FAI Case Study on Small Batch Production of Lens-Mounting Parts

In precision manufacturing, First Article Inspection (FAI) is a crucial step to ensure product quality by validating the stability and reliability of the production process before full-scale manufacturing.

WayKen recently carried out a small-batch project for a precision aluminum part used to secure a lens after assembly. Despite its small size (10.3mm × 9.9mm × 5.0 mm), the part features multiple deep cavities. CNC machining alone posed a high risk of tool breakage, so we combined CNC and EDM to process the part efficiently while maintaining precision.

We first conducted FAI by producing 5 initial articles and measuring them with a CMM, confirming all dimensions met the required tolerances. However, during FAI, several issues emerged that were not captured through dimensional inspection alone.

Issues Identified Through FAI and Their Solutions

FAI verifies dimensional accuracy and uncovers assembly or functional issues that measurements alone may miss.

1. 2D Measurements Passed, but Assembly Failed → Clearance Added + Custom Gauges

For the groove structure shown in Figure 1, we used the centerline as a reference and measured the groove width at three critical positions—top, middle, and bottom—all within tolerance. The part, therefore, passed inspection and proceeded to first article assembly.

However, the customer reported difficulty in assembling the part smoothly. Investigation revealed the cause: the grooves were formed using EDM, which leaves a microscopically rough surface. While the CMM measurements were within specification, local high points on the groove walls interfered with the mating part, increasing assembly resistance.

To resolve this, we conducted a video conference with the customer to review machining methods, functional requirements, and inspection techniques. Together, we designed custom “Go/No-Go” gauges (Figure 2) to simulate actual assembly conditions. Additionally, we added 0.1 mm clearance on both sides of each groove to compensate for surface roughness, ensuring smooth assembly.

This “Go/No-Go” approach effectively prevented jamming caused by local high points during assembly.

2. R Radii Interfering with Assembly → Add Clearance Features

Another issue observed was interference from R radii at part edges, which blocked proper lens installation. Analysis showed the cause was electrode wear during the EDM process: the copper electrode gradually erodes, rounding intended sharp corners into R radii.

In collaboration with the customer’s engineering team, we implemented two solutions:

• Modify the copper electrode design to include sacrificial “ears” at the corners, which absorb wear without affecting critical features.
• Add clearance grooves at the inner corners of the part to structurally eliminate interference.

These adjustments preserved the intended right-angle geometry and resolved the interference issue.

3. Uneven Bottom Surface → Optimize EDM Process

The final issue was an uneven bottom surface, which hindered proper installation. Investigation showed the height mismatch originated from combining CNC milling and EDM: the EDM-processed area naturally has a rougher surface, causing it to sit slightly higher than CNC-machined sections.

To address this, we slightly recessed the EDM area below the CNC-milled surface. This eliminated the height mismatch and ensured a smooth, flush assembly.

The Value of FAI and Project Outcome

Through this FAI process, we identified and addressed several key issues: discrepancies between 2D measurement and actual assembly, R-radius interference, and bottom surface unevenness. Timely process optimization and structural adjustments helped avoid costly rework and loss.

For WayKen, FAI is not just about measuring dimensions or filling out forms. It’s a critical opportunity to validate design rationality, identify production risks, and refine manufacturing processes.