Types and Uses of Plug Gauges

Plug gauges are available in several different types. These include:

Go plug gauges

No-go plug gauges

Combination go/no-go plug gauges

Go plug gauges and no-go plug gauges are called single-ended gauges and consist of a handle into which a precisely machined pin or shaft has been inserted (sometimes called a member) and which is used to verify an aspect of the dimensional tolerances of the 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 represents the maximum amount of remaining material on the part or workpiece. For an acceptance condition, the go plug gauge should fit in the hole. If the go plug gauge will 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. Table 1 below summarizes these conditions and the resulting accept/reject status of the part.

In a combination go/no-go plug gauge (also called a double-ended plug gauge), the gauge handle has a go plug gauge extending outwards from one side of the handle and a no-go plug gauge extending outwards from the opposite side. This simplifies the use of the tool and allows for increased speed in the inspection process. In some models, the handle may be color-coded with the go side painted with green paint and the no-side painted with red paint. Even without color-coding, for plug gauges, the go plug gauge can always be distinguished from the no-go plug gauge by two observable conditions:

The go plug gauge is always smaller in diameter than the no-go plug gauge, and

The go plug gauge is always larger in width (thickness) than the no-go plug gauge

Plug gauges provide fast results and have the advantage that users can be easily trained on their use as plug gauges rely on a simple working principle. As a result, no strong knowledge of scientific methods and metrology is needed, making training simpler and open to a wider pool of users. Their simplicity also means that they are inexpensive relative to other forms of measuring instruments.

One drawback of plug gauges is that through use they tend to wear out over time and therefore require replacement. They should also be checked periodically to assure that they are still within tolerance.

Plug gauges that are used for the accept/reject screening of parts are called working gauges.