4-Axis CNC Milling

4-axis CNC milling adds a rotary axis to machine multiple sides and wrap-around features in fewer setups, maintaining tight tolerances on prismatic parts.

Overview

4-axis CNC milling is CNC milling with an added rotary axis (typically A-axis) that indexes or continuously rotates the part during cutting. It enables machining around the part’s circumference, accessing multiple faces without repeated re-fixturing, and improving feature-to-feature alignment.

Choose 4-axis when your part has side features that need consistent angular relationship, when you want to reduce setups for prismatic parts, or when wrap-around pockets, slots, and hole patterns drive cycle time on a 3-axis. It’s common for medium-complexity geometry where full 5-axis simultaneous motion isn’t required.

Tradeoffs: programming, workholding, and probing are more involved than 3-axis, and rotary-axis clearance limits tool length and reachable areas. Surface finish and accuracy on “wrapped” toolpaths depend on rotary calibration and part balance. Very deep cavities, undercuts, or compound-curvature surfacing may still require 5-axis or secondary operations.

Common Materials

  • Aluminum 6061
  • Aluminum 7075
  • Stainless Steel 304
  • Stainless Steel 17-4 PH
  • Alloy Steel 4140
  • Brass C360

Tolerances

±0.001"

Applications

  • Couplings with cross-holes and keyways
  • Manifold blocks with multiple side ports
  • Gear blanks with bolt circles and slots
  • Valve bodies with radial features
  • Housings requiring 4-sided machining
  • Pulleys with wrap-around grooves

When to Choose 4-Axis CNC Milling

Choose 4-axis CNC milling for prismatic parts that need multiple faces machined with tight positional relationships, especially radial hole patterns or wrap-around features. It fits prototypes through medium volumes where reducing setups lowers cost and improves repeatability. It’s a strong option when geometry is more complex than 3-axis access allows but doesn’t justify full 5-axis capability.

vs Manual Milling Machine

Choose 4-axis CNC milling when you need repeatable angular indexing, consistent feature-to-feature alignment, or more than one or two parts. The rotary axis and CNC control reduce setup variability and cut cycle time on multi-face or radial features.

vs 3-Axis CNC Milling

Choose 4-axis CNC milling when a 3-axis process would require multiple re-clamps to reach different sides or to machine around the OD. Fewer setups typically improves true position between faces and reduces labor on bolt circles, cross-holes, and wrap-around slots.

vs 5-Axis CNC Milling

Choose 4-axis CNC milling when features can be reached with indexing/rotation around one axis and you don’t need tool tilt to avoid collisions. It often delivers most of the setup reduction benefits at lower programming and fixturing complexity than full 5-axis simultaneous work.

vs CNC Gantry Milling

Choose 4-axis CNC milling when the part benefits from rotary indexing/continuous rotation more than it benefits from a very large work envelope. For smaller to mid-size parts with radial or multi-face features, a 4-axis vertical can be faster and simpler to fixture.

Design Considerations

  • Define the rotary axis datums and critical angular relationships on the drawing (bolt circle clocking, zero angle reference).
  • Leave clearance for tool and holder around the rotary axis; avoid features that force excessive tool stick-out.
  • Prefer standard hole and slot sizes; minimize tiny end mills on wrap-around features to control cycle time and deflection.
  • Keep critical features on as few setups as possible; call out which surfaces must be machined in one setup if alignment is tight.
  • Add generous internal corner radii on pockets and keyways to use standard tooling and reduce chatter.
  • Provide stock and workholding surfaces (pads, bosses, or a sacrificial area) so the part can be clamped and rotated safely.