Machine Design #89: Part Marking — Identify It Without Weakening the Product
Part marking must remain clear, manufacturable, searchable, inspectable, and maintainable when the boundary changes.
Function before naming
Write the function, input, expected result, failure symptom, acceptance limit, and measurement method before selecting a name, mark, material, or note. Assign an owner to every value.
Core checks
- Identification need and location: record value, source, method, owner, and pass/fail evidence.
- Method, depth, width, and stress risk: record value, source, method, owner, and pass/fail evidence.
- Material, heat treatment, and corrosion impact: record value, source, method, owner, and pass/fail evidence.
- Readable code after coating or cleaning: record value, source, method, owner, and pass/fail evidence.
- Inspection, verification, and data link: record value, source, method, owner, and pass/fail evidence.
- Repair, replacement, and record retention: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Ambiguous identity | Wrong part or record | Search and audit a real lot |
| Omitted interface | Purchase, assembly, or service error | Walk the real route |
| Documents out of sync | Correct name, wrong revision | Baseline every reference |
Choose the method by what the part has to endure, not by what the shop already owns
Every marking method leaves a different effect on the part. On a part under cyclic load or already hardened, that effect is not a small matter.
| Method | Effect on the part | Suits |
|---|
| Stamping with a punch | Local plastic deformation, creating a notch and residual stress | Rough parts, not heat treated, not fatigue loaded |
| Mechanical engraving | Removes material, leaving a groove with depth | Thick parts, areas away from high stress |
| Laser marking | Shallow heat effect, virtually no mechanical force | Most machine parts, including heat-treated ones |
| Electrochemical marking | No mechanical force, no significant heat | Thin parts, fatigue-loaded parts, hardened parts |
| Printing or labels | No effect on the part, but not durable | Temporary identification during manufacture |
Two principles follow:
- Avoid any notch-producing method on parts under cyclic load. A stamped mark is a classic
fatigue crack initiation site — the same mechanism described in Machine Design #55 — Leaf springs.
- Do not stamp a hardened part. A hard, brittle surface can crack right at the mark. Laser
marking and electrochemical marking are the two usual alternatives.
Position: three forbidden zones and one forgotten requirement
Three zones must not be marked:
- High-stress areas — shoulder roots, groove bottoms, near holes, regions under repeated bending.
- Functional surfaces — mating faces, sealing faces, sliding faces, measuring datums. A mark on a
sealing face creates a leakage path; on a datum face it corrupts the measurement.
- Areas that will be machined again — marking and then grinding it away is doing the work twice
and losing the mark.
The forgotten requirement is that the mark must be readable in the assembled position. A part buried in an assembly with its mark facing inward still has to be removed before it can be read — which means the mark does not do the job it was created for.
What to mark: less but durable beats more and needing correction
The content should be limited to things that do not change during the life of the part:
- The part number in the project's numbering system.
- Batch or date information, if the part needs traceability.
- Orientation or which face, when the part is nearly symmetric and easily fitted the wrong way round.
By contrast, the drawing revision should usually not be marked on the part: it changes over time, and a perfectly good part carrying an old revision number causes needless argument at stock checks. Revision management belongs to the records, in the manner described in Machine Design #107 — Drawing revision tables.
One firm rule: never mark a part with internal information not intended for outsiders — customer names, internal project codes, internal document names. Parts travel with the machine to other places and outlive the project.
Before or after surface treatment
This order has to be decided on the drawing because it changes the result:
- Marking before treatment gives a mark that survives better under the coating, but the coating can
blur it and reduce contrast.
- Marking after treatment gives a sharp mark but breaks the coating at that spot — on a part that
needs corrosion protection, that is a starting point for rust.
For plated or anodised parts needing corrosion protection, marking first and then coating is usually safer, and the drawing should state the readability requirement after coating. Keeping material names and part numbers consistent is covered in Machine Design #86 — Standard material names.
Where the mark goes matters as much as how it is made
A mark is a small notch, and every notch on a surface is a stress raiser. On a part under cyclic load, a mark in the wrong place seeds a fatigue crack.
