Compared to magnetic particle and dye penetrant inspection processes, eddy current testing systems are well suited to perform in-process inspections.

Brass hex bar stock is fed into an eddy current coil. Source: Criterion NDT

In its 2000 annual survey,QualityMagazine found that only 43.9% of manufacturers were performing “in-process” inspections of parts. Today, the same survey indicates that 95% of manufacturers are doing some sort of in-process inspection. Driving this trend is the increased importance manufacturers are placing on quality. According to the current survey, the top five reasons manufacturers are making additional investments in quality are: to increase productivity, to reduce scrap and rework, to increase production capacity, to reduce costs and to obtain tighter part quality standards. All of these also help to reduce warranty costs.

Manufacturers like in-processes inspection as it fits into lean manufacturing processes, reducing additional process steps and finding problems in real-time, before bad batches of parts are manufactured. An integrated in-process inspection system not only rejects out of tolerance components, but it also helps to identify an upstream manufacturing process that is malfunctioning, alerting production managers and even shutting down the production line. This saves in scrap costs and reduces manufacturing and shipping delays.

Also supporting the trend toward in-process inspection is a continual reduction in the price of electronics and memory. Embedded processors with full PC capabilities are a fraction of the cost they were ten years ago. These also run at much lower power, reducing heat and simplifying packaging requirements. Low cost data storage, both on-premise and in the cloud, allows quality data to be stored for future reference and tied to a specific production lot or even individual parts.

Compared to magnetic particle and dye penetrant inspection processes, eddy current testing systems are well suited to perform in-process inspections. Modern eddy current instruments not only drive and receive signals from eddy current coils, they also fully integrate with material handling systems to operate sorters, cutters and marking devices to identify defective materials and remove them from the production line. Alarms can be set to notify managers of multiple failures in a row, which may signify a process issue versus just a random failure. On-board memory can capture and store a shift’s worth of production run data, while connecting instruments to the plant network allows data to be stored off line or in the cloud.

The pictured probe contains multiple eddy current coil windings. Source: Criterion NDT

Critical Interfaces

To maximize in-process testing capabilities, it is critical for eddy current systems to be fully integrated into the production line. Figure 4 shows an in-line bearing heat-treat testing station integrated into a material handling system. Figure 5 shows a detail of the eddy current coil. The eddy current instrument interfaces directly to the station’s PanelMate controller. Communication signals passing back and forth include test enable, which lets the eddy current system know the part is in place and the test can begin, test in-process, test complete, and the actual testing results. If the bearing passes the heat-treat test, the material handling station sends the part to the next manufacturing process. If the bearing fails, it gets sent to a reject bin.

Eddy current instruments can turn a light pole indicator red, and signal operators or manufacturing managers that a reject condition has occurred. The system can also be configured to indicate/alarm when multiple rejects have occurred in a row. In advanced scenarios, this information could be relayed to the PC or cell phone of the manufacturing manager.

Having this information readily available enables the operator/manager to shut down the line to find the defective upstream process. For a bearing heat-treat line, it could include a failure of induction heating coils, induction heating power supplies, poor parts handling during the heat-treating process, or mal-functioning quench units. The eddy current tests can also identify machining issues associated with tooling wear resulting in out of tolerance parts.

Beyond setting the eddy current testing parameters of frequency, drive and alarm tolerances, the eddy current instrument operator can also set a test result output time delay or extension depending on the timing needed between PLC and instrument to meet the line’s production rate. The eddy current instrument must also be capable of being configured into a limited (restricted) access mode. This ensures that only skilled and authorized operators can adjust the testing parameters which are critical in maintaining production process consistency. This can be set as a multi-tier limited access, giving some access to operators and then increased amounts of access to supervisors and metallurgists.

A powder metal gear sorting station can inspect and sort up to 60 parts per minute. Source: Criterion NDT

Capturing the Data

Companies are being driven to capture data at every process step. While this focus has been seen in the food industry, automotive and medical manufacturing are starting to see more of this. Individual components are being serialized and tracked from initial forging to final machining. This helps to ensure traceability and recording of quality processes. The electronic nature of the eddy current test results makes it very easy to store and later review the test results. Bar-code scanning data can easily be used to tie component serial numbers to test data. Ultimately, data from different product lines and plants can be stored together.

Pictured is an eddy current in-line bearing heat-treat testing system in a material handling station. Source: Criterion NDT


A properly designed in-process eddy current system not only facilitates 100% component inspection but can decrease testing costs, decrease warranty and scrap costs and improve overall quality. Above all it can verify manufacturing processes in real-time and store data for future analysis.NDT