When the first digital electronic indicators were introduced nearly 20 years ago, trend watchers predicted that these gages quickly would dominate the market and render mechanical dial indicators obsolete. But that didn't happen, and mechanical gages still perform yeoman's work in the majority of precision manufacturing companies. Despite great improvements in speed and accuracy permitted by microelectronics and personal computer applications to gaging during the past decade, "old faithful" still provides adequate performance for many inspection tasks, often at a lower cost.

However, there are many good reasons to believe that dimensional inspection finally will evolve into a predominantly electronic regime during the next 20 years. That's not to say mechanical gages will disappear from the scene -- far from it. They continue to undergo technical advancement and perform well in the marketplace. But the reasons for using electronic indicators will become more compelling as more manufacturers are required by their customers, regulatory agencies or their own ISO 9000 compliance programs to capture, manipulate and report on huge masses of quality-related data.

The digital shift

History gives us every reason to expect greater demands for precision during the next decade. Since the 1940s, typical machining tolerances have become about 50 percent tighter every 10 years, continually shifting the accepted meaning of "high precision" -- from 0.002" (0.05 mm) in the 1940s to 0.001" (0.02 mm) in the 1950s, all the way to 50 µ" (1 µm) today. Some metal-cutting industries already are working to 20 µ" (0.5 µm) tolerances, and single digit tolerances may be considered normal before long.

This trend will give impetus to the shift from mechanical dial indicator gages to gages equipped with digital indicators or electronic amplifiers and transducers. Most digital indicators already offer 50 µ" (1 µm) resolution, and 20 µ"       (0.5 µm) resolution is readily available. In contrast, few mechanical dial indicators resolve finer than 0.0001" or 1 µm. Digital gaging amplifiers offer 1 µ" (0.1 µm) resolution, and some transducers are capable of such accuracy, but this performance level can't be achieved yet on the production floor because of environmental instability. Future gages may address this issue, however, as discussed below.

Further promoting the shift, electronic indicators already are cost-competitive with mechanicals. If pricing trends follow those of other popular digital electronic goods that seek to compete directly against analog mechanical devices (e.g., wristwatches and telephone dials), the electronic versions soon may become cheaper. That in itself might be enough impetus to effect a large-scale conversion.

End-users will become increasingly familiar with the advanced dynamic functions that some digital indicators offer: automatically capturing a minimum, maximum or TIR reading, and making inspection operations easier, faster and less error-prone. Perhaps the greatest advantage of electronic indicators is their ability to output data to data loggers or directly into PCs running SPC software.

With more gages tied to PCs in one way or another, we'll see an increase in gage results networked plantwide, as well as a trend toward centrally processing that data and generating feedback to the production process. Preliminary work is underway to establish a standardized file transfer format for measurement data.

New sensing functions

Improvements in transducer technology will allow gages to combine high resolution accuracy with long measurement range, generally an either/or proposition in today's gages. Digital indicators with resolution of 50 µ" (1 µm) and ranges of 1/2" or 1" (12.7 mm or 25.4 mm) are now on the market. Expect ranges to double or triple and resolution to improve to 20 µ" (0.5 µm) or less in simple indicators over the next few years.

In some applications, this will precipitate a shift from comparative to absolute measurements (i.e., gages will display the feature's actual dimension rather than its deviation from the nominal specification) and a great increase in gage flexibility. For example, an inspector who currently uses four indicator gages and masters to measure four different dimensions on a workpiece may soon be able to use a single indicator gage and no master. More powerful microprocessors may permit users to program multiple presets to accommodate a series of different dimensions, but this may require additional improvements in the user interface.

Some advanced laboratory gages incorporate nondimensional transducers and associated software to counter environmental effects on measuring. This concept will work its way into simpler instruments intended for shop-floor use, where environmental variation effects are greater. Indicator gages will have built-in sensors to measure the gage's temperature itself as well as the workpiece. An onboard processor will take that data and automatically correct it for thermally induced dimensional variation.

A focus on geometry and surface finish

Until recently, a few microinches of variation in surface finish or geometric accuracy usually could be ignored. But as dimensional tolerances become tighter, surface finish and geometric variations  represent a larger proportion of the total allowable part variation, and their measurement becomes increasingly important. Furthermore, engineers are learning more about how these variations influence functionality, and tolerances are being specified more frequently.

Both ANSI and ISO recently adopted new or revised standards that will bring needed stability to the field of surface    finish measurement. Until recently, the number of parameters for stylus-type measurements had been growing rapidly, and now more than 100 parameters are in use. Few of these are standardized, however, and some are used only on very limited bases. There also have been considerable differences between U.S. and international standards. As a result, the methods employed by one company often aren't recognized or accepted by another.

With the new standards in place, the parameter proliferation should slow considerably. There will be stronger bases for agreement between suppliers and vendors. Because of the intentional overlap between the new ISO and ANSI standards, manufacturers can now select surface measurement methods that satisfy both standards.  And it's now more profitable for design engineers to pay attention to existing parameters, learn what they mean from a functional standpoint and begin applying them intelligently.