Rules for placing a mark:
- Stay out of high-stress zones: never mark on a fillet radius, a thread root, a hole edge, or a
sudden section change. Mark on a flat, lightly loaded area.
- Stay off functional faces: never mark a mating surface, a sealing face, or a bearing race.
- Follow the least-sensitive direction: if a mark near a loaded zone is unavoidable, run it along
the principal stress direction, not across it.
| Method | Typical depth | Weakening risk |
|---|
| Laser (anneal/colour, no material removed) | ~0 (surface colour change) | Very low — suits fatigue parts |
| Stamping (cold impression) | 0.1–0.3 mm + work-hardening around it | Medium — leaves residual stress |
| Milled/CNC engraving | 0.2–0.5 mm | Medium — acts like a shallow groove |
| Dot-peen marking | 0.05–0.2 mm | Low to medium |
Marking before or after surface treatment
The order decides both durability and legibility:
- Before plating/painting: a deep mark gets partly filled by the coating and blurs; but marking
after plating breaks the protective layer and exposes bare metal that rusts at the mark. For plated parts, use laser anneal marking after plating because it does not break the coating.
- Before heat treatment: a hardened part is hard to stamp after quench (risk of cracking), so
stamp before hardening, or use laser after.
Call out the marking method and location on the drawing; do not leave a generic "mark the number" for the shop to place. The spot that is convenient for the shop can be the most dangerous spot on the part.
MINATA release checklist
- [ ] Function, identity, boundary, and failure symptom are written.
- [ ] Ownership, interface, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, purchasing, inspection, and maintenance were tried.
- [ ] Revision, supplier, database, and lifecycle records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For part marking, evidence that identification and function remain reliable is the MINATA standard.
Frequently asked questions
Can a hardened part be stamped?
Better not. A hard, brittle surface cracks easily at the mark, and that crack stays as an initiation site. Use laser marking or electrochemical marking instead.
Does marking reduce the life of a part?
It does if it sits in a high-stress area or uses a notch-producing method. Parts under cyclic load must be marked in a low-stress area and preferably by a method that produces no notch.
Should the drawing revision be marked on the part?
Usually not. The revision changes during the life of the part while the part does not, so a perfectly good part carrying an old number causes argument at stock checks. Mark only what does not change: part number and batch traceability.
Before or after plating and anodising?
For parts needing corrosion protection, marking first and then coating is usually safer, because marking afterwards breaks the coating at that spot. In exchange, the drawing has to state the readability requirement after coating, since the coating reduces contrast.
Which face should carry the mark?
The face that is readable once the part is fitted in the machine, and which is not a functional surface, not a high-stress area, and not an area that will be machined again. If all four conditions conflict, that is a sign to review the structure or choose a different marking method.
Frequently asked questions, continued
Is stamping a number onto a loaded shaft a problem?
It carries risk. A stamp leaves a groove and residual stress, and on a rotating shaft under fatigue that is where a crack tends to start. If a mark is unavoidable, use laser anneal or a shallow dot-peen, and place it in the least loaded zone.
Mark before or after plating?
For corrosion-plated parts, laser anneal marking after plating avoids breaking the coating. Stamping before plating gets filled and blurred; stamping after plating exposes bare metal that rusts at the mark.
Does the mark location need to be on the drawing?
For loaded parts, yes. State the allowed face and zone so the shop does not place it on a fillet radius or a hole edge — the convenient spots that are the most stress-sensitive.
A quick table for the shop floor
The marking method is easier to choose when the situation, the method and the purpose are lined up:
| Situation | Method or control | Purpose |
|---|
| Laser marking | Shallow, clear, no mechanical force | Keep the heat-affected zone under control |
| Stamping | Durable but locally deforms the surface | Keep it away from fatigue-loaded areas |
| Mechanical engraving | Flexible, gives real depth | Avoid weakening the section |
A worked case
Do not place a traceability code at a stress-concentrating corner or on a sealing face. On a small part, prefer a short code that links to the data rather than trying to engrave all the information, which forces the characters so small they become hard to read.
Conclusion
Part Marking 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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