Welcome to our “Notepad Series” of fast, informal instructional videos. In these short sessions we try to make surface texture understandable for those who rely on the information but are not necessarily experts in the field. Each video addresses common topics related to surface specification, measurement and interpretation, in five minutes or less.
If you like this kind of training approach and want to go deeper, contact Digital Metrology to learn more about our online and onsite training and consulting. You can also check out our additional training resources, including our Surface Notes blog.
Notepad Series Videos
1. What is Roughness?
When we talk about surface texture, what is roughness? Does the word “roughness” mean the same thing to everybody? The fact is, what we call “roughness” is entirely based on our application. This video shows an example, and why it is so important to specify the range of features that we will call “roughness” in any given situation.
2. Roughness and Waviness
In this second Notepad Series video we look at the process of separating data into features that we will consider shorter-wavelength “roughness” versus longer-wavelength “waviness” for a given application. Seeing the process drawn out, step by step, can really clarify how the roughness and waviness profiles are created, and what they really mean.
3. Bandpass Waviness
In our last video, “Roughness and Waviness,” we looked at how to separate longer wavelength “waviness” from short-wavelength “roughness.” In “Bandpass Waviness” we go a step further and also separate waviness from the long-wavelength “form” shapes. Making this distinction lets us target the waviness features that could matter most to you. For example, if you are trying to create a sealing surface, controlling the waviness-related lumps or bumps may be your biggest challenge. One surprising upshot of controlling these features separately: the added controls may actually let you loosen tolerances on the long wavelength form as a result.
4. Average Roughness (Ra)
The average roughness (or “Ra”) value of a surface is the most common number describing the “amount” of roughness on that surface. While the Ra value (or “Sa” for areal / 3D measurements) may give a general sense of the surface texture, it cannot distinguish between two surfaces of different shapes. For example, a jagged surface with sharp spikes could have the same Ra value as a smoothly plateaued surface with lots of deep porosity. As we show in this video, describing a surface using only Ra is like describing a concert only by loudness! Yet, Ra (or Sa) may still have its uses in some production settings.
5. Average Peak-to-Valley Roughness (Rz)
In “Rz (Average Peak-to-Valley Roughness)” we look at the world’s second most common surface texture parameter. Our eye can do a pretty good job of telling us the general roughness of a surface. Rz works similarly. One caveat: there are other definitions of Rz out there—we will show you the differences and what to look for.
6. The Material Ratio Curve
In “The Material Ratio Curve” we look at this rather well-known curve (historically known as the Abbott-Firestone Curve). It shows us the amount of material that we encounter as we move further down into a surface. That can tell us a lot about the surface: how durable it may be, how it could carry lubrication, how well it may wear…even how comfortable it may be to slide around on (not recommended!).
In this introduction we show how the material ratio curve is derived. Then, we show you some examples that will help you estimate the nature of a surface from the shape of its material ratio curve.
7. The Rk Parameters
In our previous Notepad Series video we introduced the Material Ratio Curve. Today we look at the “Rk Parameters” which are derived from that curve. These parameters help us describe the peaks, valleys, and core region of a surface. All three regions may play important roles in how a surface will function: how it will wear, support a load, retain lubrication, etc. A single number cannot describe all of these traits—but the Rk parameters can.
8. Specifying Waviness
If we are controlling things like sealing in gaskets, or noise in bearings and gears, the longer-wavelength waviness may be more important than the surface roughness. In this video we look at how we extract waviness from surface texture data, using a “cutoff wavelength” to determine what will be called waviness and what will be called roughness for our application. We also show how to specify waviness on a drawing so that we can measure and control it in production.
9. Fake Peaks and Filters
Are the peaks that you are seeing in your surface texture measurement real? It’s a critical question, as peaks may cause all kinds of issues for an interface. The trouble is, in some cases the peaks that you see in your graphs may not be real at all…they may be artifacts of the filtering operation. False peaks can make you think something’s wrong, which can lead to a lot of unnecessary rework or even scrapping of good parts. In this video we look at how the filtering process works, and how it can lead to the generation of peaks that aren’t there in reality. We also look at a better filter–the Robust Filter–that is able to provide more accurate representations of the roughness and waviness.
10. Stylus Tip Radius
Is a smaller stylus tip radius better for measuring roughness? Is a larger tip more durable?
The stylus tip radius influences what we see in surface texture measurement data. Since every tip is a little different, the resulting roughness values can be different even when you are trying to use the “same” radius. To make sure that the data from different instruments and styli are consistent we employ a “short wavelength” filter. In this video we will talk about how that filter’s cutoff is determined and what it means for your measurements. We also look at the common misconception that a larger radius stylus is more durable than a smaller one. It’s really not the case, and we will show you why.
11. 2D vs 3D (Areal) Texture
Is a 3D (areal) texture measurement “better” than a 2D (profile) measurement? There are strong opinions on both sides. But the truth is that there may be some common ground. In fact, there are cases in which each type of measurement may be the right choice. In this video we discuss the properties of 2D vs areal (3D) measurement systems. We talk about how the two are related and we show where one may be preferable over the other depending on the application.
12. Surface Texture and Wear
Thank you for all your suggestions for new Notepad Series topics! We’re going to get to one of the most requested topics now: how to assess wear.
If you dug a hole in your yard, you likely wouldn’t assess how deep it was by describing the roughness at the bottom of the hole. In this video we’ll show you some better ways to measure wear, both micro wear (the kind within the surface texture) and macro wear (the depth of the wear scar, or the hole in your yard!)
13. Measuring Short Surfaces
How much of a surface do you need to measure in order to accurately describe its roughness? But if you are measuring a narrow feature or part, such as an O-ring groove or washer, the surface may be smaller than the length suggested in the standards. In this video we show you how to determine the “evaluation length” for measuring roughness, introduce options for measuring texture on smaller surfaces, and point out pitfalls that can lead to incorrect evaluation of roughness and waviness.
14. Peaks, Valleys, and Skewness
Many of our surfaces have symmetric peaks and valleys that follow a normal or bell-shaped height distribution. But often we may want that distribution to be stretched, or skewed, toward the peaks or valleys.
The Rsk parameter, which is widely used to report this skewness, has some serious issues baked into the math which can lead to incorrect conclusions. In this video we examine the Rsk parameter, how it’s calculated, and why it can be unstable. We also provide alternative parameters that may be better than Rsk for production measurement.
15. Why Not Repeatability?
How good is my surface roughness gage? There’s an old tale of three blind people describing an elephant based on the small portions of the animal that they each experience. It’s a great analogy for surface measurements: we may get very different information about a surface just by slightly changing the measurement location.
In this Notepad Series video we look at how measurement location can account for poor “repeatability,” and why repeatability is not a good judge of a stylus instrument’s condition or capability. In fact, good repeatability may indicate a bad instrument!
16. Why Use a Skid?
To measure surface roughness, a roughness gage (sometimes called a stylus profilometer) traces a small stylus over the surface. A stylus on its own can be very sensitive to not only the surface but also to outside influences. Any vibration or disturbance can make it move, which will introduce errors to the measurement. To help solve this issue, a stylus is often housed within a skidded probe. But a skid can cause other measurement errors as well. In this Notepad Series video, we describe how a skid works, how it can help improve your data, and how some potential pitfalls can adversely affect your surface roughness measurements.
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