ISO 2768 vs GD&T for CNC Machined Parts: How to Choose
ISO 2768 and GD&T solve different drawing problems. ISO 2768 provides general tolerances for dimensions that do not have individual tolerances, while GD&T uses geometric symbols and controls to define how features relate in form, orientation, location, or runout. For CNC machined parts, use ISO 2768 for straightforward, non-critical dimensions and GD&T when function depends on feature relationships, such as a hole pattern aligned to a mounting face. Many drawings use both: GD&T for critical interfaces and an explicitly stated general-tolerance standard for the remaining dimensions.
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At jinhui, we recommend choosing controls from the part’s function and inspection needs rather than applying the tightest tolerance everywhere. The drawing should identify the governing standard and edition, define critical datums where needed, and make clear which dimensions are individually controlled. This helps us review manufacturability and inspection requirements before production.
Quick Summary: The Main Difference
- ISO 2768: A general-tolerance system for specified dimensions without individual tolerances; the applicable tolerance class and drawing notes determine its use.
- GD&T: A method of controlling geometric characteristics and relationships between features, typically through feature control frames and datums.
- Best practice: Apply each method where it adds clarity. Use GD&T for functional relationships and ISO 2768 for suitable untoleranced dimensions.
- Before release: Confirm the standard, units, datum scheme, critical characteristics, and inspection approach with your supplier.
What ISO 2768 and GD&T Control
ISO 2768 general tolerances
ISO 2768 is commonly called out on engineering drawings to establish general tolerances for dimensions that do not carry individual tolerances. Its tolerance class and relevant part of the standard determine which limits apply; the result depends on factors such as the nominal dimension range and the type of characteristic. A drawing note should therefore state the intended standard and class clearly rather than simply saying “standard tolerance.”
General tolerances are useful when a part has many non-critical dimensions and the designer wants to avoid assigning a separate limit to each one. They do not automatically express the functional relationship between a hole, a face, and another feature. Nor should a general tolerance be assumed to apply to every characteristic: the drawing must make its scope clear, and individually toleranced features need to be read according to their specific callouts.
GD&T geometric controls
GD&T communicates allowable variation in a feature’s geometry or relationship to other features. A feature control frame can specify a geometric characteristic and, when appropriate, reference datums that establish how the part is oriented or located for measurement. Common applications include controlling the position of a hole pattern, the flatness of a sealing surface, or the perpendicularity of a machined face.
GD&T is not one universal notation independent of standards. Drawings should identify the applicable system, such as the relevant ISO GPS standard or ASME Y14.5, and avoid mixing conventions without explanation. The datum scheme and inspection method should match the way the part functions and is assembled.
ISO 2768 vs GD&T: Key Differences
| Comparison point | ISO 2768 | GD&T |
|---|---|---|
| Primary purpose | Sets general tolerances for applicable dimensions without individual limits. | Controls geometric characteristics and, where defined, feature relationships. |
| Drawing communication | Usually stated as a general note with a tolerance class. | Uses geometric symbols, feature control frames, and sometimes datum references. |
| Functional relationships | Does not, by itself, define a datum-based relationship between features. | Can clearly describe location, orientation, form, or runout requirements. |
| Typical inspection focus | Checking applicable dimensions against the general limits. | Checking a defined geometric characteristic using an appropriate setup or method. |
| Best suited to | Non-critical dimensions on drawings with many ordinary features. | Interfaces and features whose geometric relationship affects fit or function. |
Which Method Fits Your CNC Machined Part?
Use ISO 2768 for suitable non-critical dimensions
A general tolerance can keep a drawing readable when the exact limit of every edge-to-edge or overall dimension is not essential to assembly or performance. It may be appropriate for a simple cover, bracket, or spacer when the important interfaces have separate requirements and the remaining dimensions can follow the declared class. The engineering team should still verify that the selected class is achievable and adequate for the part’s use.
