Selecting the right thread gage

In today’s demanding manufacturing environment, it’s vital that all threaded parts are properly inspected with the correct types of thread gages. Choosing the proper inspection instrument is a critical quality control decision; the wrong device or method could produce inaccurate results and affect the cost and performance of the parts inspected.

For Metric ISO threads – the most used general-purpose screw thread worldwide – the gage system specified is ISO 1502 to ensure the unlimited compatibility of workpiece threads. Modern thread production uses various tools and inspection methods, from fixed-limit gages to dial/analog or digital types. Regardless of the tools or methods, it’s important the same results are achieved. If in doubt, the gages recommended in the ISO 1502 standard will decide the inspection method for Metric ISO threads.

Gages should also meet U.S. thread standards ASME/ANSI B1.2 for unified screw threads and ANSI B1.16M for metric screw threads. Other standards may apply, such as FED-STD-H28/System 21, 22, or 23 and other MIL standards, depending on the application and industry. In addition to standards, traceability to the National Institute of Standards and Technology (NIST) may also be required.

If inspection in production is done mainly by measuring, it’s still necessary to perform random sample inspection with standardized gages. The reference temperature for the gage and work piece dimensions should be 68°F (20°C) to ensure inspection accuracy. If inspections are done at other temperatures, expansion coefficients must be considered.

When purchasing precision fixed-limit thread gages, it’s imperative to select a quality gage. Important general features and benefits to look for in a thread gage:

• Aged gage steel that’s extremely stable and true-to-dimension
• Hardness that’s noticeably above the standardized minimum requirements
• Extra-high wear resistance
• Availability of standard and special tolerance models
• Special designs that may be required
• Inspection certificates available upon request (issued by an independent or in-house inspection lab)
• Ability to laser mark custom specifications on gages
• Gage handles with double surfaces leaving sufficient space for specific marking requirements
• Incomplete threads are removed before the beginning of the first full thread for creating a stable thread start
• Plug gages provided with a dirt flute, for safe gaging even under difficult conditions
• Knurled handles for safe handling, improved gripping
• Reduced thread start for easy insertion of the NO-GO gage body
• Recessed minor thread diameter of NO-GO gages for safe function
• Gages that are ISO 9001-certified

Gages for internal threads

The GO thread plug gage and the NO-GO thread plug gage are used for gaging internal threads. GO and NO-GO plug gages are mounted on a common handle for thread diameters up to 40mm and are designated as GO/NO-GO thread plug gages. For exceptional cases, handles for GO/NO-GO thread plug gages up to a thread diameter of 62mm are standardized in DIN 2240-2. A (smooth) GO and NO-GO plug gage is recommended for gaging the internal thread minor diameter.

The GO thread plug gage checks the so-called virtual size of the internal thread and the proper screw-in fit. In doing so, it checks the smallest size of the internal thread pitch diameter D2, including certain form deviations in the thread, such as pitch and thread profile angle deviations. It also checks the smallest size of the major diameter. The minor diameter D1 of the internal thread isn’t checked.

The GO thread plug gage must be able to be screwed by hand into the full length of the workpiece thread without using force (a gage should be screwed firmly but must never be forced into the part being checked). The GO thread plug gage is subject to a higher level of wear than the NO-GO and should be checked at regular intervals defined by a quality control plan. When a thread plug gage will be subject to a high level of daily use or to inspect internal threads in abrasive material, it’s recommended to use GO thread plug gages in a TiN-coated version for enhanced wear resistance and longer service life. The acceptable wear of the GO thread plug gage is determined by measurement based on the three-wire-method.

The GO thread plug gage has a full thread profile along its thread length. Note the thread length isn’t less than 80% of the screw-in length of the workpiece thread. According to DIN ISO 1502, no so-called acceptance GO plug gages are standardized. It’s good practice to always use new plug gages for production and keep those close to their wear limit in the inspection room.

The NO-GO thread plug gage checks whether the actual pitch diameter of the workpiece internal thread exceeds the largest specified size. The NO-GO thread plug gage shouldn’t be able to be screwed into the workpiece thread by hand for more than two revolutions (from both sides) without any force.

The GO/NO-GO thread plug gage combines a GO thread plug gage and a NO-GO thread plug gage on one handle. The dimensions of the GO/NO-GO thread plug gages are specified up to a nominal dimension diameter of 40mm in DIN 2280. It provides the functionality of GO and NO-GO thread plug gages.

The internal thread minor diameter D1 is checked with plain cylindrical GO and NO-GO plug gages or a GO/NO-GO plug gage. Because the minor diameter can change during thread tapping, an inspection is required after the thread has been completed. The internal thread minor diameter should be checked before gaging the internal thread pitch diameter. The smooth GO plug gage should be able to be guided by hand through the workpiece thread without using force. It must not be possible to insert the smooth NO-GO plug gage into the workpiece thread from both sides deeper than one pitch (1 x P) from the start of the thread.

Analog thread depth plug gages

GO/NO-GO thread depth analog and plug gages permit gaging threads and the measuring of thread depth in a single operation. They are typically used for components with identical thread sizes and different thread depths, and for setting the thread depth in all kinds of internal thread production. By pushing the spring-loaded scaled sleeve into the handle, the fully cut thread depth can be read quickly and precisely.

In the industry, opinions vary concerning the full gage length of a GO thread plug gage. One of the advantages of using a thread depth plug gage is it can be easily adjusted to a number of variants without the use of auxiliary tools. For example, a gage with a scaled sleeve can be set to the face of the GO gage body or set to the beginning of the incomplete thread. Smaller contours on the workpiece surface, such as countersunk edges or elevations, and small dimensional variations of different GO gage bodies can also be compensated for with an easy adjustment.

The analog plug gage should meet all the requirements and gaging criteria as specified in ISO 1502 for standard GO/NO-GO thread plug gages. Helpful features in a plug gage include:

• Simple, safe handling – can help reduce inspection work
• A full range of sizes available
• Measurable thread depth of up to 4 x D
• Easy to adjust
• Universally applicable due to exchangeable gage bodies
• Availability of TiN-coated gage bodies for extra wear resistance
• Measuring accuracy of 0.5mm (0.020")
• Available with locking screw

In some gage designs, when a GO gage body on a thread depth analog plug gage has reached its wear limits, it can simply be extracted with a pulling sleeve and a common hexagon nut and replaced with a new gage body.

Gaging of other threads

Gaging other threads such as sealing threads, tapered threads, threads for tight fit, and EMUGE SELF-LOCK threads that are important in safety critical parts in the aerospace and defense industry, require thread gages that often deviate considerably from the normal gage design. They are usually adjusted to the special design and function of these threads.

For example, when gaging SELF-LOCK threads, EMUGE recommends using their two-piece gage system which corresponds to the usual combination of GO and NO-GO gage and is perfectly sufficient for gaging threads, provided the LK threads were produced with EMUGE true-to-profile taps.

There’s no generally applicable standard (e.g., DIN standard) for EMUGE SELF-LOCK threads, so other manufacturers may use different limit sizes for their threads. For this reason, it’s prudent to gage EMUGE SELF-LOCK threads exclusively with EMUGE SELF-LOCK gages.

Conclusion

When inspecting threads, it’s important to use the proper gage and use it correctly. Good thread performance hinges on proper thread inspection and gage calibration, as well as meeting current thread inspection standards.

EMUGE-FRANKEN USA

About the author: Robert Adamiak is gage product manager, EMUGE-FRANKEN USA.