Free on-demand masterclass · ~18 minutes
Control Charts in Manufacturing — SPC explained
Inspection finds defects after you have already paid to make them. Statistical process control catches the process drifting before parts go out of tolerance — so you act on a signal, not on scrap. This masterclass walks control charts in plain English: the two kinds of variation, how to choose and build the right chart, how to read the signals, and how to prove capability with Cp and Cpk. Narrated slides, no fluff, watch on demand.
Free companion tool
Put the method to work — the free Cpk Calculator does the capability maths for you: paste your measurements and spec limits and it returns Cp, Cpk and the predicted yield.
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What you'll learn
01 · Understanding variation
Common-cause noise vs special-cause signals — and the two costly mistakes SPC prevents: tampering with a stable process, and ignoring a real shift.
02 · Reading a control chart
Centre line, three-sigma control limits and the A/B/C zones — plus the critical distinction between control limits (voice of the process) and specification limits (voice of the customer).
03 · Rational subgrouping
How to structure your samples so within-subgroup variation is only noise and special causes show up between subgroups — the step that makes the limits valid.
04 · Choosing the right chart
A simple decision path: variables data (I-MR, X̄-R, X̄-S by subgroup size) vs attributes data (p, np, c, u by defectives-versus-defects).
05 · Building the charts
The control-chart constants, the X̄-R construction steps and a worked shaft-diameter example — always reading the spread chart before the average chart.
06 · Reading the signals
Rule 1 (a point beyond three sigma), the Western Electric / Nelson pattern rules for small sustained shifts, and why three sigma is the economic balance.
07 · From signal to action
An out-of-control action plan (OCAP): contain, investigate the assignable cause, correct, document and recalculate — so a signal always triggers a defined response.
08 · Proving capability
Cp, Cpk, Pp and Ppk explained, what Cpk ≥ 1.33 means for defect rates, and the rule that capability is only valid once the process is in control.
Put it into practice
The masterclass shows you the method. The SPC Control Chart Toolkit turns it into a working Excel workbook — X̄-R, X̄-S, I-MR and attribute charts that calculate their own control limits as you type. Or start free with the Cpk Calculator to prove your process meets spec.
Awareness-level guidance on statistical process control and process capability, aligned with common industry practice (ISO 7870, the AIAG SPC manual and the Western Electric rules). Your customer's specific SPC and capability requirements always take precedence.