The drawing on the left depicts the machine starting or 0 point for easy and automatic positioning attainable in a sinker EDM once the quill and receiver Gage Balls are set at 0. The reason the Precision Gage Balls are zeroed on the centerline of one another is that at times a sine or compound sine will be needed in setting up the component in a sinker EDM. No matter what angle is used, the common centerline means the balls rotate around one another in a constant fashion keeping the measurement from their surface constant as well. Even dimensions needed on compound angles are therefore no problem to measure very precisely from the surface of the Gage Ball to the feature EDMed. The 0s are established once the receiver is mounted on the setup sine plate.
The drawing in the center shows the electrode being moved to Precision Gage Ball generated X and Y positions in the sinker EDM. The drawing on the right shows the EDM moving/burning to the Z position, and interpolating as programmed to complete the burn.