You Have to Go Beyond Ra
Failure, fit, and form are measured in microns
By Robert B. Aronson
The answers to many manufacturing problems may lie in a detailed evaluation of a product's surface. The once "lab-only" kind of tests are now becoming more common. Manufacturers not only need to know if their products are being made to the new highly demanding tolerances, but why, if everything seems to be made to spec, a product fails.
Two basic instrument types are used for surface analysis: contacting and noncontacting. With the contacting type, usually called a profilometer, a small diamond tip is moved across the test area. The wire's oscillations as it "bumps along" the surface are recorded. This gives a 2-D picture of a small area of the surface.
According to established standards you have a data point every 20 millionths of an inch with each pass about 5-mm long. Despite the small size, one trace can supply a significant amount of data.
It's possible to get 3-D information by making multiple passes. Multiple passes can be made manually, or some instruments can do this automatically.
"The profilometer picks up two basic surface features: the variations the machine tool caused, such as motor vibration and positioning error, and what the cutting tool did to the surface," explains Scott Ledger, product specialist, Mitutoyo America Corp. (Aurora, IL). "Normally, when you are evaluating roughness you want to filter out the waviness. For finer work, you want to look at both.
"The less costly profilometers are skidded. That is, there is a mechanical block that limits stylus travel to sense only the variables caused by the cutting tool. Nonskidded units can pick up waviness. This type is also essential for curved surfaces.
"We have seen significant growth in highly automated, CNC-controlled shop-floor, nonskidded units. The automotive people like them because they are automated. This means they can be used in production situations and don't need highly skilled operators.
"There is also more interest in using our machine to evaluate tool wear. You can determine how well a tool is functioning by evaluating the surface it cuts.
"Process change is also a growing opportunity. For example, when a manufacturer wants to run faster, often a different set of parameters related to surface finish come into play. In one case, a company that injection-molded bottle caps wanted to shorten cycle time. Just speeding up the equipment didn't work because the caps did not have enough time to "relax" in the molds and were hanging up in the molds and not ejecting. We analyzed the molds and found that with a better surface finish they could achieve the desired cycle speed," Ledge concludes.
Mitutoyo America's Surftest SV-M3000 CNC has an 800-mm side traverse capability with a column-moving mechanism that virtually eliminates load capacity limitations. The 800 x 500 x 200-mm measuring envelope and long-stroke probe arm accommodate large, heavy workpieces such as crankshafts, heads and blocks, and camshafts as well as other powertrain components. The probe arm allows measurement of multiple cross sections and waviness over a wide range.
Surfpak software also enables the Surftest SV-M3000 with 800-mm traverse capability to support automatic measurement as well as a variety of evaluation parameters conforming to most world standards and analysis routines.
For greater surface detail, one of the noncontacting systems has to be used. These instruments give a 3-D view of the surface, which is becoming more important. For example, it's often important to look at a large surface, not a narrow track. When you want to check the flatness of a head gasket, you need to look at the entire surface.
With these instruments, white light or a laser beam is reflected from the test surface. Various types of surface information are gathered by focusing the beam at various distances from the source. For example, the question may be how much of the surface has variations greater than 0.1 µm. The noncontacting unit will focus on a plane and record variations beyond that distance.
Ra isn't enough. Unfortunately, many manufacturers are still hung up on this number as the only measurement needed to determine surface finish. It's fine for general evaluations, but for detailed analysis, it's important to go get more data. Ra is only the measure of average roughness. It was probably the first measurement to be developed, and is often used as a basic specification in the manufacture of many products.
"We have found that about 90% or more of our customers are interested only in Ra even though there are more than 100 parameters to characterize a surface ," says Pat Nugent, vice president, metrology system, Mahr Federal (Providence, RI). "Various parameters can be defined and put on the measurement systems so that a variety of measurements can be made."
"We find that buyers are trying to understand what the numbers are telling them, and to do that you have to get beyond Ra. That number doesn't tell the whole story. You have to look at the problem the other way around. First you need to know how the part will be used, then you can select the measurement parameters that will tell you if that part will perform as you want it to," says Nugent.
Choosing the best instrument is very part-dependent. "Initially, you have to know what you are trying to find," says Nugent. "Part characteristics, such as which parameters are required and feature size are important. It's not the kind of decision that is best made by looking through a catalog. You need to talk to those who know the industry."
"How you handle your metrology problem depends on how much pain it's causing you," says Geoff Anderson, product manager, Veeco (Tucson, AZ). "If it's a single problem, you can probably best handle it by using an independent operator. If it's an ongoing situation, you should consider buying your own equipment.
"It's tough to convince a potential customer to buy something that doesn't cut metal. We don't make good parts, but we can keep you from making bad ones.
