Finishing - January-February 2020

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22 ANODISINGAnodising aluminum cylindersTo permit the use of aluminum instead ofother, heavier metals in industrialapplications, many cylinders are anodizedto create an extremely hard surface that iswear-resistant, corrosion resistant, nonconductiveand lubricious. Because anodizedsurfaces are porous, they also improveadhesion of coatings as well as accept avariety of dyes for colouring.Given the myriad of benefits, anodizing ispopular for a variety of cylindrical itemsincluding lift mechanisms for chairs, liftcylinders for hatchbacks, shock absorbers andforks for bicycles, fuel pumps, water pumps,pneumatic and hydraulic cylinders, spoolvalves, valve stems and valve bodies.By definition, however, the anodizingprocess means the parts grow dimensionallyand increase in surface roughness.For a cylinder, that includes both anincrease to the outer diameter (OD) anddecreases to the inner diameter (ID). Thereare several different types of anodizingmethods, and each type or class reflects arange of coating thicknesses. As a rule, thickercoatings provide greater corrosion protectionand, in harsh environments like salt air, thismeans longer-lasting surfaces.As for surface finish, generally a hardcoatthat is anodized to a .002 thickness will resultin a Ra that is 2-3 times the original baremetal finish. For example, a machined Ra of16 can easily become 30 Ra or more afteranodizing.For many parts, this is not an issue.However, when the part is cylindrical andmates with another part, often using a seal,increased dimensions and rougher surfacefinish can be problematic.Anodic coatings are very hard (only slightlyless than diamond and harder than hardchrome plating) and increased surfaceroughness can abrade sealing materials. Sealwear and coating irregularities can provide apath for leaks.For this reason, parts require a fine surfacefinish for reliable sealing and long componentlife. To accomplish this, many are utilizinghoning tools as an economical approach totreating the surface before or after theelectrochemical process to control thedimensions and create a smoother surface.The result is a cost-effective approach toresolving finishing challenges in the anodizingprocess to consistently yield high-qualityproducts at a competitive price.Honing ToolsTraditionally, manufacturers have usedgrinding, lapping, and rigid honing toimprove the surface finish of anodized andhard-coat anodized parts. Machine setups aredifficult, however, and they must be extremelyprecise. There are several reasons for this.First, the anodized coating is very hard.Second, the total coating thickness is verythin. Third, the high points and low points ofthe anodized coating are not absolutelysymmetrical around the centerline of thecylinder ID.When rigid honing is used with anodizedparts, the honing stones only contact thecoating’s high points. In other words, parts ofthe cylinder ID remain untouched.There are other issues with rigid honing,too. Because anodized coatings are relativelythin, only a very small amount of materialshould be removed. Yet rigid honing workscontinues on page 24Finishing - January/February 2020
ATPull-Off Adhesion TestersMeasure the adhesion of coatings tometal, wood, concrete and moreAutomatic Featuresn Impact and scratch resistant color Touch Screen display with keypadn Wirelessly connect the PosiTest AT-A to your smart device using our free appn Record the results as pass/fail and the nature of fracture —cohesive, adhesive and glue failuresWiFiPlease comeand visit us atSurface World Exhibition at theNEC 18/19th March,Stand D9AT-AAutomaticModelAlso availableAT-M Manual ModelDFT Instruments UK LTD16A Long Drive, Meadway, Berkhamsted, Herts HP4 2PNt: 01442 879494 f: 01442 879595www.dftinstruments.co.uk e: sales@dftinstruments.co.ukProud supplier of DeFelsko Inspection Instruments
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Finishing - January-February 2020

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