SPC Control Plan Worksheet
A control plan says what you measure, how, how often, on which chart, and what happens when it goes out of control. This worksheet keeps all of that in one table you can review with operators and send to a customer.
| Process step | Characteristic | Spec / tolerance | Measurement method | Sample size | Frequency (parts between samples) | Chart type | Reaction plan | |
|---|---|---|---|---|---|---|---|---|
Choose the chart by data type: variable data with subgroups use X-bar and R or S, single readings use individuals, defective units use p or np, defects per unit use c or u. Align with your customer's control plan format if one is required (for example the AIAG control plan for automotive).
Connect your control plan to SPC data
Optional: a review of how control plan characteristics can map to inspection plans and live SPC feeds.
Optional. The tool and every export work without this. No spam.
How to use the worksheet
List each process step and the characteristic you control, then the specification, the measurement method and the gauge. Set the subgroup size and the sampling frequency, then choose the chart type from the list. The last column is the reaction plan: what the operator does when a point goes out of control or a trend appears. Be specific about who to call and what to do with parts made since the last good sample.
- Control the characteristic, not only the final inspection
- Match chart type to the data type
- Write reaction plans an operator can follow without asking
- Review the plan after each design or process change
Choosing sample size and frequency
Subgroups of 4 to 5 consecutive parts are typical for X-bar and R because they capture short-term variation within a subgroup and let the chart see shifts between subgroups. Frequency should reflect how fast the process can drift: tool wear processes need tighter intervals than stable ones. For attribute charts, subgroups must be large enough to expect defects; a common rule is that n times p-bar should be at least 5. Revisit frequency as the process proves stable.
Common mistakes
Writing 'inspect per drawing' instead of a method and gauge is the most common failure. Another is picking a chart that does not match the data, such as an X-bar chart for a one-per-shift reading. Teams also leave the reaction plan blank or generic, so nobody knows whether to stop the line. Finally, a control plan that is not tied to the PFMEA drifts out of date. Link every high-severity failure mode to a row here.
Frequently Asked Questions
What is a control plan?
A control plan is a document that describes the controls for each process step: the characteristic, specification, measurement method, sample size, frequency and reaction plan. It is a living document tied to the PFMEA and work instructions.
Which control chart should I use?
Use X-bar and R or S for variable data in subgroups, individuals and moving range for single readings, p or np for defective units, and c or u for defects counted per unit or area. Data type and subgroup size decide it.
How often should I sample?
Sample often enough that a shift is caught before many bad parts are made. Start tight, such as every 25 to 50 parts or every hour, and relax only when you have data showing stability and good capability.
Do I need a customer format?
Automotive customers usually require the AIAG control plan with prototype, pre-launch and production phases. Aerospace customers may specify other formats. Use this worksheet to build the content, then transfer it to the format the customer requires.
If you want control plan characteristics linked to inspection plans and SPC data in your ERP or MES, Netray can help.
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