What Is a Thread Gauge?

The component production specifications are routinely checked and are often recorded. In terms of production, an array of metrology equipment is on standby to measure diameters, lengths, surface finishes, and more. When producing a threaded component in particular, there are several methods and tools available to verify thread dimensions. For example, acquiring a thread’s effective pitch diameter is achieved by using a thread micrometer, which is fitted with a vee and conical anvil. However, thread wire sets and a standard flat face micrometer may also be used.

Another example of a commonly used tool is a thread gauge. While one may assume that thread gauges are used to measure a thread, they are actually used to check that the thread dimensions fall within an upper and lower dimensional limit. Thread gauges are incredibly accurate, allowing you to meet the required standards specification. Below, we will provide an in-depth look into thread gauges and their uses.

Thread gauges can verify the specific thread form angle, pitch, and diameter, making it a widely used tool for many applications. A plug gauge, for instance, is another type of measurement tool that is used to check internal threads, and a ring gauge is utilized to check external threads. You may ask yourself: “What is a gauge composed of and why does it vary in design?”

A gauge consists of two elements: a GO gauge and a NO GO gauge. Each part should be used to check the thread. If you are machining a component with an external thread, both GO and NO GO ring gauges should be used. The GO gauge will screw onto the part and the NO GO gauge should not be screwed onto the part. For a thread to meet the acceptable dimensions, a GO plug gauge should be screwed into an internal thread and a NO GO should not.

Gauges are available for a wide range of thread forms and sizes, including a 60 degree form, Trapezoidal form, UNF form, and many others. For instance, tapered threads, or NPT and BSPT threads, must be checked with a stepped plug gauge or ring gauge. These gauges check the effective diameter of the tapered thread at a standard length from the component face. When mating components are affixed together, a seal forms on the taper at a given position.

It is important to note that any type of thread gauge is either screwed into or onto the component. Meanwhile, the start of the thread/component face should stop between the upper and lower limit on the gauge. Pre-plated gauges are a subtype of thread gauges that are used to check the thread size before coating is applied to a thread. Different components will necessitate different coating thicknesses, so having this information handy is critical.

To ensure the appropriate gauge is selected, the finished thread size and thickness of the coating is required. This allows you to make a reliable calculation for the new major and minor diameters of the gauge you need to carry out your measurements. Similarly, there are various tolerances of thread gauges, depending on how loose or tight your thread must be. There are DIN standard thread gauges, those of which include tight fit (4H), standard fit (GH), fit with some movement (6G), and loose fit for treatment or coating (7G).

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