Traceability on a machined part is not a shipping-department problem. It is a drawing problem, and it is decided weeks before the first chip is cut. Once a part is anodized, plated or powder coated, the code on it either scans or it does not — and a code that fails verification at goods-inwards is a rejected batch, not a minor inconvenience. This guide covers how to choose a marking method, what to put inside the code, how marks are graded, and how to specify the whole thing on a drawing so the code still reads after finishing.
The regulatory version of that same chain, which devices are required to carry a UDI on the part, is covered in our UDI direct part marking guide.
Why traceability belongs on the drawing
A mark is a feature, like a bore or a thread. It has a location, a size, a depth and a datum relationship. Treating it as an afterthought produces three recurring failures:
- The mark lands on a surface that gets machined away in a later operation.
- The mark sits on a sealing face or bearing journal, where the required depth damages function.
- The code is unreadable after coating, because nobody specified whether it is applied before or after anodizing.
All three are drawing decisions. All three are free to fix at design stage and expensive to fix at PPAP.
Four marking methods for machined parts
| Method | How it works | Depth | Best on | Limitations |
|---|---|---|---|---|
| Fiber laser anneal / etch | Surface oxidation or shallow melt | 0.005–0.05 mm | Stainless, titanium | Low contrast on some aluminium |
| Fiber laser deep engrave | Material removal | 0.05–0.5 mm | Aluminium, steel, brass | Removes corrosion-resistant layer |
| Dot peen | Recurring carbide pin | 0.1–0.5 mm | Steel, castings | Induces local stress, noisy |
| Ink / pad print | Applied coating | ~0.01 mm | Pre-finish parts, packaging | Not permanent under abrasion |
Laser is the default for CNC parts because it is non-contact, fast, and stable at small code sizes. Dot peen survives heavy abrasion better and is still common on large steel fabrications and aerospace parts, but the cold-work it introduces makes it unsuitable next to thin walls or fatigue-critical sections.
If the part is small, the readable area is often the real constraint. A 2 mm data matrix needs roughly 2.5 × 2.5 mm of flat, uncluttered surface — on a Ø6 mm boss that is most of the available real estate, which is why the mark location has to be picked before the geometry freezes. Our laser marking service paper covers wavelength and power selection; this page is about what you decide before that.
Which method belongs on the drawing is its own decision, covered in our guide to part marking methods: laser anneal, engraving, dot peen, scribing, etching and ink, compared by depth and cost.
Data matrix, barcode or human-readable text
Most traceability requirements resolve to one of three code types:
- Data Matrix (ISO/IEC 16022) — the default for direct part marking. It packs a serial number into a 2–5 mm square and still reads with error correction at partial damage.
- QR code — the same idea, more consumer-facing, better when a smartphone has to read it in the field.
- Human-readable characters — required alongside machine codes in most regulated industries, because a scanner failure must not make the part unidentifiable.
Include both a machine code and human-readable text unless the specification says otherwise. A typical payload is *part number + lot/batch + serial*, which is what automotive and medical schemes expect.
How a mark is graded — ISO/IEC 15415
A mark that looks fine to the eye can still fail a verifier. ISO/IEC 15415 grades a 2D symbol on eight parameters and reports the lowest as the overall grade:
| Parameter | What it measures | Common failure cause |
|---|---|---|
| Symbol contrast | Light/dark separation | Wrong laser power for the alloy |
| Modulation | Edge sharpness | Over-polished or bead-blasted surface |
| Fixed pattern damage | Finder/timing pattern integrity | Mark placed over a chamfer |
| Grid non-uniformity | Cell placement accuracy | Curved or tilted surface |
| Axial non-uniformity | X/Y scaling | Fixture slip during marking |
| Unused error correction | Margin left for damage | Code too dense for the surface |
| Decode | Can it be read at all | Contrast plus geometry combined |
| Print growth / reflection | Cell size drift | Coating thickness variation |
Grade A or B (4.0 / 3.0) is the usual acceptance threshold; below C, expect scanner failures in the field. Verification should be done on the finished part, after coating — not on the bare machined part.
