Metal Parts Quality Inspection Checklist: What to Check Before Accepting a Shipment

I have watched a lot of shipments come through a loading dock over the years. Some go straight to stock without anyone looking twice. Others sit in quarantine for a week while someone argues about a 0.05 mm deviation. Neither approach is right. What you need is a checklist. Not a vague guideline. An actual list of things to check, in order, before you sign the receiving paperwork.

The cost of skipping inspection is not hard to calculate. A batch of out-of-spec components that reaches your assembly line means rework, line stoppage, or a recall. I have seen a single missed burr on a gear tooth destroy a transmission test rig. The checklist below walks through dimensional verification, material conformance, surface condition, and functional checks. These are the four gates that catch most problems before they become someone else's problem.

1. Dimensional inspection: measure what matters first

Start with the print. Every dimension on the drawing has a tolerance block, and not all of them carry the same risk. The features that mate with other components (bearing seats, seal diameters, locating faces) are where you spend your inspection time. A cosmetic radius on a non-functional edge? You can sample that. The bore diameter that takes a press-fit bushing? Check every single one in the first article and at least one per cavity or fixture position in production.

For parts produced by CNC machining, dimensional inspection is relatively straightforward. The process is repeatable and the surface is clean. You still check it. But I worry more about castings and MIM parts, where shrinkage and distortion come with the territory. Investment castings are a good example. You cannot assume every cavity in the tree behaves the same way. They do not.

What to check and how:

• Critical dimensions: CMM or height gauge with dial indicator. Hand calipers are not enough for anything under ±0.05 mm.
• Threads: Go/no-go gauges, every threaded hole. A single oversized tapped hole ruins the mating fastener. It also wrecks the assembly sequence.
• First Article Inspection (FAI): Every dimension on the print, measured and recorded. AS9102 if you are in aerospace, or the equivalent PPAP dimensional layout for automotive.

I learned the hard way that skipping FAI on a "repeat" order is a bad idea. Tooling wears. Operators change. One shop I worked with ran the same aluminum housing for three years. On batch 41, the fixture had shifted just enough to push a dowel hole 0.08 mm out of position. The parts assembled fine at the shop. The customer's automated line jammed on the first pallet. A 20-minute FAI would have caught it.

2. Material verification: certificates are only the beginning

Every shipment should come with a material certificate. A 3.1 certificate per EN 10204 is the minimum I expect. The certificate gives you the heat number, chemical composition, and mechanical properties. Here is the thing: I have seen certificates that do not match the parts.

That sounds alarming. It is. It happens less often than it used to, now that traceability requirements in automotive and medical supply chains have tightened. But it still happens. The fix is Positive Material Identification (PMI) using handheld XRF or OES. For critical applications, PMI is not optional. For less critical parts, I still spot-check one or two pieces per batch, especially when the material is something expensive or safety-related like stainless steel 316L or titanium.

Material verification checklist:

• Match the heat number on the certificate to the heat number stamped or tagged on the parts. No match, no acceptance.
• Verify chemistry via PMI for at least one sample per heat. Pay attention to chromium and nickel in stainless steels. These drive corrosion resistance.
• Check mechanical properties on the cert: tensile strength, yield strength, elongation. If the cert says the material meets ASTM A240 for 316L, confirm the tensile values fall within the standard's range.
• For investment casting parts, request a separately cast test bar from the same heat. A test bar poured from the same ladle tells you more about the actual casting quality than a mill cert alone.

Hardness testing is another quick check you can run at incoming inspection. A Rockwell or Vickers test takes 30 seconds and tells you whether the heat treatment hit the target. A batch of 4140 parts that was supposed to be hardened to HRC 38-42 but comes in at HRC 28 is a problem. It will surface when the parts wear out in half the expected life.

3. Surface finish and visual inspection: your eyes are a gauge

Surface finish matters for more than appearance. A rough surface on a sealing face leaks. A rough surface on a bearing journal increases friction and wear. For parts that move or seal, the Ra value on the print is not a suggestion.

