Machining Ti-6Al-4V: the parameters that survive contact with a real part
Grade 5 titanium — Ti-6Al-4V — is not hard. It measures around 36 HRC, softer than plenty of steels that cut without complaint. What makes it difficult is that it refuses to move heat away from the cutting edge.
Titanium’s thermal conductivity is roughly 6.7 W/m·K. Aluminium is about 150. Every joule the tool generates stays in a tiny volume at the tool-chip interface, and carbide starts giving up above 600 °C. That single number explains nearly everything about how the alloy behaves.
Starting parameters that are worth trusting
These are conservative starting points for solid carbide in a rigid setup with through-tool coolant. Treat them as a first cut, not a recipe — your machine’s rigidity matters more than any number in a table.
| Operation | Surface speed | Feed per tooth | Notes |
|---|---|---|---|
| Roughing, end mill | 40–60 m/min | 0.05–0.10 mm | Radial engagement 10–15 % of diameter |
| Finishing, end mill | 60–80 m/min | 0.04–0.06 mm | Light axial passes, constant engagement |
| Drilling | 15–25 m/min | 0.05–0.12 mm/rev | Peck. Never let the drill dwell |
| Turning, roughing | 45–70 m/min | 0.15–0.30 mm/rev | Depth of cut 1.5–3 mm |
| Turning, finishing | 70–100 m/min | 0.08–0.15 mm/rev | Sharp positive geometry |
Two rules underpin all of it. Keep the feed up, and keep the tool moving.
Why low feed rates destroy titanium tooling
The instinct when a cut sounds bad is to slow the feed. In titanium that makes things worse. Below roughly 0.04 mm per tooth the edge stops cutting and starts rubbing, and rubbing puts all that heat straight into the tool instead of carrying it away in the chip.
The chip is your heat-removal system. A thick chip leaves with the energy in it. A thin one leaves the energy behind.
Same logic applies to dwell. A drill that pauses at depth, a tool that sits at the end of a pass waiting for the next move — that is a small furnace held against your cutting edge. Program the retract.
Coolant pressure, not coolant volume
Flood coolant washing over the outside of the cut does very little in titanium. The heat is at the interface, under the chip, and low-pressure coolant never reaches it.
Through-spindle coolant at 20 bar or better changes the picture, for the reasons set out in high-pressure coolant. At 70 bar it changes it dramatically — chips break instead of stringing, and tool life often doubles. If the shop quoting your titanium job does not have high-pressure coolant on the machine that will run it, that is worth knowing before the purchase order, not after.
The four ways titanium parts go wrong
Work hardening. Titanium hardens in front of the tool. A pass that lifts off and comes back over the same surface is cutting hardened material. Program continuous engagement and avoid re-cutting the same face.
Galling and built-up edge. Titanium is chemically reactive and welds itself to carbide above about 500 °C. Once material sticks to the flute, surface finish collapses and the edge chips off, taking your coating with it. Sharp uncoated or AlTiN-coated carbide, high pressure coolant, and a feed that never drops.
Springback. The elastic modulus is roughly half that of steel, so thin walls push away from the cutter and spring back behind it. On a 2 mm wall you can be 0.05 mm out without doing anything wrong. Machine thin features last, take light finishing passes, and support the wall where you can — thin-wall titanium parts covers the wall ratios and fixturing in detail.
Fire. Fine titanium chips ignite. It is rare in a well-run shop and it is a real reason to ask how a supplier handles chip evacuation on dry or minimum-quantity-lubrication setups.
What to put on the drawing
Titanium quotes swing wildly because the same geometry can be cheap or ruinous depending on details buyers often leave off. Specify:
- Grade and condition. Grade 5 and Grade 23 (ELI) machine similarly but are not interchangeable, and ELI costs more. Say which, and say annealed or STA. If the part is not structural, check whether Grade 2 would do the job for less — Grade 2 or Grade 5.
- Where the tight tolerance actually matters. A blanket ±0.01 mm across a part is the single most expensive line on most titanium drawings. Tolerance the two features that mate and open the rest to ISO 2768-m.
- Surface finish by face, not globally. Ra 0.8 µm everywhere on a titanium part can double the cycle time for no functional gain.
- Material certification requirements. Mill certs, heat-lot traceability and chemistry reports are normal for titanium and cost nothing to ask for upfront — but they cost real money to reconstruct after the fact.
- Whether stress relief is required between roughing and finishing. Large titanium parts move. If flatness matters after machining, say so.
The honest version of lead time
Titanium stock is not always sitting on a shelf. Grade 5 plate and bar in common sizes usually is; ELI, large diameters and unusual sections often are not, and mill lead times of six to ten weeks are normal when stock has to be ordered.
When you send a drawing, send the target date with it. A shop that knows the date can tell you whether the constraint is the machine or the metal — and those two problems have completely different solutions.
If you have a titanium part that has been quoted badly or delivered late, send the drawing and we will tell you which of the two it was.