What is an RA Finish

Ra is calculated as the Roughness Average of a surface’s measured microscopic peaks and valleys.

RA is typically measured in either microns (µm) or micro-inches (µ-in, µ”). One micron equals roughly 40 micro-inches (µ”). As a frame of reference, 100 microns may represent the thickness of human hair, or a sheet of paper. Both, of course, are dependent on the individual, and the specific sample of paper being measured.

Ra is measured a few ways. One is with the use of a profilometer. This is an instrument with a stylus that travels across the surface of a part, measuring the difference in height between the peaks and valleys of the surface profile. ISO standards use the term CLA (Center Line Average), which is interpreted identically to Ra.

Non-contact means involve the use of an optical light, lasers, and even x-rays.

A low RA value represents a very flat surface, with little deviation. Higher RA values represent a surface that is “rougher.”

The costs associated with achieving specific finishes increase exponentially as RA numbers get lower. RA and costs are, therefore, inversely related.

As for the common stainless steel finishes, they include:

1. No. 2B – Matte finish
2. No. 4 – Brushed finish
3. No.8 – Mirror finish

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This simply means that no further processing has occurred. No. 2B finishes are dull in appearance, and, generally speaking, are used when a certain aesthetic is not required. It is a good choice for situations where appearance is not terribly important, or when further downstream finishing processes are intended.

The finish for a No.2B is produced by ‘cold rolling’ stainless steel through special rolls or dies. Cold rolling provides a surface that is smoother and less pitted. After this step has been completed, the steel is placed in an acid solution to soften and de-scale. It is then sent for a final pass on polished rolls to help enhance its smoothness.

Matte Finish

Brush Finish

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This finish can very dependent on the supplier, as well as individual batches from a single supplier. This variation is the unfortunate bi-product of varying conditions experienced during the finishing process. This includes, but is not limited to, the wear being experienced by the abrasive belts and other abrasive media being used during the process.
As a result, it is always best to always accept some degree of variation when buying product with a No. 4 brushed finish.

A No. 4 Brushed Finish produces a very distinctive look, resulting from the fine parallel lines produced. Subsequently, it has a nice decorative appeal, without being too reflective.
Some of the more common drawbacks of utilizing a No 4 Brushed finish include reduced corrosion resistance, as the scratches in the finish can make it susceptible to rust.
One way to achieve this finish is to sand the stainless steel in one direction, starting with a 120 grit belt, and progressing to a 180 grit, before finishing it off with a medium non-woven belt.

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A No. 8 finish is highly reflective finish. It is created via the mechanical abrasion of the surface of the stainless steel with, with the progressive use of finer abrasives. A common sequence of coated abrasive products can include 220, 400, 800, 1500, 3000, and 5000 grit. The final process involves buffing the surface in order to achieve the desired mirror-like highly reflective finish.

A side benefit to a No. 8 Mirror Finish is the added corrosion resistance. During the polishing process all the little crevices where corrosive particles can become lodged are removed, mitigating the risk of corrosion post abrasion corrosion.

Mirror Finish