Why do I always apply a filter and why does it have to be this one? What does Rz mean again and why do we measure profile roughness and not surface roughness? Regardless of whether you are new to the topic of roughness measurement or are already a professional in surface structure measurement, there is a lot to learn. Here you will find the most important terms in a simple and understandable way. Let's start at the very beginning.

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Your easy way to roughness measurement

Which surface is rougher? Which one is better? It all depends on the functionality it's supposed to have.

What is roughness anyway?

Roughness is the condition of an object's surface. Let's call it the "unevenness". Now, most surfaces of components are not flat, but are primarily determined by their functional form. The first step is to remove this shape so that the texture of the surface remains. After all, the shape itself may have heights and depths, but these are geometrically intended to be so. No matter how small they may be, they have nothing to do with the roughness. Therefore, the shape is subtracted for the time being. But that is not all, because the surface is characterized by high and low frequency waves. High-frequency waves are the roughness you want to measure. That means you still have to get rid of the low-frequency ones, the so-called waviness. And for that, in turn, we need a filter.

How do I get rid of the form?

This is a question you are now legitimately asking yourself when you want to perform a roughness measurement. Or not, because you already work with Bruker Alicona measurement technology and therefore know that this is done with a few mouse clicks. Examples of geometric shapes are: 

• Cone
• Level
• Cylinder
• Ball
• Polynomial (2nd - 8th degree)

After form removal, a flat surface remains for evaluation.

And what about the waviness?

In order to be able to deduct the waviness after the shape, there are two different methods in roughness measurement, depending on the preceding level of knowledge. Both procedures have one thing in common: they are based on Lc (LambdaC). Lc is the cutoff wavelength, the length that separates waviness from roughness. Or to put it more simply: higher frequencies remain in the surface, lower frequencies are filtered out. But how do you get the right Lc value? There are two possibilities:

1. Lc can be seen in the drawing.

2. The drawing does not give any information about Lc. In this case, assume your nominal roughness value and the Setting Class and look for your corresponding Lc in the table. See figure below.

Table to determine Lc 

Lc in roughness measurement - the final boss?

Now this all sounds insanely complicated. However, the good news is that Bruker Alicona's software (MetMaX) already gives instructions on how to perform a roughness measurement in compliance with the standard. The table for determining Lc according to ISO is also implemented.

Profile roughness vs. surface roughness

Actually, it should be called profile roughness measurement and areal roughness measurement, because it is about the way of measuring. They are two different measurement methods for determining roughness, although different results are obtained. The name already reveals what is involved: profile roughness measurement takes its values from a line, while surface roughness measurement evaluates an entire surface.

Profile (or linear) measurement

Profile roughness measurement is widespread and thus better established than areal surface determination. This is because roughness on the profile has been measured for decades. The result values may be Ra (mean roughness), Rq (root mean square) and Rz (roughness depth) to name just a few. The standards behind these are extensive, but we will go deeper to the standards in a different article. To get meaningful results, the metrologist must already have some knowledge of the surface and have an idea of what they are doing.

Area measurement

Surface measurement is newer. In the past, roughness was measured with tactile systems. Tactile means that a probe moves over the component and thus determines where there are highs and lows. With such a roughness measuring device, it is not possible, or possible only with extremely high effort, to perform a surface measurement. Only non-contact methods make areal surface measurement possible. The results are more complex and comprehensive. In addition to Sa (mean arithmetic height), Sq (mean square height) and Sz (maximum height), you receive many other parameters, such as the volume parameters, which also allow better conclusions to be drawn about the function of the surface. With these parameters, you can make statements about haptics, friction or sliding features. The outcome is also more reliable and repeatable. Furthermore the surface measurement is safer against user influences. The figure below shows you, why in most cases the areal roughness measurement is way more meaningful.

Same profile, but totally different roughness

Important parameters

So that you also know which values are behind which terms, we provide you with a short glossary. You have already heard some of them:

In profile roughness:

Ra, the arithmetic mean roughness value, is the integral of the absolute roughness value (shaded area) divided by the measured distance I.

Rq is the root mean square value of the roughness of the profile

Rt: total height of the roughness profile

Rmax: Maximum height of the roughness profile within a single measuring section

Rz: Averaged height of the roughness profile

Rp: Height of the largest profile peak (roughness profile) 

Rv: Depth of the largest profile valley (roughness profile) 

Rk: Core roughness depth, height of the core area

Rpk: Reduced peak height

Rvk: Reduced groove depth

 

In areal roughness:

Sa: Arithmetic mean of heights of selected area

Sq: root mean square of the heights of the selected area.

