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A depth gage is a very common hand tool used to inspect the depth of holes, slots, counterbores, recesses or the distance from one surface to another. 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.

In the beginning, a depth gage was simply a scale with a sliding head. 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, but it did not absolve the operator of his responsibility 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, so if you aren’t careful, you may be measuring along the hypotenuse instead of the actual depth of the hole.

As tolerances increased, the depth scale gave way to the vernier depth gage. This type of gage took longer to read, but with some training and experience, users of the vernier depth gage benefited from greatly improved resolution.

Both the scale and vernier type depth gages need to be set to zero. This is done by placing the measuring head on a flat surface, such as a surface plate, and moving the sliding arm or contact to the same surface. If the reading on the tool is not zero, it should be adjusted so that it is. Unfortunately, the sliding members of both the scale and vernier depth gage are quite large and are not suitable for measuring small holes (for example, sizes of 1/4 inch or less).

Another improvement was the micrometer depth gage. It uses the barrel of the micrometer as the measuring leg and allows for entry into smaller holes. Micrometers provide for absolute rather than comparative gaging. Micrometer depth gages achieve very good resolution over their entire range; however, the range itself is limited. Therefore, for deep holes, an extension and a master are required. When using extension rods, always remember to add the length of the extension rod to the measurement value shown on the micrometer.

The digital caliper also may be used for making occasional depth measurements. Most incorporate a depth extension as a standard feature. The depth extension may be square or round, and the style of choice is determined by the hole size you need to measure. For the smallest of holes (less than 2 mm), round is the way to go.

The only problem with using a caliper as a depth gage is that it’s not what the gage is really designed to do. It is very difficult to align and hold the caliper square while making the depth check. The end of the caliper is just not designed for stability.

Of course, the fastest and most accurate tool for checking depth is the digital depth gage. Made in various sizes, digital depth gages may be fitted with a number reference for covering larger diameter spans. There are even extensions for spans up to 300 mm/12 inches. Digital calipers are also self-supporting on the part and have sufficient width for good stability.

There are a number of different configurations for the actual depth contact. One version of these gages uses a small, fixed contact on the end of the slide to measure the depth. This is fine for general purpose applications where the opening is large. Another style uses a rod with a replaceable contact. This offers a lot more versatility, allowing the user to change contacts according to the surface that needs measuring.

The moral of this story: There are many ways to check a depth dimension on a part. However, if speed and accuracy are important, use a tool that was designed for those purposes—today it’s the digital depth gage.