A depth gauge is an instrument used to measure the depth of a bored hole, drilled hole or cut. Often referred to as a depth micrometer, the depth gauge can be a precision-build measuring tool that is able to give the depth reading in a variety of measurements. It also can be as simple as a length of tape wrapped around a drill bit or similar boring tool to indicate when the proper depth has been reached. Occasionally, a depth gauge is nothing more than a tool that is inserted into a hole. It features an indicator marked in red for "no go" and one in green for "go," which is pushed up and out of the tool's handle when the probe touches the bottom of the hole, thereby indicating a properly-drilled hole.
When working on a very critical project, the use of a micrometer-styled depth gauge will give an exact measurement as to the depth of a boring or cut to ensure the proper machining has been completed. This exacting measurement, however, is often lost on a production line-type of machining operation and it is the typical go/no-go type of depth gauge that is commonly used to check a machinist's work. Many machine shops and manufacturing facilities are beginning to use digital measuring devices to make it easier for laborers, who lack the ability to quickly read fine measurements on a micrometer, to check the depth of a drilling, boring or cutting step in a production run.
Understanding the critical aspect of setting a component to the proper depth is crucial in some manufacturing plants. Setting the depth of a pinion gear in relation to a ring gear in an automobile drive axle, for example, is a very critical step in the longevity of the drive axle of the vehicle. Close enough is not good enough in this type of gear installation and the depth gauge must be accurately checked to three or four positions after the decimal point in this type of application. Anything short of perfection will result in a damaged and failed drive gear within a very short distance.
The depth gauge is a critical tool used in many machining processes, from automotive engines to surgical tools and ceramics. Failure to properly read the depth gauge when creating the recess for a ceramic jar and lid will result in a loose-fitting lid or, even worse, a lid that will not fit at all. When used in gunsmithing operations, the proper reading of the depth gauge is depended upon to allow the bullet to sit deep enough in the firing chamber to prevent breech explosions.