One of the lightest metallic elements in the periodic table, titanium is also among the most important. Without this durable, high-strength material, our world today would be a far different place. Airlines would charge more for flights, houses would need painting more frequently, hip joints and dental implants wouldn't last. Unfortunately, the same characteristics that make titanium ideal for a wide range of aerospace, medical and consumer products also make it an unwelcome visitor in many machine shops.
1. The Basics
Titanium has a narrow band of machinability, with recommended cutting speeds of 60 m/min for roughing and 3-4 times that when finishing. Feedrates are entirely dependent on chip loads and other factors, but should be high enough to prevent work hardening. Significant deviation from titanium's feed and speed comfort zone can mean melted or broken tools and a pile of expensive scrap. Follow cutting tool manufacturers recommendations and don't be afraid to phone a friend if you get into trouble.
2. Hot Stuff
Titanium conducts heat at about the same rate as the hot pad you use to pull a cake pan out of the oven. During machining operations, this poor thermal conductivity traps heat in the work zone, wreaking havoc on cutting tools. If your machine setup can handle the additional load, try increasing the feedrate to push some of that heat into the chip and make tools last longer.
3. Tough as Nails
If steel were stiff modeling clay, titanium would be frozen Silly Putty. Built-up edge, notching at the cut line, galled workpieces and chips welding to the cutter are the primary failure modes when machining this gummy material. A positive rake cutting tool with a tough substrate and hard, lubricious coating keeps tools in the game longer. Also, a small T-land or slight hone on the cutting edge can help improve tool life, but don't overdo it—titanium needs a sharp tool.
4. Keep it Cool
With the high heat and stringy chips generated when cutting titanium, a copious flow of clean cutting fluid is essential. Filtration to 25-micron or better is a good idea for many machining operations, but is especially important with critical operations such as this. Increase the coolant concentration to 10 percent or more, and install a high-pressure pump of at least 500 psi to blast chips out of the work area. Always use coolant-fed cutting tools, and employ inserts with aggressive chip control to avoid catastrophic re-cutting of chips
5. The Right Stuff
Because of the extreme cutting forces involved, titanium should only be machined on rigid equipment. A machine spindle with abundant surface contact at both the taper and the face used together with CAPTO holders provide the security of multiple contact points with the machine spindle, excellent repeatability, and the stiffness needed to absorb heavy radial loads. Dense machine construction will absorb vibration and cutting loads better than one designed for light duty machining. Don't even think about running it on that old lathe sitting in the corner of the shop, the one with the sloppy ballscrews and whiny bearings. Likewise, machining titanium on commodity equipment yields results similar to entering the family mini-van in a stock car race. The bottom line is this: invest in a high-performance machine tool if you're serious about titanium.
6. Hang on Tight
Titanium tends to grab end mills under heavy loads, pulling them out of the toolholder. This leads to scrapped workpieces and broken tools. Some shops turn to Weldon shank holders as a way to secure tools, only to find cutter vibration loosens even the most tightly torqued setscrews. Shrink fit holders are a good choice, but require some small investment in an induction heating station for tool changing. For a no-fail grip, a Safe-Lock or equivalent system secures toolholders tightly and accurately. Hydraulic holders like the Sandvik CoromantCoroChuck 930 utilize the latest technology and will prevent pull out. On the workholding side of the equation, a hydraulic vise with hardened and ground jaws is the best bet for clamping titanium parts—a serrated or knife-edge jaw gives an extra bite during roughing operations.
7. Programming Techniques
The right toolpath is a big part of success when machining titanium. The same techniques as those used in high feed machining (HFM) are effective here. Roll into the cut and don't slow down in the corners. "Drive" the cutter around the corner by using programmed radius cutter movements. Trochoidal milling paths with constant cutter engagement lessen shock to the machine tool and cutters alike, extending tool life. And plunge milling can be an effective way to rough out deep cavities.
8. Be Strategic
Above all, develop a sound machining process before the first chip is ever made. Analyze all of the part features, taking special consideration of unsupported areas, tall and/or thin walls and hard to reach features. Plan your moves more carefully than a sophomore sneaking into the senior prom dance. Pick the right cutters, set the appropriate feeds and speeds, and then generate code that meets the conditions mentioned earlier.
Granted, these are general guidelines. Titanium presents a complex machining situation, one whose cutting parameters depend on the size and geometry of the workpiece, the specific alloy being cut, and the rigidity of the setup and the machine tool. Probably the best tool available for successful titanium machining is to contact your cutting tool manufacturer or equipment provider knowledgeable in this area—best of all, it's free.