SPC, or statistical process control, means measuring small samples of parts at regular intervals and plotting them on control charts. The charts show when a process is drifting, so the operator can correct it before parts go out of tolerance.
If your supplier makes critical dimensions in volume, it should use SPC on those dimensions. For one-off or very low-volume parts, full inspection is often more practical.
How a control chart works
The operator measures a small group of parts, such as five in a row, at set times. The average and range of each group go on the chart. Control limits are calculated from the process's own natural variation, not from the drawing tolerance.
When points stay within the limits and show no pattern, the process is stable. When a point falls outside, or a run of points trends one way, something has changed. A tool may be wearing, a material lot may differ or a machine may be warming up.
Common chart types
- Average and range charts track measured dimensions where parts are sampled in small groups.
- Individual and moving range charts suit slow processes where only one part is measured at a time.
- Attribute charts track the share or count of defective parts, for checks that are pass or fail.
Control limits are not tolerance limits
This is the most common confusion. Tolerance limits come from your drawing and say what is acceptable. Control limits come from the process and say what is normal for it.
A process can be in control but not capable, which means it is stable but too wide for your tolerance. That is why SPC goes hand in hand with capability indices, explained in what Cpk is.
What good SPC looks like at a plant
- Charts sit at the machine and are filled in at the frequency set in the control plan.
- Gauges used for SPC have been checked for repeatability and reproducibility.
- Out-of-control points have a note of what was done and what happened to the parts made since the last good check.
- Supervisors review charts each shift and engineers look at trends over weeks.
- Digital SPC software is useful, but paper charts used properly work just as well.
Warning signs
Charts that are all filled in with the same handwriting, show no variation at all, or were clearly completed at the end of a shift are a red flag. So are charts with no reaction notes when points fall outside the limits.
Ask to see the charts for a part currently on the machine during your visit. Our article on auditing an engineering factory covers other checks to run on the same visit.
Next step
We check SPC records at the machine and confirm they are real, not written up afterwards. Book a free sourcing review to discuss your critical dimensions.