ISO 2768 and GD&T solve related but different drawing problems. ISO 2768 provides general tolerances for dimensions that do not have individual tolerances; GD&T defines the allowable form, orientation, location, or runout of features relative to specified datums. For CNC machined parts, use ISO 2768 for suitable non-critical dimensions and apply GD&T where assembly, function, or inspection depends on how features relate to one another. Many drawings need both, alongside explicit tolerances for critical features.
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At jinhui, we recommend choosing tolerances from the part’s function and inspection method rather than applying the tightest possible limits everywhere. A clear drawing helps us assess manufacturability, machining, and measurement requirements before production. This guide compares the two systems and offers a practical way to specify them.
ISO 2768-1 covers general tolerances for linear and angular dimensions without individual tolerance indications. A drawing must identify the applicable tolerance class for those defaults to be interpreted consistently. ISO 2768-1 uses classes such as fine (f), medium (m), coarse (c), and very coarse (v), with the permitted variation depending on the dimension range and class.
For example, under the ISO 2768-1 medium class, a linear dimension greater than 30 mm and up to 120 mm has a general tolerance of ±0.3 mm. For a dimension greater than 6 mm and up to 30 mm, the medium-class value is ±0.2 mm. These examples apply only when the drawing invokes the relevant standard and class; they do not override a separately specified tolerance or automatically define geometric relationships.
GD&T is a system for specifying permissible geometric variation using symbols, tolerance zones, and, where needed, datum references. The applicable conventions depend on the standard named on the drawing, such as ASME Y14.5 or the relevant ISO GPS standards. The two systems are not interchangeable, so a drawing should not leave the governing standard ambiguous.
A position control, for example, can define the allowable location of a hole relative to datum features. A perpendicularity control can limit the orientation of a face relative to a datum plane. These controls communicate relationships that a simple plus/minus dimension may not adequately describe.
| Consideration | ISO 2768 | GD&T |
|---|---|---|
| Primary purpose | Provides defaults for eligible dimensions without individual tolerances. | Controls the geometry of features, often in relation to datums. |
| Typical use | Non-critical linear or angular dimensions when the drawing specifies a class. | Feature location, orientation, form, or runout that affects function or assembly. |
| Datum requirement | Does not, by itself, establish a datum reference framework. | Uses datums when the control needs a functional reference. |
| Drawing interpretation | Requires the standard and tolerance class to be identified. | Requires the applicable GD&T standard, symbols, and datum scheme to be clear. |
| Inspection planning | Often assessed by measuring the stated dimension against its tolerance. | May require inspection relative to datums using suitable methods and equipment. |
General tolerances can keep drawings concise when many dimensions do not require individually assigned limits. They may suit external sizes, non-mating edges, and other features whose variation will not compromise the part’s intended function. The correct class still depends on the design, process capability, and acceptance requirements agreed for the project.
Do not assume that a general tolerance is appropriate for every feature. A dimension tied to a press fit, sealing surface, or precise alignment may need its own tolerance, a geometric control, or both. Also check which features are excluded or treated differently by the drawing standard and the customer’s specifications.
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GD&T is often the clearer choice when the part must assemble with other components in a defined way. Examples include a hole pattern located from mounting datums, a bore axis aligned to a reference face, or a sealing face controlled for flatness. The control should express a real functional requirement, not simply add symbols without a clear inspection or assembly purpose.
For a CNC-machined bracket, for instance, a general tolerance may be suitable for its overall non-critical length, while position controls can define mounting-hole locations from selected datum surfaces. For a shaft, individual size tolerances may control a bearing journal, while a runout requirement may be needed for a surface that rotates relative to a datum axis. The exact callouts depend on the part’s design and the governing standard.
Tighter or more complex requirements can affect machining strategy, setup, tool selection, inspection planning, and acceptance decisions. The effect depends on the geometry, material, quantity, machine process, and measurement method, so it cannot be reliably estimated from the tolerance notation alone. A broad blanket tolerance requirement can also create unnecessary effort on features that do not affect function.
To support a useful quotation, provide a complete drawing with the governing standards, units, material, quantity, surface-finish requirements, and clearly identified critical features. If a feature requires a specific inspection report or acceptance method, state that requirement during quotation rather than after production. This gives the supplier an opportunity to review feasibility and clarify open points before machining begins.
At jinhui, we review CNC machining drawings to understand dimensions, datums, material requirements, and the features that drive part function. Where a tolerance callout or reference is unclear, we can discuss the interpretation and request clarification before proceeding. This helps align the manufacturing plan with the drawing requirements and the buyer’s inspection expectations.
For a quotation, send the 2D drawing and, when available, the 3D model, along with quantity and any quality documentation or inspection needs. Highlighting critical-to-function features helps us focus the review on the requirements that matter most. We can then discuss whether the proposed tolerance scheme is clear and suitable for the intended CNC-machined part.
ISO 2768 and GD&T are complementary, not competing choices. ISO 2768 can provide general tolerances for eligible dimensions, while GD&T is the better way to communicate geometric relationships that affect assembly or function. For most CNC machined parts, a practical drawing uses explicit limits and geometric controls on critical features, then applies a stated general tolerance class to suitable remaining dimensions.
Before releasing your next drawing, confirm the governing standards, identify functional datums, and ensure each tight tolerance has a clear reason and inspection plan. If you are sourcing CNC machined parts, share the drawing with jinhui for a manufacturing review and quotation discussion.
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