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 successfully machining titanium.
The different grades of titanium, i.e., commercially pure and various alloys, do not have identical machining characteristics, any more than all steels, or all aluminum grades have identical characteristics. Like stainless steel, the low thermal conductivity of titanium inhibits dissipation of heat within the workpiece itself, thus requiring proper application of coolants.
Good tool life and successful machining of titanium alloys can be assured if the following guidelines are observed:
- Maintain sharp tools to minimize heat buildup and galling
- Use rigid setups between tool and workpiece to counter workpiece flexure
- Use a generous quantity of cutting fluids to maximize heat removal
- Utilize lower cutting speeds
- Maintain high feed rates
- Avoid interruptions in feed (positive feed)
- Regularly remove turnings from machines
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 of titanium being somewhat harder to machine.
Specific information on machining, grinding and cutting titanium and its alloys can be found in RMI's comprehensive booklet "Machining."
Turning
Commercially pure and alloyed titanium can be turned with little difficulty. Carbide tools should be used wherever possible for turning and boring since they offer higher production rates and longer tool life. Where high speed steels are used, the super high speeds are recommended. Tool deflection should be avoided and a heavy and constant stream of cutting fluid applied at the cutting surface. Live centers must be used since titanium will seize on a dead center.
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 high speed steel. The increase in cutting speeds of 20-30% which is possible with carbide tools compared with high speed steel tools does not always compensate for the additional tool grinding costs. Consequently, it is advisable to try both high speed steel and carbide tools to determine the better of the two for each milling job. The use of a water-base coolant is recommended.
Drilling
Successful drilling is accomplished by using sharp drills of proper geometry and by maintaining maximum drilling force to ensure continuous cutting. It is important to avoid having the drill ride the titanium surface since the resultant work hardening makes it difficult to reestablish the cut.
Another important factor in drilling titanium is the length of the unsupported section of the drill. This portion of the drill should be no longer than necessary to drill the required depth of hole and still allow the chips to flow unhampered through the flutes and out of the hole. This permits application of maximum cutting pressure, as well as rapid drill removal to clear chips and drill re-engagement without breakage. An adequate supply of cutting fluid to the cutting zone is also important.
High speed steel drills are satisfactory for lower hardness alloys and for commercially pure titanium but carbide drills are best for most titanium alloys and for deep hole drilling.
Tapping
Percentage depth of thread has a definite influence on success in tapping titanium and best results in terms of tool life have been obtained with a 65% thread. Chip removal is a problem which makes tapping one of the more difficult machining operations. However, in tapping through-holes, this problem can be simplified by using a gun-type tap with which chips are pushed ahead of the tap. Another problem is the smear of titanium on the land of the tap, which can result in the tap freezing, or binding in the hole. An activated cutting oil such as a sulfurized and chlorinated oil is helpful in avoiding this problem.
Grinding
Titanium is successfully ground by selecting the proper combination of grinding fluid, abrasive wheel, and wheel speeds. Both aluminum oxide and silicon carbide wheels are used. Considerably lower wheel speeds than in conventional grinding of steels are recommended. Feeds should be light and particular attention paid to the coolant. A water-sodium nitrite coolant mixture gives good results with aluminum oxide wheels. Silicon carbide wheels operate best with sulfo-chlorinated oils, but these can present a fire hazard, and it is important to flood the work when using these oil-base coolants.
Sawing
Two common methods of sawing titanium are band sawing and power hacksawing. As with titanium machining operations, standard practices for sawing titanium are established. Rigid, high quality equipment should be used incorporating automatic, positive feeding. High speed steel blades are effective but for specific blade recommendations and cutting practices the blade manufacturers should be consulted. Cutting fluids are required.
Abrasive sawing is also commonly employed with titanium. Rubber bonded silicon carbide cutoff wheels are successfully used with water-base coolants flooding the cutting area.
Water Jet
Water jet cutting is a recent innovation in cutting titanium. A high speed jet containing entrained abrasive is very effective for high cutting speeds and for producing smooth burr-free edges. Sections up to three inches have been cut and the process is relatively unaffected by differences in hardness of the titanium workpiece.
Electric Discharge Machining
Though not common, complex titanium components with fine detail can be produced via EDM. The dielectric fluid often consists of various hydrocarbons (various oils) and even polar compounds, such as deionized water. Care must be taken to avoid or remove any subtle surface contamination in fatigue sensitive components.
Chem Milling
Chem milling has been used extensively to shape, machine or blank fairly complex titanium components, especially for aerospace applications (e.g., jet engine housings). These aqueous etching solutions typically consist of HNO3-HF or dilute HF acids, with the HNO3 content adjusted to minimize hydrogen absorption depending on the specific alloy.