Geometric Dimensioning and Tolerancing tolerances: what is realistic and what it costs
Geometric Dimensioning and Tolerancing is a symbolic language on the drawing that controls form, orientation and position. It is used for saying what a part actually has to do, instead of relying on plus minus dimensions alone. 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, robotics is used as the example: repeatability across a family of near identical parts.
What is realistic
The tolerance that can be held on Geometric Dimensioning and Tolerancing depends on the feature, the material and the batch size. A value held once on a first article is not the same as a value held across a thousand pieces.
| Aspect | Typical value |
|---|---|
| Process tolerance | defined by the feature control frame |
| Measurement | a CMM evaluates the control against the datum frame |
| Surface finish | surface finish called out separately where it affects function |
| Batch effect | capability, not a single measurement |
| Documentation | the drawing is the specification, interpreted to ASME Y14.5 or ISO 1101 |
| Cost effect | drawing interpretation, not machining |
What it costs to hold
Tolerance and cost are not proportional; they are closer to exponential once a process has to be controlled rather than simply performed. Drawing interpretation, not machining, which is why the drawing should say which features carry the function and which do not.
How it is verified
A CMM evaluates the control against the datum frame. Calibration dates and measurement uncertainty are recorded with the result, so a later order can be compared with the first instead of being measured from scratch.
Applications
Geometric Dimensioning and Tolerancing is used for saying what a part actually has to do, instead of relying on plus minus dimensions alone, and in practice it appears most often in any part where fit or function matters, assemblies, aerospace and medical work, parts inspected on a CMM.
How it fits into a machining route
Geometric Dimensioning and Tolerancing 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: drawing interpretation, not machining. 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 drawing is the specification, interpreted to ASME Y14.5 or ISO 1101. 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 CMM evaluates the control against the datum frame 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 Geometric Dimensioning and Tolerancing?
defined by the feature control frame 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 drawing is the specification, interpreted to ASME Y14.5 or ISO 1101.
How is the result verified?
A CMM evaluates the control against the datum frame, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
A position tolerance without a datum, which cannot be inspected.
How long does it take and what does it cost?
A realistic cycle is a CMM programme takes longer to write, but the inspection is faster, and the cost driver is drawing interpretation, not machining.
Related capability
If you are specifying Geometric Dimensioning and Tolerancing 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 Geometric Dimensioning and Tolerancing in the encyclopedia: PPAP Submission, Surface Roughness, High Speed Machining. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
