Chicken cracks - Grinding cracks

Chicken cracks - Grinding cracks - Machining defects

Figure 1: "Chicken cracks" caused by excessive grinding pressure and/or insufficient lubrication.

Defect name: Grinding cracks
Record No.: 1899
Type of defect (Internal/Surface): Surface
Defect classification: Machining defects
Steel name: Steel
Steel composition in weight %: No data.
Note: The stripping of electrodeposited and flame-sprayed coatings is a necessary part of surface finishing. Stripping may be required to allow for the re-plating of a part rejected due to defective finishing or as part of rework processes such as those commonly conducted by airline/aerospace facilities. This paper discusses stripping methods and operational considerations in their use.

It is important that we point out that mechanical means can be and often are also utilized to remove organic and inorganic coatings, especially when the part shape makes the application of abrasive methods practical. Mechanical finishing may also be employed in addition to chemical stripping to produce a more uniform surface finish on the stripped part and/or to remove smut and other residues that may be left behind by chemical methods.

Blasting, machining, grinding and vibratory finishing are the most commonly employed mechanical methods of stripping or treating parts after chemical or electrochemical stripping. Machining and grinding are commonly employed methods of stripping or finishing parts with simple geometries, such as shafts and cylinders, while blasting is employed on more complicated part shapes.

In the airline / aerospace industry, equipment manufacturers provide specifications that describe the approved methods of mechanical stripping/finishing. These must be carefully followed since machining and grinding in an uncontrolled manner can cause surface defects before and after re-plating. As an example, Boeing has reported that grinding the stripped surface of a part by using too much pressure and not enough lubrication can cause "chicken cracks" to appear in the hard chromium deposit (Fig. 1).
Reference: Not shown in this demonstration version.