Depth gages are among the simplest of gages, typically consisting of an indicating device mounted through a reference bar or plate. Though they may be simple, depth gages are used in thousands of critical applications to measure the depth of holes, counterbores, slots, and recesses, as well as heights or locations of some features. They are especially common in the tool and die industry. Like other hand tools, they have undergone a gradual change from mechanical scales to digital wonders.

The first depth gages consisted of a simple rule with a sliding perpendicular beam as the reference. The scale was set into the hole and the slide squared up with the reference surface. The depth of the hole was then read directly from the scale itself. This was a simple tool, requiring the operator to employ good judgment and proper technique.


To ensure consistent measurements with any kind of depth gage, it is important to adhere to some basic ground rules:

Make sure the cross bar or reference head is clean, flat and free of nicks and burrs.

Hold the manual gage flat and square to the reference surface. Any out of squareness of the head to the surface introduces error.

If you aren’t careful, you may be measuring along the hypotenuse of a triangle instead of the actual depth of the hole. 

Vernier and Micrometer-based Depth Gages

As the demand for higher resolution and precision increased, these early gages were largely replaced by vernier and micrometer-based depth gages. Over the years these basic verniers and micrometers have in turn spawned a wide variety of offshoots that make the basic measurement of depth pretty easy.

When vernier calipers came along, someone had the idea to add a rod or bar to the moving contact. And all of a sudden the basic caliper increased its measuring capabilities, adding depth to its ID, OD, and distance repertoire.

The moving rod on the end served as a long probe, which made measuring the depth of small holes fairly easy. However, the rod was so small in diameter and so long that in some cases it presented too much flex and would influence the reading. Another option was added: the depth rod version, which was heavier by design and offered a more robust solution. The problem with this was that the reference—the end of the caliper itself—became even narrower, and less effective as a stable fixturing point. Thus, as with calipers themselves, these depth gages are fairly low accuracy measuring instruments with lots of operator influence.

To address this one weak spot a new series of digital calipers, built around the reference anvil, was designed specifically for measuring depth. These used the same concept of having the movable jaw for measuring depth, but made the reference jaw into a wide reference anvil. This provided a much more stable foundation for measurement.

Of course, not all holes, slots and grooves are created equal. Thus there are many possible sizes and depths and many options for cross beam extensions and contacts to get into the smallest of holes. Offset contacts are available to measure the inside surface of the groove with the use of a hook contact.

To ensure consistent measurements with any kind of depth gage, it is important to adhere to some basic ground rules: Make sure the cross bar or reference head is clean, flat and free of nicks and burrs. Hold the manual gage flat and square to the reference surface. Any out of squareness of the head to the surface introduces error. If you aren’t careful, you may be measuring along the hypotenuse of a triangle instead of the actual depth of the hole.

The next level of accuracy above the caliper is the micrometer. Using the same concept and the same style contacts and accessories, the micrometer can also be turned into a very capable measuring system for hole and groove depths.

Indicator Depth Gages

While both verniers and micrometers remain in wide use, indicator depth gages provide even higher levels of accuracy, as well as increased speed of operation and lower dependence upon operator skill.

As with almost all indicator gages, depth gages can be readily modified to suit particular application needs, especially to make high-volume gaging tasks quicker. Depth gages are available with various styles of indicators, contact points, and bases.

The simplest and most common depth gage has a flat base or anvil, a sensitive contact that retracts flush with the base, and a radiused contact point. This is an absolute gage, measuring the full depth of a feature, from zero out to the indicator’s maximum range. No master is required: to zero the gage simply set the base on a precision flat surface.

Different contacts can be used to tailor the gage to special applications. For example, by replacing the standard radiused contact with a needle-style contact, it is possible to measure surface pits, small holes and recesses, and etched depths.

Extended contact points can be added to measure greater depths, or to turn an absolute-measuring gage into a comparative gage. Such a gage can be mastered with gage blocks by holding one end of the base firmly on top of the stack, with the spindle as close to the stack as possible without interference. Special depth masters, however, are quicker and more reliable, and are thus more practical for production gaging applications.

Special bases can also increase gaging efficiency. Counterbores may be gaged more easily if the indicator is offset from the centerline of the base. V-shaped bases are useful in applications where a standard flat base would interfere with the user’s ability to locate a needle-type contact in a small feature, such as a pit or an etched line. The V-base provides a wider viewing angle, but still has a narrow “flat” on the bottom to help orient the gage perpendicular to the part surface. The user first tips the V-base on the workpiece surface, locates the contact point in the feature, then “rolls” the gage upright until it rests on its flat.

Custom anvils can be readily designed to conform to the shape of the workpiece. Take, for example, the aerosol can. This is a metal part that is literally under pressure, and so is more liable to potential failures than most types of containers. The depth of the crimp groove is a critical quality dimension that must be carefully monitored. Depth gages designed for this application with special bases that rest securely on top of the can have proven themselves ten times faster in use than generic vernier depth gages.

