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	Learn to Machine Titanium, a Tough Metal of Choice by Dale Ernstmeyer (20-year horizontal experiece) High Productivity Titanium Machining Titanium can be tough, but machining it doesn’t have to be. With the right type of machine tool, machining can be as easy as other materials and a lot more profitable. Because of its outstanding strength-to-weight ratio, the usage of titanium is rising at almost an exponential rate. The new Boeing 787 uses more titanium than the 737, 747 and 777 combined. We are even starting to see titanium being used in automotive parts to reduce weight without giving up strength. Today you will also find it’s the metal of choice for most medical implants. Compared to steel, titanium alloys offers many advantages. It’s density is about half that of steel, so parts made from them weigh roughly half as much as steel parts. Yet titanium’s strength ex-ceeds steel’s, and they have twice the elasticity. That makes them ideal for applications that re-quire flexible materials that don’t crack or rupture. Titanium alloys resist corrosion better than the best stainless steels. And, like steel, titanium alloys can be readily cast or forged. It’s All About Tool Life Machining titanium does have its difficulties. It is a poor thermal conductor. Heat generated during cutting doesn’t dissipate through the workpiece or fixture, but instead the heat is concen-trated in the cutting area. This causes a very high tool tip temperature, in some cases up to 2,000 deg. F. This can quickly dull the cutting edges. With aluminum for instance, virtually all of the heat either goes out with the chip or into the workpiece. These high temperatures in the cutting area also cause a chemical reactivity with the cutting tools. Titanium has a strong alloying tendency that will cause galling, welding and smearing which can cause a rapid destruction of the cutting tool. As mentioned earlier, titanium has a relatively low modulus of elasticity. This can cause slender parts or light cuts to deflect more than steel. This can cause tolerance problems but more of a concern is it may cause the workpiece to deflect so that the cutting tool rubs the surface instead of biting into it. Titanium will work harder if the cutter is allowed to rub the surface. This to will cause rapid destruction of the cutting tool. Keep the followig in mind when considering machining titanium: Heat and chatter are the enemies Machine stiffnes is very important (see section below on machine stiffness) Use plenty of cutting fluid (high pressure is preferred) Nevew dwell the cutter while in contact with the workpiece Leave about 0.025 in. for the finish pass or "spring pass" Rely on tool life management data, replace tools before they are worn Technigues For Machining Titanium Some machine builders advocate using a light depth of cut with high feed rates. The advantages are less spindle torque is needed along with less machine stiffness. More machining passes are needed but they are at a higher rate of speed. The disadvanages are generally you will have a poor ratio of tool-life-to-metal-removal. Simply put, ou will remove less metal before your tool is worn out. Again, the problem is with the modulus of elasticity and the fact htat titanium will work harder if the cutter is allowed to run the surface isstead of dig in and cut. Most companies that have experience cutting titanium (and make money doing it) prefer to use the high torque with high depth of cut approach. This requires a machine with very high spindle torque along with a very rigid machine construction. However the advantages are a higher ratio of tool life to metal removal. With a high depth of cut, the cutter teeth can take a good bite out of the material instead of rubbing. But remember, besides heat, chatter is the enemy. It doesn’t mat-ter how much torque you have, if the machine structure isn’t stiff, you will have chatter. Other considerations when machining titanium are: • Climb milling should be used • Use the shortest tool possible for the maximum tool stiffness • Try to maintain a constant feed rate and good chip load • The workpiece also needs to be supported • Use plenty of coolant to wash the chips from the cut, never recut chips. • For milling, a horizontal machining center is better than a vertical. They are stiffer and they have less of a problem recutting chips. Machine Stiffness Every machine tool builder says their machine is stiff. But how do you know? Two of the largest users of titanium, Boeing and Pratt and Whitney, use what is called a Modal Analysis Test. This is also sometimes called a “Tap Test.” Essentially what this test does is measure the harmonic frequency of the machine. The stiffer the machine, the higher the frequency. The goal is to have a machine whose harmonic frequency is higher than the frequency of titanium. If a machine has a lower harmonic frequency than titanium then you will have chatter during cutting. There are many details involved in this test and that could be a subject for another article. However it should be good to know that every Doosan “HM” series horizontal machining center has passed this modal analysis test for titanium machining centers.