Stepping Up to the Plate
When nothing else will do, it is time for portable hardness testers to step up to the plate. Or, more accurately, it is time for these devices to step up to heavy or hard-to-reach parts.
Portable hardness testers are useful in a number of arenas, including the aircraft industry where they first got their start. "They're useful in the field, obviously, to test large parts that you can't bring to a benchtop tester," says William Luck, product manager for Qualitest USA LLC (Fort Lauderdale, FL). Other customers want to use portable devices for rapid testing of incoming parts, "basically as they're coming off the truck so they can determine right away what's good and what's bad," he says. In addition, they're often used when a benchtop tester cannot accept a large part and clients do not want to cut down their parts to test them.
Not bad for a device that has been around for decades. Mars Webster has watched the field evolve from a unique perspective: Mars, president of Webster Instrument Inc. (Los Angeles) also is the grandson of the man who helped develop the first portable hardness tester. The early ones were built in a workshop in the back of his grandfather's garage, Webster says. He now points with pride to the measured steps his company takes in producing these venerable devices.
"Our tester hasn't changed in recent years," he says. "It's evolved, of course, and we're always striving to make a better product, but we feel our product is tried-and-true."
A technology adaptsPortable hardness testers hardly have been static since Webster's grandfather and other pioneers began developing the devices. Luck points to the recent development of testers that do not have the minimum weight and thickness requirements associated with the popular rebound-type testers. "A lot of people now are demanding portable hardness testers that can test parts under the limits of 3⁄8-inch thick and under 11 pounds," he says.
Newer devices using the Rockwell principle can test parts below 1 millimeter in thickness, Luck says, "and you're really not concerned about the weight or the size of the part." These testers have a 5-kilogram maximum test load, rather than the typical Rockwell load of 150 kilograms. This is particularly useful for testing aluminum. "In the past that's been an issue because it's so light," creating a deflection problem, he says. Now, the aluminum extrusion industry is becoming a hot market for portable hardness testers.
Some handheld rebound testers automatically identify impact direction, Luck says. He also notes the arrival of a new principle, resistivity, which is being incorporated into portable testers for hard-to-reach areas such as the inside of nooks and crannies on the edges of gear teeth. "You complete a circuit, and as you press on the tester itself, you build a load vs. resistivity curve. First, you do this in a hardness test block or a part with a known hardness, and you assign a hardness value to this load vs. resistivity curve." He adds that the process then is repeated on the testing of the actual part, which generates a new curve for comparison. Devices also have become more compact.
As portable hardness testers have become digital in recent years, the internal conversion devices have improved, says Ed Tobolski, hardness manager for Wilson Instruments (Canton, MA), a division of Instron Corp. "So the testers are becoming easier to use. Most of the devices now have a means of identifying the kind of alloy that you're trying to test, whether it be hardened steel or soft aluminum or something else-so you have some way of selecting what the conversion would be for the particular material being used."
Here's another shift: "With most of the newer portable hardness testers, you now can turn them sideways and upside down, and they automatically compensate for the angular change," Tobolski says. Though this is not a strictly new development, he says, the feature today is being more widely used.
Applying the right toolPortable hardness testers are good at what they do. However, they cannot do everything, cautions Wally Wardzala, major instruments product manager for Mitutoyo America Corp. (Aurora, IL). "The portables are good for approximations or comparisons when there's no other way to do it."
The real issue with portable hardness testers concerns conversion. A device that uses a Rockwell principle, for example, does not perform a true Rockwell test; rather, the devices rely on converted data. "Conversions always are a question mark, and they're always considered approximate," Tobolski says. "Unless you've actually done the real testing on the actual sample loads that you're trying to determine, your measurement will be approximate."
These limitations may serve as the basis for significant change in the not-too-distant future. The American Society for Testing and Materials (ASTM) is working on new standards to define a limited number of conversions. "That will be the first official conversion table, which should end up in an ASTM document," Tobolski explains. This first standard, which may take another year or two to develop, may be followed by standards for other materials.
The current standard for the Leeb Test is ASTM A956-02-Standard Test Method for Leeb Hardness Testing of Steel Products. The task group that wrote A956 is working on new conversion tables for Leeb to Rockwell, Vickers and Brinell for a specific steel alloy. Upon completion of the study, the results will be added to ASTM E140-Standard Hardness Conversion Tables for Metals, Tobolski says.
The basic principle of metal hardness testing has not evolved, however. "It's basically reading the amount of penetration into a material and comparing that to known samples," Webster says. That, along with the self-limiting nature of the testers, puts device-makers in the position of telling customers that constant innovation is not necessarily a plus.
"Nobody is stopping anyone from coming out with a brand-new test," Wardzala says. "Are we going to have a better Rockwell tester? I would hope so. But unless something radically different happens to technology, the portables are going to remain good for approximations or comparisons, or because there's just no other way to measure hardness."
There's a certain allure to portable devices, Tobolski concedes. "They're pretty slick, and it's easy to do a test-you walk up, push a button and, bingo: You have an answer. So, people tend to want that to be the definitive answer.
"But it isn't," Tobolski adds. "That's because of the conversion issues, and because there's a certain amount of uncertainty in the testing process. Getting it straight and holding it right and the surface finish-they all contribute to errors."
That does not stop manufacturers from wanting portable hardness testers to be more than they are. "But you need to know the limitations of the devices," Tobolski says. "If you use them correctly, they can be a nice tool-but just to get a general idea if you're in the right area. If you want to use them for a definitive answer, you ought to do something else."
"Someone's always trying to build a better mousetrap. But that may not be what we need with portable hardness testers," Wardzala says. "The testers we have are about as good as you can get."