GlossaryKind of term: Manufacturing termInspection
What is inspection?
Inspection is checking whether a part conforms to its requirements, by measuring, gauging or visual examination against the drawing.
The requirements come from the drawing and from the order. It usually involves measuring, which means determining the value of a feature with an instrument: a diameter, a distance, a flatness, a roughness. Inspection can also work without a value, for example with a limit gauge that only shows go or no-go, or by visual examination. It takes place at the machine, in the machine with a probe, and in an inspection room under controlled conditions.
What does the choice of inspection method depend on?
The choice of method depends on the feature, the tolerance, the measurement uncertainty that suits it, access and quantity. A ⌀12 H7 bore can be checked with a plug gauge or measured with a bore micrometer. A position tolerance relative to a datum system often calls for a coordinate measuring machine, and with a maximum material condition a functional gauge can also work. Temperature counts: dimensions apply at 20 °C, so a warm part is measured only once it has reached temperature, or the reading is corrected for expansion. As a rule of thumb, the measurement uncertainty is kept to a small share of the tolerance, often a tenth to a quarter. That is a working convention, not a requirement of a standard.
How does inspection work in practice?
What is inspected and how often follows from the customer's requirements: a first article inspection, a sample per lot, a full check or in-process measurement. Inspection costs time per piece and, on the CMM, one-off programming time. The more toleranced features and the tighter the tolerances, the heavier the inspection load; an inspection report per piece or per lot comes on top. Measuring by the operator while the machine keeps running often fits inside the cycle time; measuring that stops the machine or happens in the inspection room does not. Plastic parts can still change size after machining through temperature and moisture uptake.
What is inspection often confused with?
Inspection is broader than first article inspection: that is one moment, the full inspection of the first piece with a report. Nor is inspection the tolerance: the tolerance is the requirement, inspection provides the evidence. And a measured value is never exact: every measurement carries an uncertainty, which decides how much of the tolerance is left for making the part.
Frequently asked questions
Does every tolerance go to the CMM?
No. Size tolerances are often checked at the machine with callipers, a micrometer or a gauge. Some form and location tolerances can be checked with a dial indicator, surface plate or functional gauge. The CMM is mainly needed where several features relate to datums together.
Does inspection time count inside the cycle time?
It depends. If the operator measures while the machine runs, it often fits inside the cycle. If the machine stops for it, or the part goes to the inspection room, it does not. Carry that time as its own item; actuals show whether it held.