(Above: Isotropic Micro-Finishing, Photo by Mark Riley, BV Products)
Excerpted from SME Technical Paper MR79-569 by J. Bernard Hignett
edited and updated by Dave Davidson, firstname.lastname@example.org
[ed. note: Harperizer and Harperized are trade names that refer to a specific OEM’s brand of centrifugal barrel finishing equipment]
Metal fatigue is the most common cause of fracture in metal components. It is usually caused by numerous repeated applications of low stress—stress much lower than that needed for fracture in a single application. The higher the stress, however, the fewer applications are needed to cause failure. It follows that fatigue failure will be most cannon in components that are highly stressed and subject to repeated applications of stress in their functioning. A fatigue crack usually starts because the tensile component of stress at the surface of the material is too high. It is thus beneficial to impart compressive stress of components to oppose any tensile stress to which the component may be subjected in service.
Edge and surface finishing improvement can reduce the risk of fatigue failure. Surface imperfections can act as stress raisers and removal of these will invariably improve performance of any highly stressed part. For critical components it is desirable to achieve very high surface finishes to facilitate inspection for stress raises. Removal of burrs and uniform radiusing of all sharp edges and corners will similarly improve performance.
As has already been discussed, the CBF process can simultaneously deburr, edge radius and surface finish an immense range of components. It can also impart very high compressive stresses uniformly to the parts while edge .land surface finishing so offering unique capability of improving the resistance to fatigue failure of many highly stressed parts. The capability to improve resistance to fatigue failure is demonstrated by the results of some tests made by a manufacturer of stainless steel coil springs which were taken from a standard production run, Half of the components had the conventional finishing process of barreling, followed by shot peening, and the other half were processed in a centrifugal barrel machine for 20 minutes. The springs were tested to failure by compressing them from 1.104″ length to .730″, corresponding to a stress change from 9 to about 50,000 psi. The results were that all springs finished by the conventional method failed at between 160,000 and 360,000 cycles. The springs that had been Harperized failed at between 360,000 and 520,000 cycles, an average performance improvement of 60%.
Below: Centrifugal Isotropic Finishing for precision machined parts at MacKay Manufacturing
The tests indicated a benefit potentially greater than mere improvement of resistance to fatigue failure. It was noted that some of the parts that were processed in centrifugal barrel equipment had clearly visible surfac e defects or inclusions. Such defects were not visible in the parts that had been barreled and peened. It was the parts with the visible defects which were always those that failed below 400,000 cycles so that if in inspection department were instructed ot to accept parts with such defects, then performance of the springs put into service would be of consistently much higher quality.
Even more striking results were observed during a series of tests in another set of production springs of a somewhat different type. The springs which were not processed in CBF equipment all failed life tests before 600,000 cycles. None of the springs which were Harperized had failed at 800,000 cycles, the limit of the test.
Using CBF markedly to improve resistance to fatigue failure by a combination of edge and surface finishing, together with imparted very high compressive stresses, is cheaper than finishing by conventional means and then shot peening. There are opportunities to improve the ultimate resistance to fatigue failure of many parts, and, of prime importance, enable much better quality control by facilitating inspection. There is no longer need for components to be designed to allow a proportion of parts to fail prematurely due to surface defects. Of course, the technique has wide use for spring components, for instrument parts, for bearings and throughout the aerospace industry. There are also many opportunities to utilize improved and more consistent performance to design some of the cost out of many more mundane components within the metalworking industry, in particular, some automotive parts.
Free Sample part processing available. Contact: Dave Davidson ( email@example.com )