Resonant inspection (RI), the general classification of Resonant Acoustic Method (RAM) nondestructive testing, is commonly used for 100% quality assurance testing of ductile iron cast, powder metal (PM), and metal injection molded (MIM) components, providing a volumetric whole body approach that detects both external and internal structural flaws and anomalies. This technique measures a metal component’s mechanical resonances by impacting a part and analyzing the acoustic ringing produced against a template generated from a statistically significant sample of “good” parts. These mechanical resonances are defined by material, mass and stiffness—with sensitivity to both structural flaws and process consistency. Process variations due to manufacturing variables can present as much of a significant challenge, if not more, as structural defects themselves. NDT-RAM allows for both online quality assurance testing for structural flaw and anomalies and benefits manufacturing consistency as a process gauge, including the ability to meet or exceed the user’s Six Sigma quality goals and drive for continuous improvement.
Dr. W. Edwards Deming’s “Fourteen Points for the Transition of Management,” states that a focus on quality throughout every level of a company drives expenses and costs down while increasing productivity and market share. A singular dependence on inspections has the opposite effect—costly, unreliable systems that never eliminate the source of the variations. As illustrated in the case study below, resonant “inspection” based on NDT-RAM has been used successfully as a continuous process monitor, resulting in the elimination of the sources of variation and serving as a guidepost to process improvement.
Introduction to NDT-RAM Technology
NDT-RAM is a volumetric resonant inspection technique that measures the structural integrity of each part to detect defects, anomalies and inconsistencies on a component level. This technique automates easily, eliminating human errors and subjectivity with fast throughput, providing cost-effective 100% inspection with minimal disruption to production. With a large number of global successes on the production lines of powder metal and cast parts, NDT-RAM has proven to be the simple and effective solution to a manufacturer’s Zero PPM challenge.
RI-based NDT techniques all vibrate a part mechanically, exciting the given part’s resonant frequencies, and detecting defects based upon measurable changes within this spectral frequency pattern. These techniques have become most commonly used for quality inspection in the manufacturing of sintered powder metal (PM), metal injection molded (MIM) and ductile iron cast metal parts. NDT-RAM specifically uses the acoustic signature of the metal component. As shown in Figure 1, NDT-RAM techniques vibrate a part by mechanically impacting it, exciting the given part’s resonant frequencies across a broad frequency range, then measuring the part ringing acoustically. By analyzing the resulting spectral frequency pattern (a typical response spectrum is shown in Figure 2), possible defects, anomalies and inconsistencies are detected and used for sorting.
Process Variation Within the Powder Metal Manufacturing Process
Dr. W. Edwards Deming stated, “We should work on our process, not the outcome of our process.” Deming emphasized the importance of measuring and testing to predict typical results. If a phase consists of inputs + process + outputs, all three should be inspected to some extent. Inconsistency problems with inputs are a major source of trouble, but the inconsistencies with the process using those inputs will also cause problems. Inspecting both the inputs and the process and applying continuous improvement practices accordingly results in outputs that are more consistent and predictable. Using PM as an example, the process variables shown in Table 1 may affect final part performance.
The better controlled the process, the more consistent the output. Since resonant frequencies of the manufactured parts exhibit sensitivity to variations in many of these processes, NDT-RAM provides excellent feedback to the control of that process. This data can be utilized to optimize the process and as the basis for continuous improvement.
From a manufacturer’s perspective, this capability translates to the ability to rapidly detect a shift in the process and prevent a potentially non-conforming part or batch of parts from being shipped to a valued customer. The technology also permits a team to objectively measure the downstream effect(s) of planned changes as the basis for kaizen events, waste reduction, and product and safety improvement programs.
While NDT-RAM technology is a 100% inspection system when installed inline with the production system, it has also served successfully as an early warning process monitor. As shown in Figure 3, an undesired shift in a typical PM manufacturing process or supply chain caused a significant shift in the final part’s resonant signature of more than 100 parts. Each potentially defective part that failed to meet the established criteria was automatically sorted and segregated and a visual indicator from a light tower was triggered to notify operations of the shift in performance. This permitted the operations team to immediately investigate any unusual conditions and make the necessary repairs or modifications to prevent the continued production of potentially defective parts and minimize the cost impact of these parts on both the manufacturer and its customers.
PM Case Study: Differences in Production Lines Contribute to Final Part Inconsistencies.
A PM manufacturer had two separate production lines producing the “same” PM part, although one line was significantly older than the other. As such, the lines were assembled from components of different ages and performance characteristics, such as the press tooling, pressure sensors, and sintering furnace temperature zones. Even though the part exhibited the “same” general resonant frequency characteristics, as a result of the differences in the process, the parts produced on Line B showed about 33% to 35% less variation in resonance frequency than parts produced on Line A. This would allow for a much tighter criteria range (shown by the green boxes in Figures 4 and 5) to be used during inspection. The parts were also tested by the metal department, capturing the part weights and densities. Likewise, Line B exhibited about 43% to 46% less variation than Line A.
Monitoring and controlling the process from the powder metal to the finished product produces a better quality and more consistent product. The resonant signatures on finished parts from Line B exhibited less than 35% of the variation of parts from Line A. Deming’s “Plan-Do-Check-Act” approach requires a manufacturer to analyze its process, identify opportunities for improvement and then take action to realize continuous improvement. Here, the NDT-RAM technology not only provided a performance metric for the final part but it also provided a method to detect and quantify the effect of different and critical process variables on the final part’s performance. The results of this case study showed that three process changes yielded the greatest improvements on final product quality:
1. The use of automatic mass measurement (AMM) on molded parts to ensure they fall within specifications.
2. Carbide tooling, as used in Line B, produced a more consistent product although it is more expensive than various grades of steel.
3. The added capability of a closed-loop monitoring system with additional thermal zones for a more controlled sintering process.
Inherent in Deming’s Fourteen Point philosophy is the focus on “building quality into the process from the start to the finish.” As stated previously, he was commonly quoted, “We should work on our process, not the outcome of our process.” He encouraged the use of statistical analysis to verify the process performance and as the benchmark for continuous improvement. As evidenced by the 1970s Japanese companies who adopted his techniques, superior quality and low costs enable a company to obtain a global leadership position. These highly successful companies, regardless of their size, products or industry, had a common focus on their production processes.
Within many metal component manufacturing industries, NDT-RAM technology provides instantaneous and objective feedback by focusing on the variables within a production process. Resonant frequency ensures consistency of process, leading to consistent products. This process focus and NDT-RAM technology can serve as one of the bases for true success under Deming’s total quality vision.