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Surface Roughness in Metal Finishing


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SVF Flow Controls manufactures industrial control valves fulfilling special application needs throughout industry. Their product line includes specialty materials on construction and very specific finishes on those materials. This white paper provides some explanation of how surface roughness is measured and how various levels of finish are achieved.

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Surface Roughness in Metal Finishing

  1. 1. “What do you need today?” TM Phone: 562.802.2255 • Fax: 562.802.3114 • • The Art of Ra SVF is the recognized leader in providing the pharmaceutical marketplace with hygienic valves for a range of applications. A common requirement is to provide our valves with various levels of surface finish to meet specific design specifications. So what is Surface Finish? Surface finish is the nature of a surface as defined by the three characteristics of lay, surface roughness and waviness. It comprises the small local deviations of a surface from the perfectly flat ideal (a true plane). Each manufacturing process (such as the many kinds of machining) produces a surface texture. The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process is added to modify the initial texture. This may be grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, buffing or other processes. Two methods for achieving desirable surface finish is Mechanical Polishing and Electro-Polishing. The Mechanical Polishing Process Mechanical polishing covers grinding, polishing, and buffing, - all processes for improving the surface conditions of a product for decorative or functional purposes. The process includes; grinding first, polishing second, and buffing third. In general, grinding permits far more aggressive abrading action than polishing. Likewise, polishing is a far more aggressive abrading action than buffing. In grinding, polishing and buffing, the requirement is for highly skilled labor with years of experience and a thorough knowledge of the art of their craft. Electro-polishing Electropolishing is a reverse plating procedure that entails the electrochemical removal of metal (including carbon, silica, iron, and other impurities) from a stainless steel surface. Prior to electro-polishing, parts are mechanically polished to ensure optimal results - selected surfaces are mechanically buffed to a smooth finish. Next, the part is fitted with electrodes, immersed in an electrolyte solution, and subjected to a direct electrical current. During this electrolytic process, the metallic surface of the anodic part is removed ion by ion, yielding a nickel and chromium-rich surface free of microscopic “peaks” or “valleys” that could lead
  2. 2. “What do you need today?” TM Phone: 562.802.2255 • Fax: 562.802.3114 • • to metal fatigue or contamination. Optimal results depend on careful control over the current density, the precise chemical composition of the electrolytic solution, the temperature and agitation of the bath, and the duration of current exposure. Unlike mechanically finished stainless steel, electro-polished surfaces feature no fine directional lines and hence offer less friction and surface drag. The chromium-rich surface offers excellent light reflection, yielding a bright, smooth and uniform polish. What is Ra? Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surface’s peaks and valleys. ASME/BPE calls out a Surface Designation to indicate the required Ra (Surface Finish) for process contact surfaces. Basically, these surfaces are inside the valve flow path (wetted parts).