Fishtail Gauge

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As explained by Wikipedia: Center gauges and fishtail gauges are precision tools used in lathe work to check angles when grinding profiles of single-point screw-cutting tool bits and centers. The gauge on the left is a fishtail gauge or center gauge, while the one on the right features another style of center gauge design. These versatile gauges are primarily utilized during hand-grinding threading tool bits on a bench grinder; however, they may also be used with tool and cutter grinders. Once the tool bit has been ground to the precise angle, these gauges can then be employed to set the tool perpendicular to the workpiece. They come in a variety of sizes and types, including metric or UNS at 60° and BSW at 55°, and some gauges even cater specifically to the Acme thread form.