Greater than zero defects, uncontrolled processes and lack of process improvements should not be tolerated.

“We should work on our process, not the outcome of our processes,” said Dr. W. Edwards Deming.

It is undeniable that production among automotive suppliers is down and bankruptcies are up. The immense pressure on tier suppliers has forced many out of business, to consolidate their operations or, at a minimum, to contract their output and labor force. Those who not only survive but thrive will consistently deliver high quality efficiently and on time. Intimately knowing all aspects of their manufacturing processes will be the key.

Three quality process sins can kill automotive suppliers:
1. Shipping greater than zero defects
2. Uncontrolled (or minimally controlled) processes
3. Insufficient process improvements

Product defects cannot be tolerated even in million-piece quantities. Now more than ever, there is a growing requirement for zero defect supply chain commitments. To achieve zero defect output, automotive manufacturers are making the commitment to move to on-line nondestructive testing (NDT). This type of on-line inspection requires accuracy, reliability and high throughput.

Resonant Acoustic Method

The NDT resonant acoustic method provides a proven technique exhibiting these performance requirements, evolved from methodologies used by NASA to test flight hardware and automotive manufacturers to validate new vehicle designs. Part quality is tested on 100% of the product, while the manufacturing process is continually monitored for consistency. This allows for closed loop feedback, providing data necessary for continuous improvement by refining and optimizing manufacturing processes. Utilizing structural dynamics and statistical variation, resonant acoustic method NDT provides mature, laboratory-proven technology in a robust, process-friendly manner.

Providing confidence in the quality of shipped product for the high-volume manufacturer, resonant acoustic method NDT offers reliable inspection, with quantitative, objective results. This technique is easily automated to eliminate human error with fast throughput for 100% inspection, with minimal disruption to production. It is a volumetric resonant inspection technique that measures the structural integrity of each part to detect defects on a component level.

The principle of resonant inspection is simple: every part has a unique resonant vibration signature, or spectrum, that reflects its structural integrity. A deviation from the expected signature of resonant frequencies can indicate the presence of a flaw. For example, consider a bell or tuning fork. When someone strikes either part, it vibrates and emits a sound. A device that rings true produces consistent sound, which correlates to the structural integrity of that device.

This is the basis for resonant acoustic method NDT technology. When struck by an impactor, parts such as gears, brake anchors and rotors emit many resonant frequencies as part of their structural response. This unique, measurable signature of resonant frequencies is measured using a microphone and processed with a smart digital controller, comparing its spectrum against a template defined from a statistically significant batch of good and not good product.

Just as a cracked bell will not ring true like a structurally sound bell, if a part such as a gear is cracked, lacks correct density or misses other characteristics of a structurally sound product, the anomaly will be exposed when its resonant signature deviates from what has been established as a good product. The smart digital controller compares the test results against the acceptable limits and accepts or rejects the part accordingly in real time moving along a conveyor.

Resonant acoustic method NDT is most commonly applied to larger volume manufacturing of cast, forged and powder metal parts, as well as some ceramic components. The technique detects frequency shifts that can be caused by imperfections such as cracks, chips, porosity and voids, as well as variations in nodularity (for ductile iron castings), geometry, weight, density and manufacturing processes.

One hundred percent resonant inspection of all automotive parts being produced eliminates the three deadly sins of production: part quality, process monitor and process improvement.

1. Part Quality

The ability to inspect 100% of parts produced allows manufacturers to catch nonconforming parts as they are produced. Not shipping a bad part has obvious financial benefits: avoiding additional quality audits, containment and resorting costs, as well as more costly recall expenses or loss of contracts.

Non-financial benefits cannot be overlooked either. Impeccable quality keeps customers happy, and happy customers do not search for alternative suppliers. On a human level, knowing that every part is inspected lowers stress levels on quality managers, production managers, plant managers and vice presidents of manufacturing and allows them to use their time and intelligence more productively on improving the manufacturing process rather than chasing the latest part quality fire.

2. Process Monitor

One hundred percent inspection allows for an early warning process monitoring system to identify changes in a process that lead to bad parts being produced. The manufacturing line can be shut down, root cause investigated and repaired prior to making a large number of bad parts. When there is an undesired change to a manufacturing process, the results of that change often manifest themselves in poor quality parts being produced. The earlier an undesired change in a process is detected, the sooner it can be corrected-resulting in fewer nonconforming parts produced.

Obvious savings come from lower scrap rates. Less obvious advantages come from being able to identify the cause of the process change soon after it happens, thereby increasing the likelihood of pinpointing root cause and applying proactive action to prevent it from happening again in the future thanks to continual process improvement.

3. Process Improvement

There is no better way to improve per piece quality than by continually improving the process by which the parts are made. Continuous improvement is not a new concept and few people argue its merits. One hundred percent inspection is simply a tool to identify process improvement candidates and then measure the success of continuous improvement efforts. Part-to-part variations are easily seen by plotting the resonant frequencies of each part. The closer the frequencies match from part to part, the more consistent the process is running. A high-consistency process leads to a high consistency of parts, which leads to consistently high quality parts.

This allows data to be collected to perform statistical analysis on production processes using actual structural integrity data on 100% of parts produced to help engineers identify manufacturing processes that can be improved.

Resonant inspection via the resonant acoustic NDT method serves quality-minded manufacturers who realize that measuring and controlling the manufacturing process is a major key to end part quality and are dissatisfied with time-consuming, costly and subjective manual inspection techniques. The resonant acoustic method allows for simple integration of a turnkey system that is a reliable, fully automated method for quality control, real-time monitoring and process improvement. This rapidly growing technique creates an on-line, 100% inspection system that provides for zero defect product supply.

Unlike previous implementations of resonant inspection that are excessively complicated and costly to automate, resonant acoustic method NDT is fast, simple and easily re-configurable. As a result, powder metal and casting automotive component manufacturers around the world have proven the benefits of resonant acoustic method NDT inspection over their traditional inspection techniques.

Tech Tips


  • 100% inspection is a tool to identify process improvement candidates and then measure the success of continuous improvement efforts.



  • Part-to-part variations can be easily seen by plotting the resonant frequencies of each part. The closer the frequencies match from part to part, the more consistent the process is running.



  • A high-consistency process leads to a high consistency of parts, which leads to consistently high quality parts.