The machine coordinate system is the inherent coordinate system of the machine. The origin of the machine coordinate system is called the machine origin or the machine zero. After the machine has been designed, manufactured and adjusted, this origin is determined and it is a fixed point. 3PCS/4PCS Shield Anchor 3PCS/4PCS Shield Anchor Hebei Qianmu Fastener Manufacturing Co., Ltd , https://www.qmjgjfasteners.com
When the CNC is powered on, the machine zero point is not known. In order to correctly establish the machine coordinate system when the machine is working, a machine reference point (measurement starting point) is usually set in the movement range of each axis. When the machine starts, it usually takes place. Jog or manual reference point to establish the machine coordinate system.
The machine reference point can be coincident with the machine zero point or not, and the distance from the machine reference point to the machine zero point can be specified by parameters.
When the machine returns to the reference point position, it knows the zero position of the coordinate axis. When the reference point of all coordinate axes is found, the CNC establishes the machine coordinate system.
The mechanical travel of the machine axes is defined by the maximum and minimum limit switches. The effective travel range of the machine axes is defined by software limits, the values ​​of which are defined by the manufacturer. The relationship between machine zero (OM), machine reference point (Om), machine axis and effective stroke is shown in the figure below. 
Figure machine zero point OM and machine reference point Om