Measurement: The Classic Gage
The BasicsThough the gages themselves have not changed dramatically, they are still a staple. James Meyer of Meyer Gage Company Inc. (South Windsor, CT) compares them to the number-two pencil: uncomplicated and inexpensive. But don’t underestimate them. In “The Handbook of Dimensional Measurement,” Francis Farago and Mark Curtis write that “Fixed gages constitute one of the pillars on which interchangeable manufacturing has rested since its early origins.”
The surrounding environment of the shop floor has changed, however, and plug gages are now part of a supporting cast of inspection equipment that could include coordinate measuring machines (CMMs) and more advanced electronic gages with statistical process control.
Demusz Manufacturing Company Inc. (East Hartford, CT) mainly manufactures parts for turbine engines. Functional gages are the most common item for inspection: the company has about eight complete pin sets, with about 1,000 pins in stock. “Functional gages are the one thing when the part fits on there you know it’s going to fit on the engine,” says Wes Demusz, vice president and quality manager. But the pins often work best in tandem with the company’s other inspection equipment. He cites a problem from a few days earlier. Their CMM measured a part and found that the true position of the pattern was good but the diameter of the holes was oversized. But when they tried inspecting with a pin, it wouldn’t fit. It didn’t make sense that the part would be too big, but not allow a plug gage to enter. But they noticed that they could see light coming through. “Right away we suspected that the holes were out of round,” Demusz says. They went back to the CMM to check for roundness and found that was the issue. Using two pieces of equipment allowed them to solve the mystery.
But sometimes the plug gage can handle issues on its own. “The lowly plug gage is often modified to provide more information,” says Hill Cox, president of Frank J. Cox Sales Ltd. (Brampton, Ontario) and Quality columnist. “For example, gage members are often modified to look for roundness variations. Roundness can be determined by changing their shapes to a triangular form for odd shaped lobing on the bores being inspected or an oval shape for two-point roundness variations. Once inserted in the part, a slight twist of the gage will reveal their presence.”
Gage SelectionIf you’re not sure how to select a gage, the gagemaker will be able to help. Hemco Gages (Holland, MI) works with “just about every industry there is that has a thread,” including nuclear, oil, automotive, medical, aerospace, defense, even hydraulic fracturing (fracking), says Michael Hop, Hemco vice president and general manager. The customer question he hears most is: What do I need? “In the past, they had their own in-house experts to determine what they needed,” Hop says. “Gaging is a lost art.”
Don Moors, president and chief executive officer of Western Gage (Camarillo, CA), says, “With go/no-go plug gages, operator clearly understand if the no-go goes in, it’s a bad part. The go must go in in a good part. What I think many quality people don’t understand, particularly if they haven’t had a lot of practical experience, is the uncertainty of measurement when it comes to close measurement features.”
Hill Cox once devoted a three-part Quality series to measurement uncertainty. He wrote, “It is an indicator of how exact a specific measurement process is. The smaller the value, the closer the reading will be to the true size. Remember that every measurement process has uncertainty attached to it; no one is exempt or perfect. Because different processes and equipment produce different values, you can see which is best for your application when compared to the tolerance involved.”
When selecting a gage, consider the 10% rule of gage tolerance, says Mike Oddy, technical resource director of Vermont Gage (Swanton, VT). The gage cannot use more than 10% of the product tolerance. “In many cases, they need to understand the intent of the rule,” he says. “It’s a guideline to help the gage user begin the decision making process.”
And beware of artificially tightened tolerances. Using a tighter tolerance pin may simply be overkill, costing more money, taking longer to receive, and wearing out faster. Len Pernecky, president of Dundick Corp. (Cicero, IL), a company makes plug gages, thread gages and ring gages, has seen this happen over the years. “There are a lot of people who are ordering gages with a closer tolerance than is really needed because they have no idea how it is really used,” he says. “They try to make plug gages sophisticated.”
Misunderstanding the tolerance can lead to unnecessary quality issues, says Mark Curtis, who is working on a fifth edition of “The Handbook of Dimensional Measurement” that will be published later this year. “At the far end of the limit, they might be rejecting a part that is technically good,” he says.