Visual inspection checklist:

• Surface roughness: Use a profilometer on functional surfaces. Compare against the print requirement. Ra 0.8 µm is a common machining finish. Ra 1.6 µm is typical for general-purpose surfaces. Anything above Ra 3.2 µm on a sealing surface should raise questions.
• Burrs and sharp edges: Run a fingernail along edges. If it catches, the deburring was incomplete. A burr on a threaded hole entry will cross-thread the fastener.
• Cracks and porosity: Dye penetrant inspection (PT) for surface-breaking defects on castings and welded assemblies. Magnetic particle inspection (MT) for ferromagnetic materials. Both methods are inexpensive and easy to train someone on.
• Surface contamination: Oil residue, cutting fluid, rust preventative. Acceptable if specified, but if the print calls for clean and dry, verify it.

For frequently asked questions about surface finish standards and inspection methods, check our FAQ page.

One thing I have noticed over the years: shops that do good visual inspection tend to do good dimensional work too. It is not a guarantee, but it is a leading indicator. If a shop ships parts with obvious cosmetic issues (scratches, dents, oxidation), the odds that they paid attention to the tolerances are not great either.

4. Functional testing: make it prove itself

Dimensions can be right and the part can still fail. Functional testing closes the loop between "matches the print" and "works in the assembly."

The type of functional test depends entirely on the part. Here are the ones I reach for with common part types:

• Threaded components: Run a fastener through every threaded hole by hand. Power tools hide cross-threading. Your fingers will feel it immediately.
• Press-fit features: For interference-fit bores, measure the actual interference. A 0.02 mm interference on a 20 mm bore is not the same as a 0.02 mm interference on a 200 mm bore. Use Go/No-Go plug gauges for production checks.
• Moving assemblies: Rotate, slide, articulate. Whatever the part is supposed to do, make it do it ten times. If it binds on cycle seven, the print will not tell you that.
• Sealing surfaces: Pressure test or vacuum test per the design specification. A flatness check with a surface plate and feeler gauge is the minimum, but it does not catch porosity.

In the automotive industry, functional testing is baked into the PPAP process. You do not just measure the part. You bolt it onto a fixture that simulates the mating component and verify fit, torque, and function. I think this is the right approach for any industry where a failure stops a line or risks safety. The cost of building a simple functional test fixture is almost always less than the cost of one line stoppage.

5. Paperwork review: what to keep and what to skip

Not all documentation is worth the folder space. Some of it is essential for traceability and for protecting yourself if something goes wrong later.

Documents you should have before signing off:

• Certificate of Conformance (CoC): The supplier's statement that the parts meet the drawing requirements. Signed and dated. This is the baseline.
• Material certificates (3.1 or 3.2 per EN 10204): As discussed above. File these by heat number.
• Inspection report: Dimensional results for the critical characteristics. If the supplier uses a CMM, ask for the CMM report file, not just the summary page.
• FAI report: For new parts or significant process changes, per AS9102 or equivalent.
• Process certifications: If the parts require special processes (heat treating, plating, welding), the certs for those processes should be traceable to the part batch.

Documents you can usually skip: general quality manuals, ISO certificates you have already verified during supplier qualification, and marketing brochures. You are inspecting parts, not auditing their QMS for the third time this year.

I store everything digitally now. A batch folder with the PO number, date, part number, and all inspection records. If someone in three years asks whether batch 47 of that stainless bracket was PMI-checked, I can answer in 30 seconds instead of digging through filing cabinets.

The checklist, distilled

Here is the one-page version I would tape to the inspection bench:

1. Print and PO in hand. Check revision level matches.
2. Quantity count. Sounds obvious, but short shipments are depressingly common.
3. Visual: cracks, burrs, oxidation, contamination. Dye penetrant or mag particle if specified.
4. Critical dimensions: CMM or height gauge. Every critical-to-function dimension on the FAI. Sampling plan for production lots.
5. Threads: Go/no-go gauge on every threaded feature.
6. Material: Certificate matches heat number. PMI on at least one sample.
7. Hardness: Rockwell or Vickers, especially after heat treatment.
8. Surface finish: Profilometer on functional surfaces.
9. Functional check: Fit, motion, sealing. Whatever the part is supposed to do.
10. Paperwork complete: CoC, material cert, inspection report filed by batch.

This is not theory. Every item on this list comes from a real problem that reached a customer or a production line because someone skipped that check. Including me, more than once. The checklist does not make inspection foolproof, but it makes it repeatable. Repeatable inspection is the difference between a quality program that works and one that only works when the senior inspector is in the building.