Fishtail Gauge

As explained by Wikipedia:

Center gauges and fishtail gauges are gauges used in lathe work for checking the angles when grinding the profiles of single-point screw-cutting tool bits and centers. In the image, the gauge on the left is called a fishtail gauge or center gauge, and the one on the right is another style of center gauge.

These gauges are most commonly used when hand-grinding threading tool bits on a bench grinder, although they may be used with tool and cutter grinders.

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