How Does a 4 Axis CNC Machine Work?

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  The short answer
• 2
  How a 4 axis CNC machine work: the mechanics
• 3
  From CAD model to rotary G-code
• 4
  What 4 axis buys you, and what it does not
• 5
  Tolerances, surface finish and inspection
• 6
  Setting up a 4 axis job, step by step
• 7
  3 axis vs 4 axis vs 5 axis: which one fits the part

Start with a 3 axis mill. The table moves in X and Y, the spindle moves in Z, and every cut comes from the tool pointing down along one fixed direction. A 4 axis CNC machine adds a rotary axis, named A when it turns around X and B when it turns around Y. On a typical vertical mill the rotary unit is a table at one end of the machine, so the part spins about a horizontal axis while the tool stays vertical.

The common point is one rotary axis on top of X, Y and Z. That axis can be a bolt-on rotary table on a 3 axis machine, or a built-in trunnion on a dedicated 4 axis mill. Either way the control now interpolates four values per block of G-code: X, Y, Z and A. The tool path is no longer limited to one face of the workpiece.

Two modes matter. In indexing mode the rotary axis turns to an angle, clamps, and stays there while the tool cuts. This is how you drill four sides of a manifold block without touching the vise. In simultaneous mode the control moves the rotary axis and the linear axes at the same time, which produces continuous cuts around a cylinder, a cam profile or a helical groove.

The rotary table sets the size limit. A common unit here has a Ø400 mm table, which handles a part roughly 350 mm across and 150-250 mm tall depending on the fixture. Heavier parts need a tailstock on the far end of the table, or the part will deflect under cut and the rotary axis will lose its position.

Accuracy comes from how the rotary axis is driven. A worm gear set gives high torque and good holding, but it wears and can drift over thousands of indexes. A direct-drive or roller-cam unit holds tighter indexing accuracy over a long run. For most parts the achievable tolerance is ±0.005 mm on the linear axes, with the rotary axis contributing a small angular error that turns into a linear error at the part radius.

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  A axisRotary motion around X. Most common on vertical mills and horizontal machining centers.
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  B axisRotary motion around Y. Seen on some vertical mills and on multi-tasking lathes.
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  IndexingTurn, clamp, cut. Best for holes, slots and faces that sit at fixed angles.
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  SimultaneousTurn while cutting. Best for cylinders, cams, helixes and wrapped contours.

The part starts as a solid model. Before any tool path is written, decide the work coordinate system: where the rotary axis sits relative to the part datum. If the CAD origin is not on the rotary centerline, the CAM software has to offset it, and any mistake there shows up as a part that is machined off-center around the A axis.

In CAM, a 4 axis job is either a set of indexed operations or a wrapped tool path. Indexed work is easier to verify: set the rotary angle for each face, pick a 3 axis tool path, and post it with the A value locked. Wrapped work takes a 2D contour and wraps it around a cylinder, or uses a true 4 axis path for a cam or impeller. Wrapped paths need a stock model that matches the real bar diameter, not a nominal size.

The post-processor is the step people skip. A standard 3 axis post will not output A values, and a post for a different machine will output the wrong rotary direction. Test the post on a scrap blank before the first real part. Check that A0 points where you expect, that positive A turns the way the drawing assumes, and that the machine honors the clamp command between indexed operations.

Simulation is not optional. Verify the full program, including the rotary moves, in the CAM simulator or on the machine's dry-run mode. Look for three things: tool holder collisions with the rotary table or tailstock, rapid moves that swing the part through the tool, and any cut where the tool reaches past the table edge. Those three cause most 4 axis crashes.

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  Set the originPut the CAD origin on the rotary centerline, or record the offset in the setup sheet.
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  Choose index or wrapIndexed faces are simpler to prove out. Wrapped paths need true 4 axis output.
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  Prove the postCut a scrap blank first. Confirm rotary direction, A0 position and clamp behavior.

The main gain is fewer setups. Every time a part comes off the table and goes back on, you lose position and add an hour of proving. On a manifold block with holes on four sides, a 4 axis machine drills all four faces from one program. Position error between faces drops, and the operator has less to get wrong.

The second gain is access to cylindrical features. Cross holes, wrapped slots, keyways around a shaft, and helical oil grooves are all natural 4 axis work. On a 3 axis machine these need a custom angle plate or a lathe operation. On a 4 axis mill they are just another indexed or wrapped path.

The limit is direction. The tool still points down along Z on most 4 axis vertical mills, so it cannot reach an undercut that faces up at 45° on the back side of a wall. A 5 axis machine tilts the tool and reaches those faces. If your part has sculpted surfaces, deep pockets with five-sided access, or impeller blades, 4 axis will not finish it.

The other limit is stiffness. Rotary tables add a compliant joint between the part and the machine bed. Heavy interrupted cuts can chatter more than on a 3 axis machine with the part bolted straight to the table. If your part is short, dense and needs heavy roughing, sometimes a 3 axis setup on a rigid fixture cuts faster than a 4 axis setup.

Cost per hour is higher than 3 axis and lower than 5 axis. Programming time is the hidden variable: a wrapped 4 axis path can take two to three times longer to program than an indexed one. For low quantities, keep the number of indexed faces small and let the operator flip the part instead of paying for wrapped paths.

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  Good fitShafts, manifolds, valve bodies, brackets with angled faces, housings with cross holes.
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  Poor fitThin-walled parts that vibrate, undercut faces, twisted blades, parts bigger than the table swing.
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  Watch the tailstockLong parts need support at both ends, or the rotary axis loses position under cut.

Linear axes on a well-maintained 4 axis mill hold ±0.005 mm (about ±0.0002 in) on size and position. The rotary axis adds an angular error, and that error grows with part radius. A 20 arc-second error is roughly 0.01 mm at a 100 mm radius and 0.02 mm at 200 mm. On a 300 mm part, that error can push hole-to-hole true position past 0.03 mm.

Surface finish depends on the path. Indexed operations finish like a normal 3 axis cut: Ra 0.8–1.6 μm is routine, and Ra 0.2–0.8 μm is reachable with a fine stepover and a sharp tool. Simultaneous 4 axis paths leave a different pattern because the tool contact point moves along the surface, so program a smaller stepover than you would on a flat face.

Inspection has to catch the rotary error, not just the linear size. Measure hole-to-hole true position across at least two faces, and check any feature that was cut at an indexed angle. On a CMM, align to the part datum, then report angular position as well as linear position. A part that measures good on one face can still be rotated by 0.05° relative to the next face.

For production runs, keep the setup the same across the batch and re-check the A zero every 20-30 parts. Thermal growth in the table and the machine frame moves the rotary centerline slowly. On tight parts, a quick re-indicate of a gauge feature takes two minutes and catches drift before it becomes scrap.

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  Size and position±0.005 mm on linear axes. Re-check the A zero during long runs.
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  Rotary errorGrows with part radius. Measure true position across faces, not just on one face.
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  FinishRa 0.8–1.6 μm routine; use a smaller stepover on simultaneous paths.