What MSA/GR&R targets are acceptable for aerospace parts?

In aerospace CNC machining, precision verification is as critical as the machining process itself. Measurement System Analysis (MSA) — particularly Gauge Repeatability and Reproducibility (GR&R) — determines if the inspection process is statistically capable of verifying tight tolerances. Acceptable GR&R levels for aerospace parts must comply with AS9100 and customer-specific quality management standards, such as those established by Boeing, Airbus, or Pratt & Whitney.

Understanding MSA and GR&R

MSA evaluates the entire measurement system, including gauges, fixtures, operators, and environmental conditions. Within MSA, GR&R quantifies the variability introduced by the measurement process compared to the total process variation. In precision environments like precision CNC machining, even minor measurement inconsistencies can misrepresent actual part quality. Thus, GR&R acceptance criteria are far stricter than in general manufacturing.

GR&R Acceptance Criteria for Aerospace Applications

The aerospace industry typically follows these GR&R interpretation ranges: * **Less than 10%:** Excellent. The measurement system is highly capable. This is mandatory for critical aerospace dimensions, such as turbine hub fits, bearing seats, and sealing interfaces. * **10–20%:** Acceptable with justification. Often allowed for less critical dimensions or features inspected under environmental variability. * **Above 20%:** Not acceptable for aerospace applications. Requires gauge redesign, operator retraining, or environmental control improvement. These values align with the guidance used in advanced programs that integrate CNC machining processes, CNC grinding services, and multi-axis machining, where tolerances can reach ±0.002 mm or finer.

Key Influencing Factors

The ability to maintain GR&R below 10% depends on both hardware and material response. For instance, superalloys such as Inconel 718 and Rene 80 require temperature-controlled inspection rooms due to their thermal expansion. * Hard coatings like PVD coatings or heat-treated surfaces affect probe interaction during contact measurement. * High-performance alloys such as titanium Ti-6Al-4V or stainless steel SUS316L demand stable fixture setups to minimize measurement drift. For reflective materials like aluminum 7075, optical or laser scanning systems must undergo calibration traceable to NIST standards to ensure reproducibility.

Integration with PDCA and SPC

MSA/GR&R results directly feed into the PDCA (Plan–Do–Check–Act) quality cycle and SPC (Statistical Process Control) systems. * During **PLAN**, GR&R studies define which gauges and inspection stations are qualified for use. * In **CHECK**, ongoing SPC charting confirms the system remains capable. * **ACT** ensures that corrective actions are documented when the GR&R index trends upward, thereby maintaining traceability during AS9100 audits.

Industry Expectations

Aerospace and defense customers—such as those in aerospace and aviation, power generation, and nuclear sectors—expect 6σ (Six Sigma) capable measurement systems. GR&R less than 10% ensures statistical confidence that the measurement system’s error does not mask true product variation, guaranteeing airworthiness-critical reliability.