C.A.S.E Isotropic Superfinishing

The technique of C.A.S.E.™ (Chemically Assisted Surface Enhancement) isotropic superfinishing has been developed as a metal surface treatment that provides components with excellent bending and contact fatigue strength with resistance to high loading.

Superfinishing Gears Chemically Assisted Surface Engineering, or CASE, is a two-stage process developed by Curtiss-Wright to extend a component’s surface fatigue life beyond that provided by shot peening alone. CASE consists of shot peening followed by isotropic finishing. The isotropic finishing step of the CASE process improves traditional vibratory honing techniques by the addition of mild chemicals, which removes the asperities left from shot peening and retains shallow micro-indentations for lubricant retention.

How superfinishing works

Controlled shot peening is followed by chemical surface superfinishing which gently removes any metal asperities but retains valleys for optimum oil retention, reducing surface friction and operating temperatures.

How isotropic superfinishing works

The component is firstly shot peened, which is a cold working process creating multiple indentations in the surface using spherical media called shot. As each indentation is made, the surface will try to yield but is restrained by the substrate, introducing a residual compressive stress and removing any prior manufacturing tensile residual stresses. This proven technique makes the surface resistant to crack initiation and propagation. The intensity and coverage of the shot peening process is carefully specified prior to any work being carried out.

Secondary processing by shot peening (dual peening) at a lower intensity and modified shot size will have the effect of reducing surface roughness and increasing the compressive stress and cold working of the near surface providing additional benefit.

Final machining is then carried out in a controlled and gentle manner using non-abrasive finishing stones together with oxalic acids where the most positive or peak surface areas are progressively removed producing a mirror-like surface finish.

Applications and key benefits of isotropic superfinishing

CASE is ideal for surfaces that require both excellent fatigue strength and surface finish due to contact loading. CASE has proven effective in improving resistance to pitting and micro pitting of gear teeth flanks, failure mechanisms that limit the performance of many gear designs. For example, transmission gears utilized in aerospace, automotive racing and off-highway applications are required to run for many years under high root bending loads and tooth flank contact loads. CASE has proven successful for use in all these industries.

Surface finishes of 10 micro-inches (Ra) or better are attainable with CASE on carburized gears. The figure below shows a typical roughness profile of a carburized gear after shot peening and also after the isotropic finishing portion of CASE.

CASE Graph
Click image to enlarge
The “peak to valley” of the shot peened finish is ~ 2.9 microns. After isotropic finishing, performed as part of CASE, this improves to ~ 0.6 microns.

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