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Facilitating Honing of Hard Anodized Aluminum Coatings

The aluminum casting market is growing at a healthy pace due to increased demand for lighter-weight aluminum components and structures in the automotive, transportation, and industrial sectors. According to market research firm Grand View Research, the global aluminum casting market is expanding at a compound annual growth rate of 7.8% and will be valued at $97.36 billion by 2025. For many applications, the aluminum (hydrated aluminum oxide) is anodized to protect it from corrosion and to create a ceramic-like hard surface that is wear-resistant, non-conductive, and lubricious. Unlike painting and plating, which are mechanically bonded to the aluminum surface, anodizing results in the formation of a surface coating (anodic aluminum oxide layer) that is chemically bonded to the aluminum base material. Anodizing also brightens the surface and, because anodized surfaces are porous, they have improved adhesion to coatings and readily accept a variety of dyes, allowing the development of a vibrant appearance.

Anodizing is commonly used on cylindrical parts such as lift mechanisms or lift cylinders for hatchbacks in cars, shock absorbers, and forks for bicycles, fuel pumps, water pumps, pneumatic and hydraulic cylinders, spool valves, valve stems, and valve bodies. It creates some difficulties, however, because the dimensions and surface roughness of the parts are changed as a result of anodization. Specifically, the outer diameter increases while the inner diameter decreases, with different types or classes of anodizing methods reflecting a range of coating thicknesses. Thicker coatings generally provide greater corrosion protection and thus longer-lasting surfaces. The surface roughness of anodized parts also increases. Typically, a hardcoat that is anodized to a 0.002 thickness will result in a value for the surface profile roughness parameter Ra (a dimensional unit reported in micrometers or microinches) that is—two to three times times greater than that of the original bare metal finish. For example, a machined Ra of 16 can easily increase to 30 or more after anodizing.

Although these changes are not an issue for many components, for cylindrical parts that are mated with other components, often using seals, the increased dimensions and rougher surface finish can be problematic, according to Jonathan Borden, national sales manager of Brush Research Manufacturing. “The very hard finish provided by anodic coatings, which are harder than hard chrome plating and only slightly less hard than diamond finishes because of their rough surface, can abrade the sealing materials,” he explains. This seal wear combined with the surface irregularities can ultimately result in leaks.

Anodizing also brightens the surface and, because anodized surfaces are porous, they have improved adhesion to coatings and readily accept a variety of dyes, allowing the development of a vibrant appearance.

To overcome this problem, it is often necessary to refine the finishes of anodized parts to ensure reliable sealing and a long performance life. The approach typically taken for industrial parts, largely due to its economic feasibility, involves the use of honing tools to treat the surface before or after the electrochemical process to control the dimensions and create a smoother surface. This technique consistently yields high-quality products at a competitive price. The most common honing methods include grinding, lapping, and rigid honing.

While economically attractive, machine setups for honing are difficult. Borden notes that they must be implemented precisely because the total coating thickness is minimal, yet the anodized coating is very hard. In addition, the high points and low points of the anodized coating are not absolutely symmetrical around the centerline of the cylinder inner diameter.

There are other issues as well, particularly with rigid honing. “When rigid honing is used with anodized parts, the honing stones only contact the coating’s high points, leaving parts of the cylinder inner diameter untouched. It is also a challenge to remove only a very small amount of material from the very thin anodized coating, because rigid honing works best with heavier cuts and greater material removal,” Borden observes. Fine cuts combined with tool loading can contribute to smeared surfaces.

The approach typically taken for industrial parts, . . . involves the use of honing tools to treat the surface before or after the electrochemical process to control the dimensions and create a smoother surface.

Brush Research Manufacturing has developed a tool designed to provide greater control and facilitate honing of anodized and hard-coated aluminum cylinders. The Flex-Hone tool contains flexible nylon filaments to the ends of which abrasive globules are permanently laminated. Because the diameter of the tool is greater than diameter of the bore, the Flex-Hone

is used in an oversized condition and is self-centering, self-aligning, and self-compensating for wear, according to Borden. “The Flex-Hone tool’s abrasive globules ‘float’ to ensure that all parts of the bore—and not just the high spots—are surface finished. In addition, unlike with rigid honing machines, Flex-Hone setups are simple, and surface finishes can be consistently improved with just a few strokes of the tool,” he notes.

Notably, these tools can be used prior to anodizing to control the size in anticipation of the shrinkage of the inner diameter, according to Borden.  At this stage, honing with the tool also removes “fuzz”—any sharp edges or amorphous material that might adhere to the surface and affect the quality of the anodized coating. They are most commonly used after anodizing, however, to correct unanticipated size and surface finish issues. “In some cases—particularly when the quality of the final anodized finish is of the utmost importance—part manufacturers use the tool both before and after anodizing,” comments Borden.

The Flexible-Hone tools are available in sizes ranging from 4 mm to 36 in. When used to improve the quality of anodized coatings, it is recommended that aluminum oxide (400, 600, 800) and levigated alumina (extra fine only) grits be employed with the Flex-Hone tools. The specific choice of grit depends on the type and thickness of the anodized coating and the desired properties and specifications for the final surface finish.


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