Metrology
Metrology
Depth Gages
EN-US

Depth Gages
George Schuetz. Mahr Federal Inc.

 Depth gages are among the simplest of indicator 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.

 The first depth gages consisted of a simple rule with a sliding perpendicular beam as the reference.  As the needs for higher resolution and precision increased, these were largely replaced by vernier devices and micrometer depth gages.  And 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 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, 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 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.  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.

 All of the gages described above are a portable or hand-held design, 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.  Bench top 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.

 Rules for depth gage use are straightforward.  Both workpiece and anvil must be clean and free of burrs.  For portable gages, the base must be held firmly against the workpiece, and it must be positioned flat and square.  As with all indicator gages, accuracy also requires a rational mastering schedule, the frequency of which depends upon the amount of use, as well as the conditions in the gaging environment.