For example, a drawing may show a nominal length of 50 mm and rely on its stated general-tolerance note if that length is not a critical interface. The permitted variation cannot be determined from the nominal value alone; it depends on the applicable standard, tolerance class, and drawing requirements. This is why a supplier should not infer a class from a title block that omits it.
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Use GD&T for critical relationships
GD&T is often the clearer choice when a part must locate against other components, maintain a sealing surface, or keep several holes aligned to a functional datum. For instance, a hole pattern may need a position tolerance of ⌀0.2 mm relative to defined datums; this is an illustrative callout, not a recommendation for every design. The appropriate value depends on the mating parts, assembly clearance, manufacturing process, and inspection plan.
Geometric controls can also prevent ambiguity that separate coordinate dimensions may leave unresolved. A flatness requirement on a sealing face describes a form condition, while a perpendicularity control can relate a face to a datum. These controls are useful only when their purpose, datum references, and verification method are understood by both the design and manufacturing teams.
Cost, Lead Time, and Sourcing Considerations
Neither ISO 2768 nor GD&T is automatically the cheaper option. Cost is affected by the tolerance values, part geometry, material, quantity, setup, measurement method, and risk of rejection or rework. A blanket tight requirement can add machining and inspection effort to features that do not need it, while an unclear drawing can cause supplier questions and delay quoting.
For a more predictable quote, identify critical-to-function features and distinguish them from ordinary dimensions. Provide the drawing units, material and finish requirements, expected quantity, and any required inspection documentation. When a tolerance is especially important, explain the mating or performance requirement so the supplier can assess whether the proposed control and measurement approach are practical.
Common Drawing Mistakes to Avoid
- Leaving the general-tolerance class unspecified: State the applicable standard and class, including the relevant edition or drawing convention where needed.
- Using GD&T without functional datums: A geometric callout may be difficult to interpret or inspect if the datum references do not reflect how the part is located in use.
- Applying tight tolerances everywhere: Reserve demanding controls for features whose fit, function, or assembly depends on them.
- Mixing standards without clarification: Identify the governing geometric dimensioning system and avoid ambiguous or conflicting notes.
- Ignoring inspection feasibility: Discuss how critical features will be measured before production, especially when access or part setup affects the result.
How to Select the Right Approach
- Start with function. Identify which surfaces, holes, and dimensions determine fit, motion, sealing, alignment, or safety.
- Choose controls for those features. Use individual dimensional limits or GD&T where they communicate the requirement clearly; define datums when relationships matter.
- Set a suitable general tolerance. Apply the stated ISO 2768 class to eligible dimensions that do not need individual controls.
- Review inspection. Confirm that the supplier can access and verify the specified characteristics using an agreed measurement approach.
- Resolve ambiguity before ordering. Ask for a drawing review and clarify conflicting notes, missing classes, or unclear datum references before approving production.
How Jinhui Supports Drawing Review
When reviewing CNC machining drawings, we focus on the relationship between specified tolerances, part function, machining access, and inspection requirements. We can discuss which features appear critical, whether a note is clear enough to quote, and whether the requested controls need further clarification. Final acceptance criteria should remain defined by the customer’s released drawing and agreed inspection requirements.
To request a review or quote, send jinhui your 2D drawing or 3D model together with material, finish, quantity, and delivery requirements. Mark critical interfaces and note the applicable standards or any questions you want us to assess. This gives our team a practical basis to discuss manufacturability and sourcing before you place an order.
Conclusion: Use Both Where Each Adds Clarity
ISO 2768 and GD&T are complementary, not interchangeable. ISO 2768 can define general limits for eligible untoleranced dimensions, while GD&T is better suited to clearly controlling geometric form and relationships that matter to function. For many CNC machined parts, the most useful drawing combines a clearly stated general-tolerance class with targeted geometric controls on critical features.
Your next step is to identify the part’s functional interfaces, specify the governing standards and tolerance requirements, and review whether each critical characteristic can be inspected as intended. If you are unsure how to communicate a requirement, share the drawing with jinhui for a manufacturability discussion before production.