditions. In practice, engineers often use it for cylindrical features, holes, shafts, pins, bushings, and two parallel opposite surfaces where functional assembly matters.

Like ISO 2768, ISO 286 applies to relevant unmarked features only when the drawing has not already specified a separate tolerance. The source article focuses on IT6, IT7, and IT8. Lower IT numbers represent tighter tolerance bands and generally require more careful machining and inspection.

Useful ISO 286 terms include:

• Nominal size: the design size defined by the drawing.
• Actual size: the measured size of the manufactured feature.
• Upper limit size: the maximum permitted feature size.
• Lower limit size: the minimum permitted feature size.
• Tolerance: the difference between the upper and lower limits.

ISO 286 IT6, IT7, and IT8 Deviation Bands

The following table keeps the source article’s tolerance-band data for IT6, IT7, and IT8. Values are shown in millimeters and should be interpreted with the correct fit system, drawing requirement, material behavior, machining process, and inspection method.

Corresponding ASME Fit Classes

ISO standards are common internationally, while many US engineering teams also use ASME standards. The source article notes the following relationship for ISO 286 fit grades and ASME B4.1 grades.

How to Choose the Right Tolerance Level

A practical tolerance strategy starts with the part function. General tolerance classes are suitable for ordinary geometry that does not drive assembly performance. Functional fits, bearing surfaces, precision alignment features, press-fit areas, sliding components, sealed interfaces, and critical inspection datums need closer review.

For CNC machining, a good drawing usually separates three categories of features. First, normal unmarked features can follow the general tolerance note. Second, functional features receive explicit dimensional tolerances, fit classes, or GD&T controls. Third, cosmetic or non-functional features are kept realistic so the project does not pay for precision that adds no value.

When in doubt, discuss tolerance intent before machining. A manufacturing partner can often suggest where tighter control is necessary, where a general tolerance is enough, and where a design change would reduce cost without weakening function.

Conclusion

ISO 2768 and ISO 286 help engineers communicate CNC machining tolerance requirements without overloading every drawing with individual notes. ISO 2768 is useful for general unmarked dimensions, angles, radii, and chamfers. ISO 286 is better suited to holes, shafts, and fit-related features where assembly performance depends on controlled deviation bands.

For production-ready parts, the best approach is not simply to choose the tightest available class. The best approach is to match tolerance control to function, inspection method, material behavior, and cost target. DEBAOLONG supports CNC machining, inspection planning, DFM review, and drawing-based manufacturing support for custom precision parts.

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