Balancing Gaging Efficiency with Flexibility
Manufacturing Engineering Magazine August 2019
In the world of metrology, lack of variety is not an issue. "You can think of a thousand ways to measure a hole," said George Scheutz, director of precision gages for Mahr Inc., Providence, R.I.
Techniques could include an operator equipped with a simple, custom-built go/no-go gage up to a robot loading a part onto a CMM with a pre-loaded program, and everything in between. Which to choose as "best" requires asking lots of questions about the production and the part. "Dozens of questions can lead to the right path," he said. What is the tolerance, size, production volume, length of production run, and is measurement data recorded and used in a system? What is the skill of the operators and what is the budget?
If you took a brief glance at the catalog of any company that offers gages, you might think the company could indeed offer a thousand ways to measure a hole—or length, surface roughness or most anything else. Choices include hand-held precision gages such as calipers up to high-end automated measuring systems. The latter combine optics and tactile probes for sub-micron, high-volume measurements of specific types of parts.
Begin with the Basics
The least capital-intensive devices are those used directly by an operator. These are ideal where production volumes are low, tolerances are not overly tight, but capital budgets are. Schuetz defines the range of human-operated gages as good, better and best.
"A good solution is a fixed gage that can tell you if a part is good or not," he said. These include plug gages, pin gages, or ring gages designed for a single measurement, often on a single part. "These are fast and easy to use as well as resistant to operator variability," he said. But the information is limited. "There is no data available from those parts, just if it is good or bad," he said.
The better solution is getting measurement data from handheld gages like micrometers, calipers or hand-held dial indicators. "These are versatile, can measure a wide variety of parts and are accurate. Micrometers can go down to a micron in accuracy," he said. Many of these devices are equipped with wireless data transfer, so they can feed process data wherever the operator wants. However, they too have issues—repeatability from one person to the next is one.
"Operators can vary in skill and in their strength and gaging force," he said. Another is speed. Even skilled operators need to take their time to get accurate measurements. Digital readouts and updated mechanicals have improved handheld gage usability. For example, Mahr's Micromar 40 digital micrometer features a 25-mm measuring range, with a 2-µm error limit. Still, there are human limits.
"The best operator-used gages are fixed variable gages," said Schuetz. Fixed variable gages tend to have a narrow measuring range and are built to a specific size. This makes them as easy to use as go/no-go gages. But they supply actual measurement data and provide fine accuracy, sometimes to sub-micron levels, according to Schuetz. Fixed variable gages include air plugs, air rings, or dial indicators mounted on a bench. There is no adjustment that an operator can make, so they are fast. "A shop floor guy can make hundreds of measurements in no time," said Schuetz.
While variable gages, like air gages, are easy to use and can be extremely accurate, in the sub-micron range, they sacrifice range for accuracy. "You cannot measure an 80 millionths of an inch accuracy over a ten thousandths of an inch range," explained John Castle of Universal Gage Corp., Hawthorne, Calif. The company is a dedicated supplier of ID and OD air gaging. "We measure poles and holes," he said.
Another key misconception is that many people think air gages can measure close to edges of parts. "An air gage is a nozzle with a face. It is like a hole with a doughnut around it. It covers an area, so it cannot work like the ball of a tactile probe," he said. As it nears an edge, part of the measurement area will move past the edge.
Yet the advantages of air gages are clear. Consistency of measurements is key to production, and air gaging provides that. Castle attested to how such consistency can also be accurate and easy to obtain with air gaging. "We did a gage R&R at a customer's site where there was some disagreement on the consistency of the measurements," he said. They asked an administrative assistant to perform the test using a set of written instructions and got the same result as the technician on the floor.
Another key advantage of air gaging is they perform well in wet, dirty environments. "The air blows away any debris while it measures," he said. He added that for small measurements, in the sub-micron range, no other practical instrument can do the job. Others interviewed for this article agreed.
Applications and Flexibility
Automotive applications, especially engines and powertrain parts, illustrate the need to balance flexibility with price in a high-volume business, according to Scott Lukomski, director of sales, metrology for Jenoptik Automotive North America LLC, Rochester Hills, Mich. Jenoptik also offers gaging products using tactile, optical and pneumatic technologies. Its products range from simple manual systems to fully automated custom solutions. The company also offers custom gaging.
Jenoptik recently introduced two new optical shaft measuring devices that illustrate what might be called focused flexibility. These are aimed at high-volume turned parts, especially automotive parts. One makes high-end fixed gaging a little more flexible. The other one is a little more fixed to give a better value to a targeted application.
"The auto industry sold 17.2 million cars, [SUVs and light trucks in 2018], and each has at least four pistons, plus various shafts," he said. "We provide optical gages so they can measure them practically on the fly. But [they are programmable] to change over quickly when there is a product change," which is happening more often today.
For greater flexibility, Jenoptik has improved its Opticline "C" series systems with an optional T-3D tactile probe. Like other devices in its class, the Opticline measures turned parts like a powertrain shaft using a light source projecting onto a camera, turning the part to acquire measurements that are compared against a master.
"Adding a tactile probe lets us measure other features on the shaft part, while in the gage, that cannot normally be read with light," Lukomski explained. Users are now able to measure into machined features, such keyways or holes. In the past, they might have had to pull the part from the line and measure those features on a CMM. Now it can be done in one set-up. "The Opticline product line has a series of variants, ranging from a simple table top version to fully automated solutions," he said.
