From the Manufacturer
The purpose of these tools is to measure the depth of holes, slots, shoulders, and projections quickly and easily.
The Starrett 449 Vernier Depth Gauges, Micrometer Type, Inch, depending on the model, have a measurement range of 0 to 3" or 0 to 6", a resolution (or smallest graduation interval) of 0.001", with + or – 0.0001" of accuracy. The vernier scale enables precise measurements in fine units. The gauges in this series have non-rotating blades, or flattened probes, that are 1/64" wide by 1/8" long, and can be used to measure narrow recessions without the blade slipping off of a surface. These units include measuring rods to cover the intended range. They come with a ratchet and speeder to enable the users to apply uniform pressure and make quick adjustments for precise measurements. Depending on the model, the bases are either 2.5" or 4" long and all measuring surfaces of these units are hardened, ground, and lapped for durability and data accuracy. These units include a protective case. Depth gauges are typically used in manufacturing, machining, and mechanical engineering.
Depth gauges are precision measuring instruments used to determine the depth of any shape that has a step, groove, or slot. These gauges often take the form of sliders that travel on a measuring beam or rods, are moved to position by hand, and then adjusted with a calibrated screw. The base of the gauge rests at the top of the shape whose recession is to be measured, while the beam or rods extend to the bottom of the recession to determine the measurement. Gauge position is read from a graduated scale, dial, counters, or an electronic display.
The L.S. Starrett Company manufactures precision measuring tools, metrology and testing equipment, and saw blade products. The company, founded in 1880, is headquartered in Athol, MA.