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Plug Gages - Indispensable Inspection Tools

May 22, 2012
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A fixed mechanical plug gage provides fast, accurate and easy to use measurement to the point of manufacture. Source: Mahr Federal Inc.


Since the beginning of industrial manufacturing, periodical inspection has been required to ensure that the components or parts involved in the manufacturing process meet required specifications. This has led to the innovation and usage of several types of quality control tools. Several gages have been employed in an effort to standardize quality levels in high volume processes. The types of gages that are used vary depending on their application and function, which include height, depth, ring, bore, plug and snap gages. Gages are among the simplest equipment available for ensuring adherence to specifications. For more than half a century, plug gages have been a crucial part of maintaining tolerances of the highest standard and have been at the core of quality control applications across a variety of end-user applications.

A thread and plain plug gage. Source: Wikimedia Commons

Simple Working Principle Propagates Widespread Usage

Plug gages are commonly used in the inspection of several product lines as a result of their ease of use. A plug gage assesses the conformity of objects to the prescribed specifications. A typical plug gage resembles a rod whose ends are carefully fashioned so that one end is the upper limit while the other end is the lower limit.

As we can observe from the bottom gage in the image below, the left end is marked green and is the go-end, while the right end is marked red and is the no-go end. The go-end should fit into the component under test and the no-go end should not fit into the said component for the component to meet specifications. On account of its function, these gages are commonly referred to as “go/no-go gages” and “fixed limit gages”. Apart from these double-ended gages, single ended plug gages that contain only go or no-go measurement sides are also available. In such cases, these gages are offered in pairs.

Plug gages are available in plain or threaded form factors. Typically plug gage manufacturers tend to focus on developing only one type of solution and not both. The majority of market participants are regional, whereas less than a handful of participants have a larger geographic presence.

Plug gages can determine if a part should be accepted or rejected in a fast and relatively accurate manner. They are primarily preferred for their simple working principle, as the operator does not require a strong scientific or metrology background to use the instrument and apply the data gathered-the major advantage of plug gages over other types of measuring equipment. Also, as these gages are small, they are extremely portable (coming in easy to carry cases) and can be used for applications regardless of whether they are in the shop floor or in the field.

Another key advantage of these gages is that they are inexpensive when compared to complex precision equipment such as universal gage machines or optical comparators. The pricing of the gage is dependent on the size of the gage, in which larger gages have a greater price. The price of gages are also primarily dependent on the tolerances, with the descending order of prices being class XX, X, ZZ, and Z pins. The exception to this pricing rule is the smallest available gages, such as those in the order of 1/64ths of an inch. These are priced significantly higher than the next size, as a result of the challenging process involved in manufacturing these gages. More often than not, these gages are sold in sets, wherein a single set contains the entire batch of gages, from the minimum to the maximum size.

A chief drawback of plug gages is that they tend to get worn out from usage over a period of time. As a result, careful handling and maintenance of these gages are necessary to ensure a long shelf-life. Periodical checks are necessary to ensure that the best gages are used to ensure the highest quality. In addition, newer, more technologically advanced machines pose as the biggest threat to plug gages.



Applications - Working Gages and Master Gages

Plain and thread gages are used for entirely different applications. The thread gages are used to check tapped holes in a component. This is an extremely simple method to inspect threads and is one of the reasons why end-user industries continue to favor them in production lines. Whether it is the automotive industry, aerospace or commercial industries, such as plastics - threaded holes are present and these gages offer the most simple and effective solution.

Plain plug gages are used in a variety of roles. Their primary application is the measurement of diameters of various components. Inspection of the size of the hole is crucial in end-user industries such as aerospace, automotive and transportation and medical devices. For example, within medical devices, plug gages are crucial in the manufacture of implants, orthopedics and other devices, despite the shift in customer preference towards more advanced precision systems such as profile projectors, multi-sensor systems and three-dimensional scanners.

Darryl McIntosh, national sales manager of Vermont Gage, manufacturers of plain and thread gages, notes that “It basically comes down to this – do you want to perform a measurement on the part or do you want to ensure if the part is assemblable or not. As long as the customers have to be concerned about assemblability, fixed limit gages is the easier option”.

Another primary use of these plain plug gages is in the calibration of other precision equipment. Metrology systems such as calipers, micrometers and other gages are calibrated in regular intervals by plug and ring gages. The gages used in these applications are known as ‘master plugs’ and ‘master rings’.



The Way Forward

Despite the onslaught of newer and more advanced precision measurement systems such as coordinate measuring machines (CMMs), scanners and vision systems, plug gages continue to remain the ideal solution for a wide range of users. Even small and mid-sized companies prefer these simple gages to other more complex equipment.

“There will always be a need for low end gages in the market,” according to Hassan Itani, manager of Ideal Thread & Gage Manufacturing Company, Inc.-another company that specializes in making thread plug gages. “Several small and mid-sized companies primarily depend on this for their everyday measurement needs.”

These tools are being used less as work gages, but remain being used as master gages. In an era where tolerances have become ever so tight, plug gages continue to set the standard for measurement. For instance, air gauging remains one of the most accurate methods of measurement even when compared to using sensors. “While some sales volumes may have been lost on the measurement side, we have gained sales on the standards side,” notes George Schuetz, director of precision gages at Mahr Federal, Inc. “These plug gages which are used as setting standards are actually of a higher cost than the work gages.”

Large corporations spend huge sums of money acquiring advanced systems with cutting edge technology and integrating them into the manufacturing process. Such equipment is better suited for large volume inspection and the entire process is automated. Nowadays, these advanced inspection systems are also being adopted by mid-sized customers. “The inspection system that is used is predominantly dependent on its suitability to the end-user application requirements and not as much dependent on the cost of the measuring system,” says Schuetz. In today’s changing world where automotive companies outsource component production to several third party vendors, there is a market for both variable gaging and fixed limit gaging. “Half a century ago, an automotive manufacturer would manufacture and assemble all the required components, but in today’s world, it’s cheaper to buy the necessary components from somewhere else and then assemble it,” said McIntosh. “Here, these plug gages are highly important as they provide a quick and simple solution to check if these components are assemblable or not.”

Conclusion

Despite the advent of advanced precision solutions, plug gages have managed to survive in the 21st century market. The most visible trend in the market over the past few years has been the decline in the use of plug gages as a working gage and instead as a master gage. Nonetheless, these easy-to-use gages remain the best fit for several applications, as they provide vital information on the components usability. Small and mid-sized companies continue extensive use of these gages, while larger companies continue to use them, albeit in a limited scale. In several applications, these gages perform a complementary function to equipment that measures other parameters of the component under test. As a result of their simplicity and high accuracy levels, plug gages are expected to remain an ideal part of manufacturers across the globe.

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