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Roughness, Profiles and Surfaces—Getting Them Right

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media
The Mahr PS 10 portable surface roughness measurement device in action. It features an inductive skidded pick-up with the feel and ease-of-use of a smartphone.

Engineers are getting smarter in specifying and creating surfaces that meet performance requirements, from a cosmetic shine to a surface intended to hold paint or lubricant. Making sure they meet design intent is where the metrology industry enters the picture using a variety of precision devices.

The most popular of these are diamond-tipped stylus systems, according to Pat Nugent, vice president of metrology systems at Mahr Federal (Providence, RI). “You drag a probe across the surface, and you watch what happens to the needle,” he explained. There are two types, skidded and skidless. Skidded stylus systems are great for simple measurements of high-frequency surface roughness. Skidless systems measure low-frequency ripples, or waviness, and surface profiles.

People and Devices

One of the most important trends is the question of who, exactly, are using these devices—and what they need to know to use them correctly. “If we go by sheer numbers, more profilometers are used in production than in quality labs because of the need to measure at the point of production,” stated Nugent, a trend that has been growing for all aspects of metrology.

Can machine tool operators and production workers measure surfaces accurately? “The issue is that he or she is probably an expert CNC machine operator and one day they are told to use a surface finish gage. Now they have to be an expert at surface finish,” he said. This is trickier than it sounds, since measuring surfaces can be fairly involved to ensure engineered surfaces meet precise specifications.

There is more than one way of measuring surface roughness. Arithmetic average, or Ra, is the most common, but there are others such as root-mean-square, or Rq, or maximum valley depth, Rv, or peak height, Rp. Rz is the average maximum peak to valley of five consecutive sampling lengths within the measuring length. “The vast majority of our customers are measuring Ra, and most of them do not know there is more than one measurement; they talk about THE surface finish, as if it was one thing,” he said. Simply varying parameters and filters on the device itself may fool the operator into thinking the surface meets specifications if they do not understand the basics.

In response, Mahr has focused its efforts on delivering units that are as automated as possible. One example is the new PS 10, a hand-held unit with a display that resembles a smartphone. It was first introduced in September 2016. “You can plug the PS 10 into a PC via a USB port, and download data or create PDF reports—we want this device to be used by someone who is not lab-trained,” he said. That is done via an integrated, removable roughness standard as well as an automatic cut-off selection.

Surface Measurement Applications and SPC

There are cautions about using surface measurements in Statistical Process Control calculations, according to Tom Stewart, president of Q-DAS (Rochester Hills, MI), a Hexagon-branded company. As he describes it, calculating common SPC parameters, such as process capability index measures Cp and Cpk, depend on data that is both normally distributed about a mean and is stable.

That assumption may not hold for surface measurements such as profile and roughness. “They are unilateral, because they cannot fall below zero,” he explained. This means, according to him, that they are biased towards zero and not distributed about a mean. “Our experience shows that 98% of all surface measurement data points do not fall into a normally distributed data model set; they are mixed distribution data sets,” he stated. “You can transform the data to get a capability index, but that is counterproductive because you have changed the data to suit the analysis method.”

Recognizing this issue, Stewart makes the point that his company’s Q-DAS statistical software package finds the best fitting distribution model for any given data set and uses that in quality reporting. “We fit the analysis method to suit the data, rather than transform the data to fit a pre-determined method,” he said. “You need to use that accurate statistical data model to set proper upper and lower data limits for control of your process.”

In an interview, he showed surface measurement data that had been modeled in Q-DAS using the percentile method, which is equivalent to standard deviation, but not dependent on fixed ranges. “They are not equal distance and they get smaller and smaller as you go towards the zero because the data set is tending towards zero anyways,” he explained. “Visually it makes more sense.”

Using a percentile method for modeling surface data instead of a Gaussian method, as shown here with actual test data, is more accurate, according to Stewart from Q-DAS.

The Spread to Area Measurements

While the measures for roughness, waviness, and profile for stylus systems are predicated on a single line, the growth of optical surface measurement systems has led to development of areal surface measurements. “There are many different advantages to optical methodologies,” claimed Michael Schmidt, market development manager of Zygo (Middlefield, CT), an Ametek company. Faster scan times, a much larger data set that provides an area rather than a line, and the fact that it is noncontact means it won’t deform or mar the surface of a part.

He noted that many of Zygo’s offerings are based on coherence scanning interferometry, or CSI, that features a white light source rather than a laser and produces 3D topography maps of a surface. He believes it has become even more powerful. “CSI in the past has been often challenged in performing gage-capable measurements of complex surfaces in machine-shop environments. However, recent advancements in CSI-based tools have met these challenges and exceeded the capabilities of many other optical metrology technologies. For example, robust solutions exist for measuring highly-sloped surfaces or rough surfaces,” said Schmidt.

He believes noncontact scanning is becoming especially important in a number of manufacturing applications. One is in measuring surfaces created using additive manufacturing techniques. Another is in measuring surfaces generally used in all aspects of powertrain—gears, pistons, liners. As these need to accept higher densities of power as engines have downsized to meet fuel economy regulations, their surface roughness specifications are even more critical to get just right.

With areal measurements come areal specifications. “For example, Sa is the equivalent in areal terms to Ra, the arithmetic average over a surface,” explained Schmidt. The symbol V is also used to explain volume terms. The specification generally used in industry for area surface measurements is ISO 25178 for 3D surface texture.

