In today’s demanding manufacturing environment, it is vital that all threaded parts are properly inspected with the correct types of thread gages.
For Metric ISO threads, the most commonly used type of general-purpose screw thread worldwide, the gage system specified is ISO1502, used to ensure the unlimited compatibility of workpiece threads. In modern thread production, there are various tools and inspection methods, from fixed limit gages to dial or digital types.
Regardless of the tools or methods, it is important to make sure that results are consistent. When in doubt, the gages recommended in standard ISO1502 will decide the result of the inspection for the Metric ISO thread.
Gages should also meet US thread standards such as ASME/ANSI B1.2 for unified screw threads and ANSI B1.16M for metric screw threads. Depending on the application and industry, other standards may also apply, such as FED-STD-H28/System 21, 22, or 23 and other MIL standards.
In addition to standards, traceability to the National Institute of Standards and Technology (NIST) may also be desired or required. If the inspection work in production is done mainly by measuring, it is still necessary to perform random sample inspection with standardized gages.
When purchasing precision fixed limit thread gages, it is imperative to select a quality gage. Some important general features and benefits to look for in a thread gage include:
- Aged gage steel that is stable and true-to-dimension
- Hardness that exceeds the standard minimum requirements
- Extra high-wear resistance
- Availability of standard and special tolerance models
- Inspection certification (issued by an independent or in-house inspection lab)
- The ability to laser-mark custom specifications on gages
- Incomplete threads that 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, and improved gripping
- Reduced thread start for easy insertion of the NO-GO gage body
Gages for Internal Threads
The GO thread plug gage and the NO-GO thread plug gage are used for the gaging of internal threads. GO and NO-GO plug gages are mounted on a common handle for thread diameters up to 40 mm and are designated as GO/NO-GO thread plug gages.
With a full thread profile along its thread length, the GO thread plug gage checks the "virtual size" of the internal thread and the proper screw-in fit. It checks the smallest size of the internal thread pitch diameter D2, including certain deviations in the thread, such as pitch and thread profile angle deviations. It also checks the smallest size of the major diameter.
The NO-GO thread plug gage checks whether the actual pitch diameter of the workpiece internal thread exceeds the largest specified size.
The GO/NO-GO thread plug gage is a combination of 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 40 mm in DIN 2280. It provides the functionality of both the 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.
Analog and Digital Thread Depth Plug Gages
GO/NO-GO thread depth analog and digital plug gages permit the gaging of 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.
One of the advantages of using a thread depth plug gage is that it can be easily adjusted to a number of variants without the use of auxiliary tools. Smaller contours on the workpiece surface, like countersunk edges or elevations, and small dimensional variations of different GO gage bodies can also be compensated for with an easy adjustment. ME
This article was first published in the September 2013 edition of Manufacturing Engineering magazine. Click here for PDF.