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 bit A tool bit is a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool, as distinguished from other cutting tools such as a saw or water j ...

s and

centers Center or centre may refer to: Mathematics * Center (geometry), the middle of an object * Center (algebra), used in various contexts ** Center (group theory) ** Center (ring theory) * Graph center, the set of all vertices of minimum eccentrici ...

. 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

s on a

bench grinder A bench grinder is a benchtop type of grinding machine used to drive abrasive wheels. A pedestal grinder is a similar or larger version of grinder that is mounted on a pedestal, which may be bolted to the floor or may sit on rubber feet. These t ...

, although they may be used with

tool and cutter grinder A Tool and Cutter Grinder is used to sharpen milling cutters and tool bits along with a host of other cutting tools. It is an extremely versatile machine used to perform a variety of grinding operations: surface, cylindrical, or complex shapes ...

s. When the tool bit has been ground to the correct angle, they are then used to set the tool

perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It ca ...

to the workpiece. They can incorporate a range of sizes and types on the one gauge, the two most common being

metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement * An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement Mathematics In mathem ...

or UNC and UNF at 60°, and

BSW British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world's first national screw thread standard, and is the basis ...

at 55°. Gauges also exist for the

Acme thread form Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are r ...

References

Dimensional instruments Metalworking measuring instruments {{tool-stub