Zygo is known for noncontact surface measurement while its companion brand in the Ametek umbrella, Taylor-Hobson, is known for its stylus-style systems. “We are working with our sister company on a convergence of our respective technologies such that customers will no longer need to second guess if the results of a tactile or noncontact system will correlate,” he said. “Even as the optical systems can do things that others cannot, there remains a place for both.”

Speed and Ease of Use

Jonathan Tedesco of Keyence (Charlotte, NC) also believes that easy-to-use hardware and analysis software is important for a simple reason—customers are demanding it. “Our customers are always asking for faster data collection,” he said. “They also want to know what the numbers mean and how to use them to make better decisions.”

Acknowledging that noncontact surface systems are generally more costly than contact-stylus systems, their advantages also go beyond better speed and coverage. Picking the right spot to start a stylus can be subjective and prone to error, according to Tedesco. “Noncontact systems cannot mar the surface, as might be possible with a stylus system, especially on soft materials” or where the surface has to look pristine, he stated. The width of the stylus may also be an issue in measuring surfaces with a very fine roughness structure. “We use lasers with a 400-nm spot size to measure and resolve very fine structure,” he said.

With area data comes flexibility. “We can provide four-million data points and even if the customer wanted a linear R measure, they can decide how and where to draw that profile through the data,” he said. “Maybe they are looking for the deepest point, or the highest point; they can use an assistance tool in our software to find those lowest or highest points to draw their profile.” The software can also draw multiple lines as well as provide full area S and V measurements per ISO 25178. The newest Keyence systems also import CAD models for data overlays.

An example of surface roughness data derived from area scanning systems.

Tedesco also made a point about users today—often manufacturers only have one or two truly knowledgeable professionals who understand surface measurements. “That can be a risk, so we provide a system that is as easy to use as putting a part on a stage, clicking a button and getting a repeatable measurement,” he stated.

Keyence offers two different systems for area roughness measurements, the VR series for larger area measurements, up to 4 × 8″ (102 × 203 mm), and the VK series for nanometer resolutions over much smaller fields of view.

Diverse Customer Base

Another supplier of noncontact surface topography systems that provides areal results is Polytec. Their TopMap Pro.Surf and Pro.Surf+ equipment is also based on coherent scanning interferometry, according to Manas Lakshmipathy, business development manager for Polytec (Irvine, CA). He agrees that today, surface measuring equipment must be easy to use. However, the shop-floor user is only one of a spectrum of potential users. “The customer base is diverse,” he explained. “From someone engineering a part to one checking 1000 parts.”

He believes, like all others interviewed for this article, that software is the biggest enabler in making Polytec’s equipment accessible to a wide range of users. “To accommodate the busy shop floor, there must be options for one-button operation and a logical, even linear logic flow,” he said. It needs to make clear what to look for in the monitor and the program of the software as it goes through a sequence, with as few button clicks as possible. “However, while on the shop floor is where you know if you are going to be successful; it is almost impossible to plan for every variance,” he said. Thus, they need to allow power users, such as quality professionals, access to a rich set of analysis features, which he notes their TMS software does.

When asked what new feature his customers are asking for, his answer was a bit surprising. “For most surface metrology devices, especially noncontact systems like ours, more speed is not as big a request as other desires,” he answered. “Noncontact systems are already pretty fast.” What ranks higher on their wish list, according to Lakshmipathy, is even easier to use equipment and software, followed closely by documentation that shows correlation between the latest 3D technologies and the trusted decades-old stylus tools that are prevalent in the industry. “They want to know that results between various vendors and suppliers and principals that use both stylus and noncontact tools are harmonious. As institutional providers, we have to prove correlation to a standard stylus system, prove our system is easy to run and get the right answer,” he stated.

Practical Uses without Parameters

While precise measurement of parameters such as Ra or Rq are valuable, there is also a niche for a hand-held surface gage looking at localized defects that may affect performance and long term durability. These nicks and dents are outside of statistical measurements, like Ra, which can calculate the same statistical answer for surfaces that are visually quite different.

The TopMap Pro.Surf, a precision, noncontact topography measurement system designed for surface characterization of precision mechanical components, incorporates a traceably calibrated scanning white-light interferometer.

Enter the 4D InSpec Surface Gauge, introduced in September 2016, by 4D Technology (Tucson, AZ). It produces areal surface maps to about 2 µm of resolution, according to Erik Novak, director of business development for the company. This is not enough resolution for the typical roughness calculation. “Instead, what it is doing is replacing a visual check, scribe, or even a ‘calibrated fingernail’,” he explained. “It measures and calculates defects such as pitting, scratches, dents, or nicks and determines if they are functional or cosmetic. A feature that is 50-µm deep might lead to a high prevalence of corrosion on a turbine blade, for example.” Novak related that aerospace and stationary power turbine blades are a particularly useful application of the technique.

If it does not calculate the standard roughness parameters, like Ra, how does the operator know if a part is out of spec? “Most companies have specs on their drawings about surface defects, their prevalence, and depth and we can input that directly into the device,” he said. Advanced uses include mounting the device on a robot for 100% inspection and exporting data for use in a company’s part lifeing models.

He also pointed out that having this level of data is sometimes having an impact on upstream engineering. “Many of the specs companies use are guesswork; now one can measure [these defects] and then observe the actual lifetime of the component,” he said.

What makes this hand-held gage particularly useful in a shop-floor setting is that it is not a scanning device. Data is taken instantaneously by exploiting linear polarization filters mounted on the camera. It takes a single digital camera image to extract its depth map. The single image capture makes it insensitive to noise and vibration, so it is both rugged and flexible, according to the company.

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