Machining titanium: a working guide

Why titanium behaves the way it does

Ti-6Al-4V conducts heat at roughly 6.7 W/m·K. Aluminium is about 150, carbon steel around 50. Every joule the cut generates stays in a few square millimetres at the tool-chip interface, and carbide begins to give up above 600 °C.

Three consequences follow, and between them they explain almost every titanium machining problem you will meet.

Tool life is governed by heat, not abrasion. The tool does not wear out so much as overheat and fail. That is why cutting speeds look low and why coolant pressure matters more than insert grade.

Titanium is chemically reactive. Above about 500 °C it welds itself to carbide. Built-up edge forms, breaks away taking coating with it, and the surface finish collapses.

It is only half as stiff as steel. The elastic modulus is around 114 GPa against roughly 200 for steel, so thin features deflect twice as far under the same cutting force, then spring back behind the tool.

None of this makes titanium unmachinable. It makes it unforgiving of the process shortcuts that steel tolerates.

Which grade your part actually needs

The most expensive titanium decision is usually made before anyone touches a machine, and it is the grade.

	Grade 2	Grade 5 (Ti-6Al-4V)	Grade 23 (ELI)
Yield strength	~275 MPa	~830 MPa	~795 MPa
Structure	Alpha, single phase	Alpha-beta	Alpha-beta, low interstitials
Weldability	Excellent	Good, needs shielding	Good
Corrosion resistance	Best of the three	Very good	Very good
Relative machining cost	~0.5–0.7×	Baseline	Baseline, tighter stock
Where it belongs	Chemistry, not load	Structure and fatigue	Implants, cryogenic, fracture-critical

Grade 5 is the default in most engineers’ heads, and it is specified on plenty of drawings where Grade 2 would do the job for less money and less trouble. Grade 2 is both cheaper and more corrosion resistant — alloying slightly reduces corrosion performance — and it machines roughly twice as easily.

The question that settles it is short: is the part being protected from load, or from chemistry? Chemistry starts at Grade 2. Load starts at Grade 5.

The rules that actually control cost

Keep the feed up. Below about 0.04 mm per tooth the edge stops cutting and starts rubbing, and rubbing puts all the heat into the tool instead of carrying it away in the chip. The instinct to slow down when a cut sounds bad is wrong here.

Never dwell. A tool that pauses while still engaged is a small furnace held against the cutting edge. Program the retract, peck the drills, avoid spring passes.

Use pressure, not volume. Flood coolant barely reaches the interface, because the chip is in the way and because coolant flashes to steam before it arrives. Through-spindle coolant at 70 bar changes tool life by a factor that no insert upgrade will match.

Tolerance only what mates. A blanket tight callout across a titanium part is the single most expensive line most buyers write without intending to. Tighten the two features that locate or seal, and open the rest to ISO 2768-m.

Let the part reach temperature before inspection. A titanium part measured warm off the machine and the same part measured at 20 °C are different parts.

Where titanium jobs actually go wrong

In rough order of how often it happens:

1. A blanket tolerance that forces finishing passes on twenty faces which needed three.
2. Thin walls that measure correctly under the cutter and spring back afterwards.
3. Material availability, because the drawing calls for a size or a grade that is a mill order rather than shelf stock.
4. A missing condition callout, so estimators guess between annealed and solution treated and aged, and the quotes come back incomparable.
5. A shop without high-pressure coolant quoting a price that will not survive contact with the part.

The first two are design decisions. The third is a planning decision. The last two are sourcing decisions — and all five are cheaper to fix before the drawing is frozen than after the first article.

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