This is part 1 of a three-part series. 

 The measurement of inside diameters can be achieved by a number of instruments but the principles on which each is based can make picking the right one elusive. This series of columns will look at a variety of options available to you—a Metrology 101 series for lack of a better description of what is to follow. My remarks assume the hole or bore in question is a machined one with a continuous surface requiring the use of a portable instrument. Unlike my last column on overall considerations to be dealt with when selecting instruments, this series focuses on technical matters with a few random thoughts thrown in for free.
Hole measurements become problematic because of repeatability problems and correlation issues i.e.: results from one method are different than another. The reasons for some of these situations will become apparent as we go through the various choices and the principles behind them.
As their name implies, the business end of these devices is a modified ball at the end of a shank which is split in half along most of its length. The spherical ends are sprung outwards to contact the wall of the bore and act on a pin or needle running up the center of the shank which in turn contacts a dial indicator (usually) attached to the handle that holds both units.
As the diameters involved become larger, the spherical shape morphs into a split disc the edge of which has a precisely finished radius. Blind bores are accommodated by the ball shape or disc modified so the radius is closer to the end of the gage head.
Each gage head has a small size range it is suited to measure and because of the simplicity of the mechanism, they are available to suit very small diameters. Unlike dial bore gages, each head covers its range without the need for interchangeable contacts and extensions.
Accurate measurements using them require setting to a ring that is usually provided when they are supplied in sets where one ring typically covers two heads but can be improved if a setting ring of the nominal size being measured is used.
The shape of the gaging element assists in centralizing during measurement but skill is required on the users’ part to find the correct diameter normal to the bore axis by rocking it in the bore.
Short run precision machining, toolmaking and general inspection work are areas where one or more sets of these gages are kept on hand—especially when a number of hole sizes must be dealt with.
This type of gage has an adjustable contact so the basic instrument can cover a range of sizes. They are a cost-effective choice and can be quite accurate most of the time. However, they are short range comparators and as such, will only be as accurate as the master to which they are set. Some folks use a micrometer to set them but this method is only good for coarse tolerances. A more accurate choice would be to use a gage block build-up in a holder. However, neither of these choices can match the use of a nominal size setting ring as a master.
Their mechanism employs a right angle crank or other device in the foot of the instrument which acts on a rod in the stem on the unit which in turn acts on the indicator. These mechanics can stick and/or wear, affecting repeatability and linearity. There are other systems with better mechanics that employ strip steel or lath spring systems to replace the right angle system noted but are usually limited to larger models.
A certain level of skill is required to manipulate the instrument to find the actual “diameter” of the bore, meaning more time is required to use it. Similarly, a lot of time can be spent setting these gages when sizes are changed. And to make things more frustrating, someone is always losing one or more of the lock-nuts required to keep adjustable contacts in place.
Putting a digital indicator or electronic probe on these devices in lieu of the dial indicator usually supplied with them will not improve their accuracy very much if at all. If you require better precision, a different gage is in order.
The next column will look at some other choices for bore measuring applications.