Aerospace Titanium Bracket Surface Roughness Compliance Audit

Scenario

An AS9100-certified aerospace component manufacturer in Seattle, WA, conducted a post-delivery audit for titanium alloy (Ti-6Al-4V) structural brackets supplied to a major airframe integrator. The contract specified surface finish per AMS 2482B: Ra ≤ 1.6 μm on machined faces, but the supplier’s QC report listed only Rz = 6.8 μm and RMS = 2.1 μm—no Ra measurement. FAA Part 21.303 compliance requires documented Ra traceability. Constraints included zero destructive testing (brackets already installed in wing spar assemblies), tight 72-hour response window for non-conformance resolution, and no access to original profilometer raw data.

Given Data

Supplier’s final inspection report (verified via digital certificate):

• Rz = 6.8 μm
• RMS = 2.1 μm
• No Ra reported

Material: Ti-6Al-4V, finish: CNC-milled + vibratory deburred (no secondary polishing).

Calculation

Using the Surface Finish Conversion Calculator:

• Input Rz = 6.8 μm → tool applies Rz/Ra ≈ 4.8 for Ti-6Al-4V after milling/deburring (calibrated to NIST SRM 2166a + 120 in-house Ti samples)
  • Raequiv = 6.8 / 4.8 ≈ 1.42 μm
• Input RMS = 2.1 μm → tool applies RMS/Ra ≈ 1.12 for this process/material combo (validated per ASTM E2907)
  • Raequiv = 2.1 / 1.12 ≈ 1.88 μm

Calculator fuses inputs using weighted least-squares regression (Rz weight = 0.7, RMS weight = 0.3, per historical uncertainty analysis). Output:

• Converted Ra = 1.52 μm
• Converted Rz = 6.80 μm
• Converted RMS = 2.10 μm

Result and Decision

The converted Ra (1.52 μm) falls within the AMS 2482B limit of ≤ 1.6 μm. The auditor accepted the conversion as compliant under FAA Advisory Circular 21.303-2 §4.2.2 (allowing validated empirical conversions when primary measurement is unavailable and process is stable). No corrective action was required—but the supplier committed to adding Ra reporting to all future Ti-6Al-4V milling work instructions.

Lesson

For regulated industries, maintain a documented, statistically validated conversion matrix per material-process combination—not generic textbook ratios—because surface topography dependencies (e.g., Ti-6Al-4V’s low thermal conductivity causing micro-welding during milling) significantly alter Ra–Rz–RMS relationships.