Titanium has a tendency to gall causing premature tool wear due to the welding of chips to the cutting edges. Frequent sharpening of tools or replacement must be considered before they dull. Continuous feed of the tool into the work piece will prolong tool life. Problems with chatter, tool contact and tolerances can occur due to titanium’s low modulus of elasticity. Thin walled parts can deflect more than comparable parts in steel.
As a manufacturer of titanium castings, Ti Squared Technologies has found that the machining characteristics of titanium castings on cast surfaces containing alpha-case. Machining operations such as broaching, drilling, end milling and tapping can present problems. In such cases, carbide tooling may be required. High -speed steels are widely used for machining titanium because of their flexibility and lower cost than cemented carbides. When it comes to true tool economy however, we have found that often the tooling that costs least to buy ends up being the most expensive on a cost-per-cut basis. For best tool economy, the cutting tool should be matched to the material being machined.
While titanium presents a unique set of machining problems, many of those problems can be alleviated or eliminated by adhering to the following set of guidelines:
These recommendations should be used as a guide and may vary slightly with various machines and material input.
Titanium does not work harden like stainless steels but is similar to carbon steel. Titanium is difficult to machine due to its physical properties. Titanium is a poor heat conductor so the heat can build up quickly on the cutting tool. Because titanium is a reactive metal it can alloy and chemically react with material in the cutting tools which cause galling, welding, smearing and rapid destruction of the cutting tool. Due to its relatively low modulus titanium has a tendency to move away from the cutting tool. This can be avoided using heavy cuts or fixturing to provide proper backup.
Because of the lack of a built-up edge ahead of the cutting tool, a high shearing angle is formed. This causes a thin chip to contact a relatively small area on the cutting tool face and results in high heat on a very localized portion of the cutting tool causing excessive break down and wear.
Commercially pure and alloyed Titanium are soft and can be turned with little difficulty. Carbide tools will offer higher production rates and longer tool life. If high speed steel cutters are used, the very high speeds are recommended. Tool deflection can be avoided using a heavy and constant stream of cutting fluid at the cutting surface. Live centers should be used to avoid seizing.
The milling of titanium provides more challenges than 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 becomed damaged. This problem can be alleviated to a by employing climb milling, instead of conventional milling. Climb milling allows the cutter to be in contact with the thinnest portion of the chip as it leaves the cut which minimizes chip “welding”.
We have found that a neutral to negative rake angle is preferred to prevent chip loading on the cutting edge. When 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. 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.
For easy cleanup, we have found a dilute solution of dishwashing fluid works as well as some commercial cutting fluids.
Using sharp drills of proper geometry and maintaining maximum drilling force will ensure continuous cutting. Avoid having the drill ride the titanium surface which will result in work hardening and make it difficult to cut. The length of the unsupported section of the drill should be no longer than necessary and still allow the chips to flow unhampered through the flutes and out of the hole. This will prevent breakage and allow maximum cutting pressure as well as rapid drill removal to clear chips and allow drill re-engagement. As is the case with milling and turning, an adequate supply of cutting fluid is recommended. For low hardness alloys such as commercially pure titanium, high speed steels are preferred but carbide drills are best for most titanium alloys and for deep hole drilling.
We have found the best tool life is obtained with a 65% thread. Chip removal is a problem which makes tapping one of the more difficult machining operations. As is the case with milling, we have found that a neutral to negative rake angle on the leading threads of taps, inside the flute will prevent loading and provide maximum tap life.
The smear of titanium on the land of the tap can result in the tap freezing or binding in the hole. An activated cutting oil such as a sulfurized and chlorinated oil can help avoid this problem.
Both aluminum oxide and silicon carbide wheels are effective for grinding titanium. Lower than conventional wheel speeds are recommended to maximize wheel life. Belt grinding of titanium is most effective using alumina-zirconia coated abrasives. Slower speeds around 1200 SFPM will provide maximum belt life.
Band sawing with a carbide tipped blade and liberal use of cutting fluid is recommended. Abrasive sawing is also commonly employed using rubber bonded silicon carbide cutoff wheels either dry or with water-base coolants flooding the cutting area to minimize oxidation of the titanium.
Water jet cutting using a high speed jet entrained with abrasive is very effective 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.