In surface texture, Average Roughness (Ra) is still the most widely used parameter, even though it has serious limitations. But what other tools are there that can tell us more about our surfaces? We’ll be exploring that topic in a series of posts, so watch this spot for updates every week!
Looking for more info? Check out our original “What’s Really Beyond Ra” post!
Let’s start with our favorite was to move “beyond average roughness:”
1. SEE the data.
For most of us, music notes on a page aren’t really a song until we hear them played. It’s the same with surface texture: parameters are just numbers until we can see what they’re trying to tell us.
A 2D profile shows a lot more about a surface than just a few numbers. A 3D image takes it to still another level, showing trends in all directions. And these days you can even 3D print data so you can hold it in your hand.
But, of course, our eyes can play tricks on us, too. Are those actual spikes or maybe noise in the measurement? Is the surface really tilted, or is it skewed by a few bad pixels? Seeing a surface is a powerful aid, but there are many more tools in the toolkit as well…
2. Waviness
Average roughness (Ra) describes the finer features in surface texture. The larger shapes that we call “waviness” can be just as impactful. Waviness may be the source of noise, vibration, and harshness in gears, as well as the cause of leaky gaskets, premature wear, stress concentration, and many other issues.
The trouble is, roughness parameters like Ra don’t tell us anything about the waviness. It’s like describing a roller coaster by talking about the grain of the wood: you won’t get any sense of the humps and dips that make the roller coaster fun! Check out our Notepad Series videos, Roughness and Waviness and Specifying Waviness to learn more.
3. Wear Analysis
To gauge how much a part is wearing, it’s tempting to measure the average roughness (Ra) of the surface before and after use, then treat the difference as “how much the part has worn.”
That’s a lot like digging a hole and trying to judge its depth by measuring the am0unt 0f roughness at the bottom!
To measure wear properly, we need to have a reference from the original surface. In the case of a hole, we can use the “ground” around the hole as the reference and measure the difference between the “before” and “after” depth.
In the case of “micro-wear”— the wear within the original texture—we need to match up the unworn valleys. We can do that by shifting the worn surface up and down relative to the unworn surface.
Got a wear issue? Talk to us about how to approach it! Want more details about analyzing “macro” and “micro” wear? Read more in this wear analysis blog post.
4. Ra by Wavelength Bands
Average roughness doesn’t always help to describe how a surface looks. Appearance may be affected by features of one particular scale (i.e., wavelength) more than others. In a paint finish, for example, excessive roughness at very short wavelengths may result in haze, or dullness.
Even though the total Ra may not be helpful, comparing the average roughness (Ra or Sa) over narrow bands of wavelengths can help differentiate “good” from “bad” finishes. Monitoring Sa over several wavelength bands can help manufacturers dial in an exact finish. Digital Metrology’s Bandify and Bandify3D software are made for this kind of multi-band analysis.
