The Custom Fabrication QC Checklist: 6 Things to Verify Before Approving Parts

If you've ever approved a batch of custom-manufactured parts, slept on the decision, and regretted it later — this one's for you. I'm the quality/compliance manager at Star Micronics (star-micronics.com), and I review somewhere around 200 unique fabrication orders a year: CNC machined components, laser-cut enclosures, 3D printed prototypes, sheet metal work, and the occasional injection molding run. Over 4 years of doing this, I've put together a checklist that catches most problems before they become expensive rework.

This isn't a theoretical framework. It's the actual checklist I run here, in the order I normally do it. First time through, give yourself an hour. After that, it's faster.

Step 1: Verify material certifications before touching the parts

Paperwork isn't exciting, but it's the most expensive step to skip. Ask for the mill certificate or material test report on every batch you receive. For food and beverage CNC machining, this is non-negotiable: parts that contact food need FDA-compliant materials like 304 or 316 stainless steel, and you need the paper trail to prove it. If a vendor says "we'll send the cert later," treat that as a red flag.

Checkpoint: Does the grade on the cert match your purchase order? Is the heat number traceable to the parts on your bench?

Step 2: Measure critical dimensions yourself

I learned this one the hard way. A few years back, I assumed "same specification" meant identical results across two vendors. Same part number, same tolerance callouts, same material. Turned out each vendor had a slightly different interpretation of "within tolerance." Their parts passed incoming inspection on paper, then failed a simple fit test on our side.

I'm not 100% sure why their report said 4.52mm when my calipers read 4.61mm, but after that, I stopped trusting inspection reports alone. Don't hold me to those exact numbers—the point is, spot-check critical dimensions. Pick at least three per part and measure with calibrated tools.

Checkpoint: If you don't have measurement tools in-house, ask the vendor for their inspection report and the raw dimensional data, then verify with a nearby shop if needed.

Step 3: Look at the finish — actually look

Surface finish is where "acceptable" gets fuzzy. CNC parts can carry tool marks that don't affect function but look bad to your end customer. Laser-cut parts have edge conditions to check: fiber lasers give cleaner edges on thin metals, while CO2 lasers handle thicker material well but may leave more dross on the bottom edge. Both can be within spec, depending on what you asked for.

Quick tangent: CO2 laser technology shows up well beyond the fab shop. The same principles power medical devices like the DEKA SmartXide CO2 laser used by dermatologists — you'll find practices offering treatments in NYC's Flatiron District and elsewhere. Different power levels, different beam delivery, but the underlying physics is the same. The takeaway for buyers: a laser's quality depends on its configuration, so ask your fabricator what laser source and settings they used for your job, rather than just accepting "laser cut."

For food and beverage CNC parts, finish also isn't just cosmetic. A rough surface can harbor bacteria, so a 32 Ra finish might be just as important as the material grade.

Checkpoint: Compare the actual finish to your approved pre-production sample. If it looks different or feels different, it's different.

Step 4: Clean and inspect resin 3D print vats

If you work with resin 3D printers, a lot of quality problems start with a dirty vat. People often ask me how to clean resin vat 3D printer film without damaging it. Here's the routine we use in our shop:

1. Wear nitrile gloves and make sure the area is ventilated. Uncured resin isn't something to play with.
2. Inspect the vat for cured resin bits stuck to the FEP film. These are the usual cause of sudden layer lines.
3. Pour the resin back through a mesh filter into its bottle.
4. Use a plastic scraper to remove any remaining residue, then wipe the film with a lint-free cloth and isopropyl alcohol (85% or higher works).
5. Check the film for scratches or creases. A damaged FEP film will show up on every subsequent print.

We do this after every dark resin print — about 15 minutes of work. It prevents the majority of print failures we see, but people skip it because it's boring. Then they blame the settings when the real culprit is a piece of cured resin stuck to the film.

Checkpoint: If your last print has a defect at the same layer height repeatedly, clean the vat before you change anything else.

Step 5: Fit-test before you pay the final invoice

You'd expect this to go without saying, but I've watched projects rush right past fit testing because the timeline slipped. In one case, two mating parts looked perfect on screen but didn't align when produced, because different print orientations led to slightly different shrinkage.

The urgency was real, and the temptation was to ship the parts and fix it later. But shipping a wrong part on time is still a wrong part. A rushed reorder with expedited shipping always costs more than the $400-500 you might save by skipping the check. In my view, when a deadline is tight, paying for rush service is reasonable — you're buying certainty, not just speed. An uncertain schedule at a discount is a bad bet when your launch date is on the line.

Checkpoint: Physically assemble or test the part in its intended use condition, not just on a bench where it sits fine.

Step 6: Write it down and get sign-off

After 4 years of reviewing fabrication orders, I've come to believe that if it isn't written down, it doesn't exist. Before production, a spec sheet should be signed by both parties, covering material grade, dimensions, tolerances, surface finish, packaging, and shipping method.

This also matters for what you claim to your customers. Per FTC guidelines (ftc.gov), advertising claims must be truthful, not misleading, and substantiated with evidence. If you market your part as "food-safe" or "precision-machined," your signed spec sheet and material certs are what back those claims. Keep them on file.

Checkpoint: Does your purchase order reference the signed spec sheet? If a vendor says "industry standard," ask them to define it in writing.

Mistakes I won't repeat

I keep a file of lessons learned, mostly from painful experiences. Here are the recurring ones:

• Assuming the prototype represents the final product. Production runs sometimes happen on different equipment than the prototype. Verify the actual production parts, not just the sample.
• Choosing the lowest quote on a tight deadline. I once saved $120 by picking a cheaper supplier for a prototype run. The prototype arrived three days late with wall thicknesses all over the place. The reprint and expedited shipping cost $470 more than the original "premium" estimate — which happened to be the estimate I tried to avoid. That's a pretty expensive lesson in favor of paying for reliability.
• Discarding shipping packaging too early. We received a batch of machined fittings via USPS and one had a visible dent. The box looked intact, but because we kept the packaging and packing list, the claim and replacement went through in days rather than weeks. USPS's claim procedure, documented on usps.com, asks for photos and the original packaging — easy to provide if you leave the box alone until inspection is done.

Everything connects back to the same idea: good parts come from good process. A clean resin vat produces better prints. A signed spec sheet prevents arguments about tolerance. A documented inspection catches the cracks before you explain them to your customer.

Final note

This checklist has saved me from signing off on bad work more times than I can count. It's basically the difference between hoping you got what you ordered and knowing you did.

If you'd like to see how we handle quality control at our shop, the star micronics official site has a process overview section that explains our documentation flow. The Star Micronics website also lists our equipment and capabilities—CNC machining, laser cutting, additive manufacturing, and more. Or just reach out through star-micronics.com; happy to talk through your project.

Your mileage may vary — every shop is a bit different. But in my experience, these six steps cover roughly 95% of the quality checks that matter. Take it from someone who's paid the price for skipping them.