Structured light in the forge and foundry

Forging and casting is where industrial structured light grew up, and the connecting rod explains why better than any other part. As it leaves the die, it lives in a world of half-millimetre tolerances, mismatch and die wear — full-field territory. After machining, it lives at thirteen microns of bore roundness — hard-gauge territory. One part, two metrological lives, and the boundary between them is exactly where optical scanning earns its keep or doesn't.

How this report was built. Findings were researched from primary sources — standards documents, peer-reviewed studies, vendor technical documentation, foundry trade press — and each carries its confidence honestly: claims marked reportedly or vendor-sourced come from trade press or marketing material I could not independently verify; unmarked claims trace to the sources listed at the end. The 3D models are AI reconstructions for illustration — they are not scans of real parts, and no dimensional claim rests on them.

1 · Why rough parts scan well — and what actually fails

The intuition most people bring — rough surface, bad measurement — is backwards for structured light. A shot-blasted or as-cast skin scatters the projected fringe pattern diffusely, which is precisely what a fringe-projection sensor wants. The failure cases on a casting are usually the machined patches: bright, specular faces that mirror the projector instead of scattering it, and dark glossy areas that swallow it. A Czech forging-machine builder, Šmeral Brno, bought a portable optical-tracking scanner — per Creaform’s case study — specifically because it handled the shop's mix of glossy and rough surfaces with no preparation at all — something its laser tracker and CMM could not do. On the same logic, an iron foundry in Wisconsin — Willman Industries — inspects castings up to 30,000 lb with a sub-$20k handheld scanner and cut large-casting dimensional workups from 7–10 days of manual layout to 6–10 hours.

What actually corrupts scans of foundry parts is geometry, not texture. Deep pockets and concave features suffer multipath interreflection: projected light bounces between opposing walls before reaching the camera, adding false optical path length. The published analysis of the effect is worth knowing because the error is not what you'd guess — mild interreflection produces a systematic bias, but past a threshold the reconstruction develops a second, competing depth solution and phase unwrapping breaks down. The result is not noise you can average away; it is a confidently wrong surface. Practitioners on the ZEISS forum trade a concrete recipe for pockets: open the allowed sensor-to-surface angle up to 85°, accept overexposure on outer regions to get light into the pocket, use a smaller measuring volume, and give the rotary table more positions. Dark and shiny together — black glossy parts were the forum’s example — is the combination practitioners describe as nearly impossible without matting.

2 · The spray argument, with the numbers on the table

Which brings us to the longest-running argument in the field: matting spray. The debate has real numbers now, and they cut both ways.

A 2023 peer-reviewed study out of Brno UT measured eight commercial sublimating sprays with an ATOS III Triple Scan and found real coating thicknesses of roughly 24–43 µm at realistic multi-pass coverage — AESUB Blue at 32.6 ± 3.0 µm over four layers, Attblime AB Zero at 43.4 ± 9.2 µm. The manufacturer's datasheet for AESUB Blue says 8–15 µm per layer, and the study's per-layer figure agrees — the gap is that nobody sprays one perfect layer on a real part. Meanwhile the old-school answer, airbrushed titanium dioxide, measures around 3 µm — an order of magnitude thinner than any convenience spray (against ~44 µm for chalk spray) — it just doesn't vanish on its own. Sublimation times ranged from 47 minutes to 7.4 hours depending on product, over-spraying cracked some coatings, and one product left hundreds of holes in the mesh regardless of layers. The UK national lab's good-practice guide for optical point clouds says it plainly: expect matting sprays to cost precision "in the order of tens of microns."

The decision rule that falls out of the data: on a sand casting with millimetre-class tolerances, spray freely — 40 µm of coating is noise. On a precision casting or any machined datum you intend to measure optically at the 25–75 µm level, the spray is a significant fraction of your error budget: scan bare if the surface allows, switch to TiO₂ airbrushing if it doesn't, and treat "we always spray everything" as a process smell.

3 · What foundries and forges actually do with scanners

Die and pattern correction is the founding use case. Scan the casting, compare full-field against nominal, correct the tooling — the deviation colormap replaced the layout table. Two published details matter more than the marketing version of this story. First, the reference isn't necessarily design CAD: Grede Foundries inspects patterns against a golden STL of the approved pattern, because tooling intentionally deviates from CAD to accommodate shrink — the colormap's zero is itself a negotiated artifact. Second, in the peer-reviewed die-correction workflow, registration uses machining datums rather than best-fit, because best-fit alignment absorbs exactly the systematic deformation the correction loop is trying to see. Grede's economics, from trade press: pattern-scan baselines eliminated three to five sample-casting iterations, and one brake-bracket job cut scrap from 5.2% to 1.0% — about $48,000 a year on that part alone. (Grede’s instrument was a CMM-mounted laser scanner rather than structured light; the workflow is sensor-agnostic.)

