5-axis machining

5-axis machining (five-axis machining) is CNC machining, most often milling, in which the tool moves relative to the workpiece along the three linear axes X, Y and Z while two rotary axes change the relative tilt between tool and workpiece. The tool can therefore approach the workpiece from almost any direction, except from the side by which the part is clamped.

The rotary axes can sit in the table (a trunnion or tilting rotary table for small and medium parts), in the spindle head (for large, heavy workpieces that should not be tilted), or be split between the two. Mill-turn centers with a tilting milling spindle also offer 5-axis machining.

There are two ways of working. In 3+2 (positional) machining, the rotary axes turn the workpiece into the required orientation and lock, and the machine then cuts in three axes. Most prismatic parts with features on several sides and angled holes are made this way. In simultaneous 5-axis machining, all five axes move at once and the tool continuously follows a freeform surface. This is used for blades, impellers, molds, implants and other freeform shapes.

The main benefit is fewer setups and therefore more accurate relative positions of features (every re-fixturing on a 3-axis machine typically adds an error on the order of hundredths of a millimeter), fewer fixtures and a shorter lead time. Tilting the tool allows a shorter cutter with less stickout, which chatters less and leaves a better finish. With a ball end mill, tilting also keeps the cut away from the tool center, where the cutting speed is zero.

5-axis machining pays off for parts with features on four or five sides, with angled holes and faces, for parts where the relative position of features on different sides is critical, and for freeform surfaces: molds and EDM electrodes, blades, impellers, shape-optimized aluminum parts or prototypes with organic shapes. For prototypes and small batches it often lowers the cost, because there is no need to design and build dedicated fixtures for each setup.

For simple parts machined from two sides (plates, simple brackets), a 3-axis machine is usually cheaper. The hourly rate of a 5-axis machining center is higher, and CAM programming is more demanding.

Even a 5-axis machine cannot reach the side by which the part is clamped. Parts are therefore often machined from stock with extra material for workholding, such as a dovetail for a dovetail vise, which is cut off afterward before that side is finished in a second operation. Tool reach remains a limit too: tilting does not solve deep narrow cavities and undercuts unless there is a straight path in for both the tool and its holder. On machines with a trunnion table, the table itself limits the size and weight of the part.

Freeform surfaces are finished with a ball or barrel cutter in successive passes, which leave scallops between the toolpaths. A smaller stepover means a better finish but a longer cycle time, so specify low roughness only where the function requires it. Simultaneous toolpaths require a simulation of the full machine kinematics in CAM and calibrated rotary axes. When requesting a quote, check whether the supplier offers only 3+2 positional machining or true simultaneous 5-axis work, and for complex shapes ask for CMM measurement against the datums on the drawing.