Surface Roughness vs. Surface Finish | AR and RMS | Astro package (2023)

Surface roughness is an often overlooked dimensional aspect of the manufacturing process. While more attention is generally paid to the composition and strength of a part, or its measured dimensions and tolerances, too rough a surface can lead to increased friction and premature failure of a part. Beyond mechanical operation, ultra-clean manufacturing requires smooth surfaces within processing equipment to prevent contamination or scale within the process equipment. Simply put, the smoother a surface, the less likely material will stick to it. And of course, the smoother the surface, the easier it is to clean.

How soft is soft?

We interact with smooth surfaces in the course of our daily lives. The glass windows are smooth, the skin is smooth, and the touchscreen of our mobile devices is smooth. Or are? Anyone who paid attention in biology class knows that skin is not smooth when viewed up close. But touch screens? They are made with a controlled roughness because too high a brightness makes the images difficult to see. Also, the screen would be too slippery to precisely interact with the virtual keyboard. This roughness can be measured in microns (µ), which is 0.001mm, or microinches (µin), which is 0.0001 inch. But while the roughness allows you to interact with the screen, it also allows you to leave smudges on the screen.

Different ways to measure roughness

Roughness itself is a series of microscopic "peaks and valleys" along a surface. This becomes clearer when viewed in cross-section. Surface roughness is calculated by measuring the average of surface heights and depths along the surface. This measurement is most commonly given as "Ra" for "average roughness" and this value is used to determine complianceEquipment with various industry standards. Formally, Ra is described in ASME B46.1 as "the arithmetic mean of the absolute values ​​of profile height deviations from centerline recorded within the rating length". Ra, Rq (RMS), Rv, Rp, Rz and some other parameters are two-dimensional in nature and only refer to "up and down" measurements in a straight line. They do not take into account other components of the surface topography such as defects, form defects or waviness (symbolized as Sa, Sq, Sz) that would be measured in a 3D assessment. Two-dimensional roughness measurements are usually carried out through any grain that may be present.

For math lovers, the formula for calculating the main height over the entire measured length or area is:

1/n * SUM(ABS[Zi-Zmean] from i = 1 to n

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Previously, surface roughness was calculated using Root Mean Square or RMS, which used the same measure of peaks and valleys but a different formula. RMS is sensitive to larger peaks and valleys, while RA is not. RMS or Rq will mostly appear on older engineering drawings as it has been dropped in favor of RA. Also, RMS is generally measured in inches in most countries except the US, while RA is generally measured in millimeters since most industries are now using the metric system. Many drawings in the US show metric measurements with English in parentheses, e.g. e.g. "0.8 (32)". It is also worth noting that the measure of the absolute distance between the highest peak and the lowest valley is given as Rz.

Ra is measured with a profilometer. This is an instrument with a needle that moves across the surface and measures 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.

Why is Ra important?

As mentioned above, a smooth surface makes it difficult for product to stick to the sides of a container or pipe in the system. If free iron or other unwanted material enters the system, it is less likely to become lodged in the metal and become a source of contamination. In high-purity processes, any single contaminant can spoil an entire batch of product. While the cost of cleaning and flushing a system can quickly add up, the cost of lost production time must also be considered. In general, the lower the Ra value, the higher the purity of the vessels in production use. Not only is it easier to clean, but a smoother surface makes it easier to empty. The time savings between batches can increase product yield, which has a positive effect on the bottom line.

Table 1. Plot of Grain vs. RA

Surface roughness vs. surface finish

It should be noted thatSurface roughness differs from surface finish. The term "conclude“Used to describe the appearance of a stainless steel plate or sheet and can be very subjective.surface roughnessit is measured objectively with calibrated equipment. Stainless steel with a 2B rolled surface is a bright, relatively flawless surface produced by a final cold rolling pass with polished rolls. It has no grain and has been likened to a "cloudy mirror" in appearance. Because thinner metal passes through the rolls more frequently than thicker sheet, thinner metal generally has a lower Ra and a smoother surface. Defects embossed in the surface become visible when the surface is electropolished.

Examples of different surface finishes

In general, the higher the required purity of the product, the finer the surface finish of the production equipment must be. For example, the 2B surface is used in bakeware, food processing, tanks and vessels, pharmaceutical equipment and vacuum drum dryers. It is considered smooth or smoother than a #4 polished finish and both meet USDA standards. The Ra for a 2B surface is typically 0.3 (12) to 1 µ (40) depending on the thickness of the metal.

Other surfaces and their roughness averages for comparison:

• A #1 finish, sometimes referred to as hot rolled, annealed and pickled (HRAP), is plate stock as it comes from the rolling mill. It is very rough, close to 3.2 (125) to 12.5 (500) Ra, and no mechanical finishing such as the application of abrasives has been performed.
• A #4 finish is a straight grain finish, commonly considered a "brushed" finish, just like the #3 and #6 finishes. A standard #4 finish can be approximately 0.8 (32) Ra while a No. 4 dairy or hygiene finish has an average roughness between 0.3 (12) Ra and 0.4 (16) Ra.
• Two more finishes; #7 and #8 are polished. The surface of #8 is almost flawless. The Ra on a #8 finish would be 0.025 (1) Ra.

Many other surfaces are of course available, but for biopharmaceutical purposes (injectables, ophthalmic solutions) 0.38(15) Ra and electropolished are often specified and coded under BPE SF-4. Powder and tablet manufacturers can use a slightly rougher surface of around 0.5 (20) Ra according to BPE SF-2 standards as it is not electropolished.

Roughness reduction

The finish of a container and its Ra will determine what product can be made in it and as mentioned above, increasing levels of cleanliness require ever finer finishes with lower Ra numbers. Every industry has specific surface standards that must be met.Food grade sanitary surfacesThey generally range from 0.5 (20) to 0.7 (27). This area eliminates places where bacteria or other contaminants can grow. Ra can be reduced using a combination of chemicals and electricity to gently dissolve the surface of the steel. This process is called electropolishing. In reality, only 5-10 µm of material is removed, and that is mainly the high peaks on the surface. The results make the valleys significantly flatter in comparison. The surface roughness can be reduced by up to 50%.

Electropolishing is not the right solution for heavily damaged surfaces, e.g. B. by mechanical impact, welding or chloride gray stains. In these casesmechanical polishingGrinding or grinding may be required to reduce Ra to near the desired range. Once this is achieved, electropolishing is performed. While electropolishing provides an overall smoother surface, it also removes any embedded residue such as swarf or metal fines that may have been polished into the surface.

Finally, the thickness of the stainless steel also affects both Ra and electropolishing, as thicker metal can withstand more machining to achieve greater smoothness.