Here’s a scary scenario. What if you’re a marine. And what if you line up a mortar to fire. You fire. But then what if the weapon explodes because the barrel has been worn from repeated use, and you had no way to check it for wear before using it.

Happily that is not a real scenario because the U.S. Navy maintains facilities designed to prevent such occurrences. Purpose of the facilities is to ensure the standardization of measurements necessary for interchangeability of weapons and combat systems subassemblies and components.

The Navy’s Product Engineering Assessment Department operates the Measurement Science & Technology (MS&T) Laboratory, which, in turn, is comprised of the Standards Laboratory and the Gage Laboratory operating out of a 39,000-square-foot facility in Corona, CA, featuring state-of-the-art environmental controls and precision measurement instruments directly traceable to the National Institute of Standards and Technology (NIST).

Advanced Technology Laboratories

The two laboratories employ some of the most accurate optical, dimensional, force and gage measurements in the Navy and play critical roles in maintaining interchangeability through gage certifications and standards calibrations. (Gages used among other things to check the barrels of weapons to prevent such occurrences described above.)
Because of the MS&T lab’s capabilities, the Navy, Marine Corps, Air Force, Army, Coast Guard, NASA, the National Science Foundation and others also come to the facility for specialized interface assessment and calibration services. In addition, calibration services are provided to fleet and field labs, shipyards, Fleet Ballistic Missile shore-based facilities and defense contractors.

Interchangeability Critical

Interchangeability is essential to ensure that weapon components work together seamlessly, without selective assembly or modifications. The Interface Assessment Division at the facility makes sure the systems they test meet interchangeability standards and insure interchangeability of weapon systems so there are no problems in combat.

“Our efforts support the warfighter by providing structured, proven interface control practices and precision measurements for maximum efficiency, flexibility and safety of battle-field weapons systems,” says Michael Wheeler, Gage lab manager. “The idea is to solve mechanical interface problems before they happen, that is, to ensure that interchangeability occurs as intended.”

Light Weapons Gages

Wheeler’s Gage Certification Laboratory is responsible for the Special Interface Gage Program that supports and provides certification of interface gages and tooling, master tooling and other associated tooling requiring dimensional certification.

From Wheeler’s lab, certified and calibrated gages are shipped daily to front-line field units, deployed ships and more than 750 Marine Corps units across the globe.

The number of light weapons gages shipped can climb to as many as 12,000 per year. The lab also supports several hundred ships and defense contractors.

As systems procured by the Navy become ever more complex, the volume and diversity of certification measurement work has risen significantly.

“We need to measure a wider variety of special interface gage sizes than ever before,” Wheeler says. “From gages a couple of inches in length that you can hold in your palm calling for a tolerance of ± 0.0001”—to large gages in excess of 10 feet in diameter and weighing 7,000 pounds and calling for a measurement tolerance of 0.0005 inch to 0.001 inch. We do first-article inspections on new procurements as well as periodic re-inspections of gages returning from the field where the end user may be a defense contractor using our gage to assemble or inspect a weapon system or subassembly,”

“Alternatively, the end user might be the warfighter, requiring a gage to validate the safe operation of a weapon,” he continues, “for example to check an infantry weapon or a large gun barrel to check for wear which would make the weapon unsafe to fire.”

Gage Life

Gage life varies, typically, but some gages have been in use more than 20 years. As a result, there is a regular recertification cycle for each gage type; most gages are not used just once, but return to the lab approximately on an annual basis to be remeasured and recertified.
Gages used for infantry weapons experience high turnover, especially those used in Iraq and Afghanistan, due to wear caused by the gritty, sandy, abrasive conditions. Consequently, when the gages return for recertification, many have to be replaced.

Other gages, such as those used in less strenuous enviornments, such as a manufacturing facility, experience less wear so the recertification cycle is longer.

Wheeler: “The need to increase gage lab throughput caused us to seek ways to increase efficiency while maintaining accuracy. In other words, we needed to increase productivity. Examining our options led to realization that an effective way to advance the lab’s metrology operations was to replace our old CMM with its 20-year-old-technology, hard-to-maintain through-hole circuit boards and vintage software, with a state-of-the-art CMM.”

CMM Hunt

The Gage Lab conducted a survey of potential CMM vendors in search for a new piece of equipment. Then they evaluated candidates on-site and in vendor showrooms, as well as entertained presentations from five different CMM manufacturers.

“As a result of our selection process, which included extensive testing, we believe the Mitutoyo Legex 12128 was the best value,” Wheeler reports, “offering optimum flexibility to measure the wide range of gage types and sizes while maintaining the ultraprecision critical to our requirements. As a matter of fact, the Legex’s ISO accuracy specification (ISO-10360-2,0.6+1.5L/1000µm) was a deciding factor. Once we made our choice, we handed off the results to our procuring agency—the Federal Industrial Supply Command—where they confirmed our purchasing recommendation with an independent review. In total, the procurement process required a nine-month effort from research through procurement and delivery.”

After installation, the Gage Lab measured the uncertainty of the Legex CMM’s probing performance, which was found to exceed the published specification.

Advanced CMM Operations

The Legex CMM is equipped with a Renishaw PH10MQ/TP7 EP indexable touch-trigger probing system and a Renishaw SP25M scanning probe. Although fitting the Legex with a vision system was an option, that option was not taken.

“Before we deploy vision systems on the Legex, we’ll want a set of methods and standards able to associate vision data together with touch-probe data,” says Ken Harrell, Gage Lab technician. “We’re still in the ‘looking’ stages on that.”

“The CMM is efficient,” Harrell adds, “because we need to compare gage features located in terms of the design datum structure vs. actual geometry. In a single setup, we get a three dimensional picture of a gage-a picture that is difficult to achieve with multiple, open setups. The advantage of the CMM is that it reduces uncertainty, especially for more complex geometries. One reason is that the staging requires less clamping pressure, which translates to less of a need to compensate for distortion. And compensation can increase the chance for error and thus, for uncertainty. Also, since the CMM ‘flies’ all around the gage, measurements usually do not require multiple stagings, again reducing uncertainty. Because the gage can be staged in the orientation in which the actual article will be used, the force of gravity will act on the gage in the same way it will on the article, again reducing, or even eliminating, compensation for that factor.”

As an example, Harrell points to a Trident submarine missile door gage recently certified. The door gage is used to measure the access hatch door to the missile loading tube used both to transport Trident missiles and to load them into a sub.

“The article has a radius of 45 inches but our gage provides only 20 degrees of arc available for measurement,” he says, “and we have to maintain the profile to less than 0.002 inch. The gage is about 38 inches across and has 14 hole-features that are at precise angular relationships to the radius, and with positions that must be located in relation to the radius with a tolerance of less than 0.002 inch.”

Productivity Results

Mitutoyo provided the Gage Lab lab with a one week training course on MCAT, the Mitutoyo metrology software package.

“MCAT software is very intuitive,” says Dave Nelson, another Gage Lab technician, “and after about a month, maybe less, we were able to get programs rolling and gages certified. Consider that now we are able to measure 50 Marine Corps M-4 rifle head space gages at one time in batch runs.”

“So far, taking into account all measurement activity undertaken on the Legex, we are calculating savings of more than 40% compared to open set-up methods,” Wheeler adds.

The Gage and Standards Laboratory team is just 17 people, but it leverages every technology to achieve the highest levels of performance.

“People’s lives depend on the work we do here,” Harrell says, “Consequently, we take it very seriously. When the fighter in the field pulls the trigger or activates a system, we want to make sure it works, to help keep our guys on the winning side. Everything that goes out of this building has to be right.”

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