Benchtops and Other Options

All of the gages described above are portable, or handheld designs, which implies bringing the gage to the workpiece. It is often convenient, however, to bring the part to the gage, especially if the part is small. Benchtop depth gages essentially turn the portable gage upside-down, and provide a wide flat reference surface—virtually a table—upon which the workpiece can be placed and manipulated. Parts can also be “explored” for flatness with this type of gage, by sliding the workpiece around on the table.

Users can also choose among indicator styles. Long-range indicators, with revolution counters, can measure depths from 0” to several inches (or their metric equivalents). Special indicator faces can be designed for “stoplight” gaging, with green, yellow, and red segments to quickly signal good, marginal, and out-of-tolerance parts. Indicators with “push-down” movements allow users to locate the contact point against the workpiece more positively than is possible with conventional “sprung-down” indicators. Gages can also be equipped with digital electronic indicators, providing opportunities for dynamic measurements (such as automatic capture of minimum or maximum readings), and data output.

Measuring Depth with Height Gages

With its long range of motion, the digital height gage can be thought of as a giant caliper, but one with the accuracy of a much more sophisticated bench top gaging system. As such, digital height gages are extremely versatile and capable of performing a wide range of measuring tasks quickly and reliably. Digital height gages consist of the base plate, the height measuring station and the control/evaluation display. The measuring station is suited for one-dimensional coordinate measurements in a vertical direction and is therefore used primarily for determining diameters and distances between points on the test piece. And with the right set of contacts they are a fast and reliable way to measure depths.

One would think that with gages capable of measuring up to 1000mm/40”, measuring depths would be a little overkill for these powerhouses. But in reality most height gages are used on measurements that fall into the 300mm/12” range—just the place where most of our depth gage requirements usually occur.

Typically a height gage is provided with a general-purpose ball probe. These are great for heights, distances and diameters, but not good for depth measurement. For depth, a right angle probe should be used, preferably the style with an adjustable contact that can be set to different depths.

In use, the contact is mounted in the probe holder and zeroed. But it is not zeroed on the reference plate as normally done but rather on the top of the part. Then the height gage is slid over so that the probe can now reach down into the hole until it touches the surface to be measured. Since the height gage was set to zero on the reference face it will read directly the depth of the hole.

Other probes and contacts are available with hooks, so that the top surfaces of grooves can also be measured for their depth relative to the top surface.

When Depth is a Diameter

When measuring tapers, such as those found on machine tools and tool holders, the idea of measuring depth is probably not the first thing to come to mind. But by measuring depth we can get a pretty good indication of how the tool holder might fit into its mating spindle.

Air gaging is an ideal way of checking the rate of taper on such parts. Because air tooling is made to closely resemble the mating part, it compares the deviations from the part to an ideal condition. In some applications it is important to measure just the rate of taper, while in others, it’s also important to check part diameters at the same time.

When only checking rate of taper, a “jam” fit air tool is employed. This air tool acts exactly as its name implies. It is a tapered piece of tooling that contains two air circuits, which measure taper diameters at a known distance apart. Because there is no reference surface, the tooling goes into the part as far as possible. If the clearance between the two diameters is the same, then the part taper matches the taper machined onto the tooling. If the taper angle is different, the readout will “see” a variation in the two diameters signifying an incorrect taper angle.

Sometimes, it’s also important to measure the two diameters for size in order to control how deeply the two parts fit together. For example, a tool holder that is undersized would have too much clearance between the mating parts, resulting in a cutter that would be loose under the stress of machining. An oversized condition would prevent the tool from being pulled to the reference surface. This could potentially cause parts to be machined at the incorrect size. Air tooling for this application is called “clearance fit” air gaging, because a reference stop is included in the design. By using the reference stop, the same two-circuit air plug can measure the taper and two diameters at a specified location.

But there is a hybrid to this type of air tool, which employs a depth gage. By using the standard jam style air plug and building in a depth gage to measure how far into the mating taper it goes, you can tell something about the size of the taper. If the taper angle is good and the diameters are a little large then the air plug will fall further into the hole. If the diameters are smaller then it will not go in as far. By using a depth gage to measure this, the operator has an indication of the relative size of the taper.

Typically a dial comparator is used like a dial depth gage to indicate how far the plug is going into the taper. By adding tolerance limits to the comparator, the taper could also be assessed for size conditions. If the dial comparator is replaced with an LVDT and a processor that can use the air plug to measure the angle and the depth of the tool, a calculation can be made for the hole size.

 Whether its IDs, ODs or lengths of depth, there are many choices of gage and different levels of performance available. The choice as always is dictated by numerous factors including speed, operator skill, tolerance, budget and ease of use. Selecting the right gage for the job takes a lot of deep thinking.