CNC 5 Axis Machining Guide

A 3-axis mill moves the tool in X, Y and Z while the part stays clamped in one orientation. Reach is limited to whatever the spindle can see from that single position, so undercuts, side walls and deep pockets usually force a second or third setup. Each re-clamp adds a datum transfer, and each datum transfer adds error.

Five-axis machines add two rotary axes on top of the linear three. The table tilts, or the spindle head swivels, or both. Now the tool can approach a face at an angle without the operator touching the vise. That single change removes setups, and removing setups is where most of the accuracy gain comes from.

The two rotary axes are named A, B or C depending on which linear axis they rotate around. A common trunnion layout pairs a tilting A axis with a rotating C table. A swivel-head layout puts one rotary axis in the spindle and one in the table. Which one suits a part depends on its size and how much of it needs to hang free of the fixture.

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  Setup countOne clamping can reach five sides; fewer re-datums means less stack-up.
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  Tool accessShort, stiff tools can reach angled walls that 3-axis cannot.
• 3
  Fixture costComplex dedicated fixtures are often replaced by a standard tombstone.
• 4
  Programming loadCAM must post collision-checked rotary motion, not just 3-axis paths.

3+2, also called positional five-axis, locks the rotary axes at an angle and then cuts with the three linear axes. The machine behaves like a 3-axis mill that can be aimed. Toolpaths stay simple, the post processor is easy to verify, and cycle times are predictable. Most prismatic parts with angled faces fall into this group.

Simultaneous five-axis moves all five axes at once. The tool tip follows a continuous path while the part rotates under it. This is the only way to cut a true ruled surface, a twisted blade, or a port with a continuously changing wall angle. It also lets the programmer keep the tool normal to the surface, which spreads wear across the flute instead of burning one edge.

Simultaneous work costs more to program and more to verify. The machine must interpolate rotary motion accurately, and the post processor has to output rotary feed rates the control can follow without chatter. If a part can be reached in 3+2, we usually quote it that way and keep the simultaneous capability for the geometry that genuinely needs it.

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  Choose 3+2Angled holes, pockets on several faces, flat sealing surfaces.
• 2
  Choose simultaneousImpellers, blisks, turbine blades, curved ports, sculpted molds.
• 3
  Mixed approachRough in 3+2, finish critical surfaces simultaneously.

The published tolerance of ±0.005 mm applies to the finished part, not to each individual axis. Rotary axes introduce a different error pattern than linear ones. Any angular error at the table center is multiplied by the distance to the cutting point, so a feature 200 mm from the rotary center sees roughly 200 times the angular deviation. That is why datum placement matters more on a 5-axis job than on a 3-axis job.

The practical fix is to put the datum as close to the rotary center as the part allows, and to keep critical features within a short radius of that center. When the geometry forces a long reach, we check the feature with a probe while the part is still clamped, so any deviation is corrected before the next operation rather than discovered at final inspection.

Surface finish follows a similar logic. A tool held normal to the surface produces a consistent Ra 0.8–1.6 μm on most aluminum and stainless work. When the tool is dragged at an angle, the effective cutting speed at the tip changes and the finish can drift. Fine finishes down to Ra 0.2–0.8 μm are achievable, but they usually need a separate finishing pass with a smaller step-over.

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  Datum near centerCuts the lever arm that amplifies rotary error.
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  In-process probingCatches drift before the part leaves the fixture.
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  Normal tool axisKeeps chip load and finish consistent on curved walls.

Aluminum grades such as 6061-T6, 7075 and 6082 cut fast on five-axis machines and hold tight tolerances well. Titanium TC4 (Ti-6Al-4V) and Inconel are different. They generate heat at the cutting edge, work-harden quickly, and push the tool away from the surface on long reaches. For those materials we reduce step-over, use high-pressure coolant and accept longer cycle times to protect the tolerance.

Stainless 316L and 17-4PH sit in the middle. They machine predictably but tend to move after roughing if a lot of material is removed from one side. Symmetrical roughing passes and an intermediate stress-relief step keep the part stable through finishing. Magnesium AZ31B and AZ91D cut easily but need chip control and a clean machine, since fine magnesium chips are a fire risk.

Before quoting, we run a DFM pass on the model. The checks that most often change a design are corner radii smaller than the tool that can reach them, pockets deeper than four times the tool diameter, and tolerances called out on features that do not need them. Tightening a tolerance costs money; leaving one off a non-critical face saves it.

• 1
  Corner radiusMatch it to the largest tool that can reach the corner.
• 2
  Pocket depthBeyond 4× diameter, expect a smaller tool and longer cycle.
• 3
  Wall thicknessThin walls deflect; add support or accept a finishing pass.
• 4
  CalloutsTolerance only the faces that mate or seal.

Every job starts with the same questions: how many setups, where is the datum, and which faces are critical. The answers decide the fixture, the tombstone angle and the order of operations. On a typical part we rough in 3+2, leave 0.3 mm on finishing faces, then run the simultaneous finishing paths once the part has settled.

Inspection is done against the same datum used for machining, so the numbers match what the machine saw. We check raw material certificates before cutting, monitor the part in process, and inspect 100% before shipment. Reports are available on request, including dimensional layouts and surface finish readings.

Our shop runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers and 12 four-axis mills, across three plants covering 7,600 m². Parts range from a single prototype to runs of 10,000 or more, with no minimum order quantity. Uploads stay confidential, and an NDA is available on request.