What is "jig borer"

n. A type of machine tool invented during the World War I era to make possible the quick-yet-very-precise location of hole centers.

The jig borer is a type of machine tool invented at the end of World War I to make possible the quick-yet-very-precise location of hole centers. It was invented independently in Switzerland and the United States. It can be viewed as a specialized species of milling machine that provides tool and die makers with a higher degree of positioning precision (repeatability) and accuracy than those general machines had provided. Although capable of milling, a jig borer is more suited to highly accurate drilling, boring, and reaming, where the quill or headstock does not see the significant side loading as it would with mill work. The result is a machine designed more for location accuracy than heavy material removal.

A typical jig borer has a work table of around 400 x 200 mm, which can be moved using large handwheels (with micrometer-style readouts and verniers) on particularly carefully made shafts with a strong degree of gearing; this allowed positions to be set on the two axes to an accuracy of 0.0001 inch (2.5 micrometres). It was generally used to enlarge to a precise size smaller holes drilled with less accurate machinery in approximately the correct place (i.e., with the small hole strictly within the area to be bored out for the large hole).