Taking a closer look at the specification for surface texture, “Ra“. How is it derived, measured and controlled? This is not a dry academic exercise but has practical relevance to the making of your parts and tooling. Knowing what this measurement means, and even its limitations, is therefore critical to specifying exactly the surface texture you want and will help you to avoid costly delays.
What is the Ra Value?
Don’t be put off by the diagram above. The math is not all that complex, but even if it were it can be boiled down to an easy concept. Imagine that you’re looking at the profile of a part. If you take a sample length and measure the high and low spots along that length, Ra is just the average height above and below the reference line, or mean. The bigger the Ra number, the more rough the surface. If the Ra value was 0, the graph would show a perfect horizontal line, meaning the surface was absolutely flat. Not possible in this universe.
There are other systems of measurement which are similar but not exactly the same. This can potentially lead to confusion and error, so it’s important to know what system you may be using and that it conforms with the factory making your parts. When in doubt, ask. There are conversion charts available to easily convert from one system to another.
Why should you care about the Ra value?
This is the industry standard for specifying to a manufacturer what the roughness should be – and should not be – on your part. This is used for prototypes, machined parts, and for tooling. It affects the parts functionality and performance as well as its appearance.
Note the surface roughness should not be confused with surface finish, although the two are related. Therefore understanding the purpose of Ra is essential to getting the part that you want, but it’s not the whole story.
What’s the rest of the story?
Remember, Ra only tells us the average deviation from the mean. But there are many ways to achieve this same number using different manufacturing techniques, and each of those techniques will impart a unique pattern on the surface. Thus, manufacturers routinely use surface roughness gauges such as these to demonstrate how different machining methods will produce very different looking textures.
The patterns on the surface of these gauges differ essentially in the spacing of the grooves, or the waves, which is one of the basic determiners of surface texture. The waves on the left are low-frequency, increasing in frequency from left to right. Again, the average height, or Ra value, may be the same but the look is quite different.
That’s why we strongly recommend that, whenever possible, you reference either a gauge such as one of these or, better yet, submit a sample to be compared with.
Textures are by their very nature very complex and difficult to define. A sample is the best way to insure customer satisfaction.
How is surface texture measured?
There are literally hundreds of possible methods, depending on the size of the part feature, part geometry and material type.
One of the most common ways is with a probe or stylus, called a surfometer, that is slowly drawn across the face of a part or part feature. Every perturbation or deviation from a nominal or reference flat line is recorded on a line graph for later interpretation.
One key use of optical photometry is with the identification of defects or asperities. These might be voids, scratches or inclusions of some contaminant in the raw material. Many mathematically-derived models tend to discount these anomalies as irrelevant, but for increasingly critical applications in aerospace, defense and scientific instruments the presence of any defect could be catastrophic. It therefore requires a combination of human and machine to make the ultimate decision about surface qualification.
Why should you care?
There’s no area of manufacturing that causes more frustration and misunderstanding between client and factory than that of surface texture. It can be difficult to describe or specify, so in all cases it’s best to discuss this aspect of your design in detail before commencing with a quotation or production. And remember that there are many different ways to impart a finish on a surface, which create different mechanical and aesthetic affects. Consult one of our experts today to discuss the methods we offer and how they can be put to use on your next project.