Definition of Plug Gauge

Plug gauge, also known as pin gauge, is a metrology tools which is used to measure the inner diameter of a hole that has been drilled or machined into a manufactured part, component, or assembly. This is an essential tool for quality control, and it enables the inspectors, machinists, manufacturing engineers, and others to quickly obtain a go/no-go decision about whether the inner diameter being measured falls within the specified dimensional tolerance. This measuring process is designed to allow accurate assessments to be made without the need for the production personnel to use an addition of more sophisticated and/or more expensive measuring instruments such as inside micrometers. In production operations, where a large volume of parts is being produced, plug gauges provide a rapid means of assessing the quality without taking time to perform actual hole diameter measurements and to set-up instrumentation.

When using plug gauges, it is important to acknowledge that the results only apply to a yes/no condition – the tool allows the quality control inspector to either accept or reject a part. It does not perform an actual measurement of the diameter, but merely assesses if the diameter is within the tolerance range established for that parameter.

Types and Uses of Plug Gauges

Plug gauge is available in several different types. These include:

Go plug gauge

No-go plug gauge

Combination of go/no-go plug gauge

Go plug gauges and no-go plug gauges are also known as the single-ended gauge. It forms with a precisely machined pin or shaft (sometimes called a member) inserted into a handle and it is used to verify an aspect of the dimensional tolerances of a hole. For single-ended gauges, they are usually offered in pairs with one go gauge and one no-gauge gauge.

In its use, a go plug gauge is designed to test the minimum dimensional limit of the hole (i.e. the lower tolerance of the hole’s diameter), which indicates the maximum amount of remaining material on the part or workpiece. For an acceptance inspection, the go plug gauge should fit in the hole. If the go plug gauge did not fit into the hole in the workpiece, then that is an indication that the hole’s diameter is too small (an insufficient amount of material has been removed from the workpiece) and the part needs to be rejected.

For a no-go plug gauge, its purpose is to validate the maximum dimensional limit of the hole (the upper tolerance of the hole’s diameter) which corresponds to the minimum acceptable amount of material remaining on the part or workpiece. For the part to be accepted, the no-go plug gauge should not fit into the hole. If the no-go plug gauge did not fit into the hole, then this is an indication that the hole’s diameter is below the upper limit and the part can be accepted (provided that the go plug gauge did fit into the hole). If, however, the no-go plug gauge fits into the hole, then that is an indication that the hole’s diameter is too large (i.e. beyond the upper tolerance limit that was specified), meaning that too much material was removed during the machining operation, and therefore the part must be rejected.