Star CNC Advanced Processing Solutions: How Multi-Axis Cutting Actually Works

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  What a rotary axis changes in the cut
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  Where the tolerance actually goes
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  Matching the alloy to the operation
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  Part size and machine selection
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  From file to first article
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  Which setup fits which part

A three-axis mill moves the part under the tool in X, Y and Z. The tool stays vertical. That is fine for a flat pocket, a drilled hole pattern, or a part you can reach from one direction. The limit shows up when a feature sits on a face that is not perpendicular to the spindle. You either reposition the part, or you accept a second setup.

Adding one rotary axis, usually A, tilts the work. Adding a second, B, spins it. When the control interpolates both while the linear axes move, the tool tip follows a path that a three-axis machine cannot reach without stopping. The cutter stays normal to the surface through a compound curve instead of chattering across it at an angle.

The practical result is fewer setups. A housing with features on five faces can be cut in one fixturing. Each setup you remove is a datum you no longer have to re-establish, and that is usually where the tolerance budget was leaking. On the 16 simultaneous 5-axis machining centers in our shop, the rotary table runs to Ø400 mm, which sets the ceiling on what can be tilted in one pass.

One caution. Simultaneous motion is not the same as 3+2 positioning. In 3+2, the table indexes to an angle and locks before the cut. That is rigid and fast, and it covers most prismatic parts. Simultaneous is for contoured surfaces, impellers, and blended fillets. The control has to keep five axes coordinated every millisecond, so the post-processor and the CAM strategy matter as much as the iron.

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  3+2 indexedTable locks, then cuts. Best rigidity, simplest toolpaths.
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  Simultaneous 5-axisAll axes move together. Needed for compound curves and undercuts.
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  Fewer datumsOne setup means less stack-up error between operations.

A tolerance callout on a drawing is a sum, not a single number. Machine positioning is one term. Thermal growth of the spindle and the part is another. Tool deflection under load is a third. Fixture rigidity and the datum scheme add more. When a print says ±0.005 mm, all of those terms have to fit inside the band at the same time.

On thin walls this gets harder. A wall under about 1 mm deflects away from the cutter, so the finished thickness varies along the depth of cut. The usual fixes are a lighter radial engagement, a sharper edge geometry, and sometimes a support wax or a sacrificial rib that comes off in a later op. Thermal and stress simulation during toolpath planning helps here, because you can see where the part will move before you cut it.

Surface finish follows the same logic. A Ra 0.8–1.6 μm finish is a normal machined target for most metals with a controlled feed per tooth. Ra 0.2–0.8 μm needs a finishing pass with a small stepover, a balanced tool, and often a different insert grade. Getting to the fine band on a deep cavity is mostly about reaching the surface without a long, flexing tool holder.

Inspection closes the loop. We check incoming stock, monitor in process, and inspect before shipment, with reports on request. The point of that sequence is not paperwork. It is catching a drift at part 40 instead of part 400.

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  Thin wallsUnder ~1 mm, expect deflection; plan a support or a finishing pass.
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  Deep cavitiesTool reach drives finish more than spindle speed does.
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  Thermal driftWarm up the spindle; let the part settle before final inspection.

Aluminum is the default for a reason. 6061 and 6061-T6 cut fast, hold a good finish, and take anodizing well. 7075 is stronger but gummier and more prone to a built-up edge, so it rewards a polished flute and a generous coolant flow. 2024 machines cleanly and is common in aerospace brackets where fatigue life matters more than corrosion resistance.

Stainless behaves differently. 303 is free-machining and easy to run. 304 and 316 work-harden if the tool rubs instead of cuts, so a light feed and a dwell in the cut is a recipe for a hard skin and a chipped edge. 17-4PH (SUS630) is a precipitation-hardening grade used for shafts and valves; machine it in the annealed condition, then heat treat, then finish grind if the tolerance is tight.

Titanium and nickel alloys are the slow end. TC4 (Ti-6Al-4V) has low thermal conductivity, so heat stays in the edge. Inconel is worse. Both need lower surface speeds, higher coolant pressure, and a rigid setup. Magnesium AZ31B and AZ91D cut very fast but demand chip control and fire-safe handling, which is a shop discipline rather than a CAM setting.

Plastics round out the list. PEEK and carbon fibre composites are common in satellite and medical parts. PEEK wants sharp, uncoated tooling and a fast feed to avoid melting. Carbon fibre abrasive wear is severe, so diamond coating pays for itself quickly on a production run.

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  Aluminum 6061 / 7075Fast, good finish, anodizes well. 7075 needs sharp edges.
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  Stainless 304 / 316Work-hardens. Never dwell; keep the edge cutting.
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  TC4 and InconelLow speeds, high coolant pressure, maximum rigidity.

Not every part belongs on a five-axis center. A simple plate with holes belongs on a three-axis mill, where the setup is quick and the hourly rate is lower. A long extrusion with a few cross-features fits a four-axis mill, which indexes the part while the tool works the side. The rotary table on a four-axis machine is a workhorse for shafts and manifolds.

For large work, the travel envelope decides. Our largest platform reaches 4,000 × 400 × 150 mm, which suits long structural rails and beams. Medium platforms cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, a range that handles most housings and brackets. Compact platforms at 500 × 500 × 450 mm and 500 × 310 × 200 mm are where small, high-feature-density parts run best.

The 4,000 mm maximum processing size is a real constraint, not a marketing figure. If a part is longer than that, it has to be split or moved to another process. Knowing the envelope early saves a redesign later. It also affects fixturing: a long part sags in the middle, so support placement becomes part of the tolerance plan.

Mill-turn centers are the other option to keep in mind. When a part is mostly cylindrical with milled flats, cross-holes and slots, a mill-turn center does the turning and the milling in one program. That removes a chucking operation and the concentricity error that comes with it. For a valve body or a motor shaft, that is often the difference between a two-op part and a one-op part.

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  3-axisPrismatic parts, one accessible face, lowest cost.
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  4-axisIndexed side work on shafts and extrusions.
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  5-axisContoured surfaces, undercuts, five-face work in one setup.
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  Mill-turnCylindrical parts with cross-features; fewer chuckings.

The process starts before metal is cut. A STEP or native CAD file goes through a DFM review: wall thickness, tool reach, corner radii, and datum choice. The output is a quotation and a free DFM analysis within 12 hours, and the notes usually point at one or two features that will be expensive or unstable as drawn. Changing a corner radius from 1 mm to 3 mm can remove a whole EDM operation.

Once the design is settled, programming builds the toolpath and the fixture plan. For a five-axis part, the post-processor has to match the specific machine kinematics, or the tool will gouge a surface the simulation said was clear. Verification runs against the stock model, not just the nominal part, because the stock is what the tool actually touches.

Production can start within 24 hours of a released order, and parts ship in 3–5 days for typical work. Our historical late-delivery probability is below 2%. Those numbers come from a shop running 127 high-precision CNC machines across three wholly-owned plants, with 150 technicians and 7,600 m² of floor space in Dongguan plus a site in Singapore.

First-article inspection is where the loop closes. Dimensions are measured against the print, and the report can be sent with the shipment. If a feature is drifting, we adjust the offset and re-cut rather than shipping a marginal lot. That habit is cheaper than a returned batch.

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  DFM firstQuote and free DFM analysis within 12 hours.
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  Verify against stockSimulate the real stock model, not the nominal solid.
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  Report on requestInspection data travels with the parts.