In previous blog posts, we have touched upon critical raw material characteristics and methods. Metal strip can be rolled and annealed. These manufacturing processes allow for a great range of mechanical properties. Next month, we will talk about slitting which is the icing on the cake: the proper material width. This month, we will talk about something subtle and confusing: surface roughness measurements. View our Capabilities Before we dive into a world of terms and definitions, we must ask why surface roughness is important anyway. Well, let’s break roughness into two practical characteristics of metal. On the functional side, roughness influences the flow of oils in forming processes. Think of roughness as tiny channels between a forming machine and the raw material. Having the proper roughness means having the proper oil contact. This can make the difference between being able to make a part and disaster. On the aesthetic side, roughness provides reflectivity, either shiny or dull. The new trend in home appliances is stainless steel. Robust machines as if from the future inherit a new level of perceived quality thanks to their steel clothing. Next time you look at one, take a careful look at this metal. Its dull appearance is due to its roughness. But for what purpose? Imagine if the outer metal was a mirror finish. Every fingerprint and smudge would stand out as a dull aberration on its sheen surface. The dull surface, i.e. high roughness, makes fingerprints less noticeable. Quite an important feature for a refrigerator under constant attack from drowsy, late-night expeditions for a perfect snack.
A Blizzard of “R” Let Us Quote You Now!
A brief web search of surface roughness parameters will yield hundreds of abbreviations and formulas. Ra, Rq, Rsk, Rku Rp lead way to the obscure descriptions of arithmetic average, root mean squared, skewness, kurtosis and maximum peak height. And these only represent a few of the many values! Attempting to follow the mathematical formula to gain a better understanding is even more cumbersome. Each value has importance, so where do we even start? The real question we need to ask is: what is important to us? Most application requirements can be discussed with just a few common values. What we really need to define and communicate is how rough the surface is on average, and how to define unwanted surface conditions.
Surface Roughness: Ra
When we spoke about the dull refrigerator, we were speaking about Ra. Metals are not completely flat when we look at the surface under a microscope. There are peaks and valleys much like a mountain range over the entire surface. The Ra value expresses the average of the peaks and valleys of a surface. The image to the right shows how this value is determined. Values are typically expressed in microinches or micrometers. Micro means one millionth, so the values we are speaking in are very small. For example, Ulbrich defines our typical #2 finish as 12 microinches maximum. That means 12 millionths of an inch! Average roughness is important because it best defines the surface. Qualitative statements such as “I want smooth material” or “I want rough material” are the metallurgical equivalent of going to a restaurant and saying “I want something yummy.” We must be specific in how we talk about surface because the surface has specific needs. Saying “I need a surface between 15 and 30 microinches Ra because I want this fridge to hide fingerprints” is akin to “I need a meal without peanuts because I have allergies.” Well, it’s a close analogy although fridges are not allergic to fingerprints.
Condition Measuring: Ry
Condition is a precursor for the term defect. Defect is the conclusion that a condition is not acceptable. How do we determine if a surface condition is a defect? With surface roughness of course! This time we will employ the use of the roughness term Ry. This is defined as the maximum height of the surface profile. It is the difference between the highest peak and the lowest valley of a sample. This value is beneficial to us because it will show us conditions in the surface that are far from the average. The most common conditions that are not close to the average roughness are scuffs and scratches. The use of profilometry allows Ulbrich and customers to speak in similar terms. It is very difficult to determine acceptance criteria for scuffs and scratches using subjective means. Imagine attempting to talk about a scuff by how it looks or how it feels? Industry sometimes defines a scratch as a depressed condition greater than Ry 100 microinches. A scuff is therefore often defined as any similar condition below Ry 100 microinches.
The Hybrid: Rz
Getting right to the definition, Rz is the distance between the peaks and valleys of a sample. This sounds quite like Ry! However, Rz is the sum of the average of the 5 tallest peaks and the 5 lowest valleys. Curious, this part sounds a lot like Ra. This hybrid aspect raises some concern when using Rz in place of Ry or Ra.
Rz is less sensitive to surface conditions when compared to Ry. This means that if there is a big scratch and a few smaller scuffs, the severity of the big scratch is diminished when averaged with the scuffs.
Rz is more sensitive to overall roughness when compared to Ra. Rz is only averaging the top 5 highest and top 5 lowest sections. Ra is averaging the entire measured distance. To illustrate, let’s assume that all the workers at Ulbrich are standing with 5 of the tallest people to ever live. Their heights are all above 8 feet tall so let’s round their heights all to 8 feet tall. We can make an estimate that most of the people at Ulbrich are around my height, so we’ll say 6 feet. If we take the Rz approach, the average height of the top 5 people in this group are 8 feet tall, so therefore everyone at Ulbrich must be about 8 feet tall. Wow! What an unlikely group of tall folks! Now, if we take the Ra approach with just one hundred of the 6 foot Ulbrich employees plus the five 8 foot folks, the total average comes to 6.095. This value is much more representative of the total population.
Ulbrich prefers to use Ry and Ra for these very reasons.
A brief look into “How”
The values discussed above are measured using tools called profilometers. The methods range from contact styluses, lasers and visible light. International standards have been developed to account for the sensitivity of measurement to ensure that different machines can be used somewhat interchangeably. Remember, we are speaking in millions of an inch! This means that we should arrive at similar measurements even if I use a Brand-X meter and you use a Brand-Y meter, assuming that we use the same standards. These standards define characteristics such as how much material is measured. In the case of contact stylus methods, characteristics such as speed of measurement are also defined.
Keep your eyes open for our November blog concerning surface finishes. In this blog post we will talk more about profilometers and also about how we get material to be a particular Ra in the first place!