Quality Test & Inspection: Beyond the Basic Bench Top
Bench-top gages sure have changed! The effects of the electronics revolution on them were on full display at the recent International Manufacturing Technology Show (IMTS) held in Chicago, Sept. 8 to 15. Twenty years ago, bench-top gages consisted of such things as mechanical micrometers, vernier calipers, 6-inch rulers and dial indicators in test stands. Today's versions of those same devices contain digital readouts and can transmit their measurements to computer networks at the touch of a button.
The changes don't stop there, however. The range of metrology equipment that fits on a bench top has grown tremendously and has expanded the notion of what constitutes a bench-top gage. Miniaturization, information technology and applications engineering have allowed manufacturers of measurement devices to repackage technologies previously confined to large systems. Profilers, vision systems and coordinate measuring machines (CMMs) all come in bench-top models.
At the most basic level, electronics have given conventional hand gages features they did not have before. Consider measuring a bolt-hole pattern with a conventional height gage that displays its readings on an indicator. "You'd have to take one dimension, referencing from a gage block or a height standard to determine a number," says Scott Robinson, technical support specialist, The L.S. Starrett Co. (Athol, MA). "Then you'd have to do the same to find the bottom of the hole, do the math to find out the dimensions of that hole and write the position on a piece of paper."
The computers and trigger probes on high-end height gages today do all of that automatically. The touch-screen computers on these gages run Windows CE and have software that allows users to program measurement routines and run full statistical analyses. Most also have USB and printer ports on them for connecting to off-the-shelf printers, so users can print hard copies of the statistics or generate validation reports directly from the gage.
The electronics on these and other gages have communication ports for connecting to any PC and dumping the data directly to an Excel spreadsheet or other software. Although wires are still the most common means of establishing such a connection, wireless data transmission could become an option for replacing them soon, especially on hand tools like calipers and micrometers. "These products allow the operator to move freely and collect data on a timely basis, but not be tethered down by a wire," explains Kurt Braun at Mahr Federal Inc. (Providence, RI). Wireless transmission avoids not only the worry of ensuring that the wires are long enough to accommodate every contingency, but also the bother of preventing the wires from becoming tangled in fixtures.
Mahr Federal introduced its wireless technology to North America at IMTS. The tiny transmitter fitted to a caliper, for example, sends signals to a receiver connected to a PC that can be as far as 200 feet away, depending on the electromagnetic fields and obstructions in the shop. Because these exist in real shops, the engineers at Mahr designed the wireless set to be a two-way system. When the operator pushes the button to send a measurement to the PC, the PC verifies receipt of the information, and a small green light illuminates on the gage. A red light indicates that the PC did not receive the data and that the operator should try again.
Simplicity through versatility
Versatility is another ramification of putting modern electronics on bench-top gages. The electronics and some simple mechanical enhancements have increased the functionality of these gages tremendously. Height gages, for example, can take a variety of measurements-from diameters to slots-in succession easily and quickly. "One instrument can do the job of maybe five or six other instruments," says Fred Fowler III at Fred V. Fowler Co. (Newton, MA). "High-end height gages can measure in 2.5-D and now incorporate many functions of a CMM in a much smaller space." Rotating a special fixture 90 degrees is all that is necessary to establish a new axis.
The result is that manufacturers often can get away with buying fewer gages. "There are two ways to go," explains Fowler. "You can have a myriad of hand tools on a bench top to cover all your measurements, or you can go with a more expensive and sophisticated solution to reduce your equipment numbers significantly." Opting for fewer gages can reduce not only the initial outlay in some applications, but also maintenance and calibration costs.
Another advantage of this choice is that it improves the reliability of the measurements in two ways. First, it adds consistency to the measurement process by reducing the options that operators have at their disposal for making measurements. Second, it shrinks the number of sources for error. Relying on fewer instruments gives the operator more practice in using an instrument and reduces the chances of misreading and miscalculating from it.
Fowler claims that these gages not only will do the work of several gages, including that of a CMM, but also have the potential for offering better accuracy. "Accuracy can stack up much better [on a high-end height gage] because you're dealing with only one axis [rather than three]," he says. Accounting for error along one axis is easier than in three-dimensional space.
Despite the advantages, Robinson at Starrett urges users to think the concept through carefully to avoid taking it too far. After all, each gage exists for a purpose and has its limitations. "CMMs are used primarily for odd shapes, curvatures, cones and bolt positions," he says. "There are some things that a height gage is never going to be able to measure." Besides, he adds, the cost of a small CMM today is very close to the price of these sophisticated height gages.
