The main differences between a right-handed and left-handed end mill is the direction in which they rotate and the direction in which the cutting forces are applied.. A right-handed end mill rotates clockwise when viewed from the top, while a left-handed end mill rotates counterclockwise.
The advantage of using a left-handed end mill is that it can reduce the tendency of the cutter to lift the workpiece off the table. This is because the twist of the helix of the cutter pushes the workpiece down onto the table. On the other hand, right-handed end mills are more commonly used and are generally more readily available.