Castings follow their own rules: our cast part identification marking guide covers where the code goes and how deep it has to be on iron and steel.
Keeping codes readable after coating
This is where most programs fail, and the fix is a specification, not a machine setting.
- Anodized aluminium. Anneal marks survive Type II anodizing well because the mark is an oxide, not a pit. Mark before anodizing and the dye will change contrast; mark after and you break the sealed surface. Both work if the drawing says which one. See anodizing aluminium for how dye colour affects contrast.
- Powder coat and paint. A mark under 60 µm of powder will not scan. Engrave deeper than the coating build, or mask and mark after coating.
- Plating. Electroless nickel and zinc add 5–25 µm; a shallow laser mark can be partially filled. Compensate depth or mark post-plate.
- Passivated stainless. Laser anneal is the standard here, because it does not remove the passive layer the way deep engraving does.
Whatever the finish, verify after it. A pre-coat verification result is not evidence.
Industry rules that force the issue
| Sector | Driver | What the mark must carry |
|---|---|---|
| Medical devices | UDI (FDA / EU MDR) | Device identifier + production identifier, human-readable too |
| Automotive | PPAP / IATF 16949 lot control | Part number, batch, supplier code; IATF 16949 expects full genealogy |
| Aerospace and defence | UID / MIL-STD-130 | Unique item identifier, often dot peen |
| Electronics | Internal serialisation | Serial, date code, revision |
Regulated programs also expect the identifier to link back to material certificates and inspection records, which is why the mark format should be settled alongside the ISO 13485 documentation package rather than at first article.
We break the measurement side down by tolerance band in our guide to CNC inspection methods, from calipers through to written uncertainty budgets.
That number is what ties the part to its material certificate and its dimensional results inside a first article inspection report.
Design rules: where to put the mark
- Put it on a flat surface, at least 3 mm from any edge, hole or bend.
- Keep it off sealing faces, bearing journals and thread crests.
- Keep it off the clamping area, or the fixture will cover it.
- Orient it so a scanner has line of sight in the assembled state.
- Allow for coating build in the depth callout.
- Specify the datum relationship if the code location is functionally controlled.
Rules 1 and 4 are the two most often violated, and both are caught by a five-minute check before the drawing is released.
What to write on the drawing
A marking callout needs eight fields: method, code type, payload structure, size, depth, location, whether marking happens before or after finishing, and the acceptance grade. Written compactly:
LASER ANNEAL, DATA MATRIX 3 mm, PAYLOAD PN-LOT-SER, DEPTH 0.02 mm, LOCATION PER VIEW A, MARK BEFORE TYPE II ANODIZE, GRADE ≥ B PER ISO/IEC 15415
That is a specification a shop can quote and verify. "Mark with serial number" is not.
Cost and lead time
For machined parts, marking is usually a small line item — a laser cell adds seconds per part, and the real cost is the fixture and the verification step, not the beam time. Costs rise when the part needs a dedicated rotary fixture, when the mark must be applied post-coat with masking, or when every part needs a verified grade printout for a regulated dossier.
If you are still at prototype stage, mark the prototypes too. Serialising early is the cheapest way to find out that your chosen location gets covered by a bracket in assembly — a discovery worth far more at prototype than at low volume production.
CNC pop marking: the locating dot made in the same setup
Not every mark on a part is a serial number. A pop mark is a locating dot — a shallow point the machine puts on the workpiece to show where a secondary part sits. In NC data it is written as a zero-diameter hole; on the part it is a small dimple left by the spindle or a small drilled point. Fabrication software exports pop-mark positions together with the part geometry so the shop can place stiffeners, gussets and shear tabs without measuring anything by hand (Tekla Structures writes pop-marks directly into NC files).
