Titanium Grade 5 CNC Machining: How Ti-6Al-4V Actually Behaves

• 1
  Why titanium grade 5 CNC machining fails at the edge, not the spindle
• 2
  Carbide grades, geometry and coatings that survive the cut
• 3
  Speeds, feeds and depth of cut starting points
• 4
  Coolant, chip evacuation and the fire risk nobody plans for
• 5
  What tolerances and surface finish are realistic
• 6
  Residual stress, stock strategy and thin features
• 7
  Inspection and the non-conformances we see most
• 8
  Grade 5 parameter bands by operation

Grade 5, or Ti-6Al-4V, sits around 900 MPa tensile and roughly 110 HRB. Those numbers look mild next to a hardened tool steel. The mistake is to read them as an easy cut. Hardness is not the constraint here. Heat is.

In a normal steel cut, most of the heat leaves with the chip. In Grade 5 only about 15-20% leaves that way. The rest goes into the cutting tool and into the workpiece. Thermal conductivity is near 7 W/m·K, so the heat does not spread out and dissipate. It stacks up within a millimeter of the edge.

The result is predictable. Tools do not wear down slowly on the flank the way they do in steel. They chip, micro-fracture and fail at the corner. A tool that ran well for 40 minutes can break down in the next 5.

There is a second effect. Titanium is chemically reactive at cutting temperature. It welds to the tool edge, and the welded layer breaks off taking carbide with it. That is why a dull tool smears the surface instead of cutting it, and why the finish degrades suddenly rather than gradually.

Use fine-grain tungsten carbide in the K10-K20 range. Uncoated grades work well because there is no coating to spall. Tough, cobalt-rich grades feel safer but tend to gall and pick up material, so stay away from them on finishing passes.

Geometry matters more than grade. Two-flute or three-flute variable-helix end mills with a 10-15° helix and positive rake cut cleanly. A sharp edge is essential. Specify a honed-but-not-rounded edge, about 0.02-0.03 mm hone, which is enough to stop micro-chipping without blunting the rake.

Coatings are a trade. AlCrN and TiAlN help in continuous cuts where the load is steady. Under interrupted load, such as cross holes or interrupted pockets, they can spall and take the edge with them. AlTiN is a reasonable middle ground for milling.

For drilling, use carbide or HSS-Co with a 130-140° point and heavy web thinning. The drill must not rub. Rubbing work-hardens the surface, and the next pass cuts a harder skin than the first.

These are starting numbers, not setpoints. Every machine, holder and part stiffness shifts them. For side milling with a 10 mm solid carbide end mill, start around 40-60 m/min surface speed and 0.05-0.08 mm feed per tooth. Axial depth of cut runs 0.5-1× diameter with 5-10% radial engagement in a trochoidal path.

Turning runs faster: 60-90 m/min, 0.15-0.25 mm/rev, 1-2 mm depth of cut. Keep the tool engaged. A dwell or a spring pass at the same depth rubs and work-hardens the surface.

Drilling is the slowest operation at 12-18 m/min with 0.08-0.12 mm/rev. Peck if the depth exceeds 2× diameter. Deep holes in Grade 5 trap heat and chips, and a packed flue is how drills snap.

Climb milling is the default for finishing. It pushes the tool away from the thin section and keeps the chip load positive through the cut.

Flood coolant at high pressure is the default. For deep pockets and drilling, 50-70 bar through-tool coolant is the target. It does two jobs: it cools the edge and it clears the chip before the chip can be recut.

Neat oil or a high-oil-content emulsion gives better lubricity than a lean synthetic. On finishing cuts where the surface finish matters, that difference shows.

Never run Grade 5 dry. Titanium chips are fine, and they are pyrophoric. A dry pocket with a pile of fine swarf and a hot tool is a genuine ignition source. Keep the enclosure clean and do not let fines accumulate in corners.

Use a fire-rated extraction or a water-based suppression system near the machine. This is not a formality. Shops that cut titanium regularly have a plan for a small chip fire.

A competent shop holds ±0.05 mm on milled features and ±0.025 mm on turned diameters without special effort. That is the routine band for Grade 5.

Tighter is possible. ±0.01 mm is achievable on a rigid machine with temperature control, but it adds cost and inspection time. Our own floor holds ±0.005 mm where the geometry and setup allow it, and we will say so up front when a feature cannot get there.

Surface finish lands at Ra 0.8-1.6 μm from a good finishing pass. Ra 0.4 μm requires small stepovers, sharp tooling and often a spring pass with a fresh edge.

Below Ra 0.4 μm, expect polishing or a secondary operation. Machining alone rarely gets there in Grade 5 because the material smears before it cuts cleanly.

Bulk removal releases residual stress, and the part moves after the clamps come off. Buy annealed stock. For parts with tight flatness or bore alignment, specify stress-relieved plate.

Rough with 0.5-1.0 mm of radial stock and 0.3-0.5 mm axial. Then semi-finish and finish in separate operations. For critical parts, insert a stress-relief cycle at 540-650 °C in vacuum or argon for one to two hours between roughing and finishing.

Thin walls below 1 mm deflect under cutting force. Support them with sacrificial webs, machine from both sides, or leave the wall thick and step down progressively with light radial cuts at high spindle speed.

Pocket floors under 2 mm bow when the stock underneath is removed. Leave a floor allowance and finish that floor last, after the surrounding material is gone.

First-article inspection typically uses a CMM for position and profile, a micrometer or bore gauge for diameters, and a surface roughness tester for finish.

Titanium's low modulus means clamp force distorts thin parts during CMM fixturing. Use light-touch or non-contact probing where possible, otherwise the inspection measures the clamp, not the part.

The recurring non-conformances are chatter marks on walls, smeared or torn surfaces from dull tools, and out-of-tolerance flatness after unclamping. Each one traces back to a decision made earlier: too much radial engagement, a worn edge kept in service, or stock removed without a stress-relief step.

We inspect 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Reports are available on request.