Whether they involve simple cosmetic requirements on a plastic part or essential characteristics for a highly stressed cylinder wall, correct and optimal surface texture are critical elements in a part’s ultimate success or failure. Precise surface texture control, measured in millionths of an inch, affects factors ranging from component wear and fatigue to machining an effective bearing surface for lubricants. When used at different points in the production cycle, surface measurement serves multiple ends. Measuring surface texture at the end of the production process allows machinists or lab technicians to assess a part’s performance ability. Surface measurement taken during manufacturing allows machining systems and processes to be controlled and optimized. Thus, surface measurement is not just for after-the-fact pass/fail testing; it also can prevent errors and/or bad parts in the first place. Texture measurement: purpose and principles Every part’s surface includes some type of texture created by multiple factors. These include the material’s microstructure, the cutting tool’s action and instability, errors in tool guideways and deformations caused by stress patterns in the component. The resulting texture, known as surface geometry, is actually a combination of three features -- roughness, waviness and form -- that can be likened to the characteristics comprising a desert’s surface. Individual grains of desert sand constitute surface roughness, which is the dominant surface feature. In manufacturing, roughness is caused by a material’s microstructure and the cutting tool’s action on the material. This is where variables like tool shape, speed, feed and cutting fluid come into play. Ripples in the desert’s surface represent surface waviness, the second most prominent contributor to surface texture. Waviness is caused by cutting tool instability, such as a grinding wheel’s imbalance, as well as errors in machine tool guideways. A desert’s dunes are examples of surface form, more commonly known as straightness error. This type of texture results from tool guideway errors and stress factor deformations. In addition to their varying origins, roughness, waviness and form also differ in how they affect part performance. As a result, conventional surface analysis requires separating the three texture types in order to isolate the factor being measured or controlled. This process is called filtering. For example:  A roughness filter is used when measuring to control component performance such as wear characteristics, friction, reflectivity, resistance to stress failure and lubrication properties. Because surface texture measurement is most often concerned with roughness, waviness and form textures are filtered out so that the roughness can be observed with maximum detail.
A waviness filter is used to monitor and control machine tool performance and certain types of component performance, including noise or vibration. A profile filter is used to control assembly criteria or general performance. Surface measurement advances During the first quarter of this century, surface measurement methods were entirely manual -- and subjective. The tool of choice was a comparison block. A part’s surface was compared to varying, identified surfaces machined into a metal block. Running a fingernail across a surface known to be 0.0001” told the machinist whether the part’s roughness was within an acceptable range. The current generation of automated surface measurement technology, where results are electronically calculated, is traceable to tools that began emerging in the 1930s. As recently as the 1980s, even the most sophisticated machines measured only a few roughness parameters. In some cases, roughness average was the only parameter. Although roughness average remains the most widely used parameter, it does have its limitations. During the last 10 years, silicon chip technology has improved texture analysis to the point where even the most basic measurement instruments use up to six parameters to characterize a surface more fully. Enhanced processing technology routinely provides capabilities such as Gaussian filtering, semiautomatic calibration routines, results tolerancing and SPC output. At the high end of the systems spectrum, surface measurement machines include PCs and the full power of software-based filtering technologies. Today’s top-end systems can calculate a virtually unlimited range of roughness, waviness and form parameters. The data capture phase of surface measurement technology, however, remains relatively unchanged: It basically involves dragging a stylus across a surface. The breakthrough gains in surface measurement are the result of PC-based processing, which has significantly expanded a system’s capacity to manipulate, analyze and understand texture data. Single measurements can be reanalyzed against additional parameters and features by using different filters. Various stylus tip sizes and materials can create more optimal and enlightening measurement conditions. As a result, it’s now possible to measure, analyze and control features that weren’t known to exist a few years ago. Advances with high impact Several key advances are shaping the impact that new and emerging surface measurement technology will have on product design and engineering, manufacturing capabilities and part/product performance. Enhanced engineering -- Certain automotive engineering applications provide convincing evidence of how sophisticated surface measurement adds precision to the entire production cycle, beginning with the design phase and continuing through to manufacturing. Until recently, for example, a vehicle owner was advised to “run-in” a new engine for about 2,000 miles. This was to permit components such as cylinder walls to wear-in progressively. Auto manufacturers know they can minimize this requirement by plateau-honing, which essentially knocks down the “peaks” but leaves the “valleys” that aid in retaining lubricating oils. Initially, however, the solution achieved during this two-step machining combination was an engineering option that couldn’t be employed with confidence. Previous surface measurement technology proved insufficient for