First Article Inspection for CNC Machined Parts: What Should Be Checked?

First Article Inspection (FAI) is a quality control process used to verify that the first CNC machined part meets its approved drawings, dimensions, material specifications, and other technical requirements before production continues.

Even when a CNC machine produces consistent results, an incorrect setup or outdated drawing can lead to an entire batch of defective parts. First article inspection helps identify these problems early, reducing the risk of scrap, rework, and production delays.

At Kaierwo, first article inspection is an important consideration in CNC machining quality control, particularly for precision parts with tight tolerances, complex geometries, or specific assembly requirements.

This guide explains what should be checked during first article inspection, which measuring tools are commonly used, and how manufacturers can document inspection results.

Why Is First Article Inspection Important in CNC Machining?

CNC machining is known for its accuracy and repeatability. However, producing identical parts does not necessarily mean those parts meet the design requirements.

For example, an incorrect tool offset may cause every component in a production batch to have the same dimensional error. Similarly, an outdated CNC program could produce parts that no longer match the latest drawing revision.

First article inspection helps manufacturers identify these issues before full production begins.

The main benefits include:

• Early error detection: Identify incorrect dimensions, machining setups, or material specifications before producing additional parts.

• Reduced scrap and rework: Prevent repeated manufacturing defects that increase production costs.

• Improved dimensional accuracy: Verify critical features against engineering drawings and specified tolerances.

• Better process control: Establish an approved reference for subsequent machining and inspection activities.

• Clear quality documentation: Provide customers with traceable evidence of the initial inspection results.

FAI is one part of a broader quality management process. Manufacturers also need in-process and final inspections to maintain consistent quality throughout production.

For a more detailed explanation of these processes, see our guide to CNC machining quality control.

What Should Be Checked During First Article Inspection?

A complete first article inspection should cover the applicable requirements specified in the latest engineering drawing, purchase order, and inspection plan.

The inspection scope depends on the part's complexity, application, and customer requirements. However, most CNC machined components require the following checks.

1. Drawing Revision and Part Identification

Before measuring the part, inspectors should confirm that it was manufactured according to the correct engineering documents.

Important information includes:

• Part number and product identification

• Latest approved drawing revision

• CNC program and setup documentation

• Customer specifications

• Applicable manufacturing and inspection requirements

Drawing revision control is particularly important when a component has undergone multiple design changes.

A part manufactured to an outdated drawing may be dimensionally accurate but still fail to meet the current design.

2. Material Grade and Specifications

Material verification ensures that the machined component uses the material specified in the design.

Inspectors should check:

• Material grade and alloy designation

• Material condition or temper

• Material certificates, when required

• Heat or lot traceability

• Specified heat treatment and surface treatments

  
  

For example, aluminum components made from 6061-T6 and 7075-T6 may have similar appearances but different mechanical properties.

Using the wrong material can affect strength, durability, and performance even when all dimensions are correct.

Material verification may involve reviewing supplier certificates or performing additional tests if required by the customer.

3. Dimensions and Geometric Tolerances

Dimensional inspection is one of the most important stages of FAI.

Inspectors compare actual measurements with the values specified in the engineering drawing.

Typical inspection items include:

• Overall length, width, and height

• Hole diameters and hole positions

• Internal and external diameters

• Wall thickness and pocket depth

• Center-to-center distances

• Flatness and perpendicularity

• Position and profile tolerances

For precision components, geometric dimensioning and tolerancing (GD&T) may also be required.

GD&T defines how features must relate to specified datums and helps ensure that a part functions correctly during assembly.

For example, a mounting hole may have an acceptable diameter but still be incorrectly positioned. This can prevent the component from fitting with another part.

Tight tolerances also require suitable machining and measurement methods. A tolerance of ±0.01 mm, for example, should only be specified when the part design, manufacturing process, and inspection capability can support it.

These considerations are especially relevant when designing CNC machined prototypes that need to meet production-level dimensional requirements.

4. Holes, Threads, and Machined Features

Many CNC machined components contain threaded holes, precision bores, slots, and other functional features.

These details require additional attention because small machining errors can affect assembly.

