The precise location of holes is a problem that is frequently encountered in tool and die work. The coordinate dimensioning system was developed to accurately measure the location of holes. This system is used on drawings for jig-bore work and is designed to be compatible with the coordinate axes of movement available on the jig-boring machine. For jig-boring work, the coordinates are generally referred from the intersections of lines that are tangent to the hole-circle at the top and at the left , as shown in Figure 22.9. Thus, the horizontal coordinate dimensions are taken from a vertical line passing through the center of the hole at the extreme left and the vertical coordinate dimensions are taken from a horizontal line passing through the center of the hole at the top.

F IGURE 22.9 Jig bore coordinates

This problem is also frequently encountered on numerically controlled machines. In the system used on these machines, the table movements must be compatible with the coordinate axes of the dimensioning system. The base or reference tangents in this system are usually below and to the left of the hole-circle as shown in Figure 22.10.

F IGURE 22.10 Numerical control coordinates