Digital Metrology has always been committed to helping you see, explore and understand your surfaces. With the latest release of OmniSurf3D you can now use your sense of touch as well! OmniSurf3D now includes the ability to export a 3D solid STL model of your surface for 3D printing:
Use OmniSurf3D STL Maker to scale your surface. The interactive plot shows you exactly what will print.
Just like that, you have a model ready for 3D printing.
Better yet, in the latest Windows 10, you can simply right click on the file and get your surface printed online.
For more information on seeing, feeling and better understanding your surfaces, contact Digital Metrology today!
How much has this surface worn? The answer can be more complicated than it seems…
Accurate wear analysis is critical for designing surfaces in contact. Unfortunately, mistakes are common when it comes to assessing the actual wear depth for an interface. The attributes that we measure and report are often not actually related to the amount of wear.
Macro vs micro wear
Let’s start by considering two types of wear. “Micro” wear occurs at depths similar in scale to that of the overall roughness. It is often due to the ongoing wear typical of a system operating within designed parameters, such as in well-lubricated engine components.
“Macro” wear is typically comprised of a worn region that is deeper than the original surface texture. You will often find macro wear as the result of many tribological tests (e.g., pin-on-disc, ball-on-disc, etc.) in which we attempt to simulate wear at a vastly accelerated pace. This typically results in making a macro wear scar, as the testing conditions are amplified to shorten test times and reduce testing costs.
In many cases, understanding the subtle changes in micro-wear generated within the actual operating parameters is preferable. The good news is that, with modern analysis tools such as OmniSurf and OmniSurf3D, we can analyze both macro and micro wear accurately in order to explore the effects of design options and decisions.
Macro wear analysis
The general rule we will follow is this: once you have wear areas deeper than the surrounding texture, you need to use macro wear analysis. Comparing roughness in virgin material with the roughness at the bottom of a wear scar offers no information as to the depth of the scar. It’s like digging a hole in your yard, then comparing the height of the grass at the top to the size of the rocks at the bottom, in order to estimate the depth of the hole!
For “macro” wear analysis we need to consider the overall volume of material removed (or the area of material removed, in profile measurements).
As the OmniSurf3D image below shows, we can fit a reference geometry through the unworn areas to bridge across the worn area. On the right side, we see a red reference line that was created based on a 4th order polynomial. This reference was chosen based on the curved nature of the virgin surface areas, but it could be of any form (line, arc, polynomial, etc.). The wear depth and cross-sectional area are reported based on the shaded, blue region on the right side of the graphic above.
micro wear analysis…parameters can lie!
All too often, people will measure a roughness parameter before and after some period of wear, and then use the reduction in the roughness parameter as a measure of the amount of wear. For example, they may simply look at the change in average roughness (Ra) and call that the wear amount.
Consider the two surfaces we showed earlier (and again in Figure 4 below). The unworn surface has an Ra value of 0.61 µm. The worn surface has an Ra value of 0.16 µm. This could lead one to assume that the surface experienced 0.45 µm of wear.
This would be a very wrong assumption!
The problem with most traditional parameters like Ra, Rz, Rpm, etc. is that they are based on the surface’s meanline. And, when a surface wears, the meanline moves as well. Thus, there is a new reference line and results are not comparable. If we plot the worn profile on top of the unworn profile, we get the graph below:
Looking closely at the above figure reveals a problem: the bottoms of the valleys appear to have moved up after testing. In the physical world, the valley bottoms should have remained the same while the peaks moved downward. The wear of the surface should look more like the figure below:
Here we have adjusted the unworn and worn profiles to match up the nominal valley structures. In doing, so we have a very powerful graphic. We can clearly see the worn surface has peaks sitting much lower than the original peaks. In fact, we could look at these superimposed profiles and get an estimate of the wear depth of 2.2 µm—very different than the (wrongly) estimated value based on the 0.45 µm change in Ra!
Read More About Wear Analysis
There is a lot more to tell about analyzing wear. You can read our entire “Advanced Wear Analysis” paper. In it we continue the discussion with topics including:
- Beware of the Rk parameter family—they can lead you to erroneous conclusions.
- The “Material Probability Curve” — a classic technique now available in software for more robust and reliable assessment of wear depth.
- How to easily generate the material probability curve in both OmniSurf and OmniSurf3D software.
- Possible measures of wear based on these curves.
All plateau honed surfaces are not created equal. The image below is the heart of the story, and the focus of this article. In this image you see three honed cylinder bore surfaces with very different characteristics. As it turns out, each is best controlled using different sets of surface texture parameters.
Three types of honed cylinder surfaces. We will see that each requires different parameters to describe them.
Conventional height parameters are sufficient for single honed surfaces
The top profile in the image shows a conventional surface made with a single honing operation. The texture is made up of peaks and valleys of all depths. The valleys might be slightly deeper than the peak heights–but nothing too significant. Over the initial run-in period the action of the engine wears away the higher peak material, leaving a surface with smooth plateaus and valleys in between to retain lubricant.