For many years, engineers have specified circular geometry tolerances for very large parts, such as power generator turbine shifts, but there has been no method to inspect these parts because they couldn't be placed on the turntables of existing geometry gages. New methods are being developed to measure the roundness of very large parts, using a gage that comes to the workpiece rather than vice versa.

Because of tightening specifications for part geometry, gage design will become by necessity more sophisticated, and end-  users may find it less feasible to design and build fixture gages for simple dimensional measurements in-house. The days of do-it-yourself gage building may be nearing an end.

Processing power will simplify
complex procedures

As production requirements demand more complex measurements -- especially surface finish and geometry -- on the shop floor, gage makers will respond with deceptively sophisticated gages that do more work for the user. Increased data storage and processing power will be built into gages, and more gaging functions will be built into computers. Computer-aided gages will guide procedures and setups, establish datums and correct or account for geometric and environmental variation.

Some advanced geometry gages already in use are driven by Windows-based software that guides users through complex setup routines. One system even uses artificial intelligence to tell the user when a part is out of tolerance and to suggest possible approaches to analyzing the condition. Future systems will go beyond that, making specific recommendations on how to adjust or repair the process that caused the problem.

Enhanced programmability will further automate complex geometry setups. Quality control or metrology specialists will be able to write setup programs offline and save those programs to disk. These programs will take step-by-step setup guidance to the next level, including highly specific instructions on the order of   "which knob to turn in which direction." The disk, along with the part to be measured, will be provided to the gage operator, who will thus be relieved of the responsibility for reading or understanding the part print. The use of such straightforward software applications will improve the accuracy, efficiency and utility of many similarly complex or time-consuming gaging tasks.

Gage block calibration presents another example of how complex procedures will be simplified through software, although not in a shop-floor environment. Traditionally, calibrating a set of gage blocks consisting of dozens of blocks in graduated sizes required great care on the operator's part to keep the measurements in order, record the results systematically and do it all efficiently. New software specifically written for gage block calibration systems walks the operator through the process step by step, greatly reducing the possibility of error and cutting total time for the procedure by several hours.

In the future, the comparator may be equipped with an automated parts-handling mechanism to further speed the process and reduce the possibility for error. Such a system will manipulate gage blocks more gently and with greater repeatability than can human operators. And it will eliminate one subtle but important source of variability in these highly precise measurements: the gage operator's body heat.

With increasing use of graphics-based gaging software will come more user-friendly input devices. Touch screens have been used on geometry and surface finish gages for several years, and they certainly will be adopted for other applications. Joysticks, trackballs, mice and similar positioning devices all will become common gaging accessories.

More capable gages per dollar

Costs for many types of gages will drop, due to a combination of factors. As certain procedures are specified more often and the market for these gages increases, simple economies of scale will take hold. Replacing precision-machined components with electronics and software also will affect pricing. And in a few cases, creative engineering and sophisticated marketing will result in well-positioned instruments that fulfill a specific need without offering every bell and whistle.

As an example, there are now portable surface finish gages that measure just a few of the most common parameters, out of the hundreds available, that cost less than $2,000. For a large number of users, this limited functionality is all that's needed, and the product makes much more sense than a full-featured system that might cost $40,000 or more. Similar evolution has occurred in circular geometry gages, where pricing is now as low as $15,000.

Improving machine capability

Machine tool calibration will come into much wider use. The rationale behind MTC has always been compelling: to assess and improve a machine's capability for accuracy before trying to produce parts. But the complexity of the equipment and procedures has hindered MTC's acceptance. Even knowing what to do with the mass of data that MTC generates has remained somewhat of a problem.

Once again, improved software that takes the burden off operators and essentially walks them through the processes will make the difference and drive industry acceptance. When MTC becomes easy, it will no longer be a technical specialty but a standard procedure that machinists use to improve their work. A few flagship companies in the aerospace and medical implements fields already are seriously committed to MTC, and this could evolve into demands on suppliers upstream.

The future of "traditional" contact-style dimensional gaging, then, will be largely a process of "digitalizing" and computerizing many tasks that currently are performed with mechanical instruments. As this occurs, a number of benefits will accrue. Some measurements will become more precise. Many routine inspection tasks will become easier and/or quicker to perform, while some highly complex tasks will become routine. Gages will be integrated increasingly into feedback-controlled manufacturing processes and into companywide networks. Overall, gaging will play an even more important role than it does today in quality-oriented product development and manufacturing.

About the authors

The four authors are with Federal Products Co. in Providence, Rhode Island. Federal Products is a leading manufacturer of precision dimensional gaging products. Duane Christy is manager of research and technology; George Schuetz is marketing manager for gaging products; Alex Tabenkin is marketing manager for instrumentation; and Paul Tullar is marketing manager for machine tool calibration and laboratory gaging products.

For more information, contact Federal Products Co., 1144 Eddy St., P.O. Box 9400, Providence, RI  02940-9400, telephone (800) 333-4243, fax (401) 784-3246, e-mail info@fedprod.com or visit Federal Products' Web site at www.fedgage.com.