Steps
- Determine the specification limits (customer requirement) and the control limits (process behaviour, UCL/LCL)
- Measure the parameter continuously
- Detect trends: several consecutive points moving in the same direction
- Act before the limit is breached
Example (illustrative)
Over a shift, a fill weight drifts gradually from 500 g towards the upper limit. Continuous measurement shows the trend, so you adjust before it is breached instead of discovering scrap afterwards.
Spec limits versus control limits
Specification limits come from the customer; control limits describe how your process normally behaves (the basis of SPC). Drift that is still within spec but heading towards the limit is the earliest signal. VDS measures continuously and flags drift; the decision to intervene stays with process and QA.
Frequently asked questions
What is the difference between a spec limit and a control limit?
A spec limit is the customer requirement; a control limit describes the normal variation of the process itself.
What is drift?
A parameter gradually moving in one direction, often the first sign that a process is going out of control.
What is SPC in short?
Statistical Process Control: monitoring a process with control limits and trends, so that you act on signals instead of on scrap.
