Why would I need a 4th Axis unit?
A typical CNC machine is a 3 axis system, meaning that the tool will move left to right, front to back and up and down. (XYZ axis) This means that you are limited to using flat stocks such as wood, plastic sheets, metal sheets or blocks, etc. However, sometimes you want to expand the capabilities of your CNC and machine round objects like table legs, or chess pieces. For this you would need to install a 4th axis system on your CNC.
The 4th axis is usually installed along your Y axis (in the front to back direction). When using the 4th axis you zero your gantry so the X axis is set to the center line of the 4th axis system and the X axis will no longer move while the job is being machined. Instead the X axis commands are now being transferred to the A Axis. The A Axis is the rotation of the 4th axis. So as the machine is carving or cutting the stock, The gantry still moves front to back (Y axis) and up and down (Z axis), but now you add in the rotation axis (A Axis) which will rotate the stock as the machine cuts/carves the material.