Aerospace CNC alloy processing: mechanics, limits and trade-offs

• 1
  What makes aerospace alloys different to machine
• 2
  Aerospace CNC alloy processing strategies for titanium, Inconel and aluminium
• 3
  Why five-axis matters for aerospace CNC alloy processing
• 4
  Holding ±0.005 mm on alloy aerospace parts
• 5
  When aerospace CNC alloy processing is the wrong choice
• 6
  Alloy behavior, cutting window and where it is used

Most aerospace parts are not hard because of their shape. They are hard because of the material. Titanium Ti-6Al-4V, Inconel, 17-4PH and 7075 aluminium all combine high strength with low thermal conductivity, which means heat has nowhere to go. It piles up at the cutting edge instead of leaving with the chip.

That heat does two things. It softens the tool coating and it grows the workpiece. A 300 mm titanium rib can move 0.05 mm from thermal expansion alone during a roughing pass. If the finishing pass runs before the part cools, the dimension you measure at the machine is not the dimension you get at inspection.

There is also the spring-back problem. These alloys are stiff, so a thin wall pushes back against the cutter. A 1.5 mm titanium web will deflect under a heavy radial cut and then relax, leaving a wall thicker at the bottom than the top. Light radial passes and a sharp, positive rake geometry reduce it, but they do not remove it.

The practical consequence is that aerospace CNC alloy processing is a thermal and stiffness problem before it is a programming problem. Feeds and speeds matter, but coolant strategy, fixturing and pass sequencing usually decide whether the part lands inside tolerance.

• 1
  Low conductivityHeat stays in the cut zone; carbide grades and coatings must tolerate 900 °C+ edges.
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  High strength-to-weightThin walls deflect instead of yielding, so chatter shows up before the part moves.
• 3
  Work hardeningInconel and 17-4PH harden under a rubbing tool, so never dwell in the cut.

For Ti-6Al-4V, the standard approach is high-pressure through-spindle coolant at 70 bar or more, cutting speeds of 40–60 m/min with coated carbide, and a trochoidal tool path that keeps radial engagement low. Titanium also reacts with the tool at high temperature, so any dwell longer than a fraction of a second causes welding and a built-up edge.

Inconel 718 is slower. Surface speeds drop to 25–35 m/min, tools are usually ceramic or whisker-reinforced carbide for roughing, and depth of cut is kept shallow. The reason is not just hardness. Inconel keeps its strength at 700 °C, which is exactly the temperature range where most tool coatings fail.

Aluminium is the opposite problem. 7075 machines fast, but it is gummy at low speeds and it is prone to chip welding on the flute. Cutting speeds of 300–600 m/min with polished flutes and generous rake angles work well. The risk with 7075 is residual stress, not tool wear: hogging out a pocket in one pass releases internal stress and the part bows.

The alloy choice is usually fixed by the drawing, not by the machine shop. What the shop controls is the sequence: rough, stress-relieve where the alloy allows, semi-finish, let the part reach thermal equilibrium, then finish. That sequence is the difference between a part that measures well on the machine and one that measures well in a metrology room.

• 1
  Titanium40–60 m/min, 70 bar coolant, trochoidal paths, no dwell.
• 2
  Inconel25–35 m/min, ceramic or whisker carbide, shallow passes, rigid setup.
• 3
  7075 aluminium300–600 m/min, polished flutes, rough and finish in separate setups.

A three-axis machine positions the tool in X, Y and Z. The part stays fixed. On a part with a 15° sloped face or a contoured pocket, that means either multiple setups or a ball nose tool running at its tip, where surface speed approaches zero. Both options cost time and accuracy.

Five-axis machining adds two rotary axes, so the tool can tilt to meet the surface. The benefit is not just fewer setups. It is that a flat end mill can cut a contoured surface at its full diameter, which improves surface finish and lets the cutter remove material efficiently instead of rubbing.

There is a second benefit that engineers often miss. When the tool can tilt, it can approach a deep pocket from an angle where the holder does not collide with the wall. That allows shorter, stiffer tools. A shorter tool deflects less, which directly improves the dimensional result on thin ribs and deep cavities.

The trade-off is setup and programming time. Five-axis toolpaths need verification, and a rotary table adds a positioning error source. For a simple prismatic bracket with three holes, three-axis is faster and cheaper. For a contoured structural rib with compound angles, five-axis is usually the only route to a repeatable part.

• 1
  Use five-axis whenCompound angles, contoured pockets, deep cavities, tight wall thickness.
• 2
  Use three-axis whenPrismatic parts, flat faces, through-holes, simple profiles.
• 3
  Simultaneous vs positional3+2 indexing is cheaper; simultaneous 5-axis suits continuous contours.

A tolerance of ±0.005 mm is achievable, but not on every feature. It depends on the feature size, the material, and whether the measurement happens at 20 °C. A 0.005 mm band on a 500 mm titanium beam is a different problem from the same band on a 30 mm aluminium boss.

Temperature is the first control. Aluminium expands about 23 μm per metre per °C. A 500 mm part that is 5 °C warmer than the inspection room is 0.057 mm longer than the drawing calls for. That is already outside a ±0.005 mm band, so temperature-controlled finishing and a cool-down period before final inspection are not optional.

The second control is the machine itself. A five-axis machining center with a Ø400 mm rotary table has a positioning error stack that includes the table, the trunnion and the spindle. Thermal compensation and regular ballbar checks keep that stack predictable. Without them, the tolerance is a claim rather than a result.

The third control is measurement. A tolerance that tight needs a CMM or a laser interferometer, not calipers. For parts that will be inspected by the customer, we run 100% inspection before shipment and can supply reports on request. Raw material certificates, in-process checks and final dimensional reports form the traceable chain.

• 1
  20 °C referenceAluminium moves 23 μm/m per °C; let parts stabilise before final cuts.
• 2
  Machine geometryBallbar and thermal compensation keep the rotary stack predictable.
• 3
  Inspection methodCMM or laser measurement, not hand tools, for sub-10 μm bands.

Machining is not always the cheapest route to an aerospace alloy part. If the geometry is a thin, large shell with uniform wall thickness, sheet metal forming or casting may be faster and cheaper. If the part is a complex internal lattice, additive manufacturing followed by finish machining can remove more material in one step than a mill ever could.

Machining also struggles with parts that are too large for the available travel. Our largest travel is 4,000 × 400 × 150 mm, and the medium platforms cover 750 × 1,150 × 550 mm. A part beyond those envelopes needs either a different process or a split design with joints, which adds weight and failure points.

There is also an economic boundary. For a single prototype in titanium, machining is usually the fastest path because no tooling is required. For 10,000 identical small brackets, die casting or forging plus finish machining often wins on unit cost. The crossover depends on geometry, alloy and finish, and it is worth checking before committing to a process.

Finally, some alloys are better left to specialists. Beryllium copper dust is toxic, magnesium chips can ignite, and Inconel wears tools fast enough that the tooling cost can exceed the machining cost. These are not reasons to avoid the alloys. They are reasons to confirm the shop has handled them before.

• 1
  Choose forming or castingUniform thin shells, high volumes, simple cross-sections.
• 2
  Choose additive plus machiningInternal lattices, conformal channels, topology-optimised shapes.
• 3
  Check the envelope first4,000 × 400 × 150 mm is our largest travel; beyond that, redesign or split.