Fishtail-Center gauges, 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. Metal lathe or metalworking lathe are generic terms for any of a large class of Lathes designed for precisely Machining relatively They are most commonly used when hand grinding on a bench grinder although they may be used with tool and cutter grinders. A bench grinder or pedestal grinder is a machine used to drive an abrasive wheel (or wheels A tool and cutter grinder is used to sharpen Milling cutters and Tool bits along with a host of other cutting tools
When the tool bit has been ground to the correct angle they may then be used to set the tool perpendicular to the workpiece. The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, Shapers and planers. In Geometry, two lines or planes (or a line and a plane are considered perpendicular (or orthogonal) to each other if they form congruent
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°. The Unified Thread Standard ( UTS) defines a standard thread form and series—along with allowances tolerances and designations—for Screw threads commonly Gauges also exist for the Acme Thread Form