4-Axis CNC Milling

4-axis CNC milling machines add a rotating axis to 3-axis milling, enabling multi-face features, better access, and fewer setups with tight, repeatable tolerances.

Overview

4-axis CNC milling adds a rotary axis (usually A or B) to standard 3-axis milling, allowing the workpiece to rotate for machining multiple faces in a single setup. This improves access to side features, simplifies fixture design, and reduces stack-up error from multiple re-clamps. Shops typically use 4-axis for prismatic parts with features on several sides, as well as basic cylindrical components that do not justify full 5-axis.

Use 4-axis CNC milling when you need accurate features around a part’s circumference, such as holes on bolt circles, flats, keyways, and pockets on multiple faces. It balances flexibility and cost: more capable and efficient than simple 3-axis for multi-face work, but with lower machine cost and programming complexity than full 5-axis. Limitations include restricted access to fully undercut or highly contoured freeform surfaces and the need to design around a single continuous rotary axis. For best results, design parts that can be completed with one or two indexed rotary positions rather than continuous simultaneous 4-axis motion.

Common Materials

  • Aluminum 6061
  • Aluminum 7075
  • Stainless steel 304
  • Stainless steel 316
  • Alloy steel 4140
  • Titanium Grade 5

Tolerances

±0.001"–±0.003"

Applications

  • Manifolds with side ports and radial holes
  • Shafts with keyways, flats, and cross-holes
  • Housings with features on multiple faces
  • Indexable fixtures and rotary tooling components
  • Small impellers and radial fan hubs
  • Gearbox and transmission end caps

When to Choose 4-Axis CNC Milling

Choose 4-axis CNC milling for prismatic or cylindrical parts that need features around the circumference or on multiple faces in tight positional relationship. It suits low to medium production volumes where reduced setups, better alignment, and improved feature-to-feature accuracy justify modestly higher programming and fixturing cost. Aim for parts that can be completed primarily through indexed positions about one rotary axis, not complex organic surfaces.

vs Manual Milling Machine

Pick 4-axis CNC milling instead of manual milling when you need repeatable multi-face features, tight positional tolerances, or more than a handful of identical parts. CNC 4th-axis indexing removes operator variability, compresses cycle time, and enables complex bolt circles and angular patterns that are slow and error-prone on manual equipment.

vs 3-Axis CNC Milling

Choose 4-axis CNC milling over 3-axis when the part has critical features on multiple sides or around a diameter and would otherwise require several re-clamps. The rotary axis improves concentricity and true position, reduces fixture count, and cuts setup time, especially for shafts, manifolds, and parts with radial hole patterns.

vs 5-Axis CNC Milling

Use 4-axis instead of 5-axis when the geometry mainly needs indexed rotations about a single axis, not full simultaneous multi-axis contouring. 4-axis machines are usually cheaper to run, easier to program, and more available, making them ideal for multi-face prismatic parts and simple cylindrical features where 5-axis capability provides little extra benefit.

vs CNC Gantry Milling

Select 4-axis CNC milling over gantry milling for smaller parts that benefit from rotary positioning rather than large travel and table size. 4-axis VMCs or HMCs typically offer better rigidity, accuracy, and spindle utilization for detailed multi-face work, whereas gantry mills shine on very large plates, molds, or frames.

Design Considerations

  • Align key part features around a common rotary axis so the 4th axis meaningfully reduces setups and maintains concentricity
  • Keep most machining within one or two index angles to simplify programming and fixturing and to reduce cycle time
  • Avoid features that require undercuts or compound angles not reachable by rotation about a single axis unless you confirm tool access with the shop
  • Dimension critical relationships in polar or angular coordinates (e.g., bolt circles, clocking angles) to match 4-axis capabilities and ease inspection
  • Provide clear datum structure tied to the rotary axis (e.g., bore and face) so the shop can fixture and probe consistently
  • Standardize corner radii and depths so the same tools can cut features at different rotary positions, minimizing tool changes and cost