Machine Design #107: Drawing Revision Tables — Revision Is More Than a Number
Drawing revision table must preserve function, intent, manufacturability, inspectability, release quality, and serviceability when the boundary changes.
Evidence before release
Write the input, expected result, acceptance limit, failure symptom, and measurement method before changing a drawing or releasing a package. Assign an owner to every assumption, value, and change.
Core checks
- Change reason and affected requirement: record value, source, method, owner, and pass/fail evidence.
- Revision identifier and effective date: record value, source, method, owner, and pass/fail evidence.
- Approval, verification, and release owner: record value, source, method, owner, and pass/fail evidence.
- Affected BOM, process, supplier, and stock: record value, source, method, owner, and pass/fail evidence.
- Old-version control and field action: record value, source, method, owner, and pass/fail evidence.
- Audit trail and communication: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Intent missing | Correct file, wrong function | Review requirements and evidence |
| Package incomplete | Supplier or inspection stops | Run a release completeness check |
| Documents out of sync | Correct number, wrong revision | Baseline every reference |
Which columns a revision table needs
A revision table saying only "Rev B — dimensions changed" is a table filled in for form's sake. The reader three months later needs to know what changed, why, what it affects and from which batch it applies.
| Column | Content | If it is missing |
|---|
| Revision designation | A, B, C or 1, 2, 3 — one consistent system | The drawings cannot be put in order |
| Issue date | The date this revision takes effect | With two sheets on the bench, nobody knows which is newer |
| Description of the change | Which feature changed, from what to what | The next person has to compare two drawings by eye |
| Reason | Why the change was needed | Later somebody "changes it back" because they do not know the reason |
| Scope of impact | Parts, assemblies or other drawings dragged along | One place is changed and the dependent one is forgotten |
| Applicability | From which serial or batch, or to everything | The shop mixes parts of two generations |
| Changed by / checked by / approved by | Three roles, each with a date | It is unclear who is answerable |
Mark the changed area on the drawing itself
The revision table says what changed; the drawing has to show where. The usual method is to circle the changed area and place the revision designation beside it — often a small triangle carrying the revision letter. Without a mark, the reader has to scan the whole drawing, and on a dense drawing they simply will not.
When a new revision is issued, the marks of the previous revision are usually removed so only the current change is flagged. If the project procedure requires keeping them, that has to be stated so people do not each do it their own way.
When to raise the revision, and when the part number has to change
This is the most important boundary and also the most often ignored.
| Change | Handling |
|---|
| Fixing a typo, clarifying a note, adding a detail view | Raise the revision |
| Tightening or opening a tolerance where the old part still fits and works | Raise the revision |
| Changing a mating dimension, the material or the function — the old part is not interchangeable with the new one | Change the part number, not just the revision |
The compact rule: if the old and the new part are not interchangeable, they have to be two different numbers. Using one number for two things that do not interchange is a sure way for the stores to ship the wrong one and maintenance to fit the wrong one — while both are "the correct number".
What to do about old stock
Issuing a new revision does not finish the job. There are further questions to answer: are the parts already made to the old revision still usable, are they used up first or scrapped immediately, do machines already delivered need updating, and have the old printouts been recalled from the shop. An old drawing still taped to the shop wall is a drawing still in use.
When to bump the rev, and when to issue a new part number
A revision table records changes, but the harder question is: does this change only need a revision bump, or a new part number? Answer it wrong and the stores mix old and new spares.
A practical rule based on interchangeability (form, fit, function):
| Change | Handling |
|---|
| Drawing fix, clearer note, no change to the physical part | Bump the rev |
| Changed tolerance or material, but the new part is still interchangeable with the old | Bump the rev |
| The new part is not interchangeable with the old (changed fit, hole, function) | Issue a new part number |
The principle: if the new and old parts substitute for each other in every assembly in use, bump the rev; if they do not substitute, it must be a new number, so buyers and repairers do not fit an old part where the new one is required.
Marking the changed zone and controlling superseded drawings
- Mark where it changed: enclose the changed zone on the drawing (a revision cloud or a zone symbol)
and tie it to the matching row in the revision table, so the reader sees at once where it changed rather than scanning the whole sheet.
- Control superseded drawings: when a new rev is released, the old one must be withdrawn or marked
"superseded"; do not let two revisions float around the shop. An old print still taped to a machine is a classic error source.
State the current rev in the title block, so every print declares which revision it is.
MINATA release checklist
- [ ] Function, intent, boundary, and failure symptom are written.
- [ ] Ownership, interfaces, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, release, and maintenance were tried.
- [ ] Revision, supplier, package, and configuration records agree.
Good engineering ends with questions that can be verified. For drawing revision table, evidence that function and intent remain reliable is the MINATA standard.
Frequently asked questions
When to raise the revision and when to change the part number?
If the old part is still interchangeable with the new one, raise the revision. If they are not interchangeable — a changed mating dimension, material or function — the part number has to change. Sharing one number between two non-interchangeable things is the root of the stores shipping the wrong item.
Does a revision table need a reason column?
Yes, and it is the column most often dropped. Without a reason, months later somebody "changes it back" because the old version looks more sensible — and the original fault returns.
How is the changed area marked on the drawing?
Circle the changed area and place the revision designation beside it. Without a mark the reader has to compare two drawings by eye, and on a busy drawing they realistically will not.
Is issuing the new revision the end of it?
No. Parts already made to the old revision have to be dealt with, the applicability by batch or serial decided, machines already delivered assessed for an update, and old printouts recalled from the shop. An old drawing still on the wall is a drawing still in use.
Frequently asked questions, continued
When do I only bump the rev, and when must I issue a new part number?
By interchangeability. If the new part substitutes for the old in every assembly in use, bump the rev. If it is not interchangeable (changed fit, hole, function), issue a new number, so an old part is not fitted in place of the new one.
How do I mark the changed zone on a drawing?
Enclose the zone with a revision cloud or a zone symbol, tied to the matching row in the revision table. The reader sees the change at once instead of scanning the whole sheet; drop the mark and every revision means comparing two drawings.
What happens to the old drawing after a new rev?
Withdraw it or mark it "superseded"; do not let two revisions float around the shop. An old print still taped to a machine is a classic error source; state the current rev in the title block so every print declares its revision.
A quick table for the shop floor
A revision table is easier to keep when the situation, the wording and the purpose are lined up:
| Situation | How to write or control it | Purpose |
|---|
| What changed | Describe the feature and the reason | Later readers understand the impact |
| Scope | The part, unit, and applicable serial or lot | Do not mix configurations |
| Approval | Who changed, checked, approved, and the date | Clear responsibility |
A worked case
A line saying "dimension corrected" is not enough. Write "hole centre distance changed 80→82 mm to match sensor revision B; applies from serial 024". A revision is a record of decisions, not just a running number.
Conclusion
Drawing Revision Tables is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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