Most shops machining Ti-6Al-4V drop the speed and drop the feed with it. The second move is the mistake.
Walk into a shop machining titanium and the sentence you hear most often is: “We dropped the speed and the tool still died.”
Dropping the speed is right. Dropping the feed along with it is exactly what kills the tool.
The heat has nowhere to go
The root of the problem is thermal conductivity. For Ti-6Al-4V it sits around 7 W/m·K. For comparison: ordinary structural steel is about 50, aluminium is over 200.
That means most of the heat generated in the cutting zone neither leaves with the chip nor spreads into the workpiece. It stays at the nose of the insert. Carbide begins losing strength around 800 °C, and reaching that temperature in titanium is not difficult.
On top of that, titanium becomes chemically reactive at high temperature. It reacts with the tool material, chips weld to the edge (built-up edge), then break away taking the coating with them.
Why you should not cut the feed
Lowering cutting speed lowers temperature — good. But if you lower the feed as well, the chip gets thinner. A thin chip does not have the mass to carry the same heat away, so the heat stays on the insert.
Worse, once chip thickness drops below the nose radius of the insert, the tool stops cutting and starts rubbing. Rubbed titanium work-hardens. The next pass meets a harder surface. Once you are in that loop, tool life falls away quickly.
With carbide tooling, roughing typically runs somewhere in the 30–60 m/min band. Coating, coolant and machine rigidity shift that window — but carry a figure like 150 m/min over from steel out of habit and the insert will be finished in minutes.
Coolant: where, and at what pressure
Flood coolant from an external nozzle usually achieves little in titanium. The chip curtain keeps the fluid out of the cutting zone, and what does arrive evaporates before it reaches an already hot surface.
Use through-coolant tooling. Pressure matters: systems above 20 bar both cool and evacuate the chip from the flute. In deep pockets and hole operations that second job is worth as much as the first.
Running dry is generally a poor idea in titanium. Intermittent coolant is worse: as the insert heats and cools it develops thermal cracks.
Thin-walled parts
Most aerospace parts are thin-walled, and titanium's modulus of elasticity is roughly half that of steel. The part deflects more under cutting force and springs back once the tool has passed.
The result: dimensions drift, chatter marks appear on the surface, noise rises. What helps:
- Variable-pitch cutters, which break the self-reinforcing cycle of vibration.
- Reducing radial depth of cut while increasing axial depth, so the full flute length works.
- Shortening tool overhang. Rigidity falls with the cube of overhang, so even 5 mm makes a difference.
- Stepping the wall down from the top, leaving support material below at each stage.
Don't overlook the holder
Runout bites harder in titanium than in steel. If one flute takes more load than the others it fails first and the rest follow. A shrink-fit or hydraulic holder makes a visible difference over a standard collet.
Decide from measurement, not catalogue
Titanium settings are not copied from a catalogue. The same geometry with the same coating behaves differently on another machine. Pick a reference part, record parts per insert, and change one parameter at a time.
After five trials you will hold something far more valuable than any catalogue: a table for your own machine.