Inspectors should verify:

• Thread diameter and pitch

• Thread depth and specified tolerance

• Hole location and alignment

• Counterbore and countersink dimensions

• Slots, pockets, chamfers, and radii

• Sharp edges and burrs

For threaded features, go/no-go gauges are commonly used to check whether the threads meet the specified acceptance limits.

Precision holes may require bore gauges, plug gauges, or a coordinate measuring machine (CMM), depending on the tolerance and geometry.

When hole size and surface quality are critical, additional machining operations such as reaming may be necessary.

5. Surface Finish and Workmanship

Surface quality can influence how a CNC machined part looks, fits, and performs.

During first article inspection, inspectors should check for visible defects such as:

• Burrs and sharp edges

• Excessive tool marks

• Scratches or dents

• Surface damage

• Corrosion or contamination

When a drawing specifies a surface roughness value, such as Ra 1.6 μm, inspectors should use an appropriate measuring instrument rather than relying on visual inspection.

Surface treatment requirements also need to be considered.

Processes such as anodizing, plating, or polishing can affect the final dimensions and surface properties of a part.

For this reason, manufacturers should confirm whether dimensional requirements apply before or after surface treatment.

6. Inspection Reports and Supporting Documents

FAI is not complete until the inspection results have been properly documented and reviewed.

Depending on customer requirements, an inspection package may include:

• Ballooned engineering drawings

• Recorded dimensional measurements

• Material certificates

• Inspection equipment identification

• Surface treatment or special-process records

• Nonconformance reports

• Inspection approval records

A ballooned drawing assigns identification numbers to inspection characteristics. These numbers correspond to results in the inspection report, making it easier to verify whether each requirement has been checked.

Inspection documents should clearly identify the part, drawing revision, actual measurements, and acceptance status.

This makes the results easier to review and supports traceability when future production or quality issues arise.

A Practical First Article Inspection Process for CNC Machined Parts

A structured inspection process helps manufacturers identify problems before they affect additional parts.

The following six steps provide a practical approach to FAI.

Step 1: Review Engineering Requirements

Start by reviewing the latest engineering drawing, 3D model, purchase order, and applicable specifications.

Identify critical dimensions, material requirements, geometric tolerances, and surface finish specifications.

Any unclear requirements should be resolved before machining begins.

Step 2: Prepare the Inspection Plan

Create an inspection plan that identifies each characteristic to be checked.

The plan should include:

• Required dimensions and tolerances

• Measurement methods

• Suitable inspection equipment

• Applicable acceptance criteria

• Documentation requirements

For complex parts, a ballooned drawing can help organize the measurements and prevent important features from being overlooked.

Step 3: Machine the First Part

Manufacture the first part using the intended production setup, including the approved CNC program, cutting tools, workholding, and material.

The first article should represent the process that will be used for subsequent production.

This is particularly important in low-volume CNC production, where setup errors can affect a significant portion of a small batch.

Step 4: Perform Dimensional Inspection

Measure the required features using appropriate inspection equipment.

Common tools include:

The measuring equipment should have appropriate calibration, resolution, and measurement capability for the specified tolerance.

For example, a standard caliper may be suitable for a general dimension but may not provide sufficient measurement capability for a very tight tolerance.

Step 5: Record Results and Correct Problems

Inspection results should be compared with the drawing requirements and recorded in the FAI report.

If a dimension falls outside its specified tolerance, the manufacturer should investigate the cause before releasing the process.

Potential causes include incorrect tool offsets, fixture alignment, tool wear, or programming errors.

After corrective action, the affected features must be rechecked. Additional inspection may be necessary if the adjustment could influence other dimensions.

Step 6: Approve the First Article

Once the applicable requirements have been verified, the inspection results can be reviewed for approval.

Depending on the quality plan, approval may be provided internally or require customer confirmation.

After approval, production can proceed under the established process controls.

However, passing FAI does not eliminate the need for ongoing inspection. Tool wear, material variation, and other production changes can still affect subsequent parts.

First Article Inspection Checklist for CNC Machined Parts

The following checklist summarizes the main items that should be reviewed during FAI.

This checklist can be adjusted based on the component's geometry, intended application, and customer requirements.

What Standards Apply to First Article Inspection?

First article inspection requirements vary by industry and customer.

Several standards may be relevant to CNC machining quality control.

AS9102

AS9102 is widely used for first article inspection in the aerospace industry. It establishes requirements for documenting and reporting inspection results.

