, the left image is a fishtail gauge
, the right image is a center gauge
Both gauges are used in metalworking and serve the same purpose, for checking the angles when grinding the profiles of lathe screw cutting tools. They are most commonly used when hand grinding on a bench grinder although they may be used with tool and cutter grinders.
When the tool bit has been ground to the correct angle they may then be used to set the tool perpendicular to the workpiece.
They can incorporate a range of sizes and types on the one gauge, the two most common being metric or UNS at 60°, and BSW at 55°. Gauges also exist for the Acme Thread Form