A 4th axis adds rotary motion to a 3-axis mill — usually a rotary table or indexer mounted on the machine table that turns the part around an axis (the A axis on X, or B on Y). It lets the machine cut around the circumference of a part or present several faces without an operator re-clamping it.
A simple indexer rotates and locks between operations; a true 4th axis rotates simultaneously with the linear axes for continuous contouring.
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