What is a depth gauge

A depth gauge is a measuring instrument designed specifically to measure the depth of holes, slots, recesses, and the distance from a reference surface to a step or shoulder. Depth gauges come in various forms: depth micrometers (using micrometer screw mechanisms for 0.01 mm precision), vernier depth gauges (incorporating vernier scales for 0.02-0.05 mm precision), dial depth gauges (with mechanical dial indicators), and digital depth gauges (with electronic readouts). All share the basic design of a flat reference base that sits on the workpiece surface and a sliding probe or blade that extends into the feature being measured.

To use a depth gauge, place the reference base flat against the workpiece surface surrounding the feature, then extend the measuring probe or blade until it contacts the bottom of the hole or recess. The instrument reading indicates the depth from the reference surface to the point of contact. Depth micrometers offer the highest precision for critical applications like valve seat depths, bearing bore depths, and precision machining verification. Vernier and dial depth gauges provide adequate accuracy for most workshop applications while offering larger measurement ranges. Unlike calipers attempting depth measurements (which can work but are less reliable), dedicated depth gauges provide perpendicular probe alignment and stable reference surfaces specifically engineered for depth measurement accuracy. Common applications include measuring keyway depths, counterbore depths, pocket milling depths, and step heights on machined components. For blind holes or recesses with irregular bottoms, ensure the probe contacts the intended measurement point rather than debris or a tool break-out point.