As any machinist knows, when you drill precision holes, you should always measure at least the first one or two to determine if the tool you are using is cutting the correct size. Normal measuring tools, say a dial or digital caliper, may work just fine when the hole is over 1/2 inch, but what do you do when the hole is smaller?Today's tool is one that my father used in his machine shop when I was a teenager. In fact, the tool pictured here is the actual one that he used years ago: a set of expanding small hole gages made by Mitutoyo. Each gage has a fixed hole size range that it can expand into and provide accurate measurements. The small hole gage does not directly show the measurement; a micrometer or other measuring tool must be used in conjunction with the small hole gage to get an actual measurement.
In operation, the user selects the correct gage for the hole size that is being measured. The upper knurled part of the handle is grasped with one hand and the expanding split spherical end is placed into the hole. With the other hand, the knurled knob on the end of the tool is turned clockwise or counterclockwise to expand or contract the spherical end until it contacts the hole sides. When the tool is extracted from the hole, a measuring caliper or micrometer is placed over the spherical end and the measurement of the hole is determined from the caliper or micrometer. 
There are several U.S. patents describing this measuring tool, number 1,806,893 issued to H.G. Cunningham of Los Angeles and number 4,136,456 issued to Mineo Yamauchi and Masao Nakahara of Kawasaki, Japan (this patent was assigned to Mitutoyo). Surprisingly, this very precise tool set is still available under the same part number from a number of sources. One of those is from Grainger for around $100. Other stores on the internet also sell this set for slightly more or less. If you drill small holes and need to precisely measure them, get a set of these gages to make your job easier.
