Columns From: 2/17/2009 Modern Machine Shop, George Schuetz
The first depth gages consisted of a simple rule with a sliding, perpendicular beam as the reference. Vernier devices and micrometer depth gages replaced the simple rule gages as the needs for high resolution and precision increased. Today, both verniers and micrometers have been replaced by digital versions, and indicator depth gages provide even higher levels of accuracy, increased speed of operation and lower dependence upon operator skill.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 absolute gage measures the full depth of a feature, from zero to the indicator’s maximum range, without using a master. 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, recesses and etch 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, so they are more practical for production-gaging applications.
Special bases can also increase gaging efficiency. Counterbores can be gaged more easily if the indicator is offset from the center line 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 tips the V base on the workpiece surface, locates the contact point in the feature and “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 an aerosol can as an example. This is a metal part that is literally under pressure, so it is liable to more potential failures than most types of containers. The depth of the crimp groove is a critical quality dimension that must be carefully monitored. The depth gages designed for this application have special bases that rest securely on top of the can, and they have proven themselves to be 10 times faster in use than generic calipers or digital depth gages.
However, when applications call for longer range, various generic digital depth gages have made these types of checks a lot easier. Most caliper-style depth gages typically include a 6-inch caliper and a depth bar. Yet some calipers have been specifically designed as depth gages and have the same type of reference bar seen on dial indicator depth gages. These hand tools start at 6 inches in length and can range to 24 inches or more.
Similar to the dial-indicator version, different contacts are available for such gages to allow for a host of different measurements. These digital-style depth gages are available with three basic contact types. The simplest is nothing more than a 0.060-inch diameter pin. Another style incorporates two “hooks” machined into the measuring beam for measuring away from or toward the reference beam. Finally, the most universal digital depth gage is similar to a dial indicator in that it has an interchangeable contact point. This type of gage is useful for the same applications as the dial indicator version but has the added flexibility of significantly longer range.
All of theses gages are designed to be portable or hand-held, which implies bringing the gage to the workpiece. Sometimes though, it is convenient 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.
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