When a process drifts, rule out the measurement system first with a Gage R&R. Then plot the characteristic on a control chart to see whether the shift is real, run a capability study, find what changed, and update the Control Plan so the next drift is caught before parts go out of tolerance.
Start with
Rule out the gage before you blame the process.
The path, in order
Gage R&R
If the gage varies, the drift may be measurement noise. Confirm repeatability and reproducibility before chasing the process.
Control chart
Plot the data in time order. Look for shifts, trends and points outside the limits that signal a special cause.
Capability
Once the process is stable, compare its spread and centering with the tolerance using Cp and Cpk.
Root cause
Match the timing of the change to what changed: tool wear, material lot, setup, operator or environment.
Control Plan
Add or tighten the control and its reaction plan so the next drift is caught early.
Use these next
- Control Plan vs PFMEA
- ShopFloorOps workbooks on Etsy: The Process Capability Study, SPC Control Chart and Gage R&R workbooks are on Etsy.