The value of CNC pop marking is the setup. The dot is produced in the same clamping as the machining, so it inherits the accuracy of the machined datums instead of a tape measure. It adds very little cycle time, needs no consumable, and survives handling, blasting and coating. We use it on welded assemblies where a bracket has to land in the same place on every single unit.
CNC layout marking: scribing the layout instead of measuring it
CNC layout marking is the same idea applied to lines. Instead of an operator marking out centrelines, hole positions and part outlines by hand with a grease pencil, the machine scribes them: the centreline on a stiffener, the corner locations of a part still to be placed, or a partial outline that tells the fitter where a component goes. Machine builders variously call this scribing, layout marks or contour marks, and on some equipment the scribing head is a laser.
Like pop marking, CNC layout marking removes a manual step and the variation that comes with it. It pays off most where the assembly is error-prone. A full outline is the easiest to read, a partial outline is faster to scribe, and the choice is a straight trade between legibility and cycle time.
FAQ
What CNC machining services does Ruijin CNC offer?
Ruijin CNC provides CNC turning, CNC milling (3-axis and 4-axis), 5-axis CNC machining, sheet metal fabrication, metal stamping, and surface treatment including anodizing, plating and powder coating.
What tolerances and certifications does Ruijin CNC hold?
We hold tight tolerances down to ±0.01mm across all processes, and are certified to IATF 16949, ISO 9001 and ISO 13485.
What is the minimum order quantity and lead time?
We accept prototypes from 1 piece with no strict MOQ, provide free DFM review, and return a quote within 24 hours of receiving your CAD drawing and requirements.
How do I get a quote from Ruijin CNC?
Send your STEP file and drawing to gongtianbao@xhlmarketing.com or via WhatsApp +86 17620300785. We reply with a DFM review and a firm quote within 24 hours.
What is CNC pop marking?
A pop mark is a small locating dot the machine puts on a part to show where a mating part sits, written in the NC program as a zero-diameter hole. It is used for assembly and welding location rather than identification, and it is made in the same clamping as the machining.
What is CNC layout marking?
CNC layout marking is machine scribing of centrelines, hole positions and part outlines onto a workpiece, replacing manual layout with a tape measure and marker. Depending on the machine it is called scribing, layout marking or contour marking, and it locates parts for welding or assembly.
How is complex part marking handled?
Complex part marking is decided by geometry: the mark has to sit on a face a scanner can still see in the assembled state, clear of sealing faces, thin walls and clamping areas. Where no suitable flat area exists, we move the code onto a protected boss or fall back to a tagged bundle instead of a direct mark.
Can you do heavy part marking on large weldments?
Yes. Heavy part marking means bringing the marking head to the part instead of the part to a bench marker. Large weldments and castings are marked either in the machining setup or with a portable head on the floor, which also avoids a second handling and the re-fixturing error that comes with it.
What is the best alternative part marking method if laser marking is not suitable?
It depends on why the laser fails. On reflective or highly conductive metals where a fibre laser struggles, dot peen marking is usually the next choice, because it displaces material rather than colouring it and therefore survives paint and heat. On parts that cannot accept any stress concentration, inkjet or pad printing gives a readable code with no material removal at all. On soft or thin parts where a stylus would distort the wall, we use chemical etching or a low-force scribe. Every alternative part marking method trades legibility against surface damage, and that is exactly what we review at DFM stage.
Conclusion
Traceability is a drawing decision that happens to be executed by a laser. Choose the method against the alloy and the finish, size the code against the flat area you actually have, specify depth against coating build, and verify on the finished part to an ISO/IEC 15415 grade.
Ruijin Fenghui marks and verifies parts in-house under IATF 16949, ISO 9001 and ISO 13485, with 23+ years of precision machining behind the process. Send the drawing with the eight-field callout above and we will confirm the method, the grade and the finish interaction within 24 hours — contact us to start, or read how to specify finishing if the coating sequence is still open.