The other new product, called Opticline CS, trades a bit of flexibility for better value. Jenoptik bills it as a starter system for optical shaft measurement. "It's still in the Opticline family but it allows us to put the CS in shops that are smaller, with smaller production runs and limited budgets, but which still need maximum flexibility," Lukomski said.
Non-contact sensors used as gages are gaining in importance, according to Matteo Zoin, senior manager and head of new market development for Marposs Corp., Auburn Hills, Mich. These include lasers, optical, confocal imaging, X-ray tomography, and interferometry—all now packaged as gages, according to Zoin. Strictly speaking, he pointed out, how these sensors are used is what makes them a "gage"—by comparing the differences against a master part rather than recording absolute measurements. "[Comparative measurement] allows us to achieve high accuracy, better resolution and better repeatability," he said, as well as speed.
Zoin also touched on why choosing a gage can be a complex process. "Gaging is a niche market; you cannot learn it at college, and it is something you learn by experience," he explained. There are no standardized, simple rules, or a chart or table that you can follow to make the best choice. "The possible combinations and configurations are almost infinite," he said. Like other gage providers, Marposs meets this need with a variety of handheld, bench, and automatic gaging. It also provides a line of flexible gages that use optics for measuring shafts, called Optoquick and Optoflash.
Because there is so much variety in applications, Zoin thinks these non-contact, flexible gages represent the best choice for users. They are adaptable within a range of parts. The Optoquick is for high-precision measurements of cylindrical parts on the shop floor, such as gear, crank, cam, or drive shafts. Measuring uncertainty for lengths is (4+L[mm]/200) μm and for diameters is (1.5+L[mm]/200) μm. It has an imaging shadowgraph and a touch probe for measuring additional features, such as keyways or holes.
"Parts are rotated and measured with an optical system" against the measurements of a master part, he said. The Optoquick's part loading feature has an open and clear loading area with no obstructions. The Optoflash is a smaller, more economical version with side-by-side imaging. By eliminating any need for Z motion, measurement on the Optoflash takes place faster. More importantly, a robot can load either device.
Flexible, High-End Gaging
Companies that made their marks in the CMM world have also recognized the value of comparative gaging and have developed flexible gaging solutions. For example, Zeiss Industrial Quality Solutions, Maple Grove, Minn., developed two solutions with the look and feel of general purpose, tactile-equipped CMMs but are designed for gaging applications on the shop floor. "The flexibility of our DuraMax and GageMax is an important advantage," said David Wick, manager of product management for Zeiss. He quickly noted that, like a CMM, each requires a unique part measurement program, but getting that unique program in place is the only effort needed to adapt to a new part. Wick said the devices are ideal for the shop floor.
The DuraMax advertises temperature stability from 18-30oC (64.4-86oF), no need for compressed air, and four-sided loading for easy access. These are not your typical CMMs, but they still deliver decent accuracies. In the range of 15-40oC (59-104oF), the DuraMax HTG (for high temperature) still provides length measurement error of (E0 3.9 + L/100) μm. The GageMax offers (3.2 + L/200) μm. Both errors are measured against the ISO 10360 standard for CMMs.
Wick added that the machines are automation friendly and can be integrated directly into machining cells. "Many of the questions we get are about how to automate our systems, not how accurate they are," he said. "I get way more questions on productivity than measurement uncertainty. Also, they want them bigger."
CMMs like the Zeiss models that are adapted to comparative gaging make sense for complex, high-volume parts. "Our GageMax is primarily used in automobile powertrain applications," he said, "where there are a number of complicated measurements and where productivity is high." The DuraMax offers similar advantages in a smaller measuring volume. "The ideal customer for these machines needs to measure 100 of a complicated part, then 50 of another, and then 70 of something else," he said.
A unique device that has been in the market now for a few years is the Equator gage from Renishaw Inc., West Dundee, Illinois. The Equator, based on a parallel kinematic machine, comes in two sizes, the larger 500 and the smaller 300. It is a comparative gage that requires a master part to zero out the values before measuring production parts; the gage reports only the differences between the production part and the master part dimensions. According to the company, it is easily remastered in production to compensate for any change in thermal conditions.
Typically outfitted with a SP25 scanning analog probe, the Equator can also be fitted with a TP25 touch trigger probe. Keenly aware of the wide variety of available gaging solutions, Renishaw saw an unmet need. "The scanning probe means it is a gage that collects lots of data and is easy to reprogram," said Mark Adams, business manager, gauging systems for the company. "The Equator allows engineers to cope with any design changes in the development of products or parts." The data density means it measures form as well as size and position. "For example, with a hole we can measure not only its diameter but also its concentricity and form," he said. This opens gaging to non-prismatic shapes, like compound curves, as well. However, Adams noted, fixed gaging can always be faster, so a flexible gage is ideal where speed of measurement is not paramount. "People now understand that it is not a cheap CMM, unlike when it was introduced, but rather a shop-floor, flexible gage," he said. General acceptance to date has been in high-volume manufacturing in automated environments, especially in machining cells or around machine tools in general.
"A common application is to measure parts in-line as they come off the machine and use the measurements to feed tool offsets back to the machine's controller to compensate for tool wear or thermal changes," Adams said. The richness of the data set from a flexible tool like the Equator is what enables this function, as well as its ability to fit into an automated system. This is well beyond what simple go/no-go gages can provide, and shows the growing utility of truly flexible gaging. "Flexible gages like the Equator are process control devices, not quality devices," he said.