For example, we may have a surface finish requirement on a casting that ensures the finish (think of it as the deviations of the surface of the casting from an ideal) is good enough that the allowance made for extra material that will be machined off in a future step is sufficent. There are a variety of considerations, and they may even vary for different stages in the manufacture of a particular part. How do product designers decide what surface finishes to require, and how should CNC machinists think about surface finishes? For example, a surface may be warped or deflected from the ideal. Surface Roughness speaks to fine detail imperfections, but there may also be much coarser irregularities. Surface roughness is usually what machinists refer to when talking about “surface finish.” When talking about all three characteristics, they may use the term “Surface Texture” more properly. Surface roughness is a measure of finely spaced surface irregularities. Surface Finish Lay Patterns… Surface Roughness The Lay is the direction of the predominant surface pattern, and is usually determined by the production method used to process the surface. The relationship of Waviness, Lay, and Roughness… Lay This diagram gives an idea of how to think of the relationship of Waviness, Lay, and Roughness: Another term, analagous to Surface Texture, is Surface Topology. Surface Finish when it is intended to include all three characteristics is often called Surface Texture to avoid confusion, since machinists often refer to Surface Roughness as Surface Finish. Surface Finish is a measure of the overall texture of a surface that is characterized by the lay, surface roughness, and waviness of the surface. Aside from dimensions and tolerances, another important callout is Surface Finish. Engineering prints call out a great many things in their attempt to make sure the part that gets made matches the designer’s intent. Before we get on with Surface Finish Symbols, let’s understand how Surface Finish is defined.
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