A SanJun customer once approved 800 pieces of galvanized steel control-cabinet bodies for a North American utility project. The geometry was right, the powder coating looked right, the prototype had passed every visual check. Six weeks after delivery, the assembly line started rejecting parts: the gland cutouts on 23 enclosures were off-position by 1.2 mm — enough to push the IP gasket out of spec. None of those 23 had been individually dimensionally-checked before they shipped. Sheet metal quality control is not paperwork. It is the inspection step that decides whether a program ships clean or ships a recall.
In this guide we will walk through how an OEM/ODM fabricator like SanJun actually runs quality control on a custom enclosure program — from the day raw coils arrive to the morning a pallet is loaded onto a truck. You will see the four inspection stages, the defects that show up most often, the measurement tools that catch them, and the documentation every B2B buyer should require before signing a PO. Use it as a checklist for your next supplier meeting.
Why QC Is Where Sheet-Metal Programs Die
Most sheet-metal failures are not catastrophic — they are quiet. A bend that runs 0.5° over tolerance is invisible to the eye, but it racks the assembly stack by 1 mm over a 600 mm span. A weld that is 0.2 mm under-spec on a stainless seam does not fail in the shop, but it corrodes within two winters. By the time the buyer notices, the production batch is on a truck, the customer is installing it, and the cost of poor quality has compounded.
The fix is not a longer ISO manual. The fix is inspections tied to specific production stages, executed by named people, with named acceptance criteria, against named records. If your supplier cannot show you that chain in a single page, the chain does not exist. For the upstream design work that prevents most of these issues in the first place, see our DFM guide; for the document trail at the back end, our prototype-to-production guide covers it from the other side.
The 4 Inspection Stages of a Sheet-Metal Program
Every program — whether 50 pieces or 50,000 — goes through the same four-stage inspection map. What changes between low-volume and mass-production is the sample rate and the records kept, not the stages themselves.
| # | Stage | When | What is checked | Sample rate | Records |
|---|---|---|---|---|---|
| 1 | Incoming (IQC) | When raw coils/sheets arrive | Mill cert, gauge, alloy, surface defect, flatness | 100% of coils (visual), 1 sample per coil for chemistry & gauge | Mill cert, MTC, incoming log |
| 2 | In-process (IPQC) | After each fabrication step | Cut edge quality, bend angle, weld profile, hole position | First-piece + every 25–50 pieces | Operator self-check, patrol inspection log |
| 3 | First Article (FAI) | Before production batch releases | Full dimensional + functional against the drawing | 1 part (often 3) per lot | AS9102 / PPAP / ISIR report |
| 4 | Outgoing (OQC / pre-shipment) | After finishing & assembly | Cosmetic, dimensional, function, packaging | AQL sampling (typically 1.0–2.5) | Dimensional report, CoC, packing list |
If any of these rows is missing, you are buying on trust. Sometimes that works; usually it does not.
Incoming Material Inspection
The cheapest inspection is the one that catches a problem before it is cut. We check three things on every coil that comes off a truck:
- Mill test certificate (MTC) — chemistry, mechanical properties, traceability heat number. Cross-checked against the purchase order.
- Gauge — micrometer measurement on three points per coil edge. Cold-rolled steel at 2.0 mm ordered will arrive between 1.95 mm and 2.05 mm (per our tolerance guide, ISO 2768-m). Outside that window: reject the coil.
- Surface condition — scratches, rust, oil staining, edge damage. A scratch that is invisible on a flat sheet becomes a visible mark after powder coating.
For stainless we also verify the finish grade (2B, BA, No.4) by reference sample. Mixed batches of 2B and BA will look identical under shop lighting but very different after forming. For material-specific selection, our stainless vs carbon steel guide walks through when each grade makes sense.
In-Process Inspection
Each fabrication step has its own checkpoint. We pattern the checkpoints off the same press-brake operators and welders who actually run the parts — because inspection only works when the person doing it knows what "good" looks like in their hands, not just on a print.
