4 axis CNC milling explained

A 3-axis vertical mill moves the spindle in X, Y and Z against a fixed part. Add a fourth axis and the part itself rotates. In most shops that axis is A, rotation about X, because X on a vertical mill is the long table direction. A trunnion-style rotary table clamps the part on a faceplate or between a chuck and a tailstock, so features on several sides can be cut in one setup.

Indexed 4-axis work stops at fixed angles. The table rotates to 90°, clamps, and the cutter machines that face. This is common for parts with pockets or bolt patterns on four sides. It is fast to program and rigid, since the table locks during cutting.

Continuous 4-axis work rotates while the tool is in the cut. A cylindrical boss, a cam profile, or a helical slot can be finished in one pass. The controller interpolates the A and X axes together, which is where the programming effort goes up and the cycle time usually goes down.

The rotary table on our 4-axis machines is Ø400 mm. That size sets the practical limit as much as the travels do. A part that swings past the table radius, or one that is long enough to whip when indexed, needs a different setup.

The CAD model is the same model you would send for 3-axis work. The difference appears in CAM. A 3-axis toolpath assumes a single tool direction, so the software only has to avoid gouges in one orientation. With rotation in play, the tool axis moves relative to the part and the software has to keep the shank and holder clear the whole time.

For indexed work, the programmer creates several 3-axis setups inside one file and posts them with A moves between them. The post processor must output the rotation and the clamp command, then re-zero the work offset. Get that wrong and parts come out mirrored on the second face.

For continuous work, the operation is a surface or swarf path wrapped around the rotary axis. Feed rate becomes the hard part. The surface speed at the outside of a Ø120 mm boss is roughly four times the speed near a Ø30 mm hub, so a single feed value is either slow at the rim or too aggressive at the core. Most CAM tools let you drive feed from the rotary rate, but the programmer still has to check the result.

Stock models and fixtures have to be modeled too. A rotary table, chuck jaws and tailstock that are not in the CAM file will not appear in the simulation, and a collision that the simulation never saw shows up on the machine. We model the fixture for every 4-axis job.

Rotation moves the part, so the fixture has to hold it against cutting force in every indexed position. A vise that is fine for one face may not be fine at 90°. Three-jaw chucks, collet blocks and dedicated tombstone fixtures are the usual answers. Which one you pick shows up in the drawing as concentricity and perpendicularity.

Concentricity is the number that changes most. When a part is turned in a chuck and indexed between operations, runout at the chuck becomes runout at the feature. We hold ±0.005 mm on 4-axis work, but the setup has to earn it. A part with a 200 mm overhang from the chuck will not hold that number at the far end without a tailstock.

Indexing repeatability matters more than absolute accuracy. If the table returns to 90° with a 0.01° error, a feature 100 mm from center moves about 0.017 mm. That is acceptable for most brackets and not acceptable for an optical mount.

Thermal drift is the quiet one. A rotary table that has been indexing for two hours is warmer than one that just started, and the center height shifts. On tight-tolerance runs we let the machine warm up and cut the first article after that, not before.

A fourth axis is not free. It adds setup time, fixture cost and programming hours. If a part has features on two faces and the volume is low, two 3-axis setups are often cheaper. The break-even is usually around the third face or the first curved feature that would need a ball cutter run in several orientations.

Some geometry simply needs five axes. A deep undercut, a twisted blade, or a port drilled at an angle that the tool cannot reach without shanking out will not be solved by rotation about one axis. Forcing it onto a 4-axis machine means a special cutter or a second operation, and both cost more than the 5-axis hour would have.

Very large parts are another limit. Our 4-axis mills cover a Ø400 mm rotary table. A 4,000 mm part goes on a large gantry or a mill-turn center instead. Long shafts with features at both ends are usually better on a mill-turn machine, where turning and milling happen in one setup.

Finally, consider the finish requirement. Continuous 4-axis milling leaves a scallop pattern that follows the rotary path. If the drawing calls for Ra 0.2–0.8 μm across a curved surface, plan on a finishing pass at a fine stepover or a secondary polish. Bead blasting and polishing are available in-house.