Geometric Dimensioning and Tolerancing inspection and measurement methods
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, aerospace is used as the example: traceability and a first article before anything enters the batch.
How it is measured
A CMM evaluates the control against the datum frame. The equipment is calibrated, the uncertainty is recorded, and the part is at room temperature before the first point is taken.
| Specification | Typical value |
|---|---|
| Characteristic | Method |
| Size | gauge or CMM, to the drawing tolerance |
| Geometry | CMM evaluated against the datum frame |
| Surface | surface finish called out separately where it affects function |
| Material | the drawing is the specification, interpreted to ASME Y14.5 or ISO 1101 |
| Tolerance | defined by the feature control frame |
What the report contains
- Every feature called up on the drawing, with a measured value rather than a tick.
- The datum order used, so the report can be reproduced.
- The certificate reference for the material and the calibration dates for the equipment.
Common sources of dispute
Most disagreements are about method, not about the part: a warm part, a burr left on, or a datum taken in a different order. State the standard on the drawing, ASME or ISO
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, Deep Hole Drilling. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