"Customers are often unaware of the number of things you can find out about a part. 'I didn't know you could do that!' is a frequent comment. For example, appearance is a big issue with plumbing fixtures and medical parts. Often the root cause of those problems is the effectiveness of a coating, which tests done with our instruments can determine," Anderson concludes.
On the shop floor, ruggedness and ease of operation are critical when it comes to using equipment. Many of the instrument manufacturers have products that function in the shop environment. "We have worked hard to simplify our instruments," says Nugent of Mahr Federal. Our goal on our most recent system was 'no more than two button pushes to make any measurement.' "
Outside specialists are often called on to evaluate specific problems related to surface texture. Michigan Metrology (Livonia, MI) is one such company. The formal explanation of the company's function is "3-D surface microtexture/wear measurement and analysis." But company Managing Member Don Cohen explains, "A more practical statement of purpose is 'If it squeaks, leaks, wears or looks odd, maybe I can help.'"
Cohen sees these trends in his industry:
- "More problem solving. How does a part look? What is causing corrosion? These are some of the issues that surface analysis can resolve. For example, adhesion, a big issue in many applications, particularly when dealing with coatings, may be linked to surface texture. Surface finish is an important factor in evaluating lubrication problems. The micron-size grooves on a surface determine the volume of lubricant a surface can carry.
- "Globalization and outsourcing have brought a new series of problems. Parts from several sources may not all function the same way, even though they were apparently made to the same specifications. Often it's because the suppliers did not measure the same features or have the same standards of measurement.
- "Not everyone wants to do their own testing, particularly those who recognize their production problems may be related to surface issues, but don't want the high costs of the instrument, personal salaries, training, and equipment maintenance. When visiting a new client, I sometimes find a fairly new measuring instrument such as a CMM carefully wrapped off in a corner, unused. Often the reason is that someone bought the unit with intentions to set up a part monitoring system, then discovered they did not have the in-house talent, or finances, to buy the ability to run the thing properly."
Metrology problems are further complicated when dealing with large parts that are impractical to ship to a service center. These have bred a separate type of "house call" metrologist.
- New standards are being developed. The "R" standards are now being supplemented by "S" standards. R figures are chiefly concerned with 2-D data while the "S" number is for 3-D situations. There are S figures paralleling each of the R numbers, plus a few that are relevant only to 3-D work. Currently, the S standards are making their way through the ISO approval system. But instrument makers are well aware of them and are designing for them.
On the issue of standards, Jack Clark, applications development manager, Zygo Corp. (Middlefield. CT), explains: "With the unification of ISO, JIS and B46 (DIN is now combined into the ISO Standard), most of the world's industries are in agreement with respect to 2-D surface metrology and evolving to a better 3-D standard, There are, and always will be, a few pockets of manufacturing that need industry-specific parameters.
"But there still must be discussions between buyer and seller that define measurement techniques and parameters. This is particularly true when contacting versus noncontacting methods are used. With the growth of noncontacting, there is sometimes an overlap between those using a light or laser system and those with profilometers." The issue of correlation is well defined, understood, and easily addressed in all applications.
Interest in optical gaging is growing. "We use chiefly scanning white light interferometry [SWLI] for surface analysis," says Zygo's Clark. "For example, we do a lot of work inspecting fuel injectors because of the tight tolerances and other constraints. With injector bodies, for example, flatness and roughness are issues because of potential leakage in the metal-to-metal seals that have to endure pressures of 30,000 psi (207 MPa) or more—flatness tolerances of one micron or less with a roughness of less than ½ micron Rz. Utilizing SWLI, one can perform a roughness or flatness test in a single pass of 2 or 3 sec on the production floor.
"Laser-based instruments generally are not used for this type of work. You must use SWLI for these applications that require flatness, roughness, parallelism, thickness (size), step heights, form deviation, radiuses, and even burr detection. Typically, varying and non-homogeneous materials are not an issue allowing this type of metrology to be applied to new cylinder bore/piston configurations.
"Our units can be automated and merged with production equipment. They integrate well into grinding and superfinishing cells. When an instrument goes to the shop floor we have to offer a level of automation that is compatible with the operator's skill. Laboratory metrologists are not usually operating a grinder or turning center, so you have to build the capability into the instrument.
"Manufacturers often don't know what is causing part failure, and therefore, assembly failure. It may not be a machining operation, but wrong part feeding or handling. Often slight changes in a process will influence a part's function. This happens frequently in the US because of the transition, particularly in the automotive industry, to outsourcing utilizing multiple suppliers. In Europe, this is not as big a problem because those manufacturers still maintain much of their manufacturing functions internally. Today it is even more important to identify what metrology system is to be used for inspection, adding another level of control for part quality and functionality," concludes Clark.
This article was first published in the January 2006 edition of Manufacturing Engineering magazine.