Machining Strategies for aerospace material: Titanium
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| Posted : Jul, 29, 2009 | Author [titanengg]
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As more job shops move toward higher-end work, they must deal with the challenge of cutting titanium-a hard-to-machine metal.
As they take on higher-end work, more and more machine job shops will have to master to mill titanium, a lightweight metal that's hard to machine. That's because high-end parts, such as aircraft components and medical devices, are often made of this corrosion-resistant material. In fact, titanium and its alloys have already claimed a wide range of aerospace, industrial, marine and commercial applications. In addition, learning how to handle titanium is important because it provides insight into how shops can boost productivity without having to increase cutting speed.
Raising cutting speed is a big no-no when milling titanium because of two reasons. First, even a small increase in cutting speed can significantly exacerbate edge wear. And second, it can cause heat to build up quickly because of the metal's low thermal conductivity. In fact, excessively fast milling may even result in combustion.
But rest assured that you can still increase the speed of production without boosting cutting speed. To increase your metal removal rate while keeping the cutting speed steady and selecting tooling with two important traits. First, it must be able to fully utilize the power of the current machine, and second, it must be able to offset any limitations the machine may have in terms of rigidity.
To choose the right tool, the first thing you must do is to consider the cutting tool material. Carbide-often a shop's go-to material when it comes to difficult jobs-is not necessarily the best choice. Newer generation high-speed steel can be a more suitable selection. That's because carbide's superior wear resistance comes at the cost of bulk toughness. In other words, carbide is not very good at resisting fracturing and chipping-both of which can result in tool failure in titanium milling. A tougher tool-such as one made of high-speed steel-can allow deeper cuts to be taken without the edges chipping. This more tolerant tool material-especially on a less rigid machine tool-will enable a shop to reach a higher metal removal rate through cut depth as opposed to cutting speed.
But carbide should not be ruled out entirely in milling titanium. It can be used for low-radial-immersion cuts, for example, in which cuts have a relatively light depth to control heat. In such applications, it is recommended using a coated carbide tool. In particular, a carbide tool coated with titanium aluminum nitride (TiAlN) is effective because it excels in maintaining its integrity and properties as the temperature in the cut rises. Heat actually activates its protective mechanism; the energy produced during machining frees the aluminum, which aids in the formation of a protective layer of aluminum oxide. Coated carbide tools could also be used when making heavier cuts. In such cases, a stronger coating such as titanium carbo-nitride (TiCN) can be utilized. This coating can resist micro-chipping.
Another effective strategy in milling titanium is increasing the number of effective edges to boost the metal removal rate. You can do this by selecting tools with very fine pitch or trying an approach called "plunge roughing," in which a shell mill or another appropriate milling tool, is fed into the work vertically. Additionally, job shops can also increase the metal removal rate by minimizing chatter. This can be accomplished in three ways. First, you have to make sure that both the interface between the tool and the tool holder and that between the tool holder and the spindle are kept as stiff as possible. Second, you should consider a tool with an eccentric relief or a "margin." This can provide process damping, which prevents chatter. And third, you can space cutting edges unevenly so they cannot hit the work with uniform frequency, thereby warding off chatter.
In short, with the correct tooling and a sound approach, milling a hard-to-machine metal such as titanium can be accomplished productively and cost-effectively.
The author, Ramesh Kumar is a Mechanical Engineer with twenty years of experience in engineering metal machining works.
He can be contact at info@titanengg.com.sg.
For further details, please visit http://www.titanengg.com.sg
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