How to specify ISO 2768 General Tolerances on a drawing
ISO 2768 General Tolerances is a standard that sets the tolerance for dimensions that have no individual tolerance on the drawing. It is used for keeping a drawing readable, by not dimensioning every non critical size. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, oil and gas is used as the example: materials that work harden and still have to be certified.
What belongs on the drawing
A clear drawing for ISO 2768 General Tolerances states the function, the tolerance and the verification, and leaves the route open where the function allows it.
- The feature and its tolerance: ±0.2 mm for medium over 6 to 30 mm, applied to the features that locate the part.
- The material and its condition: any, with the certificate requirement.
- The surface finish: not specified by this standard, called out only where it does something.
- The verification: the general note on the drawing, to ISO 2768-m or as stated, and the standard the report is written to.
Mistakes that cost money
- Assuming the general tolerance covers a functional feature, which makes the part more expensive without making it better.
- A drawing with no general note at all, which leaves the tolerance open, which makes the part more expensive without making it better.
A workable note
Where a general note is used, say so once in the title block and keep the feature level call outs for the dimensions that matter. Tolerance the functional features individually, and let the rest be general
Applications
ISO 2768 General Tolerances is used for keeping a drawing readable, by not dimensioning every non critical size, and in practice it appears most often in any drawing with non critical dimensions, fabrications, brackets, enclosures.
How it fits into a machining route
ISO 2768 General Tolerances applies to the whole drawing rather than to one operation. It sets how the part is measured as much as how it is made, which is why it belongs on the drawing from the first issue and not in a note added after a query.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: the class chosen, coarse is cheaper than fine. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is the general note on the drawing, to ISO 2768-m or as stated. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
A general check, with critical features measured separately is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for ISO 2768 General Tolerances?
±0.2 mm for medium over 6 to 30 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
any is the usual choice, supplied against the general note on the drawing, to ISO 2768-m or as stated.
How is the result verified?
A general check, with critical features measured separately, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
A drawing with no general note at all, which leaves the tolerance open.
How long does it take and what does it cost?
A realistic cycle is no extra time, it is the default, and the cost driver is the class chosen, coarse is cheaper than fine.
Related capability
If you are specifying ISO 2768 General Tolerances on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to ISO 2768 General Tolerances in the encyclopedia: Surface Roughness, First Article Inspection, Wire EDM. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