- After laser cutting: kerf straightness, dross level, smallest-feature integrity (holes ≤ material thickness are red-flagged for punch-tool switching).
- After bending: angle checked with a protractor or angle gauge on first piece and every 25 pieces; outside dimensions with a tape or caliper; springback verified against our bending guide K-factor tables. Those same K-factor values are what our CNC press brake bending calculator uses to predict tonnage and flat length before the first part is cut.
- After welding: visual on every weld (porosity, undercut, discoloration); dimensional check before and after weld to capture shrinkage; weld-test coupons pulled per shift for destructive testing on stainless and aluminum.
- After coating: film thickness (DFT) by magnetic gauge on steel, eddy-current on aluminum; visual on coverage and color consistency.
The point is not to inspect everything — it is to inspect each step at the moment you can still correct it cheaply.
First Article Inspection (FAI) — AS9102 / PPAP / ISIR
FAI is the inspection that links the drawing to the part. The three templates you will see in the wild:
- AS9102 — aerospace (Boeing, Airbus, Lockheed supply chains).
- PPAP — automotive (AIAG standard, 18 elements).
- ISIR — general industry / "Initial Sample Inspection Report" (often customer-defined, sometimes PPAP-lite).
All three ask the same fundamental questions: every feature on the drawing — every dimension, every GD&T callout, every note — has been measured on a real part, with a real tool, by a real operator, and recorded. The differences are in which features get photographed and how the material traceability chain is documented.
| FAI element | AS9102 | PPAP | ISIR (typical) |
|---|---|---|---|
| Material certs | Required | Required | Required |
| Full dimensional report | Required | Required | Required |
| GD&T verification | Required (CMM) | Required | Often required |
| Material verification (chemistry) | Required | Required | Sometimes |
| Process capability (Cpk) | For safety items | For special characteristics | Sometimes |
If you are not in aerospace or automotive, ask for an ISIR that follows AS9102's dimensional discipline — you get the rigor without the aerospace paperwork tax.
Outgoing / Pre-Shipment Inspection
This is the last gate. We run it as AQL sampling (Acceptable Quality Level), typically at 1.0–2.5 depending on customer risk. For a 500-piece batch at AQL 1.0, we sample 50 pieces and accept ≤1 defect. The defects that matter here:
- Cosmetic (scratches, dents, color mismatch).
- Functional (missing hardware, mis-positioned cutouts).
- Packaging (incorrect label, missing foam, wrong carton size).
Every shipment leaves with a dimensional report (the actual measured values, not just "pass"), a Certificate of Conformance (CoC), and a packing list. If your supplier cannot produce these in 30 minutes, the inspection did not happen.
Common Defects Catalog
Here are the eight defects we see most often on incoming inspection rejections. Each has a typical cause and a clear acceptance criterion.
| # | Defect | What it looks like | Typical cause | Acceptance |
|---|---|---|---|---|
| 1 | Bend angle out of spec | Angle off by >1° | Worn punch tip, wrong K-factor | ±0.5° on functional features, ±1° on cosmetic |
| 2 | Hole-to-bend too close | Hole elongates, edge cracks | DFM violation (see our stainless vs carbon steel guide for material-specific rules) | Reject; repair by plug-weld requires customer approval |
| 3 | Weld porosity | Pinholes in weld bead | Dirty base metal, wrong gas flow | No visible porosity on cosmetic welds |
| 4 | Weld discoloration (stainless) | Heat tint (straw → blue) | Insufficient argon shielding, see welding guide | Straw OK on internal, blue rejected |
| 5 | Coating orange peel | Textured "skin" on powder surface | Film too thick, oven ramp too slow | Visual standard sample; max 30% coverage |
| 6 | Coating DFT too low | Magnetic gauge <60 µm | Powder too thin, operator not back-brushing | 60–120 µm typical; per spec |
| 7 | Surface scratch on visible face | Linear mark on cosmetic surface | Handling, no protective film | Zero on cosmetic; >5 mm length on non-visible |
| 8 | Dimensional drift mid-batch | First piece passes, unit 200 fails | Tool wear, fixture loose | Daily Cpk check; trend out-of-control points |
Measurement Methods Buyers Should Know
You do not need a CMM for every part. You need to know which tool fits which feature, so your supplier is not over- or under-measuring.
