6-Axis CNC machining explained

• 1
  6-axis CNC machining explained: what the sixth axis moves
• 2
  Why part rotation changes the tool path
• 3
  Fixture design decides the outcome
• 4
  When 6-axis work is the wrong call
• 5
  Materials and tolerances on a rotary setup
• 6
  What drives cost on a 6-axis job
• 7
  Which machine for which part

A 3-axis mill moves the tool in X, Y and Z. The workpiece sits still. A 5-axis machine adds two rotary axes, usually A and B or B and C, and those rotations let the tool reach undercuts and angled faces without resetting the part. The rotary motion belongs to the spindle or the trunnion, not to the part you are holding.

In a 6-axis setup the part itself joins the motion. The common arrangement is a rotary table that indexes or turns continuously while the spindle does its three linear moves and two rotary moves. So the part can rotate under the tool while the tool tilts. That is what opens up features on the far side of a part without a second op.

There is no single industry definition. Some builders call a mill-turn center a 6-axis machine because it has three linear axes, two rotary axes and a live tool spindle that also rotates. Others mean three linear plus three rotary. Both exist, and both cut metal. What matters is which axes move the part and which move the tool, because that decides which features you can reach.

For quoting, ask one question first: does the sixth axis turn the workpiece? If yes, you can machine interrupted bores, cross holes and angled pads in one cycle, often at ±0.005 mm true position. If the sixth axis is only a second spindle or a bar feeder, the geometry limits stay close to 5-axis work.

On a 5-axis machine, the tool path is defined relative to a workpiece that is essentially fixed. The machine moves around the part. That works well until a feature sits on a face that no tilt can reach, or until the part is long and the overhang grows.

Rotate the part and the geometry flips. The tool stays in a comfortable envelope while the table brings each face into position. Long shafts, housings with bores on four sides, and parts with radial hole patterns become single-setup work. We run a Ø400 mm rotary table for exactly this reason.

The trade-off is rigidity. Every rotary axis adds a joint, and every joint flexes under load. A part that is stable on a 3-axis vise may chatter on a rotary table if the table is small or the fixturing is light. Keep the part close to the table center, support the overhang, and take lighter radial depths of cut.

Positioning accuracy also stacks. Errors from the table index, the trunnion and the tool all add up at the cutting edge. We hold ±0.005 mm on finished parts, and 100% inspection before shipment catches the drift before it becomes a scrap bin full of parts.

A 6-axis job is won or lost in fixture design. The rotary table gives you the motion, but the part still has to be held. If the fixture blocks a face, the axis count is wasted. If the fixture is too soft, the part moves and the tolerance goes with it.

The usual approach is a tombstone or a dedicated plate that leaves the cutting zones open. Clamp on a surface that is not critical, or clamp on stock that gets removed later. For thin-walled parts, add a sacrificial rib or fill the pocket with a low-melt compound before the finishing pass.

Balance matters more than most people expect. A part offset 200 mm from the table center spinning at 60 rpm puts real load on the table bearings. Keep the center of mass near the axis, counterweight when you can, and keep rotary speeds low on heavy parts.

Setup time is where the savings show up. A part that needs four orientations on a 3-axis machine can mean four fixtures and four zeroing steps. One rotary setup replaces all of them, and the first-article check happens once instead of four times.

Plenty of parts do not need a sixth axis. A flat bracket with holes on one face is a 3-axis job. A mold insert with deep cavities on two faces is a 5-axis job. Adding a rotary table to either one adds setup cost, not value.

Rigidity is the hard limit. Deep pockets in hard steel, long reach with small tools, and heavy interrupted cuts all punish rotary fixtures. If the part needs a 20 mm end mill at full radial engagement in 4140, a fixed vise on a 3-axis machine will hold tolerance better and cost less per part.

Volume changes the math too. For 10,000+ part runs, a dedicated fixture on a 4-axis horizontal mill often beats a 6-axis center on cycle time. The 6-axis machine wins on geometry, not on raw speed.

The honest rule: use the sixth axis when a feature is unreachable, when orientation count drives your cost, or when one setup removes a stack of tolerance buildup. Otherwise, spend the money on better tooling and a stiffer fixture.

Aluminium is the easy case. 6061-T6 and 7075 cut clean on a rotary table, and the low cutting force means the extra joint in the kinematic chain rarely shows up in the finish. We hold Ra 0.8–1.6 μm on most aluminium work without a separate polishing step.

Stainless and titanium are less forgiving. 316L and Ti-6Al-4V work-harden, so a dwell on the rotary table can rub instead of cut. Keep feed per tooth up, use plenty of coolant, and avoid long pauses with the tool in the cut. On Inconel, expect to slow the rotary feed and accept a shorter tool life.

Plastics behave differently again. POM and PEEK hold tight tolerances but move with temperature, so the part can measure differently after the table cools. Let the part stabilize before final inspection, and check it at 20 °C when the drawing calls for it.

Tolerance is a system result, not a machine spec. The ±0.005 mm figure assumes a rigid fixture, a stable thermal environment, and a probe check on the rotary axis. Without those, the same machine will drift, and no axis count fixes that.

Programming time is the first cost. Simultaneous motion needs a post-processor that understands the table, and a programmer who can verify the tool path with simulation. A wrong rotary move is a crash, not a scrap part.

Fixture cost comes next. A tombstone or a custom plate is real money, and it only pays back if the part runs more than once. For a one-off prototype, the fixture can cost more than the machining.

Cycle time is the third lever. Sixth-axis work is not automatically faster. It removes setups, which is where the saving lives. If the part is simple, the extra axis adds motion without removing any setup.

At GreatLight we quote 6-axis work alongside 3-axis and 5-axis options, and we say which one is cheaper. Quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. No minimum order quantity, from one prototype to 10,000+ part runs.