Die-wear tracking without pulling the die: the Wrocław forging group publishes a technique they call reverse scanning — scan sampled forgings cyclically and watch the forging envelope grow over part count. The forging becomes the gauge for the tool, and die washout maps zone by zone while the die keeps running.

Scanning inside the forge itself is no longer exotic. Forges de Courcelles — reportedly Europe's second-largest crankshaft forger — runs automated ScanBox cells at three stations inside the crankshaft forging workshop, with press operators reading color deviation maps between strokes; per-part measurement that took 10–20 minutes now feeds SPC continuously (vendor-sourced case study). The physics of hot parts has been solved separately: a published in-line system measures forgings at ~900 °C with 0.11 mm repeatability on a 27-second cycle, using blue-light projection with a blue band-pass filter — blackbody glow concentrates in red and infrared, so the filter simply removes the incandescence — plus active cooling to keep the sensor alive. For everyone without that hardware, heat sets the practical rule: a conrod-sized forging leaves the die near 1,000 °C, cools at 100–150 °C/min, and needs roughly ten minutes before thermal expansion stops swamping a 0.1–0.3 mm tolerance band.

Machining-stock verification closes the loop between foundry and machine shop: nest the finished-part CAD inside the scanned blank and confirm cleanup everywhere before the first chip. A steel foundry in one published account found 0.4–0.6 inches of unintended excess metal along a cope edge on first articles — a condition manual layout could not have mapped. And where the geometry is internal — cored passages, water jackets, porosity — structured light simply ends at the surface: that territory belongs to CT, at $100k–$1M+ capex or a few hundred dollars per scan at a bureau, which is why it stays a sampled audit tool while optical covers dimensional work.

4 · Who leads, and why the software mattered as much as the sensors

The short version of the market: ZEISS owns the cell, Creaform owns the floor, and everyone else is fighting for third.

The ATOS line — blue-LED fringe projection with stereo cameras — came out of GOM of Braunschweig (founded 1990), which ZEISS acquired in 2019; in 2021 ZEISS also bought Capture 3D, its largest US distributor, so a North American foundry buying ATOS now deals with ZEISS-owned channel either way. The automated ScanBox cell debuted in 2012 with a foundry — Eisenwerk Brühl, cast-iron crankcases — among its first named installations, and the platform now spans series sized from 500 mm parts to full car bodies. The reference installations run from VW Kassel (Europe's largest light-metal foundry, ~200 components a day through a ScanBox plus tactile CMMs, reported through PiWeb) to Doncasters Bochum (investment-cast turbine blades, measuring times cut 2–3×) up to Siempelkamp, which measures sand castings to 320 tonnes in what ZEISS calls the world's largest non-contact robotic measuring cell (vendor-sourced case studies throughout).

Creaform — owned by AMETEK since 2013, which has since also absorbed FARO, consolidating the portable field under one roof — took the other regime: handheld and optically-tracked laser scanners with dynamic referencing, so part and scanner can both move on a vibrating shop floor. Named adopters include Fonderie Ariotti (structural castings, Creaform-reported 75 µm shop-floor accuracy), GF Casting Solutions, and — on the forge side — Šmeral Brno. The practical technology split, synthesized across every case I could find: fringe-projection cells win automated serial inspection; blue-laser portables win the floor, the very large, and the mixed shiny-rough surface without spray. Hexagon trails in structured light on foundry floors — my read of the case-study record — and answers with laser-based robotic cells; Artec's sub-$20k handhelds do real problem-solving work below metrology-cell grade. Budgetary classes, cross-checked across reseller listings: $25–60k entry handhelds, ~$80–100k+ configured portable systems, $150–300k+ for an ATOS-class system, $150–500k+ for automated cells — plus software that buyers routinely under-budget.

One structural observation that rarely makes the brochures: the free viewer was a weapon. GOM Inspect's genuinely free tier meant a foundry could send a full inspection project — mesh, CAD comparison, GD&T, report — to any customer, who could open it at no cost. Deviation colormaps became the lingua franca of casting sign-off in large part because the reading software was free, and the file format lock-in pulled the hardware along behind it. Vendors understand ecosystems, not just optics.

