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Surface finish, also known assurface texture orsurface topography, is the nature of asurface as defined by the three characteristics of lay,surface roughness, andwaviness.[1] It comprises the small, local deviations of a surface from the perfectlyflat ideal (a trueplane).
Surface texture is one of the important factors that controlfriction and transfer layer formation during sliding. Considerable efforts have been made to study the influence of surface texture on friction and wear during sliding conditions. Surface textures can beisotropic oranisotropic. Sometimes, stick-slip friction phenomena can be observed during sliding, depending on surface texture.
Eachmanufacturing process (such as the many kinds ofmachining) produces a surface texture. The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process will be added to modify the initial texture. The latter process may begrinding (abrasive cutting),polishing,lapping,abrasive blasting,honing,electrical discharge machining (EDM),milling,lithography,industrial etching/chemical milling, laser texturing, or other processes.
Lay is the direction of the predominant surface pattern, ordinarily determined by the production method used. The term is also used to denote the winding direction of fibers and strands of arope.[2]
Surface roughness, commonly shortened toroughness, is a measure of the total spaced surface irregularities.[1] In engineering, this is what is usually meant by "surface finish." A Lower number constitutes finer irregularities, i.e., a smoother surface.
Waviness is the measure of surface irregularities with a spacing greater than that of surface roughness. These irregularities usually occur due towarping,vibrations, or deflection during machining.[1]
Surface finish may be measured in two ways:contact andnon-contact methods. Contact methods involve dragging a measurementstylus across the surface; these instruments are calledprofilometers. Non-contact methods include:interferometry,confocal microscopy,focus variation,structured light,electrical capacitance,electron microscopy,atomic force microscopy andphotogrammetry.
Optical metrology plays a key role in non-contact surface roughness measurements, offering high-resolution and non-destructive analysis of complex or delicate surfaces. These techniques are particularly useful in environments where contact-based methods may damage the material or provide limited accessibility. Common optical techniques include:
These optical methods are widely implemented in industries such as aerospace, automotive, biomedical engineering, and microelectronics, where precise surface texture control is critical.
In the United States, surface finish is usually specified using the ASME Y14.36M standard. The other common standard isInternational Organization for Standardization (ISO) 1302:2002, although the same has been withdrawn in favour of ISO 21920-1:2021.[6]
Many factors contribute to the surface finish in manufacturing. In forming processes, such asmolding ormetal forming, surface finish of thedie determines the surface finish of the workpiece. In machining, the interaction of the cutting edges and the microstructure of the material being cut both contribute to the final surface finish.[citation needed]
In general, the cost of manufacturing a surface increases as the surface finish improves.[7] Any given manufacturing process is usually optimized enough to ensure that the resulting texture is usable for the part's intended application. If necessary, an additional process will be added to modify the initial texture. The expense of this additional process must be justified by addingvalue in some way—principally better function or longer lifespan. Parts that have sliding contact with others may work better or last longer if the roughness is lower. Aesthetic improvement may add value if it improves the saleability of the product.
A practical example is as follows. An aircraft maker contracts with avendor to make parts. A certaingrade of steel is specified for the part because it isstrong enough andhard enough for the part's function. The steel ismachinable although notfree-machining. The vendor decides tomill the parts. The milling can achieve the specified roughness (for example, ≤ 3.2 μm) as long as the machinist uses premium-qualityinserts in theend mill and replaces the inserts after every 20 parts (as opposed to cutting hundreds before changing the inserts). There is no need to add a second operation (such as grinding or polishing) after the milling as long as the milling is done well enough (correct inserts, frequent-enough insert changes, and cleancoolant). The inserts and coolant cost money, but the costs that grinding or polishing would incur (more time and additional materials) would cost even more than that. Obviating the second operation results in a lowerunit cost and thus a lowerprice. Thecompetition between vendors elevates such details from minor to crucial importance. It was certainly possible to make the parts in a slightly less efficient way (two operations) for a slightly higher price; but only one vendor can get the contract, so the slight difference in efficiency is magnified by competition into the great difference between the prospering and shuttering of firms.
Just as different manufacturing processes produce parts at various tolerances, they are also capable of different roughnesses. Generally, these two characteristics are linked: manufacturing processes that are dimensionally precise create surfaces with low roughness. In other words, if a process can manufacture parts to a narrow dimensional tolerance, the parts will not be very rough.
Due to the abstractness of surface finish parameters, engineers usually use a tool that has a variety of surface roughnesses created using different manufacturing methods.[7]
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