Wikipedia
A jig grinder is a machine tool used for grinding complex shapes and holes where the highest degrees of accuracy and finish are required.
The jig grinder is very similar to a jig borer, in that the table positioning and spindles are very accurate (far more so than a manual milling machine or lathe). It is almost exclusively used by tool and die makers in the creation of jigs or mating holes and pegs on dies. There are usually many peripheral elements to a large jig grinder, including separate hydraulic motors, air compressors, and various cooling systems for both the hydraulic circuit and supplying coolant to the work and machine itself.
The machine operates by a high speed air spindle rotating a grinding bit. The air spindles are removable and interchangeable to achieve varying surface speeds. Some spindles are fixed speed (60000 rpm), others are adjustable (30000-50000 rpm), and still others are very high speed (175000 rpm). The machines have a standard X-Y table with the notable exception of knee travel. All axes are indexed to .0001" via a vernier scale on the handwheels, with higher accuracy available with the use of measuring bars. The machine head has two vertical travels, one rough head adjustment and the other a precise spindle adjustment. The spindle to which the detachable air spindle mounts also rotates at a variable speed and can typically outfeed .100" while running, again with an accuracy of .0001" on the handwheel or greater, for very precise hole, peg and surface grinding. A well-kept jig grinder will reliably position work to a higher degree of accuracy than is possible with handwheels alone. These features are all critical in positioning a hole and peg system a precise distance from a reference surface or edge.
The most important factor on a jig grinder is the dual-spindle configuration. The main spindle is roughly positioned with between 1" or 2" of travel for setup, and then the .100" of outfeed is used during machine operation to outfeed into the work. A spacer bar may be used between the grinder and main spindle, allowing large (9" radius or larger) work to be completed. The main spindle has a wide range of speeds to ensure proper grinder feed rates are maintained.