Electric discharge machining (EDM) is a manufacturing process that uses electrical discharges to remove material from a conductive part, and can be useful to create test standards for test validation. In EDM electrodes create desired notch sizes on a manufactured product that can then be used for production test validation as a test or calibration standard.
EDM offers advantages over conventional machining as it can machine extremely narrow notches and can hold very tight tolerances. It can also be used to machine irregular shapes that can’t be machined with regular processes. This allows a customer to create a very precise calibration standard on complex test surfaces.
These test standards are then typically used at the beginning of a production shift, and occasionally a time or two during a shift, to ensure system detection capability has been maintained. Contact us for more information on the use of EDM calibration standards for eddy current non-destructive testing.
A rubber gasket can be inspected using eddy current, provided there is conductive metal within the component.
Eddy Current Testing is a method used to identify flaws or abnormalities in metal by inducing electrical current flow via alternating magnetic fields. We know rubber is generally nonconductive, but are there any circumstances that would make eddy current testing of rubber parts a feasible inspection option?
Yes, in fact, rubber parts can be tested using eddy current, provided there is a metal component within the rubber. For example, in the ball joint cup pictured there is a metal ring molded into the rubber. An eddy current test is able to identify if the rings are doubled up or missing, factors that could compromise the functionality of the part and that would be difficult and time-consuming to identify otherwise. This is a quick test that can be performed with a simple encircling coil to help ensure the quality and consistency of the manufactured component.
Test Configuration Screen of Criterion NDT’s CR-11 Eddy Current Test Instrument
Criterion NDT’s eddy current test instruments have sophisticated industrial I/O capabilities that help ensure proper communications with material handling systems on production lines. Our instruments have the capability to select the type of test enable, shape the output response, and define what to do when multiple rejects occur.
Having proper probe alignment is critical to getting accurate and consistent eddy current test results. The pencil probe shown above is firmly positioned next to the part under test. Moving the probe just a few thousandths of an inch back can dramatically change the eddy current signal response. Probe angle and rotation orientation are also important factors.
Figure 1 – Eddy Current Coil and Powder Metal Star
To engage the I/O on an eddy current test instrument, the test must be “enabled.” The test can be enabled from the instrument panel, or in automated testing, with an internal or external source.
In internal enable, the eddy current instrument senses that there is a part in the eddy current probe and starts the enable. The enable turns off when the part leaves the probe. The photo here shows a powder metal star entering an eddy current probe where it will initiate an “internal enable.” Data is captured and the test disables, as the part exits, until another part enters.
In external enable, a sensor indicates that a part is in position to begin the test, then a PLC asserts the enable signal.
Figure 1 – Criterion NDT’s CR-11 Consecutive Reject Configuration Screen
Eddy current test systems are designed to detect flaws in 100% of the production line parts. When the eddy current system is integrated into a material handling station, out of tolerance parts get sent to a reject chute.
Parts may be rejected due to “random” flaws or may indicate a more serious upstream process issue. Criterion NDT customers have the ability to set an alarm based on a “consecutive number of rejects.” An example of this alarm setting can be seen in the CR-11 configuration screen below. For example, if a customer set the Consecutive Rejects to “5”, then if 5 rejects in a row are detected, a System Alarm is set.
In addition, customers can also set an alarm based on the total number of rejects in a batch. This can also be useful to monitor manufacturing processes.
One of the more powerful tools in our eddy current systems is the Differential Filter. This unique filter is used in dynamic flaw testing. Via special algorithms, test data is enhanced and material noise is reduced to improve the overall Signal-to-Noise ratio. Using the Differential Filter can help to improve detection of smaller defects and also reduce false reject rate.
Figure 1 – Eddy current test response without the Differential Filter.
Figure 2 – Eddy current test response with the Differential Filter.
Selecting the proper test frequency for your eddy current test instrument depends on a number of factors:
- Flaw Types: Higher frequencies are used to find small, fine surface defects. Lower frequencies are typically used to find near-surface flaws.
- Material: Depth of penetration is very dependent on a materials alloy characteristics and whether it’s ferrous or non-ferrous. Typical testing frequencies are:
- Surface/near surface flaw detection on non-ferrous materials: 75 kHz to 3 MHz. On ferrous materials: 25 kHz to 400 kHz
- Material structure testing (heat- treatment process verification/hardness): 5 Hz to 50 kHz
- Probe Design: A smaller coil/probe has finer windings and can run at higher frequencies. Large/low frequency coils utilize separate windings within to minimize signal drift.
Calibration standards are used to verify that your eddy current system is properly set up to capture defects or product variations that might occur on your production line.
For crack testing, a sample part is usually created by adding a small “notch” using an electrical discharge machining (EDM) process. For example, you might put a .010” deep notch on a bearing surface you were trying to test.
For heat treat or material structure testing, you would create a number of samples that simulate potential defective conditions in the heat treat cycle, such as parts with no heat treat, no quench, short quench, etc.
It is important to integrate calibration standards into the daily workflow. As a part of the shift change process, many companies run their calibration standards through the eddy current system at the beginning and end of each shift.
It is also important to clearly label your calibration standards. While it may be obvious what a crack test standard is, a heat treat test standard can look like a regular part. We have seen companies disregard their own calibration standards because no one knew what conditions they represented.
An example of an eddy current crack test standard utilized regularly to ensure proper readings from the eddy current instrument.
Criterion NDT designs eddy current probes to fit your specific inspection needs.
We often receive inquiries about when to use a spot probe vs. encircling coil for eddy current crack detection. Here are the pros and cons of using each.
A single spot probe is usually configured in what is commonly referred to as a “pencil probe” (shown in the left side of the photo). These types of probes are typically used to detect localized flaws. For proper use, the probe tip must scan over the critical area of interest. This is the best way to find the smallest flaws.
In order to examine a larger surface area, multiple sets of windings can be combined into a single housing to form an “array probe” (the two center probes in the photo above).
If time is available, another option is to use a single probe to scan a complex geometry part by utilizing a robot or CNC type machine.
An encircling coil can be seen on the right side of the photo above. There are applications where these coil types are ideal for crack testing, such as small diameter tubing. Then there are other cases where larger coils can be used to quickly identify gross cracks, yet smaller defects may still be passed. Therefore, it is critical to understand the defect type, size, and orientation.
Criterion NDT will provide you with the right probe to fit your testing application. We take flaw detection criteria, material type, defect orientation, part geometry, and production line rate into consideration when designing a test, and we always offer and encourage a free feasibility study on a part to make sure we get it right.