A center gauge, also sometimes known as a fishtail gauge, is a tool used in machining to check the angle of tool bits used to cut screw threads. It is a small, flat handheld object made of metal, about 2 1/4 inches by 3/4 inch (approximately 57 by 19 millimeters) in size. Triangular notches are cut into the metal at precise dimensions and angles, and these notches are used as templates for shaping the machine tool bit. For the finished screw to function correctly, the threads of the screw must be cut at a precise and uniform angle. The center gauge helps ensure that the tool bit is the correct dimensions to cut these threads.
Screws are made using a device known as a lathe. Lathes work by rotating the work object — in this case, a cylinder of metal that will become the screw — across the point of a tool bit. As the cylinder rotates across the bit, the sharp edge of the bit cuts into the metal, creating scoring in the metal surface. A helical pattern is formed by advancing the cylinder in one direction across the bit as it rotates.
The spiral cut that is created in this process forms the thread of the screw. Threads allow screws to grip surrounding material or be screwed into other threaded components. It is important that the thread be cut uniformly and to precise dimensions for the screw to function correctly. In order to do this, the tool bit used to cut the thread must be shaped in a specific way.
The angle of the sides of the pointed tool bit used to cut screw threads must match the angle of the desired thread. This angle is determined by using a center gauge. When the bit is originally ground into shape, it is formed according to the graded notches in the center gauge, which are cut at specific angles. A center gauge will often include two or more notches of different dimensions as guides for different bits.
In large-scale industrial screw manufacture, automation takes care of the thread angles, and the use of a center gauge is generally not necessary. This instrument is most often used by home machinists and other workers who grind tool bits by hand. Center gauges may be purchased from manufacturers who offer other specialty hand-machining tools. They are available in a variety of different gradations for various thread measurements.