Depth Gauge Types and Basic Use
A depth gauge measures the distance from a reference surface to the bottom of a recess, hole, slot, or step. The correct instrument and method depend on the feature shape, expected depth, tolerance, and required traceability.
Common types
- Rule depth gauge: a graduated rule slides through a flat base. It is quick but suited to relatively coarse readings.
- Vernier depth gauge: a vernier scale resolves smaller increments than a plain rule.
- Dial depth gauge: a contact rod moves a dial indicator for comparative or direct readings.
- Digital depth gauge: an electronic display simplifies reading and may switch units.
- Depth micrometer: a precision screw advances a rod from a reference base; interchangeable rods may cover multiple ranges.
- Depth attachment on a caliper: useful for general checks, but the narrow reference surfaces and rod geometry may not suit every feature.
Basic measurement procedure
- Inspect and clean: remove chips, oil films, burrs, and dirt from the reference face, contact point, and feature.
- Check zero: place the gauge on a clean, flat reference surface and bring the rod into contact. Confirm zero or record the correction.
- Seat the base: keep the base flat against the same reference surface from which depth is defined.
- Lower the contact: advance gently until it touches the feature bottom. Use a consistent measuring force; do not flex the rod.
- Keep alignment: the rod should follow the intended measurement axis. Tilting can increase the apparent depth.
- Read and repeat: record the value and unit, then repeat at least once. For a broad bottom, check multiple positions.
Example
A digital depth gauge is zeroed on a reference block. Three readings in a counterbore are 12.42 mm, 12.44 mm, and 12.43 mm. The mean is 12.43 mm, but the spread also matters. A drawing tolerance must be compared with the instrument specification, calibration status, environment, and measurement method—not only the display resolution.
Error sources
- Burrs or debris preventing the base from seating.
- A rounded or pointed contact that reaches a different surface than intended.
- Using the wrong extension rod or failing to include its offset.
- Excessive force, rod tilt, or measuring near an edge.
- Temperature differences between the part, gauge, and reference.
- Confusing display resolution with measurement accuracy.
Choosing a suitable gauge
Select a range that covers the feature without placing the instrument at its limit. The base must bridge the opening and contact a stable datum. The contact shape must reach the intended bottom. For tight tolerances, use an instrument with a current calibration and an uncertainty appropriate to the decision.
A screen ruler cannot replace a depth gauge because a display provides a flat two-dimensional scale. Use the online ruler only for calibrated, non-critical length checks that can be placed directly against the screen.