Thread Ring gages are important tools for measuring threaded parts and ensuring that they meet tolerances. There are many different types of thread gages, each of which has a specific application and part of the thread that it measures. In this article, our focus will be on the thread ring Gage, which measures Male (outside diameter) threads – the specialty of Superior Thread Rolling.
Pitch Diameter
Before diving into the details of Thread Ring Gages, let’s briefly review the definition of Pitch Diameter, which is the essential basis for matching up male and female threads to properly fit with each other. The Pitch Diameter is defined as the diameter of an imaginary cylinder which intersects the thread such that the width of the groove between threads is equal to the width of the thread ridge along that cylinder.
Functional Diameter
That imaginary cylinder diameter we call Pitch Diameter plus the real-world cumulative effects of imperfect threads like Lead Error, Flank Angle Error, Out-of Straightness, Taper, and Out-of-Roundness combine to form the Functional Diameter. Each of these imperfections in the thread add to the functional diameter. The size of the Functional Diameter of a male thread will determine whether the part will properly mate and function with the female thread.
A Thread Ring Gage is manufactured to exactly represent the “worst case” conforming nut or mating piece female thread. A Ring Gage, when properly set, will act as a functional check to ensure the male thread will fit all conforming nuts or female mating pieces.
Selecting the right Gage
Before learning how a thread ring Gage works, ensuring you have the right Gage is the first thing that you should do.
First and foremost, make sure that the Gage size is aligned to the thread size. It’s important to match up the nominal size, thread form, pitch (threads per inch), and fitment class.
For example, the thread gages shown above are for a 22mm thread with 1.0mm pitch, and fitment class 6g. Ensure that information matches exactly with your blueprint requirement or you could cause a false negative.
Thread Ring Gages
Thread ring Gages are meant to measure the functional diameter of an external thread. It has two parts, a GO Gage and a NO GO Gage, which are both necessary for checking the thread. The GO ring Gage is meant to screw onto the part, while the NO GO Gage should not go onto the part for more than 1-2 turns at the start of the thread.
The GO and NO GO Pitch Diameters should be listed directly on the gages being used. It is common convention for many calibration technicians to use Red wax when locking the NO GO calibration, and Green wax when locking the GO Gage calibration. Additionally, many gage manufacturers will machine a groove through the knurling around the outside of the NO GO gage so it can be easily distinguished.
GO Ring Gages
The GO thread ring gage, if properly sized, can be screwed on by hand with only “2 finger pressure” using your thumb and only one finger and will screw all the way to the end – if the thread is conforming. If the GO Gage does not screw on the entire length of the part, then the Functional Diameter of the thread is too large and it will not function properly.
You will need to investigate further by measuring individual thread elements to determine why the GO Gage will not go. Pitch Diameter, Lead Error, Flank Angle Error, Out-of Straightness, Taper, and Out-of-Roundness all contribute to Functional Diameter. Once the underlying measurement causing the problem is determined, investigate for the Root Cause of that problem and take Corrective Action. “GO Gage will not Go” is not a complete investigation.
NO GO Ring Gages
The NO GO ring Gage ensures that the external thread size is not too small. If the thread is sized correctly, then the NO GO Gage will, when screwed on by hand without use of excessive force, only screw on one or two turns. If the NO GO Gage screws on any more than two turns, then the thread is too small and other instruments will be needed to determine the cause.
In order for the functional diameter of the thread to be sized correctly then, the GO Gage should screw on the part all the way to its end, while the NO GO Gage screws onto the threads 1-2 turns Max.
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