Titanium- Machining

Titanium can be economically machined on a routine production basis if shop procedures are set up to allow for the physical characteristics common to the metal. The factors which must be given consideration are not complex, but they are vital to successful handling of titanium.

Most important is that different grades of titanium, i.e., commercially pure and various alloys, will not all have identical machining characteristics, any more than all steels, or all aluminum grades will have identical characteristics. Like stainless steel, the low thermal conductivity of titanium inhibits dissipation of heat within the workplace itself, thus requiring proper application of coolants. 

Generally, good tool life and work quality can be assured by rigid machine set-ups, use of a good coolant, sharp and proper tools, slower speeds, and heavier feeds. Use of sharp tools is vital, because dull tools will accentuate heat build-up, to cause undue galling and seizing, leading to premature tool failure.

The machinability of commercially pure grades of titanium has been compared by veteran shop men to that of 18-8 stainless steel, with the alloy grades being somewhat harder to machine.

Turning:

Commercially pure and alloyed titanium can be turned with little difficulty. Carbide tools are the most satisfactory for turning titanium. The "straight" tungsten carbide grades of standard designations C1 - C4, such as Metal Carbides C-91 and similar types, give the best results. Cobalt-type high speed steels appear to the best of the many types available. Cast-alloy tools may be used when carbide is not available and when the cheaper high speed steels are not satisfactory.

Milling:

The milling of titanium is a more difficult operation than that of turning. The cutter mills only part of each revolution, and chips tend to adhere to the teeth during that portion of the revolution that each tooth does not cut. On the next contact, when the chip is knocked off, the tooth may be damaged.

This problem can be alleviated to a great extent by employing climb milling, instead of conventional milling. In this type of milling, the cutter is in contact with the thinnest portion of the chip as it leaves the cut, minimizing chip "welding".

For slab milling, the work should move in the same direction as the cutting teeth; and for face milling, the teeth should emerge from the cut in the same direction as the work is fed.

In milling titanium, when the cutting edge fails, it is usually because of chipping. Thus the results with carbide tools are often less satisfactory than with cast-alloy tools. The increase in cutting speeds of 20-30% which is possible with carbide tools compared with cast-alloy tools does not always compensate for the additional tool grinding costs. Consequently, it is advisable to try both cast-alloy and carbide tools to determine the better of the two for each milling job. The use of a water base coolant is recommended.