5 axis CNC machines like most common CNC machines have three Cartesian axes (X, Y and Z), in addition to those axes they also have two rotary axes (A and C, or B and C).
6 axis CNC machines also exist which have three rotary axes (A, B and C), however they’re less common because two rotary axes allows the same dexterity as three.
In the above diagram you can see the A axis is rotation about the X axis, the B axis is rotation about the Y axis, and the C axis is rotation about the Z axis.
The two rotary axes allow the machine to rotate the spindle (or the workpiece) so that the sides of the workpiece can be cut, unlike a normal 3 axis CNC that can only cut the top of the workpiece.
There are a few different configurations of 5 axis CNC machines:
1. The spindle is rotated on the B and C axes around the workpiece.
2. The workpiece is rotated on the A and C axes around the spindle.
3. The spindle is rotated on the B axis and the workpiece is rotated on the C axis.
With these additional axes the CNC machine becomes capable of machining much more complex parts, and also makes jobs easier as they’ll be less need to manually position the workpiece for multi-sided machining.
One downside with CNC machines that have rotary axes is the complexity it adds to the CAM (Computer Aided Machining). Many CAM software packages aimed at hobbyists and smaller businesses only support 3, maybe 4 axes. However, like most things, in time there will be more options and cheaper ones too.