Deburring & Finishing
Since its inception, turbo-abrasive machining, a method that utilizes fluidized abrasive materials, has facilitated signifi- cant reductions in the amount of manual intervention required to deburr large components. Additionally, the process has also proved to be useful in edge and surface finishing a wide vari-
ety of other nonrotational components by incorporating these components into fixturing systems.
The advantages of this method go beyond the simple removal or attenuation of burrs. The method is also capable of producing surface conditions at these critical edge areas
Understanding Functional Surfaces
utilizing orbital pressure methods with both grinding and polishing free abrasive materials in sequence. The surface profile has been reduced from the original 75–90 Ra
(µin.) to a 5–9 Ra (µin.) range. Additionally,
there has been a plateauing of the surface and the resultant smoother surface manifests a negative skew (Rsk
) instead of a positive skew. This Photo courtesy Jack Clark, Surface Analytics T
his before photograph above was taken with a scanning electron microscope at 500× magnification. It shows the surface of a raw unfinished “as cast” turbine blade. The rough initial surface finish as measured by profilometer was in the 75–90 Ra
(µin.) range. As is typical of
most cast, ground, turned, milled, EDM and forged surfaces this surface shows a positive Rsk
[Rsk –skewness–the measure of surface symmetry about
the mean line of a profilometer graph. Unfinished parts usually display a heavy concentration of surface peaks above this mean line, generally considered to be an undesirable characteristic from a functional viewpoint.]
Photo courtesy Jack Clark, Surface Analytics
This SEM photomicrograph (500× magnification) above was taken after processing the same turbine blade in a multistep procedure
Surface Characterization with Optical Interferometry. Surface topographical mapping is coming into increasing use to bet- ter quantify surfaces as they relate to part service life, function and performance. The surface shown in the top row is one that has been processed to blend in parallel rows of surface peaks left behind from fine grinding operations (as shown in the bottom row of diagrams). The resultant surface is one that is more isotropic or random in nature. This type of surface can be an important surface attribute to parts that are subjected to repeated stress or strain and parts that undergo high force loading of opposing surfaces. Also contributing to the improved functional surface is the negative skew of the surface profile and beneficial compressive stress equilibrium imparted to the parts by high-energy finishing methods.
type of surface is considered to be very “functional” in both the fluid and aerodynamic sense. The smooth, less turbulent flow created by this type of surface is preferred in many aerodynamic applications. Another important consideration the photomicrographs indicate is that surface and subsurface fractures seem to have been removed. Observations with backscatter emission with a scanning electron microscope gave no indication of residual fractures.
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ManufacturingEngineeringMedia.com | October 2013
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