accurately measuring the texture. Today, manufacturers can ensure the honing process is executed properly through computer-enabled surface analyses that leverage a much wider range of parameters and filters. Shop floor sophistication -- Until recently, the fragility and complexity of new, high-powered surface measurement systems restricted their use to the quality control lab. However, by the time a part is measured in the lab and found to be out-of-spec, several hundred bad parts can be manufactured. To be even more effective, computational muscle and sophistication is being packaged in formats that can survive the rigors of shop floor use by nonlab personnel. Machine tool operators no longer are limited by low-end, handheld instruments that can measure for roughness only; new systems provide accuracy and functional versatility on par with lab instruments. Optical systems -- Almost all surface texture measurement equipment available today operates by moving a stylus -- typically, diamond-tipped -- across the surface being measured. This contact technique is suitable for the vast majority of applications. However, this process also involves a pair of limiting factors: surface deformation and cycle time. A stylus can deform the surface as it measures. This is inconsequential when measuring relatively hard or rough surfaces, but it can be a serious concern when measuring soft or easily scratched parts. Cycle time also becomes an issue because the stylus must be moved across the part. In the future, these factors will lead to the widespread adoption of optical techniques now being used for precision form (i.e., flatness) measurement. Using laser technology, optical techniques eliminate surface contamination by scanning the part. Such instruments typically measure several thousand points in less than one minute. Currently, optical systems are limited in their ability to measure surface roughness parameters. In particular, because international standards are written around contact technology, optical measurement results don’t always correlate with stylus techniques. Multinational standards -- Given the ongoing trend in global manufacturing, standards play integral roles in ensuring that parts manufactured in different countries function and perform together as designed. Surface measurement systems, like many computerized instruments used in the manufacturing process, will adhere to standards governed by bodies like the American National Standards Institute, International Organization for Standardization, Deutsches Institut für Normung and Japanese Industrial Standards Committee. “It’s possible to measure features that weren’t
known to exist a few years ago.” Equipment selection Today’s surface texture technology presents users with a wide range of features and capabilities. The application will drive decisions on key elements like instrument accuracy, available measurement parameters, processing capacity and speed. Some basic considerations, however, are common to every purchase evaluation: Manufacturers that pursue high- capability surface texture measurement frequently discover applications and opportunities that extend well beyond the original application. Like any capital investment strategy, the buying decision on a surface texture instrument should reflect future as well as current needs. If a need isn’t driven by requirements internally, it’s likely to be discovered through constantly evolving customer demands. PC-based systems, of course, provide the greatest degree of flexibility. The system can be reconfigured with a simple software upgrade. Measurement continues to evolve as a shop floor function rather than a secondary operation performed back in the lab. Real-time measurement by the tool operator represents the emerging standard in efficient manufacturing for high quality and high performance. At the same time, lab-like sophistication loses its potential to impact those factors unless the system is shop-floor durable and designed for easy, effective use by nonlab personnel. On the shop floor, measurement systems that are difficult to use tend to be avoided or used incorrectly. There are two ways to build a surface measurement system: skidded and skidless. On low-end systems such as handheld devices, a small ruby skid is located adjacent to the stylus and acts as the reference point for the measurement. Such systems occupy the low end of the price range as well, but because they employ the least amount of technology, they are less reliable in terms of accuracy. Typically, these systems are limited to measuring roughness average and a few other roughness parameters. Nonportable, skidless systems, by comparison, have a straightness reference located in the driver unit. This significantly reduces the possibility of false measurements that can occur with skidded systems. A skidless design also is required to measure all three textures -- roughness, waviness and form. Consider part quantity and diversity. Basic systems might accommodate measurement needs that focus on a single part or limited range of parts. However, applications that involve auditing or inspecting hundreds of parts over the course of a production week might require instruments capable of programming multiple setups, storing results, creating desktop reports and interfacing with other production systems. An automated, programmable instrument easily and quickly customizes the measurement application to handle simple or complex measurements. Current surface measurement systems not only assess a part’s performance ability but allow greater control of machining processes. Surface geometry -- the roughness, waviness and form -- can be divided and conquered through sophisticated, PC-based filtering and calibration. Optical systems, with an eye to the future, bring the lab to the shop floor. More than ever before, precise surface measure control falls within the machinist’s capabilities. About the author Phil Castle is technical services manager for surface, form and geometry at Carl Zeiss IMT Corp. He manages application, service and technical sales support for the TSK and Tropel products. He can be reached at fax (248) 486-3114 or e-mail pcastle@qualitydigest.com. |