Sz: maximum height of the selected surface

 

Vvc: Empty volume of the core area of the surface 

Vmc: Material volume of the core area of the surface 

Vvv: Empty volume of the valleys of the surface

Vmp: material volume of the uppermost peaks of the surface

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Goto AJR to know more.

 

Spk: Reduced tip height, average height of the protruding tips above the core area

Svk: Reduced groove depth, mean depth of profile valleys below core area

We got your back in terms of roughness measurement

 

We hope our explanations help you to better understand the topic of roughness measurement and explains why you perform which steps in the application. Although the topic of roughness may seem very extensive and complicated, Bruker Alicona's measuring instruments are also perfect for getting started with roughness measurement. The software is equipped to make surface determination as easy as possible for you and our support team helps with problems and knowledge gaps.

Free Roughness Poster available

Would you like to optimize your measurement processes and achieve high-precision measurement results? Our poster on optical roughness measurement contains metrological terms, measurement tasks, know-how tables, ISO filters and LC filters to support a smooth measurement process. 

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International standards recommend clear rules for which Lc value should be selected given certain parameters. Image: George Schuetz

There are many measuring systems available that bring surface finish measurement to the point of manufacture. Some systems offer the bare minimum to make the operator's measurement process as simple as possible. Others provide virtually the same capabilities of a lab-grade surface analyzer in a portable, handheld surface finish gage.

Having so much power in a handheld measuring system is a great feature, unless the results are not really what the surface represents. For example, a user will perform a roughness measurement, and the value of the Rz or other roughness parameter will look perfectly satisfactory. But there is a bit of an issue when it comes to surface finish metrology: the surface finish results for a parameter such as Rz, and all other surface finish parameters, can vary decisively depending on the filter parameter (Lc), also known as the cutoff. With a different Lc value, the results can be different. While these deviations may not be very large, they can still lead to the wrong assessment of the part (as a good or a bad part) because of the surface finish result. Thus, there is a trend in the industry to define setup parameters as part of the finish call-out. But what if it&#;s a new surface, and not yet defined?

According to surface finish international standard deviations, the shape of a part can be broken down into different orders, or levels. The first order is the form or shape of the part, followed by a waviness component, line roughness and finally area roughness. These orders are not independent, but layered upon one another (for example, roughness will be superimposed on the waviness component).

This is where using Lc, or cutoffs, becomes important. Using cutoffs is a filtering method that provides the means for separating waviness from form, or roughness from waviness. With the proper choice of filtering, the measuring length should be so short that it nearly eliminates any waviness component. In many cases, by observing the measured profile filtered with different Lc (and a bit of experience), one can determine the correct value for the cutoff. But it&#;s possible to go to the extreme and use too short a filter, which will start to influence the measurement results.

In practice, it&#;s probably not up to the technician making the measurement to select the correct result. To establish a process that helps to achieve the best results, international standards recommend clear rules for which Lc value should be selected in unknown cases. However, a few basic questions need to be answered to choose the best cutoff Lc.

First, is the test surface made in such a way that the surface is periodic, or aperiodic? Periodic surfaces are characterized by equally spaced, recurring, typical profile characteristics that are clearly visible in both the depiction of the test surface and the profile (in the example, as distinct turning grooves). In contrast, aperiodic profiles do not show any visually striking, special surface structure. In many cases (on milled profiles, for example), not even a distinct working direction can be detected on the surface.

According to the standard, selection of the correct Lc value must be handled differently for periodic surface profiles than for aperiodic ones. In the case of a periodically structured test surface, the value of the mean size of grooves&#; RSm must be determined by measurement first. Then, based on widely available charts, the standardized Lc value can be assigned to the measured RSm value.

The charts also have information, should the surface be aperiodic, but it gets a little more complicated. In the aperiodic case, the most suitable Lc value is determined by one of four roughness parameters noted in the selection charts, and may require a series of measurements under different conditions to select the best Lc cutoff. By following the recipe described in the selection charts, the user can narrow down the proper surface finish results.

Fortunately, in most manufacturing environments, periodic surfaces are the norm. But as processes change, designer surfaces become more common or surface tolerances get tighter, knowing the recipe for selecting Lc is increasingly essential.

If you want to learn more, please visit our website Roughness Gauge Manufacturer.