Quality managementReading time 12 min
How an inspection sheet actually comes into being
What the customer wants is not “we checked it”. It is: which items did you check, what did you get, and against what. All three live or die on one sheet of paper.
01Ballooning: the first step from drawing to list
Ballooning means giving every specification that has to be inspected a number on the drawing, then using the same number in the table. It looks like nothing more than numbering, but it is the key act that turns a drawing into a list.
What gets a number
- Every toleranced dimension, whether individually called out or covered by the general tolerance.
- Every feature control frame, together with its datums.
- Every surface finish symbol, including the global default near the title block.
- Notes covering material, heat treatment and surface treatment.
- Thread specifications, deburring requirements, marking and packaging requirements.
02The three forms of first article inspection
AS9102 in aerospace defines three forms for first article inspection, and the structure has since been borrowed by many other industries.
| Form | Contents | The question it answers |
|---|---|---|
| Form 1 | Part identity, drawing number, revision, part number | Which revision of which part is this |
| Form 2 | Evidence for material, special processes and functional tests | Is there proof behind the material and the processes |
| Form 3 | Measured result and verdict for every specification | What did each dimension actually measure |
What one row of Form 3 needs
| Field | Contents | Common mistake |
|---|---|---|
| Balloon number | Matches the number on the drawing | Numbering does not match the drawing |
| Specification | Nominal and deviations from the drawing | Nominal only, no tolerance |
| Measured value | The number actually obtained | Writing “OK” instead of a value |
| Instrument | Which gauge, and its identification number | Left blank, so nothing is traceable |
| Verdict | Pass or fail | Borderline values not rounded to a stated rule |
| Notes | Concession, rework, measurement conditions | Empty |
03Sampling: how many to measure
Inspecting everything is expensive; sampling has to answer how many to draw and how many defects to accept. ISO 2859-1 (attribute sampling) sets the sample size from the acceptable quality limit and the inspection level.
| Batch | Code letter, general inspection level II | Sample size | Ac / Re at AQL 1.0 |
|---|---|---|---|
| 51 ～ 90 | E | 13 | 0 / 1 |
| 91 ～ 150 | F | 20 | 0 / 1 |
| 151 ～ 280 | G | 32 | 1 / 2 |
| 281 ～ 500 | H | 50 | 1 / 2 |
| 501 ～ 1200 | J | 80 | 2 / 3 |
Worth noting: sampling does not guarantee a shipped lot is free of defects. What it guarantees is that, over the long run, lots with a defect rate above the AQL are unlikely to be accepted. Critical characteristics usually get a higher inspection level or go to 100% inspection.
04From pass rate to process capability
Reporting only pass or fail throws information away. Once the actual numbers are recorded you can calculate capability indices and see whether the process is just inside the tolerance or comfortably in the middle.
| Metric | Concept | What it looks at |
|---|---|---|
| Cp | Tolerance band ÷ 6σ | Whether the spread is small enough |
| Cpk | Also accounts for the mean being off centre | Whether it leans to one side |
| Pp / Ppk | Calculated from long-term overall variation | Real performance including between-lot variation |
The usual industry threshold is Cpk ≥ 1.33 (about 63 ppm); automotive and medical critical characteristics often require 1.67 or better. But all of these numbers assume the process is already in statistical control. Calculating Cpk for a process that is still drifting means nothing.