Machining processes such as grinding, turning, and milling are inevitably associated with the risk of thermal damage, commonly referred to as „grinder burn“, due to the principle-related heat generation in the near-surface contact zone between tool and steel material. Known for decades as an undesirable companion, testing for grinding burns was mostly established as a classic laboratory-based sampling topic for detecting systematic process defects. As industry trends evolve, the demand for rapid, non-destructive testing becomes increasingly crucial to enhance the reliability of components.
Various trends have forced a reevaluation in recent years: the desire for much lighter weight components that are also subjected to higher loads, the use of faster and more aggressive machining processes to reduce production costs, and, not least, significantly increased expectations regarding the long-term stability of bearing components. This holds especially true in the global rail, wind energy and, automotive fields. Therefore various industries have come to realize that a fast, non-destructive crack and grinding burn test as a 100% inspection directly at the end of the corresponding production lines is indispensable. This procedure helps to eliminate product failures due to thermally damaged areas and flaws (cracks and pores) on the surface of relevant, sometimes even safety-critical metallic components.
The eddy current test method (ET) with an ET probe based on the differential principle is a proven approach to test for surface-open defects. In recent years, with the introduction of digital machine learning test instruments, the range of detectable defects has also expanded to include grinder burn. The eddy current test method can be applied for all metallic materials with electrical conductivity or magnetic permeability. By selecting a suitable probe design and a fitting transmitter frequency this crack and grinder burn evaluation can be adapted to many test tasks. As a matter of physical principle, mainly surface open flaws and pores or flaws close to the surface are detectable.
The well-known note “surface to be free from cracks or grinder burn” on drawings suggests a wish for the perfection of the part to be produced. However, there are physical limits of the eddy current crack detection relative to that wish. We have committed ourselves to move these limits further in the direction of smaller, detectable “discontinuities” without increases in pseudo rejects and under production conditions.
The exploitable sensitivity of the eddy current crack test depends on several parameters:
Surface roughness - detection of small defects is better the smoother the surface is. The limit is at defect depth equal to 5 times roughness depth, but not less than 50 microns for an articial EDM nut on a master part.
Material - use of differential probes generally suppresses noise inherent to different materials. But the material tested is a factor. For example, the detection limit for lamellar cast iron can increase to approx. 150 microns due to carbon needles in this material.
Distance probe to surface - increasing probe distance reduces sensitivity while decreasing probe distance enhances sensitivity to surface roughness and eccentricity of the test part. A good compromise for most applications is the ibg standard probe distance of 0.7 mm.
Direction of defects - the direction of a defect relative to the probe trace direction also influences the test sensitivity. Choosing a suitable ibg probe type Is the best approach to deal with defect orientations.