4 Axis CNC: What You Need To Know

A 3-axis mill moves the cutter in X, Y and Z. Left and right, front and back, up and down. That covers a lot of parts, and most shops run more 3-axis machines than anything else. Adding a fourth axis puts the workpiece on a rotary table or indexer that turns around the X-axis. This is normally called the A-axis.

The rotary table does two different jobs depending on the control. In indexing mode it rotates to a set angle, locks, and the cut happens. In simultaneous mode the table keeps turning while the cutter moves, so the tool path follows a curve. Indexing is far more common and much cheaper to program.

That single rotation changes how many setups a part needs. Instead of unclamping and rechucking three or four times, the operator programs angles and lets the table do the repositioning. Fewer setups means fewer places for a stack-up error to enter the part.

Three-axis work is still the fastest and cheapest route for flat plates, pockets, slots and parts with features on one face or two opposite faces. If your part is a bracket, a housing with a machined face, or a manifold with ports on one side, do not pay for a fourth axis.

A fourth axis earns its cost when features sit on several faces around a common centerline. Shafts with cross-drilled holes, cylinders with milled flats, rotary valve bodies, and parts that need slots every 30° or 45°. One setup instead of four.

Five-axis machines add a second rotary axis and let the tool tip away from the work. That suits deep cavities, undercuts, and contoured surfaces on impellers or turbine parts. The trade-off is programming time and hourly rate. For a part with simple radial features, 4-axis work is usually cheaper and just as accurate.

• 1
  Pick 3 axisFlat faces, simple pockets, features on one or two sides, tight budget.
• 2
  Pick 4 axisFeatures around a centerline, multiple radial angles, long shafts.
• 3
  Pick 5 axisContoured surfaces, undercuts, deep thin-wall cavities, one-setup complex parts.

Keep the part axisymmetric where you can. A rotary table holds round or near-round work best, because the centerline stays fixed as the table turns. If the part is a long rectangle with features on all six faces, you are asking the setup to fight gravity and reach, and the quote will show it.

Watch the ratio of part length to diameter. A shaft that is 400 mm long and Ø20 mm will deflect when the cutter pushes on it, even with a tailstock. Short and stubby is easy. Long and thin needs light passes, which adds time.

Put a clear datum on the part. A face, a bore, or a turned diameter that the operator can indicate in. When the datum is a rough casting surface, every angle after the first becomes a guess.

Threads and bores on the part centerline are a good fit for 4-axis work. Off-axis holes at odd compound angles are not. If a hole points somewhere between two indexed positions, a 3-axis machine with an angle plate or a 5-axis machine will handle it better.

Allow room for the chuck or collet. The rotary table jaws need to grip something, and that grip zone is not machined. Plan a stock allowance or a sacrificial stub that gets cut off later.

Aluminum is the easy case. 6061-T6 and 7075 cut fast on a rotary table, and the light cutting forces keep deflection low on long parts. 2024 and 6082 behave the same way. If the part is a rotating shaft, aluminum often needs hardcoat anodizing to survive wear at the bearing surfaces.

Stainless 303 and 304 are common for shafts and valve bodies. 17-4PH (SUS630) shows up when the part needs strength plus corrosion resistance, such as a pump shaft. Stainless work-hardens, so the rotary moves should stay under the surface rather than rubbing.

Steel grades like 4140 and 4340 go on 4-axis mills for drive shafts and splined components. Titanium TC4 (Ti-6Al-4V) is machinable but slow, and the rotary table helps because the part stays clamped through several operations.

Finishes are usually applied after machining. Anodizing, electroless nickel, black oxide and bead blasting are all standard. Laser marking needs a minimum character height of 1.5 mm to stay legible on a curved surface.

Angle accuracy matters as much as linear accuracy on a 4-axis part. A rotary table that indexes to ±0.01° still leaves the feature position dependent on how well the part was indicated in. The first article check should include the angular positions, not just the diameters.

We hold ±0.005 mm on critical dimensions and inspect 100% of parts before shipment. That covers raw material check, in-process monitoring and final inspection. Reports are available on request for parts that go into aerospace, medical or automotive programs.

Surface finish depends on the operation. Fine finishing reaches Ra 0.2–0.8 μm, standard high finish is Ra 0.8–1.6 μm, and as-machined surfaces sit at Ra 1.6–3.2 μm. A rotary table does not improve finish by itself; the tool path and feed rate still decide it.

For a part that needs one tight bore and several loose clearance holes, do not tolerance everything the same. Tight tolerances on non-functional features add inspection time and cost without helping the part work.