Robinson recommends applying the same caution to hand instruments, such as micrometers and calipers. "A micrometer is a micrometer," he says. "It's going to do what a micrometer does best. A micrometer can attain an accuracy that a caliper cannot. You don't want to be using a caliper, which has in the best of days an accuracy of a thousandth of an inch, to measure parts that have tolerances of one thousandth of an inch."
Fitting the bench
The electronics revolution has done more than enhance the flexibility of conventional bench gages and give users the option to put fewer gages on benches. It and other innovations have allowed instrument manufacturers to make bench-top versions of vision systems, CMMs and other stand-alone devices that were once too big to fit on benches. For example, users can buy a bench-top CMM that repeats within 60 millionths for less $15,000 from Hexagon Metrology (North Kingston, RI), the parent company of Brown and Sharpe Inc., Tesa Technology and other metrology vendors. And decades of software development have made these CMMs simple enough to use on benches beside production machinery.
Profilers are another example. Not only do modern versions do more than their shadowbox predecessors, but some are small enough to sit on a bench top beside the other gages. Consider a model from Brown and Sharpe. Using Pro Measure software, it detects edges, measures a variety of features and even calibrates itself. "On a shaft, for example, it can measure multiple technical features, whether they be different outside diameters, flats, angles or threads," says Dave DiBiasio, a spokesman for Brown and Sharpe.
Moreover, the software boosts the accuracy and repeatability of this class of devices. A thousandth was the best that a vision system-based profiler could offer, given the parallax problems and other operator errors arising from interpreting the cross hairs in old models. "Now with automatic edge detection, we can guarantee two tenths from operator to operator, no matter what you're measuring," says DiBiasio.
Marposs Corp. (Auburn Hills, MI) also has harnessed optical technology into a general-purpose bench-top gage that measures diameters and other exterior features on cylindrical parts. Called Optoquick Set, the device inspects and measures parts with infrared optoelectronic probes and the shadow-casting principle. A precisely aligned beam of light projects a shadow of the part onto a linear array of photodiodes, or a CCD camera. Sensors detect dark-to-light transitions on the CCD and correlate them with the dimensions of the dark areas.
"Optical technology gives us a semiautomatic, flexible system for measuring a variety of features on the shop floor," says Roberto Leonardi, product manager at Marposs. "Multiple optoelectronic heads can provide a measuring range of about 750 millimeters and an outer diameter of around 150 millimeters. Inside that range, you can calibrate the gage using only one set-up master and place any kind of part, as long as it's clean and dry, and the gage is going to give you an accurate reading." He adds that such optical gages can be as fast as a dedicated contact gage, yet they can be programmed to handle a variety of jobs.
Another option for replacing dedicated bench-top gages in low- to medium-volume applications is retoolable, contact gages, such as Marposs' Quick Set program. The modular nature of these gages makes them practical for shafts, bushings, hubs and other features. Either Marposs or the user can construct a gage from an assortment of components that fit together in a number of combinations, much like the pieces of an Erector set. Retooling them is simply a matter of reconfiguring or, in some cases, adding new components.
Flexibility is an important part of both Marposs' modular concept and the optical technologies offered by various vendors. Such gages can measure a variety of similarly sized parts being produced on a line or in the same cell. Moreover, they can be reprogrammed or retooled when a job ends. "A common mistake, especially among Tier One and Two suppliers in the automotive industry, is their failure to consider the life cycle cost advantages of flexible bench gages even though they may need to invest a bit more money at the beginning," says Leonardi. "It is true that dedicated gages are less expensive [per piece checked], but they are more expensive in the medium to long term because you have to throw them away when you retool the line."
Flexible and retoolable gages also support simultaneous engineering programs much better than their dedicated counterparts do. The problem with dedicated gages is that they accommodate design changes poorly. Changes only drive their cost up and delay their delivery, especially the closer they occur to the delivery date. With the flexible and retoolable gages, on the other hand, accommodating a design change is a matter of either tweaking a program or modifying modular details on the gage.
So as lot sizes shrink and competition puts pressure on maximizing returns, deciding between dedicated or flexible gages, or between conventional mechanical devices or their electronic versions, is a dilemma. "Do I take the time to engineer a specialized device to measure a widget so I can just put the widget and get all of the data that I need automatically?" asks Robinson. "Or do I save the money and buy a micrometer, caliper and indicator? Measurements might take more time, but I have the flexibility. And flexibility is what bench-top gaging is all about." Q
TECH tIPS• Today's bench-top gages contain digital readouts and can transmit measurements to a PC with the touch of a button.
• The electronics revolution and other innovations have allowed instrument manufacturers to make bench-top versions of vision systems, CMMs and other stand-alone devices that were once too big to fit on benches.
• With flexible and retoolable gages, accommodating a design change is a matter of either tweaking a program or modifying modular details on the gage.