It should be followed when required by the applicable customer contract or quality requirements.

ISO 9001

ISO 9001 provides a general framework for quality management systems.

It supports controlled manufacturing, inspection, traceability, and documented quality processes. However, it does not require the same FAI report format for every CNC machining project.

ASME Y14.5 and ISO 1101

These standards address geometric dimensioning and tolerancing.

They help define how features such as holes, surfaces, and datums should be interpreted and evaluated.

The applicable standard should be determined from the engineering drawing and customer specifications.

ASTM E18

ASTM E18 covers Rockwell hardness testing. It may apply when material hardness verification is specifically required.

Not every CNC machined component requires hardness testing.

Manufacturers should apply standards according to the actual design and contractual requirements rather than using the same inspection procedure for every project.

Common First Article Inspection Mistakes

Even with an established inspection process, several mistakes can reduce the reliability of FAI results.

Checking Only a Few Dimensions

Inspecting several critical dimensions does not automatically satisfy a full first article inspection requirement.

When the inspection plan requires verification of every drawing characteristic, the report should cover all applicable features.

Using the Wrong Measurement Equipment

Measurement equipment must be suitable for the specified tolerance and geometry.

For example, precision holes may require more than a basic caliper. Depending on the feature, a bore gauge, CMM, or another measurement method may be necessary.

Our article on reaming in CNC machining explains how machining and inspection methods work together to achieve consistent precision-hole dimensions.

Ignoring Drawing Revisions

A part may pass every measurement but still be unacceptable if it was manufactured according to an outdated drawing.

Inspectors should always confirm the approved revision before beginning FAI.

Recording Pass or Fail Without Measurements

Simply marking every characteristic as "OK" may not provide enough information for customers to verify dimensional conformity.

Actual measurements should be recorded when required by the inspection procedure.

Assuming FAI Replaces Production Inspection

First article inspection confirms the initial part and process setup. It does not guarantee that every subsequent part will meet the same requirements.

In-process and final inspections remain important for monitoring dimensional changes and maintaining consistent production quality.

Example: How FAI Prevents CNC Machining Errors

Consider a CNC machined aluminum housing with several mounting holes and a precision bore.

The engineering drawing specifies tolerances for the bore diameter and mounting-hole positions.

During first article inspection, the inspector confirms that the bore diameter meets the specified limits. However, CMM measurement shows that the mounting-hole pattern is incorrectly positioned relative to the required datum.

Further investigation identifies an incorrect work offset in the CNC setup.

The machinist corrects the offset and produces another part. The inspector then repeats the necessary dimensional checks to confirm that the revised setup meets the drawing requirements.

Without first article inspection, the same error could affect an entire production batch.

This illustrative example demonstrates why dimensional inspection should consider both individual feature sizes and their relationships to the specified datums.

How Does First Article Inspection Benefit CNC Machining Buyers?

For buyers sourcing custom CNC machined parts, FAI provides documented evidence that the initial manufacturing process can produce components meeting the agreed specifications.

It is particularly useful when ordering:

• Precision mechanical components

• Complex CNC milled or turned parts

• Components with tight dimensional tolerances

• Parts requiring controlled material traceability

• Custom components intended for assembly

FAI can also support supplier evaluation by showing how a manufacturer interprets drawings, verifies dimensions, and handles inspection records.

When requesting CNC machining services, buyers should provide complete technical documentation and clarify the expected inspection requirements.

Important information includes the latest drawing revision, material specification, critical dimensions, required tolerances, inspection standards, and reporting format.

Clearly defined requirements make it easier for the manufacturing team to select suitable processes and establish an appropriate inspection plan.

Conclusion

First Article Inspection for CNC Machined Parts helps verify that the initial manufactured component meets its engineering requirements before additional production begins.

An effective FAI process checks drawing revisions, materials, dimensions, geometric tolerances, machined features, surface finish, and supporting documents.

It also ensures that nonconforming results are investigated and corrected before the manufacturing process is approved.

For CNC machining projects, first article inspection works best as part of a broader quality control system that includes suitable measurement equipment, documented procedures, and continued production monitoring.

By establishing clear inspection requirements before machining starts, manufacturers and buyers can reduce avoidable quality problems and improve confidence in the finished components.