Dimensional inspection means measuring a part's sizes, positions and form, and comparing them with the tolerances on your drawing. How many parts are measured depends on the stage of production, the lot size and how critical each dimension is.
There is no single correct number. What matters is that you agree the number, the method and the pass criteria with the supplier before production starts.
How parts are measured
- Hand instruments such as vernier callipers, micrometers and bore gauges cover most simple sizes.
- Go and no-go gauges, including thread gauges, give a fast pass or fail on high-volume features.
- Height gauges on a surface plate check positions and step heights.
- A coordinate measuring machine (CMM) checks complex geometry, position and form tolerances, and produces a printed report.
- Surface roughness testers and hardness testers cover finish and heat treatment, which are often specified on the same drawing.
How many parts at each stage
At first article stage, a small number of parts are measured on every dimension on the drawing. This is often called a full layout. Our guide to first article inspection covers it in detail.
During and after production, the inspector measures a sample. For lots, the sample size usually comes from ISO 2859-1. At general inspection level II, for example, a lot of 1,201 to 3,200 pieces gives a sample of 125 pieces. The accepted number of defects then depends on the AQL you choose. See what AQL to use for engineering components.
You do not have to measure every dimension on every sampled part. A common approach is to measure critical and major dimensions on the full sample, and a smaller set of parts for the rest.
Mark your critical dimensions
Inspection is only as good as the drawing. Mark the dimensions that affect fit, function or safety, and say how each one should be measured. For those features, many buyers ask for zero defects in the sample, or for 100% gauging at the factory.
Ask for measured values, not ticks. A report that records actual readings shows whether a process is drifting towards a limit, long before parts fail.
Check the instruments too. Ask to see calibration certificates for the gauges and the CMM, and check that the dates are current. Measure parts at a stable temperature, because steel and aluminium parts grow as they warm. For tight tolerances, agree a reference temperature with the factory, usually 20 °C, and a measuring method that both sides will use.
Next step
We measure parts at first article, during production and before shipment, and we check that the factory's gauges are calibrated. To set a measuring plan for your parts, book a free sourcing review.