| Feature to check | Tool | Typical accuracy | When to use |
|---|---|---|---|
| Thickness | Micrometer | ±0.01 mm | Every coil, every critical bend |
| Outside dimension (flat) | Caliper | ±0.02 mm | Most sheet-metal features |
| Outside dimension (>300 mm) | Tape / long-jaw caliper | ±0.5 mm | Cabinet bodies, large panels |
| Bend angle | Protractor / digital angle gauge | ±0.5° | Every bend, first-piece + sample |
| Hole position | Height gauge + pin / CMM | ±0.05 mm / ±0.01 mm | Toleranced holes, GD&T features |
| Flatness | Surface plate + dial indicator | ±0.05 mm | Base plates, mounting surfaces |
| Surface roughness (Ra) | Profilometer | ±0.1 µm | Sealing surfaces, cosmetic faces |
| 3D GD&T (position, profile) | CMM | ±0.01 mm | Functional interfaces, mating parts |
A fabricator that uses calipers for everything is being sloppy; a fabricator that uses a CMM for every dimension is padding the bill. The right tool for a 0.1 mm tolerance on a 10 mm hole is a pin gauge. The right tool for a 0.05 mm position tolerance on a 600 × 400 panel is a CMM.
Welding Inspection Specifics
Welding is where most sheet-metal programs actually fail — not at the bend, not at the cut, but at the seam. There are four inspection methods a common buyer should know about:
- Visual — every weld, every time. Look for under-cut, porosity, crater cracks, profile. Cost: nothing.
- Dye-penetrant (PT) — surfaces and near-surface cracks. Cheap, fast. Use on stainless cosmetic welds.
- Magnetic-particle (MT) — surface and near-surface defects on ferromagnetic materials (carbon steel, low-alloy). Use on critical load-bearing welds.
- Ultrasonic (UT) or radiographic (RT) — volumetric. Usually overkill for enclosures; reserve for pressure-bearing or structural applications.
We default to visual + PT on stainless seams. That covers the failure modes 95% of sheet-metal buyers care about. For more on process selection, see the welding guide.
Surface Finish & Coating Inspection
| Finish type | Ra target | Gloss (60°) | Adhesion (ASTM D3359) | Salt-spray (ASTM B117) |
|---|---|---|---|---|
| Powder coating (outdoor) | n/a | 30–80 GU | 5B (no pick-off) | 1,000 h |
| Powder coating (indoor) | n/a | 30–80 GU | 5B | 500 h |
| Anodized aluminum | 0.4–0.8 µm | matte–satin | n/a | 336 h (seal required) |
| Zinc-plated (clear chromate) | 0.8–1.6 µm | n/a | n/a | 96–120 h white rust |
| Stainless No.4 | 0.4–0.8 µm | matte | n/a | No measurable corrosion in normal service |
The right test depends on the environment. A telecom cabinet in Arizona cares about salt-spray hours and UV. A food-grade control panel cares about Ra and cleanability. A coastal EV charger cares about salt-spray and adhesion under thermal cycling. See the surface finishing guide for the full treatment menu.
Statistical Process Control (SPC) for Sheet Metal
SPC sounds heavy, but for sheet metal it boils down to three numbers:
- Cpk ≥ 1.33 for critical features, ≥ 1.0 for functional features.
- X-bar / R chart — plot the mean and range of a feature over time. Out-of-control points (rules 1, 2, 3 in Western Electric rules) trigger a process stop.
- Control plan — one row per critical feature: spec, instrument, sample size, reaction plan.
You do not need to enforce SPC across all 200 dimensions on a drawing. Enforce it on the 3–5 features that determine fit, function, and IP protection — typically the bend angles, hole positions for fasteners, and weld gaps.
