How are the CNC four and five axes defined?

The fourth axis and the fifth axis refer to the two rotation axes of the machining center except X, Y, and Z.
Types of five-axis machine tools: cradle, vertical, horizontal, NC table + NC indexing head, NC table + 90 ° B axis, NC table + 45 ° B axis, NC table + universal horizontal Five-axis linkage CNC machine tool [1] A-axis °, two-axis NC spindle, etc.
There are two ways for the rotary axis of the vertical five-axis machining center. One is the rotary axis of the table. The table set on the bed can rotate around the X-axis. It is defined as the A-axis. 120 degrees. There is also a turntable in the middle of the table, which rotates around the Z axis, which is defined as the C axis, and the C axis is all 360 degrees. The advantage of this setting method is that the structure of the spindle is relatively simple, the rigidity of the spindle is very good, and the manufacturing cost is relatively low. However, the general worktable cannot be designed too large, and the load bearing is also small. Especially when the A-axis rotation is greater than or equal to 90 degrees, the workpiece will bring a large load moment when the workpiece is cut.
The other is to rely on the rotation of the vertical spindle head. The front end of the main shaft is a swivel head, which can surround the Z axis 360 degrees by itself and become the C axis. The swivel head also has an A axis that can rotate around the X axis. Generally, it can reach ± ​​90 degrees or more to achieve the same function as above. The advantage of this setting method is that the spindle is very flexible, and the worktable can also be designed very large. The huge fuselage of the passenger aircraft and the huge engine casing can be processed on this type of machining center.