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What Is a Standard Machining Surface Finish?

With all the technology currently available, you can take a machined part from concept to prototype to a finished piece and are now able to pinpoint and perfect specific areas of those parts that would have seemed inconceivable just a few short years ago. Whether the component in question was CNC machined, cut by a waterjet, or fabricated with any of the other popular machining processes, great detail can, and is now included, in the surface condition of these parts.

Surface finish is a broad term, with many different meanings, now being used when processing machined parts. As with any process that can deliver this number of alternative solutions, a surface finish has a beginning, or a baseline if you will, that serves as a starting point for the various surface finishes, which you can add to the machining of your particular part.

This post will answer some of the more baseline questions concerning surface finish for those looking to gain a more fundamental understanding of surface finish. Once these definitions and the science behind surface finish become clearer, look into how a company like Elemet Manufacturing can provide cutting and machining services with the desired surface finish for your next prototype or production job.

hybrid surface texture measuring system for surface roughness and contour measurement

What Is the Definition of Surface Finish?

Engineering Choice is an informational website that is a good starting point for understanding the basics of surface finish. The following definition is from EngineeringChoice.com:

“Surface finish, also known as surface texture or surface topography, 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).”

Surface finish can be as simple as the finish left behind after machining. In applications where the surface finish does not interfere with the part’s function, to save money on production of the part, no further action is needed for it to be complete and ready to ship to the customer. This type of surface finish can also be labeled as a Standard Machining Surface Finish.

For those parts that need to work in tandem with another part to create a seal, you must specify certain surface finish attributes as part of the manufacturing process. To machine the surface finish to a particular set of specs, you need to know how the surface finish is measured.

What Is Measured for Surface Finish?

The definition above of surface finish mentions three characteristics. These characteristics are lay, surface roughness, and waviness, and it is the measurement of these three characteristics that classify the surface finish. 

Another term used when defining or measuring the surface finish is Surface Topology. Think of surface topology exactly like you would the topology of a geographical map, taking into consideration all the peaks, valleys, and other surface attributes when referring to “the lay of the land.”

Let’s look at the three characteristics and how they contribute to surface topology.

Lay

Think of the surface finish lay just like you would think of the direction of the grain in a piece of plywood or lumber. Manufacturing processes produce the lay, and it can be parallel, perpendicular, circular, cross-hatched, radial, multi-directional, or isotropic (non-directional). Later, in this post, when we address the surface finish chart and how to use it, the Lay measurement will be further explored.

Waviness

Waviness is the broadest spaced variation in the surface finish. These periodic surface imperfections are small enough, short enough, and regular enough that they are not considered defects in the surface finish’s flatness.

Common causes of waviness are warping from extreme temperature changes and machining defects such as deflection from machined pieces of material, inadvertently creating minor surface imperfections.

Waviness requirements are not as common as other surface finish components, but they are integral in machining parts used as bearing races and sealed parts.

Roughness

These are minor irregularities in the geometry of the surface. Roughness is the most widely used measurement in the surface finish, and many times the actual term “surface finish” of a given part refers to its roughness.

 

inspecting standard machining surface finish with a roughness tester

Key Takeaway of Surface Finish Measuring

In the machining process of any given part, you can measure the dimensional tolerances and surface finish requirements with these three characteristics. Your component may call for a Standard Machining Surface Finish, or it may require some added texture and, therefore, additional machining steps to produce the correct lay, waviness, and roughness. You will need to specify the final desired measurements for manufacturing these parts and ensuring continuity from piece to piece.

This measurable information also determines if the surface finish is correct for the material type used. That is where the use of a Surface Finish Chart comes into play.

Surface Finish Charts: What Are They and How Are They Used?

The instrument used to make quick work of surface finish measurements is a measurement stylus that you can drag across the surface and take readings. If you cannot touch the part, for whatever reason, you can use a 3D metrology method.

Without going into extensive algebraic computations and equations, you can refer to this excellent reference manual for a Surface Finish 3D Metrology Tutorial from the US Department of Commerce and the National Institute of Standards and Technology. This 162-page PDF is the go-to document for surface finish terminology, processes, and charts.

By obtaining the correct surface finish measurements, you can compare your findings with the Surface Finish Charts to see if your part has the desired surface finish or if you need to take additional manufacturing steps to get your component to within spec.

Think of the surface finish chart as a way of giving the part a letter grade, and then compare it to the information on the chart to see how close to the CLA (Center Line Average) you are for the particular material you are using.

high precision grinding machine creating a standard machining surface finish

How To Obtain a Standard Machining Surface Finish and More

Hopefully, you now have an idea about surface finishing and the different degrees of surface finishing, from Standard Machining Surface Finish to Textured Surface Finish and beyond. Learn more about surface finishing technology and address your metal fabrication needs by contacting Elemet Manufacturing. See how your particular component specifications stand up to the latest in surface finishing methodologies.

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