Use 5-axis CNC at the same time: power and accuracy

• 1
  What 5-axis CNC at the same time really means
• 2
  Why simultaneous motion changes cutting force
• 3
  Where the accuracy actually goes
• 4
  When simultaneous 5-axis is the wrong choice
• 5
  How we set up a simultaneous job
• 6
  Choosing the machining mode

A 5-axis machine has three linear axes and two rotary axes. In 3+2 mode the rotary axes index to an angle, lock, and the cut runs on three axes. In simultaneous mode all five axes interpolate in the same block of code, so the tool tip follows a continuous path in space while the worktable tilts and rotates under it. That single difference is where both the power and the accuracy come from.

Simultaneous motion keeps the cutter in contact with the wall for a long arc instead of a series of stop-start passes. The load on each flute stays even. The machine can run a deeper axial cut at the same feed, because the engagement angle is controlled rather than fixed by the part orientation. On a deep pocket in 7075 or 17-4PH this is the difference between chatter and a clean wall.

The accuracy side is less obvious. Every re-fixture on a 3-axis machine adds a datum error. Simultaneous 5-axis machining reaches five faces in one setup, so the positional stack between features is set once by the machine geometry, not five times by an operator with a dial indicator. That is why a housing with bores on four sides holds true position more easily on one 5-axis setup than on four 3-axis operations.

It is not free power. Simultaneous motion needs a post-processor that understands the specific kinematic chain, a CAM strategy that keeps the rotary axes away from their limits, and a machine that is calibrated in volume, not just in a single plane. Without those three, the same code that looks correct on screen will leave witness marks on the part.

• 1
  3+2 modeRotary axes index and lock; the cut is still three-axis.
• 2
  Simultaneous modeFive axes interpolate in one block; tool tip follows a 3D path.
• 3
  Main gainFewer setups, so datum error is not stacked four or five times.

On a three-axis cut, the radial engagement angle changes as the tool walks into a corner. It can swing from a light 10 percent stepover to a full-width slot in a few millimeters. The sudden load spike is what breaks small end mills and what pushes a thin wall out of tolerance.

Simultaneous 5-axis lets the programmer tilt the tool so the contact point stays on the flank of the cutter. A constant engagement angle keeps chip load steady, which lets us raise the feed per tooth instead of backing off. In aluminum 6061 we often run a Ø12 mm carbide tool at 12,000 rpm and 0.15 mm per tooth with a 10 percent stepover, and the wall finishes near Ra 0.8–1.6 μm without a separate finishing pass.

The same tilt helps reach. A deep rib that would need a long, flexible tool on a 3-axis machine can be cut with a short, stiff tool held at an angle. Short tools deflect less. That is a direct accuracy gain, not a cosmetic one.

There is a limit. Tilt too far and the effective cutting speed at the tool tip drops, the flank rubs, and the surface tears. In titanium TC4 and Inconel we keep the tilt modest and accept a lower feed, because heat stays in the cut and the tool edge suffers. The right tilt is a per-material decision, not a fixed number.

• 1
  AluminumAggressive tilt, high feed per tooth, light stepover.
• 2
  Titanium and InconelModest tilt, lower feed, more coolant or through-tool air.
• 3
  Thin wallsTilt to keep force along the wall, not across it.

A simultaneous 5-axis machine carries more error sources than a three-axis mill. Rotary axis runout, the distance between the spindle centerline and the rotary center of rotation, and thermal drift in the table all enter the cut. Builders handle this with laser calibration, ballbar checks and probe-based part mapping. On our 16 simultaneous 5-axis centers we map the working volume and compensate, which is how we hold ±0.005 mm on features that sit far apart.

Part size matters. On a compact machine with 500 × 500 × 450 mm travels, the rotary error is small and the tolerance is easy to defend. On the 4,000 × 400 × 150 mm travel machine used for long extrusions and beams, the same angular error at the rotary table turns into a larger linear error at the far end of the part. Long parts get volume alignment instead of a single-point touch-off.

Thermal control is the quiet variable. A spindle that has run for two hours is not the same machine that started cold. Climate control and warm-up cycles matter more on 5-axis work because five axes of compensation are all moving at once.

We inspect 100 percent of parts before shipment, with raw material checks, in-process monitoring and a final report on request. For a first article on a new 5-axis geometry, the probe data from the machine is compared against the CMM result, and the offset between them is fed back into the setup.

• 1
  Rotary center offsetCalibrated once per machine, checked on a schedule.
• 2
  Long partsVolume alignment, not a single corner touch-off.
• 3
  Thermal driftClimate control plus warm-up before the first cut.

Plenty of parts that arrive as 5-axis RFQs should be cut on a 3-axis or 4-axis machine. If all the features face one direction, or two directions that a single index can reach, a 3+2 setup is faster and cheaper. Programming time for simultaneous toolpaths is real, and on a simple bracket it buys nothing.

Undercuts and re-entrant pockets are the classic case for simultaneous motion. So are impellers, blisks, turbine blades, orthopedic bone plates and complex manifolds where the wall follows a compound curve. If the drawing shows a surface that cannot be reached from any single tool axis, that is the signal.

Very deep, narrow cavities are another limit. A tilted tool holder needs clearance behind the cut. If the cavity is 8 times deeper than it is wide, the holder will hit the wall before the tool reaches the floor, no matter how the axes move.

Material is the third limit. Hardened tool steel above 45 HRC, and some nickel alloys, cut slowly enough that the simultaneous path does not pay back. In those cases we often rough on 3-axis, then finish the critical surfaces on 5-axis, and quote it as a two-operation job.

• 1
  Use 5-axisCompound curves, undercuts, bores on many faces.
• 2
  Stick with 3-axisFlat plates, single-direction features, loose tolerance.
• 3
  Split the job3-axis rough plus 5-axis finish on hard material.

The setup starts before the machine. We ask for a STEP or IGES file with GD&T, and we check that the model and the drawing agree on datums. Legacy PDF or DWG drawings are workable if the tolerance callouts are readable. Free DFM analysis comes back within 12 hours, and production can start within 24 hours of a released order.

On the floor, the part is probed in the fixture so the CAM origin matches the real stock position. For a casting or a forging with variable stock, we map the surface and shift the toolpath rather than cutting air or gouging. That probe step is what protects a thin flange on an investment casting.

Tool holding matters as much as the machine. A shrink-fit holder runs true and reaches into tight pockets. For long reach, we prefer a stub tool in a solid holder over a long tool in a standard holder, because deflection scales with the cube of the length.

Every job gets a first-article check. If the probe and the CMM disagree beyond a set band, the job stops and the rotary offsets are re-checked before more parts run. That rule has kept our qualification rate at 99.99 percent.

• 1
  Probe on the fixtureAlign CAM origin to real stock, not to the vise.
• 2
  Shrink-fit holdersBetter runout and reach in deep pockets.
• 3
  First-article stopProbe versus CMM mismatch halts the run.