Titanium machining processes, one by one

In steel a roughing pass takes a wide, shallow bite. In titanium that kind of cut keeps the edge in the material for half of every revolution and overheats it. The routine is the opposite: radial engagement of about 10 to 15 % of the cutter diameter with most of the flute length in the cut. The edge spends most of each revolution out of the material and in the coolant, and wear spreads along the flute instead of sitting at one point.

Toolpaths that hold that engagement constant, called trochoidal or dynamic milling, are the default for pockets and slots. A plain offset path loads the tool in every corner, and in titanium the corner is where tools break.

Climb milling, always: the chip starts thick and leaves thin, so the edge does not rub its way into the cut. Feed per tooth stays above about 0.04 mm, because below that the edge rubs, and rubbing turns into heat.

The usual tools are solid carbide end mills with variable pitch or variable helix to break up chatter, four to seven flutes, a corner radius instead of a sharp corner, and AlTiN-type coatings or sharp uncoated edges. Through-spindle coolant at 20 bar or more where the machine has it.

Turning is a continuous cut, so the edge never gets a moment out of the material to cool. Speeds for Grade 5 with carbide are about 45 to 70 m/min roughing and 70 to 100 m/min finishing. Grade 2 is softer and runs faster.

Chips are the main problem. Grade 2 in particular forms long, tough strings that wrap around the part and the tool and scratch a finished diameter. Chip-breaker geometry, high-pressure coolant aimed under the chip and a feed high enough to break it decide whether a job can run without someone standing at the door.

The typical insert failure is notch wear at the depth-of-cut line. Varying the depth of cut from pass to pass, or roughing with round inserts, spreads the wear along the edge.

Slender parts push away from the tool, and titanium pushes further than steel. Above a length-to-diameter ratio of about 4:1 the part needs support from a tailstock or a steady rest. Small, long titanium parts are usually Swiss-type work, where a guide bushing supports the bar right at the cut.

Mill-turn machines finish cross holes, flats and slots in the same clamping as the diameters, for the same reason 5-axis does: the part does not move between features.

Titanium grips a tap. The thread flanks spring back onto the tap as it cuts, friction rises, and the tap galls or snaps. A tap broken in a titanium part is usually the end of that part.

Thread milling is the default for internal threads in Grade 5. The cutter is smaller than the hole, so the chip is short and the coolant gets in. A worn cutter is changed before it causes damage, and the fit is adjusted in the program instead of by buying another tap.

Where tapping is used, usually on small threads or in Grade 2, the tap drill is often slightly larger so that thread engagement drops to about 65 to 70 % instead of the usual 75 %. That cuts the tapping torque considerably and keeps most of the strength. It changes the minor diameter, so it is agreed against your drawing, never decided quietly by the shop.

External threads are single-point turned on the lathe or thread milled.

Titanium threads that are assembled and taken apart repeatedly gall against themselves and against stainless steel. The usual answers are a Type 2 anodise to AMS 2488, a dry-film lubricant, or a helical-coil insert.

EDM removes metal with sparks, not with a cutting edge. Titanium's heat and springback stop mattering because there is no cutting force pushing a thin wall away. A wire cuts an internal corner of about 0.2 mm radius that no milling cutter reaches, and holds a profile to ±0.01 mm typical.

The trade-off is the surface. EDM leaves a recast layer, a thin skin of re-solidified metal, with a heat-affected zone under it. In titanium that layer is harder and more brittle than the base metal. Skim passes thin it to a few microns. On parts that carry fatigue loads it is removed afterwards by light machining, polishing or chemical etching.

Wire EDM removes material slowly, so it is used for the features that need it and not for the whole part. Sinker EDM, with a shaped electrode, makes blind cavities and internal splines a wire cannot reach.

Titanium anodise is not aluminium anodise. It grows a very thin oxide whose colour comes from light interference, not from dye, so the colour depends on the voltage and on the surface underneath. Colour is matched to a sample. AMS 2488 covers it: Type 2 is a grey finish used against galling and wear, Type 3 is the colour range. Either changes dimensions by far less than the tolerances on most drawings.

Bead blasting gives a uniform matt finish and hides tool marks. Glass bead is the usual medium for titanium; steel shot would leave iron in the surface. Bores and datum faces are masked so they stay as machined.

Pickling, an acid descale guided by ASTM B600, takes a thin, even layer off the surface: alpha case, EDM recast, smeared metal. It has to be controlled because the acid can put hydrogen into titanium. Medical prototypes are often passivated or prepared to ASTM F86 when the drawing calls for it.

Laser marking on titanium makes a dark or coloured oxide mark with little or no material removed. Marks stay off fatigue-critical surfaces and seal faces.