CNC First Article Inspection Checklist for Buyers
A CNC first article inspection verifies that a new part meets drawing tolerances, material specs, and process requirements. This checklist helps buyers catch errors early, prevent batch failures, and ensure the supplier can hold quality during full production.
- Verify dimensional accuracy, material certification, and surface finish before approving a full batch.
- Inspect critical features first, especially those driving assembly or function.
- Keep records of first article findings to hold suppliers accountable during production.
- Use the checklist to identify process weaknesses and correct them early.
- Communicate inspection results clearly to the supplier to prevent repeat defects.
A CNC first article is your first real check that the part works. Before you commit to a full production run, you need evidence that dimensions, material, and surface quality match the drawing. This inspection separates a good sample from a risky batch.
What belongs on a CNC first article checklist
The goal is simple. Confirm the part meets the drawing, the process is stable, and the supplier can repeat the result. A practical checklist covers three areas: geometry, material, and documentation.
Start with geometry. This includes all critical dimensions, tolerances, and surface finish values. Do not inspect every line on the drawing. Focus on features that affect fit, function, or assembly. A small tolerance violation on a non-critical surface may not matter. The same violation on a mating surface or a bearing seat can stop the part from working.
Next, check material. The drawing may specify an alloy, heat treatment, or surface treatment. Verify the certificate matches the part. A supplier can use the wrong steel grade if they are not careful. The part may look fine on the bench. It can fail in service.
Finally, review documentation. The first article should include the inspection report, material certificate, and process notes. Without these, you have a part, but not a process. If the supplier changes fixtures, tooling, or offsets, the next batch may not match the first.
Geometric checks that catch most failures
Dimensional accuracy
Measure the critical features against the drawing. Use calibrated tools. A CMM is ideal for complex parts. A micrometer and a pin gauge may be enough for simple parts. Record the actual value next to the drawing tolerance. Do not just mark pass or fail. A value near the tolerance limit is a warning sign.
Critical surfaces
Check surfaces that contact other parts. Mating surfaces, bearing seats, and sealing faces need close attention. Run a feeler gauge on a shaft and bore. Check concentricity with a dial indicator. Surface roughness matters too. A rough surface can increase wear or prevent a seal from seating.
Feature relationships
Sometimes the individual dimensions are correct, but the features do not align. A hole may be the right size, but offset from the center. A flat surface may be the right thickness, but warped. Check the relationships between features, not just the individual values.
Material and process checks
Material certificate
Ask for the mill certificate. It should list the alloy, heat lot, and chemical composition. Match it to the part number. If the supplier cannot provide it, that is a red flag. A missing certificate means you do not know what you have.
Heat treatment and surface finish
If the drawing calls for heat treatment, verify the hardness. A rockwell or Brinell test is standard. A part that is under-hardened will wear. A part that is over-hardened will chip. For surface treatments like plating or anodizing, check thickness and adhesion.
Tooling and process notes
Ask how the part was machined. Which machines were used? Which fixtures held the part? Which offsets were applied? These details matter. If the process is not stable, the next batch will drift. A supplier who uses a hand-holding fixture for a precision part is taking a risk.
Documentation and records
Inspection report
The first article report should list every measured value. It should also note any deviations. If a feature is out of tolerance, the report should say so. A report that only says “pass” is not useful. You need the data to track trends.
Material certificate
Keep the mill certificate with the part. It is your proof of material. If the part fails in service, the certificate is your first line of defense. Without it, you cannot prove the supplier used the correct material.
Process notes
Ask the supplier to document the process. This includes fixture setup, tooling, and offsets. If the process changes, the next batch may not match. A documented process is a stable process.
Red flags that stop a production run
Missing or incomplete documentation
A supplier who cannot provide a full inspection report is not ready for production. They may be cutting corners. They may not have the tools to measure. They may not understand the drawing.
Values near tolerance limits
If a dimension is inside the tolerance but close to the limit, the process is not stable. The next batch may fail. A process that produces values in the middle of the tolerance is healthy.
Unexplained deviations
If a feature is out of tolerance, the supplier should explain why. Was it a setup error? A tooling issue? A measurement error? An unexplained deviation is a warning. It means the supplier does not understand their own process.
No material certificate
A missing material certificate is a major red flag. It means the supplier cannot prove the part is made from the correct alloy. It also means they may not have the records to support a failure claim later.
How to use the checklist in a negotiation
A first article is not just a quality gate. It is a negotiation tool. If the supplier fails the first article, you have use. You can ask for a rework. You can require a process improvement. You can negotiate a price reduction.
Do not release the full production run until the first article is approved. The first article is your proof of process. Without it, you are buying a risk, not a part.
Table of common inspection items
| Feature | Tool | Tolerance |
|---|---|---|
| Critical dimension | CMM or micrometer | Per drawing |
| Surface finish | Surface roughness tester | Per drawing |
| Hardness | Rockwell or Brinell tester | Per spec |
| Material cert | Mill certificate | Match alloy |
| Surface treatment | Thickness gauge | Per spec |
Final step: approve or reject
At the end of the inspection, make a decision. Approve the first article or reject it. If you reject it, tell the supplier exactly what failed. If you approve it, keep the records. The first article sets the standard for the full run. If you approve a part with missing documentation or values near the limit, you are telling the supplier that the next batch can be sloppy.
A strong first article inspection protects your supply chain. It catches errors early. It gives you use. It ensures the supplier can hold quality during full production. Use the checklist as a tool, not a formality. The goal is a stable process, not a passed inspection.
Frequently asked questions
How long should a CNC first article inspection take?
It depends on the part complexity. A simple part may take an hour. A complex part with many critical features may take a full day. Budget enough time to measure all critical dimensions and verify material.
Can I skip the first article if the supplier has experience?
Do not skip it. Experience does not guarantee quality. The first article is your proof that the process is stable. A supplier can make a mistake even if they have done the part before.
What if the supplier refuses to provide a material certificate?
Do not accept the part. A missing certificate means you cannot verify the material. It also means you cannot hold the supplier accountable if the part fails. Reject the batch and require documentation.
Is a CMM required for a first article inspection?
No. A CMM is ideal for complex parts. For simple parts, a micrometer, caliper, and pin gauge may be enough. Use the right tool for the tolerance.
How often should I re-inspect after the first article?
Re-inspect after major process changes, tooling changes, or if a batch shows defects. Do not assume the process stays stable. Monitor the full run with periodic checks.
