Durable alloy CNC machining explained

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  What makes an alloy hard to cut
• 2
  Titanium, nickel alloys, PH stainless and 7075 compared
• 3
  How speed, feed and depth interact in these alloys
• 4
  Coolant pressure, coatings and edge preparation
• 5
  Rigidity, workholding and why 5-axis helps
• 6
  Which durable alloy fits which part

Ductile 6061 throws long chips and pulls heat away with them. Durable alloys do the opposite. Titanium, nickel-base alloys and PH stainless steels keep heat at the edge, and they push back on the tool while doing it.

Three material properties set the difficulty. Thermal conductivity decides how fast heat leaves the cut. Hot hardness decides whether the coating survives. Work hardening decides whether the next pass meets soft metal or a skin the last pass already hardened.

A useful field measure is specific cutting energy: the power needed to remove 1 cm³ of metal. 6061 sits low. Ti-6Al-4V runs several times higher at the same feed rate. That energy goes into the edge and the part, not the chip, and it shows up as flank wear and thermal growth.

So the first question is never which cutter. It is where the heat can go, and how much of it the part can absorb before it moves.

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  Low conductivityHeat stays at the edge instead of leaving with the chip
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  Work hardeningLight rubbing passes build a hard skin that dulls the next tooth
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  High hot strengthThe material stays strong at 600 °C and above

Titanium Ti-6Al-4V is the common case. Thermal conductivity is around 7 W/m·K, roughly a tenth of 6061, so heat builds fast. It also galls: a stationary edge rubs and welds rather than shears. Climb milling with a sharp, positive-rake carbide cutter keeps the engagement brief and pushes the chip clear.

Inconel 718 is harder still. It keeps strength at 700 °C, so the cutting edge gets no relief from the heat it generates. Expect low surface speeds and shallow radial engagement. If the tool squeals, the setup moved, not the material.

17-4 PH stainless in the H900 condition cuts more like a tool steel than like 304. In the annealed condition it is gummy and tears at the exit edge. Machinists usually prefer to cut it close to final hardness and finish with a light pass.

7075 aluminium is a durable alloy in the structural sense, not the heat sense. It machines fast, but residual stress in thick plate makes thin walls bow after the clamps come off. Rough, stress-relieve, then finish.

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  Ti-6Al-4VLow conductivity; avoid dwell and re-cutting
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  Inconel 718Hot hardness to 700 °C; small radial step-over
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  17-4 PHCondition matters more than grade
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  7075-T6Fast cutting, but plan for distortion

In soft aluminium the spindle sets the pace. In durable alloys the radial depth of cut sets it. A common strategy is high-feed, shallow radial engagement: 5–10% of cutter diameter radially, full flute depth axially, and a feed high enough that each tooth bites rather than rubs.

Surface speed for Ti-6Al-4V with coated carbide typically lands between 40 and 60 m/min. Inconel 718 drops to 20–35 m/min. 17-4 PH in H900 sits near 60–90 m/min. These are starting bands, not recipes. Tool coating, coolant pressure and rigidity move them.

Feed per tooth matters more than table feed. Below about 0.05 mm per tooth, the edge rubs, work hardens the surface, and the next pass wears the tool faster than the first one did. If the chips come off as dust, the feed is too low.

Depth of cut should be generous axially and conservative radially. Axial depth spreads wear along the flute; small radial engagement keeps the cutting arc short, which limits how long each tooth stays in the heat.

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  Rubbing is the enemyToo light a chip hardens the surface and dulls the edge
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  Shallow radial, deep axialKeeps engagement time short and spreads wear
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  Chip colorTan chips are normal; blue dust means the feed is wrong

Flood coolant struggles in deep pockets because the chip and the fluid compete for the same space. High-pressure through-spindle coolant at 70 bar or more does two jobs: it breaks the chip and it drives heat out of the cut zone. On a deep titanium pocket, through-tool delivery often decides whether the job finishes in one shift.

Coating choice follows temperature. AlTiN and AlCrN coatings hold up at the edge temperatures these alloys create. Uncoated carbide has its place in finishing passes on titanium, where a sharp, uncoated edge shears more cleanly than a coated one.

Edge preparation is often overlooked. A light hone or a small negative land resists chipping in interrupted cuts, but too much hone turns a sharp tool into a rubbing tool. For finishing, ask for a sharp edge.

Tool runout should stay under 0.01 mm. In a durable alloy, runout is not a finish problem, it is a tool-life problem. One flute doing the work wears out while the others idle.

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  Through-spindle coolant70 bar and up for deep pockets and long overhangs
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  AlTiN and AlCrNFor edge temperatures above 600 °C
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  Runout under 0.01 mmKeeps all flutes cutting equally

Chatter starts at the weakest point in the loop: tool, holder, spindle, fixture, part. In durable alloys the cutting force is high, so a setup that was fine in aluminium can sing in titanium. Shorten the gauge length before adding spindle speed.

Thin walls are the usual failure. Clamping pressure distorts the part during the cut and releases it after. For a wall under 2 mm, plan a semi-finish pass, release the clamps, then take a light finishing pass with low radial engagement so the wall does not deflect into the cutter.

Five-axis machining changes the problem. A rotary table lets the tool approach a contoured surface at a constant lead angle instead of sliding along a steep face at a shallow angle. On a titanium impeller or a 17-4 PH manifold, that single change can turn a chattering cut into a stable one.

GreatLight runs 16 simultaneous 5-axis machining centers, 16 mill-turn centers and a Ø400 mm rotary table. For parts up to 4,000 mm, large travels are available. The setup choice follows the geometry, not a preference for a machine type.

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  Shorten overhang firstRigidity beats spindle speed for chatter control
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  Release before finishingSemi-finish, unclamp, then a light final pass
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  Constant lead angle5-axis keeps the cutter loaded evenly on curved surfaces