Fishtail-Center gauge
Overview
 
Center gauges and fishtail gauges are gauges
Gauge (engineering)
In engineering, a gauge or gage, is used to make measurements. A wide variety of tools exist which serve such funtions, ranging from simple pieces of material against which sizes can be measured to complex pieces of machinery...

 used in lathe
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...

 work for checking the angles when grinding the profiles of single-point screw-cutting tool bit
Tool bit
The term tool bit generally refers to 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. The cutting edge is ground to suit a particular machining operation and may be resharpened or...

s and centers
Lathe center
A lathe center, often shortened to center, is a tool that has been ground to a point as to accurately position a workpiece about an axis...

. 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 bit
Tool bit
The term tool bit generally refers to 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. The cutting edge is ground to suit a particular machining operation and may be resharpened or...

s on a bench grinder
Bench grinder
A bench grinder is a type of benchtop grinding machine used to drive abrasive wheels. A pedestal grinder is a larger version of a bench grinder that is mounted on a pedestal, which is bolted to the floor...

, although they may be used with tool and cutter grinder
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 may then be used to set the tool perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...

 to the workpiece.

They can incorporate a range of sizes and types on the one gauge, the two most common being metric
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 or UNS
Unified Thread Standard
The Unified Thread Standard defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada...

 at 60°, and BSW
British Standard Whitworth
British Standard Whitworth is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, the others being British Standard Fine thread and British Standard Cycle...

 at 55°.
 
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