5 · GD&T on parts that were never machined

Dimensioning an as-forged or as-cast part is its own discipline, codified in standards most machining-side engineers never open — and it is where scan-based inspection either respects the drawing's logic or quietly violates it.

Datum targets, not datum surfaces. A full as-cast face carries draft, parting-line edges, and gate grind-offs, so the drawing specifies where to touch: point, line, or area targets per ISO 5459 and ASME Y14.8, placed by theoretically exact dimensions. A DLA-sponsored metalcasting case study shows the craft: primary targets on a single mold-half's face, secondary and tertiary targets at identical elevation on 2°-drafted sidewalls so the draft cannot shift the part's located position — and the target map doubles as an instruction to the foundry about where not to put gating and parting features. In scan software, a 3-2-1 target scheme evaluated on the mesh is mathematically the same datum frame the fixture builder would have made with pins — if the software applies targets rather than best-fitting the whole surface.

Profile of a surface is the workhorse, because one callout can control a drafted, radiused, flowing as-forged shape against a target-based frame; ISO 8062-4 builds its entire general-tolerance scheme for castings on exactly that construction. The casting standards carry traps for the unwary evaluator: ISO 8062-3's general geometrical tolerances explicitly do not apply to features with draft — which is most of an as-cast surface — and wall thickness always takes one tolerance grade coarser than the part's general dimensional grade. For steel drop forgings, EN 10243-1 determines everything from five inputs (mass, die-line shape, steel category, shape complexity, dimension type), and its most operationally important rule is that mismatch, residual flash, and centre-to-centre tolerances apply independently of and in addition to all other tolerances. Software that folds die mismatch into a single profile evaluation is misapplying the standard — mismatch is its own characteristic, measured parallel to the die line at areas least affected by wear. One more contractual sharp edge from the same standard: the agreed forging drawing — not the customer's CAD, not the finished-part drawing — is "the only valid document for inspection of the forged part."

For scale, the numbers on a car-sized conrod forging under EN 10243-1 grade F: centre distance ±0.4 mm, mismatch 0.6 mm, residual flash 0.7 mm, each assessed separately. Hold that thought for the next section, where the same part's machined bores get a 25 µm size window.

6 · The connecting rod dossier

The rod at the top of this page is the report's case study because it compresses the whole industry into one part.

How they're made now. Drop-forged steel dominates Europe and China; North America reportedly runs mostly powder-forged rods (GKN's route, over half a billion installed since 1986). The big end is no longer sawn and machined as a separate cap: fracture splitting — notch the bore, shatter the big end in a press, bolt the halves back — has become the mainstream route, deleting up to half the machining steps. The metallurgy is deliberately perverse: C70S6 crack steel keeps sulfur high (0.045–0.07%) and yield low so the fracture runs clean and brittle; the stronger second-generation grades are actually harder to split well, fracturing slower with more tear-prone surfaces.

The unmeasurable feature. The fracture face itself carries no dimensional specification and must never be touched — the manufacturer's service literature forbids reworking it, resting the rod on it, even brushing it, because the joint's location accuracy comes from interlocking crack topography. An entire functional interface, deliberately outside metrology's reach: quality is inferred from proxies — big-end roundness after bolting (the split alone deforms a C70S6 big end by ~33 µm before the crack initiates, more than twice the second-generation steels), pairing-number integrity, and the fine-boring operation that follows.

Mass is a first-class characteristic. Rods carry (or carried) balance pads whose only function is to be ground away for weight matching — OEM sets historically matched to a few grams, race sets to ±0.5 g or better, weighed end-by-end in two-pan fixtures. A 1990s Opel machining line already did 100% in-line weighing with automatic classification at a ~6-second effective takt with one operator — weight grading is a sorting operation, not a lab measurement. And flashless precision forging closed the loop from the other side: billet volume control got good enough that the small-end balancing boss — and its machining operation — could be deleted entirely.

Where scanning enters. In conventional hammer forging, flash consumes an astonishing share of the billet — a recent process study measured 61% of the charge going to flash, cut to 49% by a redesigned preform, with the improvement verified by 3D scan colormaps every hundred pieces. That's the pattern across the published record: structured light lives at the forge — die fill, die wear, mismatch, preform development, sampled full-field conformance after the ten-minute cooldown — while the machining line stays hard-gauge (air gauges and bore gauges at 6-second takt, roundness checked with the cap torqued) and assembly owns weighing and crack proxies. Three inspection regimes, one part; the scanner owns exactly one of them, and that's not a limitation — it's the correct division of labor.

