How to Use a Thread Gauge
A thread gauge is used to conduct a qualitative test on components manufactured against a given thread standard. The most common machine screw thread in Europe is a metric coarse ISO thread - for more information on how this thread type is defined, please see our ISO coarse thread lookup table.
Thread gauges come in a variety of types and sizes, and are available for most major thread standards. As a concept, a thread gauge is relatively simple, the gauge is fitted to a mating thread to confirm that the thread of the component matches the thread of the gauge. This is a relatively easy-to-understand concept, but the vast range of thread gauges available can cause confusion. A brief summary of thread gauge types can be found below:
A leaf gauge is a thread gauge with a similar design to a Swiss army knife. The advantage of a leaf gauge is that a single tool can be used to measure a large range of thread pitches, which also allows it to be used in instances where several thread diameters share the same pitch.
The leaf-style design of this type of gauge allows it to be used quickly and efficiently, but the major disadvantage of a leaf gauge is that it is unable to measure the full circumference of a thread.
Ring gauges are used to measure male threads, such as those found on screws and bolts. A ring gauge is used for its ability to precisely confirm both the diameter and pitch of a screw. Ring gauges are considerably more reliable and accurate than leaf-style gauges.
The main disadvantage of a ring gauge is that it is not as versatile as a leaf gauge as it cannot measure a range of sizes. To measure multiple thread sizes using this type of gauge requires several sizes of ring gauge to be used.
Plug gauges are used to measure female threads, such as those found within nuts. A plug gauge is useful for use in blind holes where a leaf gauge cannot be used, or for particularly small threads where a leaf gauge is inconvenient.
A ‘go’ gauge is designed to fit with conforming components. With male threads, a ‘go’ ring gauge is oversized, and shows the upper bound of the required tolerance, and for female threads, a plug gauge is undersized and shows the lower bound.
No Go Gauges
A ‘no go’ Gauge is designed to deliberately not fit with conforming components; if a component fits with a ‘no go’ gauge, it is out-of-spec. For male threads, this type of ring gauge is undersized to show the lower bound of the required tolerance, and for female threads, a plug gauge is oversized to shows the upper bound.
Use with Soft Materials
When a ring or plug gauge is used to test delicate components such as plastic screws, the gauge can easily be forced over ill-fitting threads, which can give inaccurate readings. To prevent this, the gauge can be used in reverse as a preliminary test. When the gauge is turned counterclockwise (or clockwise for left-hand components), smooth motion and a slight ‘click’ as the gauge runs over the start of the thread shows conformity. Once this test has been performed, the thread can be tested as normal.
Does a Go Ring Gauge Fit With a Go Plug Gauge?
No. Plug gauges and ring gauges are not designed to fit together, even if both devices are ‘go’ gauges. This is because threaded components, such as screws or nuts, are generally designed to be oversized or undersized to ensure compatibility with their mating components. Thread gauges, on the other hand, are designed to be a perfect fit, which means that when the two are used together, they do not have the inherent spacing which can be found in the components they are designed to test. Using a plug gauge alongside a ring gauge creates close to 100% thread engagement, which creates enough friction to prevent the two gauges from fitting together.