The taper gauges (also called taper rules) do not measure tapers! The principle is dead simple: A thin, tapered piece of flat stock is pushed into a hole (or slot, or other gap) as far as it will go, and dimensions engraved on the tool indicate its width at that point. They do not allow â€œexploringâ€?a hole for taper, but they are a quick and easy way to establish the diameter at the entry, and might even give an indication of any out-of-roundness at this point.
Two versions of this tool exist: Those that taper in width and those that taper in thickness. The first type may be graduated in either fractional or decimal inch units or in metric units. A set of four fractional inch rules measures from 1/16 to 2 1/16 inch in 1/64-inch increments; decimal inch taper rules come in sets of eight (0.100- to 0.500- inch) and 10 (0.500- to 0.100- inch), with 0.001-inch divisions. Equivalent metric reading versions, with 0.02-millimeter markings, span from 2 to 25 millimeters. The second, thickness tapering kind is calibrated in 0.001-inch increments on one face and 0.05-millimeter units on the other. The overall range of this tool is 0.010 to 0.150 inch /0.3 to 4.0 millimeters.