Documentation Trail Buyers Should Demand
A fabricator with no documentation trail is a fabricator you cannot audit. At minimum, require these per shipment:
- Certificate of Conformance (CoC) — signed statement that the parts conform to drawing revision X.
- Dimensional report — measured values, not just "pass/fail."
- Mill test certificates — for the specific heat(s) used.
- Inspection & Test Plan (ITP) — what was inspected, when, by whom, against what acceptance criterion. How those criteria get written is set out in our enclosure qualification test plan guide.
- FAI report — for new parts or drawing revisions.
- PPAP / ISIR / AS9102 — for automotive / aerospace / safety-critical parts.
If they hand you a one-page CoC with no dimensional report, that is a yellow flag. The first time you ask for the report is when you find out whether the inspection actually happened.
How SanJun Runs QC
At SanJun we run the four-stage inspection map on every custom enclosure program, with these specific practices:
- Every coil logged with heat number, gauge measurement, and surface check.
- Every batch runs through first-piece + every-25-pieces sampling in production.
- Every new part has an FAI before production batch is released.
- Every shipment ships with a dimensional report, CoC, and (for safety items) a Cpk summary.
- We are managed under ISO 9001 with documented procedures for incoming, in-process, FAI, and outgoing inspection. For outdoor / IP-rated enclosures we add a hydrostatic / spray test to the OQC step.
We do not pretend to be a 5,000-piece-per-day automotive stamping shop. We are a 5,000 m² custom-enclosure OEM/ODM partner serving B2B buyers in EV charging, energy storage, telecom, industrial control and outdoor security — and our QC system is tuned to that mix: high mix, low-to-medium volume, and tight tolerance.
Frequently Asked Questions
What is a First Article Inspection (FAI) in sheet metal?
A documented dimensional report on the first part off a production run, showing every feature on the drawing has been measured and meets spec. Common templates are AS9102 (aerospace), PPAP (automotive), and ISIR (general industry). It is the inspection that catches drawing errors, fixture mistakes, and programming bugs before the rest of the batch runs.
How often should a fabricator do in-process inspection?
First-piece on every setup, then patrol inspection every 25–50 pieces depending on part complexity and risk. For safety-critical features (IP gasket surfaces, load-bearing welds) the interval drops to every 10 pieces. The point is to catch drift before it is locked into a batch.
Do I need a CMM for my sheet metal parts?
Only if your drawing has GD&T callouts tighter than ±0.1 mm or features that interact with mating parts. For most enclosures — control cabinets, telecom housings, charging pile bodies — a calibrated caliper, height gauge, and surface plate cover 90% of the inspection work. A CMM is the right tool for a 0.05 mm position tolerance on a 30 mm boss, not for a 0.5 mm flatness on a 600 mm panel.
What salt-spray hours do I need for outdoor enclosures?
It depends on environment. Indoor / sheltered: 500 h is enough. Outdoor / non-coastal: 1,000 h. Coastal / harsh-industrial: 1,500–2,000 h, plus UV resistance. The right answer is dictated by your end-customer's warranty window, not by what the powder supplier stocks.
Can a small custom shop do real QC, or do I need a Tier-1 supplier?
A small custom shop can do real QC if it has (1) a written inspection plan per part, (2) calibrated tools with traceable certificates, (3) named inspectors, and (4) records it is willing to show you on request. Size is not the issue; discipline is. Ask to see the most recent dimensional report and the most recent CoC for a past shipment — if the answer takes longer than 30 minutes, the system is not real.
Conclusion
Sheet metal quality control is not a checkbox. It is a four-stage inspection chain — incoming, in-process, first article, outgoing — backed by named acceptance criteria, calibrated tools, and records you can audit. The cheapest inspection is the one that happens at the right stage. The most expensive inspection is the one your customer does for you after delivery.
If you are vetting a custom sheet-metal enclosure manufacturer for your next program, send us your drawing and your QC requirements. We will return a quotation, an inspection plan, and a sample FAI template within 24 hours.