A Closer Look at the Instruments Involved.
Any article on dimensional measurement will usually get around to micrometers in one form or another. This series on bore measurement is no exception even though I am only offering a brief look at the application.
These instruments are also known as ‘stick’ or ‘tubular’ micrometers as their overall appearance looks like a stick or rod—a mechanical, expensive one, that is. They are unique in that they usually have a one inch or 25mm range and very large capacity. Some use precise spacers or interchangeable rods to cover their capacity.
The tubular versions have reduced weight for ease of handling when dealing with larger measurements. All things considered, these micrometers offer a lot of capacity for their cost compared to others.
Their axis of measurement is the same as the micrometer head incorporated in them which is a golden feature for accurate measurement. The precision of this advantage is reduced however, due to the manipulation required to produce repeatable readings and the use of interchangeable anvils and/or spacers that enable them to cover very large capacities.
Of course there’s an upside to those interchangeable bits and pieces when it comes to calibration of the instrument. The bits and pieces can be calibrated more easily than a large capacity but limited range instrument of similar design.
Inside micrometers require setting and some makers offer jigs for this purpose that employ gage blocks. But, as is mentioned endlessly, it’s hard to beat a setting to get it right.
In use, they have to be adjusted while being carefully positioned in two planes to ensure they are contacting the bore diameter correctly. An indicator is included in some designs to make this centering/centralizing function easier. Obviously this means measurements using them require a skilled person with adequate time to do the job.
THREE POINT INSIDE MICROMETERS
The instruments I’ve commented on up to this point have a single common feature: they measure a bore across a diameter or between two points 180° apart. In this section I discuss those instruments that have three measuring contacts spread 120° apart that contact the bore.
These contacts or anvils contact the bore along the short length of the anvils. The anvils are precisely fitted into the instrument body and are spring loaded against the conical end of a pin that causes it to move in relation to a change in diameter and show that change on the indicator or micrometer head fitted to the instrument.
Despite the mechanics involved, these instruments are available for small diameters and can have a measuring range up to one inch or 25mm. The anvil configuration and shape means the instrument will self-center and does not need to be rocked in the bore to ‘find’ its diameter. This enables lower skilled users to obtain better results than other devices requiring skilled manipulation of the instrument to get viable readings.
There are other advantages available with this type of instrument. One is the length of the contact anvils that enable measurements of bores in thin parts or sheet metal to be measured. The contacts of other devices would normally pop out of such bores when those instruments are rocked to find size. Another advantage lies in their measuring range that is particularly useful in bores having short lands of differing diameters. Some makers offer simple modifications to the anvils enabling them to measure groove diameters inside other bores.
The benefits of these instruments create some problems however. Their anvil length means they are contacting the bore wall on a line compared to other types that have point contact. They are most accurate when the full length of the anvils is contacting a bore, making diameter measurements at the mouth of a bore a bit iffy at times.
Three-contact diameter measurements are able to detect significant bore roundness variations that remain immeasurable by two-point systems. This can lead to arguments over ‘size’ between the two methods.
Setting of these instruments requires a plain ring gage that is often—but not always— provided by the maker as standard. Calibration of them requires two or more rings to span the range of each instrument to ensure it is providing accurate readings over that range.
The next column in this series will look at more accurate methods for measuring bores when work volume can justify the expense.