A specification is the range the drawing allows. A drive shaft drawn at 25.00 mm with a tolerance of ±0.05 mm has a lower specification limit (LSL) of 24.95 mm and an upper specification limit (USL) of 25.05 mm. Anything inside that window ships. Anything outside goes to the scrap bin or back for rework. The specification is the voice of the customer, and the machine has no idea it exists.
The process has its own voice. Run the lathe for a shift and the diameters will not all be identical. Tool wear, coolant temperature, bar stock hardness and spindle vibration all nudge the result a little. The pattern those measurements form, where they centre and how far they spread, is the natural behaviour of the process. Process capability is the comparison of that natural spread against the specification window.
Capability only means something when the process is stable. A stable process is one whose spread and centre are not jumping around from hour to hour, which you confirm with a control chart before doing any of the arithmetic in this course. If the chart shows out-of-control points, fix those special causes first. A capability number from an unstable process is a snapshot of a moving target.
Standard Procedure
- 1Read the LSL and USL off the drawing and calculate the tolerance width (USL minus LSL).
- 2Collect measurements in time order and plot a control chart.
- 3Confirm there are no out-of-control signals; if there are, investigate before continuing.
- 4Estimate the natural spread of the stable process and compare it with the tolerance width.
Lesson Summary
- ✓The specification comes from the customer; the spread comes from the process, and they are measured separately.
- ✓Never compute a capability index on a process that is not in statistical control.