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 Gage Guide

Choose the Right Tool for Efficient Bore Gaging

Several factors should be considered when choosing a gage for a particular application.

by Drew Koppelmann

There are dozens of ways to make a hole to a particular size in a piece of metal: drilling, boring, broaching, grinding, turning, honing, laser machining -- and on and on. No matter what the process, the hole will probably have to be inspected for dimensional accuracy with a bore gage. The efficiency and economy of the inspection process depends upon selecting the right type of gage for the job.

Two basic types of indicator gages can fulfill almost all bore gaging needs. The fixed-size plug gage is a machined cylinder that fits tightly into a bore of a particular size, with maximum clearance of just 0.0030". Two diametrically opposed sensitive contacts are almost entirely enclosed within the body of the plug; only their very ends are exposed. An internal mechanical linkage connects the contacts to an electronic indicator.

The adjustable, or rocking-type bore gage has a more "skeletal" design and fits more easily into the bore. A pair of fixed contacts serves as a reference, and a single, relatively exposed sensitive contact is located opposite them. Keeping the fixed contacts hard against one side of the bore, the operator "sweeps" or "rocks" the gage back and forth, causing the sensitive contact to extend or retract. When the contacts are oriented exactly perpendicular to the bore's axis, the sensitive contact is depressed furthest, thus indicating the bore's true minimum diameter. The results are read on either a dial or an electronic indicator.

Several factors should be considered when choosing a gage for a particular application. The first is accuracy. Being nonadjustable, plug gages tend to produce more repeatable results at higher levels of discrimination. Put simply, that means they can reliably measure tighter tolerances than adjustable bore gages.

Because they remain centralized at all times, plug gages can be rotated in the bore, or slid lengthwise, to "explore" the bore for dimensional irregularities such as out-of-roundness, taper, barrel shape or hourglass shape. Adjustable bore gages lack this capability.

On the opposite side of the coin are the considerations of range and adjustable capacity. Rocking-type gages can typically measure tolerances as broad as +0.010", compared to just +0.0015" for plug gages. This makes rocking gages more practical for measuring parts in process, and for final inspection in applications where production tolerances are relatively coarse.

Rocking-type gages are adjustable; they can be set up and mastered to measure a wide range of nominal sizes. So where a 1.000" plug gage can only be used to measure bores with a nominal inside diameter of 1.000", a rocking-type gage can measure a 1.000" ID today, a 0.500" ID tomorrow and a 0.750" ID the next day.

The two types of gages cost roughly the same on initial purchase (typically between $400 and $600), but their economies of use are quite different. In large-volume applications, the plug gage promotes much higher throughput and is therefore more economical. It only needs to be inserted in the bore -- no rocking is required, and reading is virtually foolproof. The adjustable bore gage, on the other hand, requires a fair amount of skill and care to obtain an accurate reading. Hurried or inattentive use of a rocking-type gage leads to inaccurate results.

In low-volume applications, where throughput is not so critical, the adjustable bore gage may be more economical. It rarely makes economic sense to purchase a fixed-size gage if only a few parts of a given dimension are to be inspected. With a selection of just three adjustable gages, with capacities of 0.5" to 1", 1" to 2", and 2" to 8", almost any bore can be inspected.

No one gage is best in all possible applications. Pick the bore gage that best suits your immediate needs, but don't hesitate to change gage type if the application changes.


About the author

As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries, including automotive, aerospace, packaging and electronics. He may be reached by fax at (401) 784-3246.

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March 97 Quality Digest