Traditional roughness parameters such as Ra (Average Roughness), Rz (10-point Roughness Height) and Rmr (Material Ratio) are often sufficient to control the honing operations for a conventional, single honed surface.
Conventional parameters (Ra, Rz, Rmr) are often sufficient to describe single-honed surfaces.
Plateau honing leads to the K family parameters
While straightforward to manufacture, single honed surfaces do have drawbacks. The wear that occurs during break-in inevitably creates unwanted gaps/leakage as well as debris. Both factors can reduce engine performance.
As emissions and performance standards grew tighter, manufacturers began creating “plateaued” surfaces in order to reduce break-in and improve sealing. Plateau honing uses rough honing to make the valleys, followed by finer honing to create the plateaus. A “plateau honed” surface is shown in the second profile.
But with these improved surfaces came a new challenge: the Ra, Rz and Rmr could not robustly distinguish a plateau honed surface from a single honed surface. Very different surfaces gave similar values for the parameters when dealing with this class of surface.
In order to better describe and control these new surfaces, researchers developed and applied the Rk family of parameters. The Rk family is based on the material ratio curve, shown below on the right. The individual parameters (Rk, Rpk, Rmr1, Rvk, Rmr2) quantify the peaks, valleys and kernel regimes. They allow for better control of plateau honing as they are more targeted toward the individual geometries within the surface.
The Rk parameters are well-suited to describing plateau honed surfaces that have distinct plateaus, kernels and valleys.
High performance surfaces need better parameters: Q parameters
High-performance applications (for example, diesel and racing engines) require surfaces that are more extremely plateaued, as in the third of our profiles.
For these applications, manufacturers create very smooth plateaus with discrete valleys, to optimize friction, clearance and engine temperature, with virtually no run-in period. The primary characteristic of these surface is a strong distinction between the peaks and the valleys. There is a clear visual indication of where peaks meet valleys.
Unfortunately, again, the existing texture parameters proved insufficient to describe these surfaces. When a surface is extremely plateaued, the Rk parameter model does not fit the material ratio curve as well. This becomes apparent in the material ratio curve shown below. Looking at the green area we see that there is no exact position on the curve at which the plateaus give way to valleys (as indicated by the left corner of the large, green triangle.) Thus, the Rmr2 and, subsequently, the Rvk parameters become unreliable.
In a high-performance engine the distinction between plateaus and valleys, is almost non-existent, which makes the Rk parameters unreliable.
For extremely plateaued surface we turn to the Q parameters family. Rather than using the material ratio curve, the Q parameters are based on a “material probability” curve, shown below on the right. This curve is a representation of the material ratio curve with percentages mapped to standard deviations. When doing so, we see the two distinct distributions (plateaus and valleys) as two clearly linear regions. Furthermore, these linear regions produce a sharp knee on the curve. The Q parameters (Rpq, Rvq, Rmq) derived from this curve can robustly distinguish the surface regimes for extremely plateaued surfaces.
For extremely plateau’d surfaces the Q parameters, based on the material probability curve, are most robust.
When do I use K or Q Parameters?
Today both the K and Q parameters are specified in the ISO 13565 standard as well as other upcoming 2D (profile) and 3D (areal) standards. Digital Metrology’s OmniSurf and OmniSurf3D include both sets of parameters as well. Unfortunately, the standards leave out a “user’s guide” to tell us when to use one set of parameters or the other.
What we find is that each set of parameters has its use. The Q parameters work best on two-process surfaces with discrete plateau and valley regimes. However, the Q parameters, are less reliable when the distinction between plateaus and valleys is fuzzier. In some cases where the surface isn’t distinctly plateaued, the standardized mathematics for the Q parameters may provide no results at all.
The K parameters work well for many surfaces for which extreme plateaus are not required. This includes not only cylinder bores, but gears, bearings, transmission components and many other surfaces required a degree of sealing and/or friction control.
K parameters serve well for surfaces like the top profile, while extremely plateau’d surfaces are best described by the Q parameters.
And, let’s not forget Ra and Rz! These basic parameters still provide useful feedback for surfaces with a wide distribution of peaks, valleys and intermediate heights.
In any case, the best way to analyze your plateau honed surfaces is to explore and interact with your data. The numbers alone may not tell the whole story. Digital Metrology’s OmniSurf and OmniSurf3D software packages provide all three sets of parameters along with powerful visual tools to help you see what works best for your surfaces.
We have an excellent video here showing the Plateau Honing analysis in OmniSurf3D software. If you are honing, you should take a few minutes to check out how quickly and easily you can explore your surface data.
View our video here on OmniSurf3D’s Plateau Honing analysis.
There’s a lot more to talk about on this topic. In fact, Digital Metrology even offers consulting and seminars on surface texture analysis and plateau honing. If you have questions or would like to know more, please give us a call or email.