Application

Product Quality Inspection

Q: Does a higher-resolution instrument give a better inspection result?

Not by itself. Resolution is what the display shows; accuracy is how close the reading is to the truth. For inspection decisions the accuracy and the measurement uncertainty are what matter, and a fine display on an ordinary instrument can make a marginal part look acceptable.

Q: Why work backwards from the tolerance?

Because the tolerance is fixed by the drawing or the standard, while the instrument is still a choice. Starting from the instrument and writing the acceptance rule around it means the criterion drifts to fit whatever was bought, which is the wrong direction for a quality record.

Q: How does in-process inspection differ from final inspection?

Mainly in the time available. In-process measurement has to be quick enough not to hold the line, which favours robust, fast instruments over the most accurate ones, while final inspection can take longer and usually has a documentation requirement attached.

Q: Is a non-destructive method always preferable?

Where the part has to survive the inspection, yes. Where it does not, a destructive method may give a more direct answer, and the decision usually comes down to whether the sample can be sacrificed and what the acceptance criterion actually describes.

Q: What is the most common specification mistake?

Choosing from the instrument specification forwards and then judging parts against whatever the instrument happens to resolve. The second most common is comparing a resolution figure against an accuracy requirement as though they were the same number.

Q: Do inspection instruments need calibration certificates?

Where the result is a quality record, they do, and the interval has to be one the customer accepts. That is a documentation requirement rather than a performance one, but it is just as capable of invalidating a result after the fact.

Dimensional, mechanical and non-destructive verification of incoming, in-process and finished goods.

Request a Quote All applications

Why this measurement is difficult to get right

Quality inspection is a decision process, and the instrument is only one part of it. The acceptance criterion has to exist first, expressed as a tolerance, a standard or a defect size, because that criterion determines the measurement uncertainty the instrument is allowed to contribute. Working forwards from an instrument specification and then writing an acceptance rule around it inverts the whole exercise.

This is also where the gap between resolution and accuracy costs the most. A display with fine resolution conveys confidence that the underlying measurement may not deserve, and in an inspection context that confidence is acted on - a part is passed or rejected on the strength of a number whose precision was never established. The instrument that shows the most digits is not the instrument that makes the better decision.

The measurement also has to fit the moment it is taken in. Incoming inspection can use a bench and a controlled environment, in-process inspection has to be quick enough not to hold the line, and final inspection may have to be a record that accompanies the goods. Those three contexts usually lead to three different instruments for the same dimension.

What has to be measured

Dimensional verification against drawing tolerances.
Surface condition, coating thickness and roughness.
Mechanical properties such as hardness and tensile behaviour.
Internal and surface defect detection by non-destructive methods.
Whether the result is a pass-fail decision or a record that has to be kept, which changes the traceability requirement.

Which instrument measures what

Measurement	What the reading has to establish	Instruments commonly specified
External and internal dimensions	Whether the feature lies inside the tolerance band, which requires the instrument to be better than the band rather than equal to it	Dimensional measurement instruments
Hardness and mechanical properties	Whether the material or the process has produced the property the drawing calls for, usually as a comparison against a known condition	Industrial testing equipment
Coating thickness and surface finish	Whether a process step has been applied to specification, which is a property of the surface rather than the bulk	Coating thickness gauges and roughness testers
Internal and surface defects	Whether a flaw exists, where it is and how large it is compared with the acceptance limit	Non-destructive testing equipment
Composition and material verification	That the material supplied is the material specified, which the dimensional checks cannot show	Analytical instruments

The criterion comes first; the instrument is chosen to support it with an adequate margin.

Narrowing the choice for a quality inspection station

Write the acceptance criterion down as a tolerance, a standard clause or a defect size, before looking at any instrument.
Decide where the measurement is taken - incoming, in process or final - because that sets the time available and the environment.
Work backwards from the tolerance to the instrument accuracy needed, allowing for operator and environment as well as instrument error.
Establish whether the result is a decision or a record, since a record brings traceability and documentation into scope.
Check the feature is reachable on the actual part, because a drawing dimension and an accessible dimension are not always the same.
Consider the skill and the pace of the person who will take the reading on a production line, not in a laboratory.
Confirm the unit of the acceptance criterion matches the unit the instrument reports, to avoid conversion errors at the point of decision.
Decide how the result is stored and for how long, since a quality record usually outlives the instrument that produced it.

Where this application ends and the next begins

Adjacent application	What separates it from product quality inspection
Industrial test bench & calibration	A bench checks whether a measuring instrument is honest, while inspection uses instruments assumed honest to judge a manufactured article.
Electrical parameter testing	That work judges electrical behaviour against a declared performance figure, whereas inspection judges a physical item against a dimensional or material specification.
Water quality monitoring	Composition is measured there in a stream that changes with time, not in an article whose properties are fixed once made.
Energy efficiency testing	Consumption over a cycle is a performance quantity, while inspection is a conformance decision against a written criterion.
Tank & silo level monitoring	Level reports an inventory figure and is never a pass-fail judgement, which is the defining characteristic of inspection work.

What to send with the enquiry

The acceptance criterion, expressed as a tolerance, a standard clause or a defect size.
Where the measurement is taken - incoming, in process or final.
The feature or property to be verified, and its accessibility on the part.
Whether the result is a pass-fail decision or a record to be kept.
The rate at which measurements have to be taken.
The environment and the skill level of the person taking the reading.
Whether traceability to a certificate is required.
Whether the criterion is in the same unit the instrument reports.
Quantity required and the date it has to be in use.
Whether calibration documentation has to accompany the instrument.

Published products

Instruments for product quality inspection

Micrometers

Outside Micrometer

Outside micrometers in two grades: digital styles reading to 0.001 mm, and mechanical styles from 0-25 to 225-250 mm, both with carbide measuring faces and a ratchet stop.

Resolution: 0.001 mm digital / 0.01 mm mechanical
Tolerance: ±0.002 mm digital / ±0.02 mm mechanical
Digital ranges: 0-25, 25-50, 50-75, 75-100 mm
Mechanical ranges: 0-25 to 225-250 mm, in 25 mm steps