Gage UseOnce you understand tolerances and measurement uncertainty, you still have to know how to use the gage. “As simple as these things are, misuse causes a lot of problems,” says Oddy. He no longer brings examples of what not to do during his gage training because he says he will invariably see it on site. Take note: plug gages should not be used as a hammer. They shouldn’t be forced. The plug gage should be able to go all the way through the part. If it doesn’t, take the gage out, ream the hole or try a little forced air.
“As long as you take care of the gages, the gages take care of you,” Oddy says. “It gets more complicated than it needs to be. I once had a conference call with seven companies on five continents. The discussion was about a go/no-go plug gage and how many turns it could enter into the part.”
The company was making an expensive part that didn’t assemble, leading to problems and finger-pointing. But, Oddy says, this could easily be avoided. “Stay away from limits,” he says. “Make sure you set up your parameters.” For example, if a tool doesn’t seem to be functioning well, don’t try to run another batch of parts from that tool.
A Range of OptionsThough it’s important to get the right plug gage and treat it well, in many cases, customers may need a range of equipment to complete an inspection process. A plug gage’s function is inspection, not measurement. For measurement, turn to other gaging options, which often work in tandem with the basic instruments. A shop might have ten basic gages for every one electronic readout gage.
Rob Nemec, president of Diatest USA (Arlington Heights, IL), says indicating plug gages step in when customers want more information than just knowing if a hole is bigger than one size or smaller than another. Customers would need a variable bore gage to know the exact size of the hole, if parts are high limit or low limit, if the hole is straight or has a taper. Tight tolerances could be something better than two-thousandth.
John Felice, president of Classic Jig Grinding (Tolland, CT), works with close tolerances in his business in cutting metal and machining. Gage pins used to be plus or minus two- tenths tolerance, Felice says. Now they are coming closer to size, with one-tenth rather than two-tenths.
“The techniques are the same as they were, but it’s the way you go about it,” Felice says. “The production machinery can hold a lot closer tolerance than we could get 30 years ago,” he says. “Why not specify it? With CMMs and electronic gages, it’s easier to check. There’s been an overall improvement in the manufacturing process.”
Increasingly companies demand more from their gages, says George Schuetz, director of precision gages at Mahr Federal (Providence, RI). Operators want to measure the part by or on the machine tool. And the instrument doesn’t exist in a vacuum. “We always try to look at the big picture first to understand what the gaging process is,” says Schuetz. “There’s more to measuring than just the gage.”
The future of the industry will be focused on issues surrounding the gage. “All the changes are going to be on the data collection side and the measurement side,” Schuetz says. “The physics of the gage haven’t changed that much. It all comes down to what the user wants to do with that information. “
And if the company would like to output data to a computer for statistical process control, wireless gages could make the job simpler. Consider the ease in inspecting a long structure such as an aircraft wing or fuselage, says Bob Harman, product manager for the Testar division at Marposs Corp. (Auburn Hills, MI). With wireless gages, inspectors could walk down the structure and download measurements to a wrist-type computer.
And these tightened tolerances have improved quality, says Don Moors of Western Gage, who cites the example of his first car, a 1937 Chevy handed down from his parents. Back then, the advice was not to drive more than 25 mph for the first 100 miles and not over 50 mph for first 1,000 miles. “Now, with improved accuracy, you can take your new car out at full throttle and drive 90 mph or 100 mph, if you can find a place to do it,” he says.
Many different instruments and techniques have lead to these improvements. While the basic plug gage will continue to determine good or bad parts, it is backed up by indicating plug gages, air gages and CMMs.
“If you’re making a part, it costs a lot of money to produce scrap,” says Rob Nemec of Diatest. “It’s important to have a gage that can give you readings that you trust.” Q
Tech TipsWhile the basic plug gage will continue to determine good or bad parts, it is backed up by indicating plug gages, air gages and CMMs.
When selecting a gage, remember that the gage cannot use more than 10% of the product tolerance.
Using a gage with an artificially tightened tolerance may cost more money, take longer to receive and wear out faster.