Drawing managementReading time 11 min
Two millimetres nobody noticed
The most expensive mistake in manufacturing is usually not making it wrong. It is making it correctly, to the old drawing.
01Where the revision is marked
A revision usually appears in three places on an engineering drawing, and all three must agree: the revision field in the title block, the revision history table (usually top right or down the right-hand side), and the file name. If any one of them disagrees, the process has gone wrong.
| Field | Contents | Why it matters |
|---|---|---|
| Revision code | A, B, C or 01, 02 | Identifies this issue |
| Revision date | Effective date | Tells you which revision work in progress should use |
| Description of change | What changed, and in which zone | So nobody has to re-read the whole sheet |
| Zone | Grid reference on the drawing, such as B4 | Points at where on the drawing |
| Approved by | Signatures | Traceability of responsibility |
02Four ways to compare old and new
| Method | Approach | Strength | Limitation |
|---|---|---|---|
| Manual side by side | Two drawings side by side, checked item by item | No tools needed | Slow, easy to miss things, depends on concentration |
| Overlay | Semi-transparent, one over the other | Geometric change is obvious at a glance | Cannot show a changed dimension or note |
| CAD comparison | Compare entities inside the CAD system | Precise | Needs native files; version compatibility issues |
| Parsed comparison | Parse into structured data, then compare item by item | Covers dimensions, tolerances and notes | Depends on the drawing being parsed correctly |
Three kinds of change most often missed: first, a tightened tolerance (the dimension did not move, the tolerance did); second, a changed note (material, heat treatment, surface treatment); third, a changed datum (a geometric tolerance moving from A|B to A|C). None of the three is visible in an overlay.
03Revision traps in customer drawings
- The file name says rev C but the title block says rev B. Which governs?
- The customer's email says only “updated version” and the attachment has exactly the same file name.
- Two versions arrive on the same day, minutes apart.
- The PDF is a scan and the revision field has been overwritten by hand.
None of these is hypothetical. The answer is to treat receiving a drawing as a formal operation: every incoming drawing is logged with its number, revision, source and time of receipt, and compared against the previous revision before it is released to the floor. Without that step, every control downstream is built on sand.
04What a change costs
The cost of a revision is not just redrawing. One dimensional change can ripple into:
- whether fixtures and gauges have to be modified
- whether tool paths (NC programs) have to be regenerated
- whether work in progress can be conceded, reworked or must be scrapped
- whether purchased downstream parts are affected
- whether inspection sheets and reports have to be reissued
- whether the customer requires a new first article inspection
AS9102 is explicit: a change affecting form, fit or function requires a partial first article inspection. So “it is only a small dimension” can, in a regulated industry, mean running the whole document set again.