The market context, for anyone planning QC investment: hybrids, not battery EVs, set the conrod demand curve — every HEV and PHEV still contains rods — and one market forecast (vendor forecast, treat accordingly) has the market growing through 2036 on "demand redistribution rather than disappearance." The observable engineering response is automation of legacy forge lines rather than new capacity: recent papers study robotizing existing hammer lines at plants running millions of rods a year. Defending margin on mature volume is precisely the environment where scrap-rate and die-life economics — the things forge-side scanning measures — pay for instrumentation.

7 · Can you trust the scanner? Standards and the acceptance argument

The argument between "the scanner is certified" and "my customer only accepts CMM data" has a real technical substrate, and 2021 changed its terms.

For two decades the acceptance framework was a German guideline, VDI/VDE 2634 — probing error on a sphere, sphere-spacing error on a ball bar, flatness on a plane, sized to the measuring volume. Both relevant sheets are now formally withdrawn in favor of ISO 10360-13:2021, the first ISO acceptance standard written for self-contained optical 3D scanners — yet vendor certificates issued as recently as 2025 still say "with reference to VDI/VDE 2634 Part 3," the paperwork lagging the standards landscape by years. A real 2025 acceptance certificate for a mid-size ATOS volume shows what the numbers look like in practice: MPEs of 3–13 µm depending on characteristic, DAkkS-traceable sphere-bar artefact, measured results 3–10× inside the limits.

ISO 10360-13 is a better test — six characteristics instead of three, including distortion and concatenated-volume length errors — and it contains one clause every buyer should know: results are evaluated at both a 95th-percentile and an all-points basis, explicitly because the difference "can reveal influences of smoothing filters… not always transparently visible for users." A standard written to police spec-sheet games. Two hard caveats keep it honest. NIST showed in 2024 that even the improved length tests fail to consistently detect all systematic calibration errors in structured-light systems — a passed certificate is weaker evidence than most quality engineers assume. And the standard explicitly excludes handheld laser-line scanners — arguably the most common portable instruments on foundry floors — leaving them without a dedicated ISO acceptance route at all.

On the correlation question, the measured picture is bias, not noise: against a tactile CMM, structured-light step heights in a DOE-funded study read 23–36 µm high with non-overlapping error bars, and small-hole diameters erred by up to ~125 µm, improving rapidly with hole size. The same study demolished the standard gauge-R&R framing: a 2.9 mm hole's diameter spread collapsed from ~65 µm to ~8 µm just by scanning from 15 positions instead of 5 — the "operator" in an optical MSA is really scan-position count and meshing, which classical AIAG crossed studies were never designed to isolate. Hence the doctrine you actually see in regulated supply chains: hybrid inspection — tactile CMM holds the datums and tight primitives, full-field optical owns freeform surfaces and wall thickness, X-ray or CT owns the inside — and OEM acceptance of scan data turns out to be procedural, not physical: in the Boeing chain it's digital-product-definition process approval (measurement software verification included), and AS9102 Rev C explicitly contemplates verification against 3D model data. The scanner isn't on trial; your process documentation is.

8 · What this means if you run the QC lab

Surface logic beats instinct: as-cast and shot-blast surfaces are friendly targets; machined datums, dark glossy patches, and deep pockets are where scans fail — and interreflection fails confidently, not noisily.
Spray by the numbers: convenience sprays measure 24–43 µm at real coverage; TiO₂ ~3 µm; bare where possible. Match the decision to the tolerance band, not to habit.
Registration is a policy decision: best-fit hides exactly what die-correction loops need to see; datum-target evaluation on the mesh must replicate the drawing's target scheme. Mismatch and flash are separate characteristics by standard, not components of profile.
Know which regime you're in: forge-side sampled full-field, machining-side 100% hard gauge, assembly-side proxies. Structured light owning one regime is the design, not the deficiency.
Acceptance ≠ task uncertainty: a sphere-bar certificate — VDI 2634 or ISO 10360-13 — bounds nothing about your casting's fillets. Task-specific uncertainty needs substitution-style studies on calibrated parts, and optical MSA must vary scan strategy, not just operators.
The deliverable question is real: golden mesh, shrink-compensated tooling model, or design CAD as reference changes what a colormap means. Decide it in writing before the first scan.

Everything in that list is pipeline work — the evaluate-decide-move-deliver-defend chain that starts where the scanner stops. If your foundry or forge shop has the sensor and the colormaps but the decisions above were made by defaults, that is exactly the work I offer — starting with a fixed-fee pipeline assessment: hello [at] metrologymaven [dot] io.