What Is the 4th Axis on a CNC Machine?

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  What the 4th Axis on a CNC Machine Adds to X, Y and Z
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  Indexing vs Simultaneous 4th Axis Motion
• 3
  Chucks, Tailstocks and Workholding Limits
• 4
  Where the Accuracy Actually Goes
• 5
  Which Parts Belong on a 4th Axis and Which Do Not
• 6
  4th Axis vs 3-Axis vs 5-Axis: Which Setup Fits the Job

A 3-axis mill moves the tool in three straight lines: X left and right, Y forward and back, Z up and down. The workpiece stays bolted to the table. Every face you cannot reach from the top needs a new setup, which means unclamping, re-datuming and hoping the vise lands in the same place twice.

The 4th axis on a CNC machine is a rotary table that turns the workpiece about a linear axis, almost always X. The chuck or fixture holds the part and the table rotates it in degrees, so the spindle can reach the top, the sides and the far end without anyone touching the clamps.

That single rotation changes the geometry of the job. A shaft with six milled flats becomes one program instead of six. A part with holes on four sides becomes one program instead of four. The machine still has three linear axes, but the part now presents many more faces to the tool.

Rotation is measured in degrees, not millimeters. A 90° index is a quarter turn. A full 360° turn returns the part to where it started. That is the whole mechanical idea, and most of the engineering work sits in how the controller uses it.

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  Rotary axis, usually ATurns the workpiece about X; B turns about Y, C about Z.
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  Degrees, not distanceProgrammed as A90. or A-45. depending on the controller.
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  One datum, many facesPosition is held by the table, so re-clamping errors drop out.

Indexing is the simpler mode. The table rotates to a position, locks, and the tool cuts. Then the table rotates to the next position and locks again. The controller treats the rotary move as a positioning move, not a cutting move, so surface finish on the flat faces stays predictable.

Simultaneous mode, often called 4-axis contouring, feeds the rotary axis while X, Y or Z are moving. The tool tip then follows a curve around the part. This is how you cut a helix, a cam profile or a spiral groove in one continuous pass.

The trade-off is rigidity and speed. A rotary table is less stiff than the machine bed, so heavy radial cuts on a long overhang will chatter. Simultaneous motion also spends controller look-ahead on four axes at once, which tends to slow the feed rate.

Most production work we see is 90% indexing with a few simultaneous passes where the geometry demands it. Splitting the job that way keeps cycle time down and holds ±0.005 mm on the critical faces.

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  IndexingRotate, lock, cut. Best for flats, slots and hole patterns on multiple faces.
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  SimultaneousRotate while cutting. Needed for helices, cams and blended surfaces.
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  Mixed strategyIndex for the bulk of the stock, contour only the curved features.

A 4th axis table on its own holds one end of the part. Long shafts need a tailstock at the far end to stop the workpiece from lifting under cutting force. Without it, a 300 mm shaft will deflect and the middle of the part will come out oversize.

Three-jaw chucks are quick but repeat within about 0.05 mm. For tighter work we use a collet chuck, a custom soft jaw bored on the machine, or a fixture plate bolted to the table face. Boring the jaws in place on the same machine that will run the job removes most of the runout.

The rotary table carries a load limit, and it is a torque limit as much as a weight limit. A part hanging 400 mm off the table face puts far more load on the worm drive than the same mass held close in. Keep the part as close to the table as the geometry allows.

Our 4-axis mills run a Ø400 mm rotary table with a 4,000 mm maximum processing envelope on the larger machines. That covers most shaft, housing and manifold work, but a part that needs to swing past the table edge will not fit.

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  Tailstock for long partsAnything over roughly 4× diameter benefits from tail support.
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  Bored soft jawsCut the jaw profile on the machine to control runout.
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  Keep the part closeShort overhang protects the worm drive and the finish.

People assume the rotary table is the weak link. In practice the errors come from three places: table backlash, chuck runout and thermal drift over a long cycle. Backlash shows up as a small angular error every time the table reverses direction.

The fix is to approach every index from the same direction, so the worm gear stays loaded on one flank. Program the table to overshoot by a degree and come back, or simply order the indexes so the rotation never reverses. This is a program habit, not a hardware upgrade.

Chuck runout is the second source. If the part sits 0.03 mm off center in the jaws, every feature cut at A0 is offset by that amount, and features cut at A180 are offset the other way. A bored soft jaw set removes it.

Thermal drift matters on long runs. A spindle and table that warm up by 5 °C over four hours will move the part a few microns. On a ±0.005 mm job we warm the machine up before the first cut and check a master part at the start, middle and end of the batch.

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  One-direction indexingAvoid reversals so backlash never enters the position.
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  Bored jawsControls radial offset at every index angle.
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  Warm-up cycleRun the spindle before the first cut on tight-tolerance jobs.

The 4th axis pays off when a part has features on more than two faces and needs those features to be coaxial or angularly related. A hydraulic manifold with ports on four sides is a classic case. So is a drive shaft with keyways at both ends and a milled flat in the middle.

It also pays off when the part is long and round. Turning a shaft on a mill with a 4th axis lets you mill the flats and drill the cross holes in one setup, which avoids shipping the part between a lathe and a mill.

It does not pay off for a flat plate with pockets on one face. A 3-axis machine with a good vise will run it faster and cheaper. It also does not pay off for a part that is mostly turned, like a simple bushing, because a lathe is the right machine for that.

A useful rule: if you can reach every feature from one direction, stay 3-axis. If you need two or three directions and the part is roughly prismatic, a 4th axis or a 5-axis machine will beat multiple setups. If you need five directions at once, go 5-axis.

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  Good fitShafts, manifolds, housings, cams, parts with radial hole patterns.
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  Poor fitFlat plates, simple turned parts, very small high-volume parts.
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  Compare honestlyTwo 3-axis setups can beat one 4th-axis setup on simple geometry.