cncmachining 5axiscnc: How Simultaneous Motion Changes the Cut

• 1
  Key takeaways
• 2
  What cncmachining 5axiscnc Actually Adds
• 3
  Table-Table vs. Head-Table Configurations
• 4
  Simultaneous vs. 3+2 Positioning
• 5
  Where Five-Axis Machining Stops Making Sense
• 6
  Holding ±0.005 mm on a Rotary Machine
• 7
  Choosing Between 3-Axis, 3+2, and Simultaneous Five-Axis

A 3-axis mill moves the tool in X, Y, and Z. The part sits still. A five-axis machine adds two rotary axes, usually a trunnion table that tilts in A and a table that spins in C, or a spindle head that tilts and rotates. The cutting tool can now approach the workpiece from almost any direction while it is cutting.

That matters because of tool axis control. On a contoured surface, a ball nose cutter held at a fixed angle leaves a scallop pattern and cuts with its tip, where surface speed drops to near zero. Tilt the tool so the flank engages the surface, and you cut with the side of the tool at a consistent speed. Chips clear better and the finish improves without a separate polishing step.

The practical result is fewer setups. A housing with features on five faces might need four 3-axis operations, each with its own fixture and its own datum stack-up. On a five-axis center it is one program, one clamp, one datum. Every re-clamp adds positional error, so removing setups removes error at the same time as it removes labor.

None of this is free. Rotary axes have their own backlash, thermal growth, and kinematic error that the controller must compensate. The post-processor has to translate CAM output into machine-specific rotary moves. When those are wrong, the part is wrong in a way that is harder to diagnose than a simple offset.

Most five-axis machines fall into two families. In a table-table design, both rotary axes sit under the workpiece. The part tilts and rotates; the spindle stays vertical. This suits compact parts because the rotary table carries the mass of the workpiece, and a heavy part slows the rotary axes down.

In a head-table design, one rotary axis is in the spindle head and one is in the table. The spindle can reach down the side of a tall part without tilting the workpiece. Deep pockets, tall housings, and parts with long overhangs often fit this layout better.

GreatLight runs 16 simultaneous 5-axis machining centers alongside 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. That mix matters for quoting. A part that needs true simultaneous motion goes to a five-axis center. A part that only needs four-sided access in index mode runs faster and cheaper on a four-axis mill.

The rotary table on the compact five-axis centers is Ø400 mm. Maximum processing size across the shop is 4,000 mm, with travel envelopes of 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for boxy parts. If your part exceeds the envelope, it is a different process, not a different program.

Two modes are often confused. In 3+2, the rotary axes move to a position and lock. The machine then cuts as a normal 3-axis mill from that angle. This is also called positional five-axis. It is simple, rigid, and well suited to prismatic parts with features on angled faces.

In simultaneous five-axis, all five axes move at once during the cut. The controller continuously solves the kinematics so the tool tip follows the programmed path while the tool axis tilts. This is what lets a cutter follow a swept surface or a complex blend in one pass.

Simultaneous motion puts more demand on the controller. Feed rates must be limited by the slowest rotary axis, not by the linear axes. A program that looks fast in CAM can crawl on the machine if the C-axis has to swing through a large angle in a short distance.

As a rule, use 3+2 when the geometry is angular and the surfaces are flat or simple. Use simultaneous when the surface is continuously curved, when the tool must stay normal to the surface, or when undercut regions cannot be reached any other way.

Five-axis machining is a poor fit for simple work. A flat bracket with a handful of drilled holes and a pocket can be made on a 3-axis machine in less time, with a cheaper program, and with less risk. Adding rotary motion adds failure modes for no gain.

Very small parts are also a poor fit on large five-axis centers. The rotary axes and the machine geometry are sized for a certain part envelope. A 10 mm part on a 4,000 mm machine loses accuracy to the machine's own scale, not to the cut.

Thin-walled parts need care. Tilting the tool reduces radial force, which helps, but the part still deflects under its own clamping load. On a five-axis table, the workpiece is often held on a tombstone or a fixture that itself moves. A weak fixture is worse on a rotary table than on a static vise.

Soft materials like POM and HDPE cut easily in five axes but rarely need it. Rigid setups and sharp tooling matter more than rotary freedom. Save the five-axis time for parts where the geometry actually demands it.

Tolerance is a system property, not a machine specification. A five-axis center can hold ±0.005 mm (±0.0002 in) on a well-designed part with a rigid fixture, stable temperature, and a probing routine that verifies the datum before the first cut.

Rotary axes introduce error sources that static machines do not have. Each rotary axis has backlash, angular positioning error, and a center offset that must be dialed in. The controller compensates for these, but the compensation has to be correct for the specific machine.

Thermal drift matters more on long cycles. A spindle running for hours grows, and the rotary axes warm up. On a part with tight features on multiple faces, that drift shows up as a shift between the first face and the last. In-process probing and a warm-up cycle reduce it.

Surface finish follows the same logic. A fine finish of Ra 0.2–0.8 μm is reachable, but it depends on tool condition, stepover, and how steady the rotary motion is. Ra 0.8–1.6 μm is a realistic target for most five-axis production work, with Ra 1.6–3.2 μm as-machined where finish is not critical.