ISO 2768 vs. GD&T for CNC Machined Parts: How to Choose

For CNC machined parts, ISO 2768 is useful for assigning general tolerances to dimensions that lack individual tolerances, while GD&T defines how specific features relate to one another and to functional datums. They are not direct substitutes: a drawing can use general tolerances for noncritical dimensions and GD&T for features that control fit, alignment, or assembly. The right choice depends on function, inspection needs, and the standards named on the drawing. Before production, state the applicable standard and edition, tolerance class, units, and any critical feature requirements.

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Quick Comparison

ISO 2768 provides default tolerances for unspecified dimensions when the drawing invokes the relevant standard and class. GD&T uses defined symbols, datums, and tolerance zones to control features such as position, orientation, form, and runout. In practice, a clear CNC drawing often combines the two: general tolerances cover ordinary dimensions, and explicit geometric controls address functional relationships.

What ISO 2768 Covers

ISO 2768 is a standards series used on technical drawings to set general tolerances. ISO 2768-1 covers linear and angular dimensions for which individual tolerances are not indicated, and its tolerance classes include fine, medium, coarse, and very coarse. The class selected affects the permitted variation, so a drawing should identify the intended class rather than simply say “ISO 2768.”

For example, under the commonly referenced ISO 2768-1 medium class, a 50 mm linear dimension falls in the greater-than-30 mm to 120 mm range and has a general tolerance of ±0.3 mm. This is an example of a general dimensional tolerance, not a recommendation for every 50 mm feature. The applicable standard edition and drawing requirements should be checked, especially for existing designs or contracts that specify a particular revision.

General Tolerances Have a Defined Scope

A general tolerance does not automatically make every feature functionally acceptable. It applies only where the drawing invokes it and no more specific tolerance is given, subject to the scope of the referenced standard. A bore, locating hole, sealing face, or bearing seat may need a tighter dimensional tolerance or an explicit geometric requirement. For geometric tolerancing, verify which standard the drawing invokes; ISO 2768-2 is an older reference, and newer drawings may specify another GPS standard, such as ISO 22081.

What GD&T Adds to a CNC Drawing

GD&T, or Geometric Dimensioning and Tolerancing, communicates allowable variation in a feature’s form, orientation, location, or runout. It can define a tolerance zone in relation to datums, which are the reference features used to set up and inspect a part. On international drawings, geometric specification is commonly expressed through ISO GPS standards; ASME Y14.5 is another system used on drawings. These systems have their own conventions, so buyers and suppliers should not assume that standards are interchangeable.

Consider a plate with a mounting pattern. The hole diameters describe hole size, but they do not necessarily communicate how accurately the holes must be located relative to the plate’s mounting edges or datum faces. A position control referenced to a clear datum scheme can specify that relationship. For illustration, a drawing might specify a position tolerance of ⌀0.1 mm; that value is only an example, and the design must determine the actual requirement.

Why Datums and Inspection Method Matter

A geometric tolerance is only useful when the datum references reflect how the part functions or is inspected. If a drawing’s datum scheme conflicts with the assembly setup, a supplier may interpret the inspection differently from the design team. The feature control frame, datum order, material condition modifiers where applicable, and referenced standard should therefore be reviewed together. Clear definitions reduce avoidable questions during quotation, machining, and acceptance inspection.

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When to Use Each Approach

Use ISO 2768 for Noncritical, Unspecified Dimensions

ISO 2768 is appropriate when a design needs a consistent tolerance baseline without assigning a separate tolerance to every dimension. It can make drawings easier to read and can help avoid accidental ambiguity for routine dimensions. It is not a substitute for specifying dimensions that control assembly, movement, sealing, or safety. If a general class would permit more variation than a feature can tolerate, give that feature an individual requirement.

Use GD&T for Functional Relationships

Use GD&T when the part’s function depends on geometric relationships that simple size tolerances do not fully describe. Examples include hole patterns aligned to a datum, a face required to be perpendicular to a bore, or a rotating feature with controlled runout. It is especially useful when several features must be inspected from the same functional reference. The control and tolerance value should be selected from engineering requirements, not added merely to make a drawing appear more precise.

Cost, Lead Time, and Sourcing Considerations

Neither ISO 2768 nor GD&T automatically determines the part price or production lead time. Cost is affected by the actual tolerance values, material, geometry, batch size, machining sequence, setup requirements, and inspection plan. Tighter or more complex requirements may require additional process control or measurement, but the effect depends on the part and supplier’s equipment. Requesting unnecessary precision can add cost without improving the part’s function.

For a reliable quotation, send the complete drawing and identify critical-to-function features. Include the unit system, the governing standards and editions, general tolerance class, surface finish requirements, material and quantity, and any inspection or documentation expectations. If a feature requires a special measurement method or acceptance criterion, state it before production rather than relying on an unstated assumption.

Common Drawing and Sourcing Mistakes

• Using “ISO 2768” without a class: State the relevant part and tolerance class, and confirm the referenced edition.
• Applying a general tolerance to a critical feature: Add an explicit dimensional or geometric requirement where function demands it.
• Mixing standards without clarification: Identify whether the drawing uses ISO GPS or ASME Y14.5 conventions and avoid assuming equivalence.
• Specifying GD&T without functional datums: Choose references that match assembly, operation, or inspection intent.
• Over-tolerancing every feature: Reserve tight tolerances for requirements supported by design function and verification plans.

How to Choose: A Practical Framework

1. Identify the function. Mark features that locate, mate, seal, guide, rotate, or carry load.
2. Set critical dimensions individually. Define size limits where fit or clearance depends on feature size.
3. Add geometric controls where relationships matter. Use appropriate datums and controls for alignment, orientation, form, or location.
4. Apply a general tolerance to the remaining eligible dimensions. State the applicable ISO 2768 part, class, and edition, or the relevant alternative standard.
5. Agree on verification. Confirm inspection references, measurement approach, and acceptance criteria with the supplier before machining.

How Jinhui Supports Drawing Review

At jinhui, we review CNC machining drawings with attention to the relationship between specified tolerances, part function, and inspection requirements. If a requirement is unclear or appears to affect manufacturability, we can raise questions during quotation so that the buyer and production team can align before work begins. This review is not a replacement for design approval, but it can help identify missing tolerance classes, datum definitions, or acceptance details.

When requesting a quote, share the 2D drawing, 3D model if available, quantity, material, and any critical inspection requirements. We can discuss whether the drawing communicates the intended requirements clearly and identify points that may need confirmation. Contact jinhui with your part files and project details to begin a manufacturing review.

Summary and Recommendation

ISO 2768 is best used as a general tolerance framework for eligible dimensions that do not have individual tolerances. GD&T is the better tool when a part’s function depends on defined geometric relationships, datum references, or controlled feature location and orientation. For many CNC machined parts, the most practical drawing uses both: explicit dimensional and geometric controls for critical features, with an identified general tolerance for the rest.

Before release, check that the drawing names the applicable standards and editions, gives the ISO 2768 class where used, defines functional datums, and states inspection expectations for critical features. If you are unsure whether a tolerance is necessary or measurable, discuss the requirement with your design and manufacturing teams before placing the order. That step helps align the drawing, quotation, machining plan, and acceptance inspection.

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