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What is Surface Finishing?

By M.J. Casey
Updated May 17, 2024
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Surface finishing in manufacturing applies to any final procedure or series of procedures that result in a change in a desired property of a face of the manufactured item. Physical or chemical properties may be altered. The metalworking, textile, and polymer industries often use surface finishing to improve the characteristics and value of their products.

The roughness of a surface is a common physical feature that needs to be smoothed in a manufactured item. Grinding, sanding, polishing, and buffing are steps used sequentially to increase smoothness. A smooth surface may be desirable for material handling, the close mating of machined parts, or for aesthetic reasons. Reduction of friction between the object and another material is a frequent reason to smooth surfaces.

Surface roughness, expressed as Ra, is a measurement of the arithmetic mean of the peaks and valleys of a surface. It may be measured directly by a contact profilometer, a device in which a diamond needle on a stylus rides up and down the profile, following a programmed grid while recording the results. These devices measure Ra from 0.1 to 0.4 microinches (3 to 10 nanometers). Non-contact profilometers and interferometers measure Ra by a wide variety of optical measurements, comparing the angles at which light is reflected and interference patterns. These machines can measure roughness in the range of 0.012 to 0.02 microinches (3 to 5 angstroms).

The surface properties of the mirrors used in telescopes largely determine the quality of the images obtained. The measurement of the smoothness of these mirrors becomes as challenging as the surface finishing techniques themselves. The surface of the Hubble telescope's primary mirror has a total variance of less than 0.04 microinches (10 angstroms).

Metal sheets or objects are often coated with polymers or paint to protect the material from corrosion and pitting. These finishes may be applied by spray or sputtering, or by vapor or powder deposition. A mirror finish may be achieved by these methods. The final step may involve a curing, annealing, or baking to set the finish and ensure adhesion to the underlying material. Embossing or etching may also be employed to modify the surface of the finished object.

Electroplating is the practice of bonding a surface coating of one material to another by electrochemical methods. Typically, these are metals or metal alloys bonded to other metal substrates. Tough, corrosion-resistant, and attractive surface finishing is a requirement for many car, airplane, and boat components. Likewise, medical components use electroplating to achieve a surface that may be sanitized or sterilized.

In the textile industry, fabrics may go through surface finishing steps that add sheen to the material, starch to aid in handling, or an embossed pattern. The texture of the material may be enhanced by a brushing or scraping technique. Items made from polymers are often subjected to surface finishes to alter their texture, gas or liquid permeability, or stiffness.

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