- September 4, 2014
Machining superalloys calls for super tools and more
Everyone agrees: titanium use is on the rise. With much of the world's airliner fleet showing its first touch of grey, aircraft manufacturers are fielding orders for new planes to replace their aging fleets. To meet competitive pressure and an unspoken mandate for greener air travel, these planes must be fast and fuel-efficient. This makes titanium the material of choice for many aircraft parts and an increasing number of structural components.
Titanium shares many of the same attributes as other heat resistant superalloys (HRSA). But where most of these are strong and heavy like an Olympic weightlifter, titanium is a gymnast, lightweight and flexible. They all have one thing in common, though—they're a real bear to machine. Because of this, cutting tool manufacturers are stepping up with super tough carbide grades and high-tech coatings designed to slice through this arduous alloy with relative ease.
Into the shredder
Tom Hagan, milling product manager for cutting tool manufacturer Iscar Tools Inc., Oakville, ON, recommends tools with a sharp edge and tough substrate for titanium machining. Because of the high heat generated during machining, he explains, thermal cracking and edge chipping are two of the primary failure modes with this material, making good coolant flow essential. Also, a positive rake insert with light edge preparation and a TiAln or TiCn coating is your best bet for success.
One challenge with titanium machining is chip control. Iscar has developed a series of specialty inserts, of which the roughing version carries a serrated cutting edge that makes workpiece penetration easier. "The P290 cutter looks like a high speed steel roughing tool," says Hagan. "It generates very short, manageable chips, making it ideal for machining very deep cavities. Also, the cutter itself tends to dampen vibrations, making it effective at reducing chatter even at very long overhang positions."
Good inserts are important, but how you hold them is equally critical. "Milling titanium creates tremendous pressure—without a secure method to hold the insert, slippage is a real concern," says Brian MacNeil, milling products and application specialist for Sandvik Coromant Canada Inc., Misssissauga, ON.
To prevent this potentially catastrophic insert movement, Sandvik Coromant has developed its iLock technology, which uses a series of grooves and mating islands to secure the insert in the toolholder pocket. The result is a gripping system sure to calm the fears of anyone who spends their days whittling away at titanium blocks more expensive than a mid-size luxury sedan.
None of that's much good, however, if the cutting tool moves in the toolholder. MacNeil says the forces generated when machining titanium can be several times that of comparable steel cutting. As a result, tool pullout is a common problem. The best way to prevent this is through the use of shrinkfit or hydraulic toolholders equipped with the patented Safe-Lock system, which Sandvik Coromant and other manufacturers license from German toolmaker Haimer GmbH. Says MacNeil, "you simply can't risk ripping the tool out of the holder."
Another way to mitigate that risk is good planning. MacNeil pushes his customers to develop an effective machining approach and the programs to support it before the first chip is made. "It's all important to keep the heat out of the cutting zone." This is accomplished, explains MacNeil, by using techniques similar to those of high-feed machining—constant cutter engagement, relatively high feedrates with low depth of cut, trochoidal cutter paths and ramping in and out of the workpiece all serve to minimize shock to the entire cutting system.
This last point—the system—is key. A recent whitepaper published by cutting tool provider Kennametal Inc. and machine builder Mitsui Seiki USA Inc. suggests that "optimum hard metal part production lies in the ability of the machine tool system to perform low-frequency machining without chatter, hold tools tightly with heavy-duty tool tapers, and deliver the necessary power."
That's a mouthful. The takeaway is commodity equipment doesn't cut the mustard when machining titanium and other HRSA materials. These require a rugged harmony between cutting tool, work and tool holding, machine tool and manufacturing process. Tom Dolan, vice president sales and marketing for Mitsui Seiki, says shops that want to be successful in the titanium game can't afford to skimp on any part of the machining process. "Titanium machining requires an investment in good technology. Always buy high quality cutting tools and holders, robust workholding, and the best machine tool you can."
The reason is simple: titanium takes longer to cut than steel and aluminum, often far longer. As Dolan explains, longer cycle times means labour, machine downtime, floor space and energy become commensurately more expensive per part. To counter these higher operating costs, shops need to squeeze out every bit of machining efficiency possible if they're to compete in this market. The best way to accomplish this is with a top-notch machine tool.
"It's estimated that cutting tools represent about 10 per cent of the total cost of manufacturing with titanium," Dolan says. "Reducing that number depends heavily on the choice of machine tools. If you can double tool life by using a stiffer, more powerful machine, you immediately attack
one of the key cost components of hard metal machining."
Dolan contends that spending more on a machine tool can save considerable money for a shop, especially those machining tough materials. By keeping tools in the cut longer between changes, high performance equipment reduces scrap and lost machine time. And greater rigidity in the machine tool means heavier cutting is possible, which also increases productivity. Both of these factors are good news for the bottom line, but to those of us boarding airplanes every week, the principal deliverable of efficient titanium machining is part quality—with aerospace components responsible for millions of lives, only the best and most accurate machine tools will do.
Business is taking off
All of this will become increasingly significant over the next few years as aircraft and engine manufacturers launch new programs. "The uptick in activity is significant," explains Dolan. "Within the next six months, the major aerospace OEMs will be issuing large orders for hard-metal applications. Add to that the 45 to 47 aircraft produced each month by both Boeing and Airbus, with calls for production increases of 10-20 percent, and the entire supply chain will soon become quite interesting."
Dolan points out that the backlog of aircraft is huge right now, and ticks off a handful of aerospace programs that will require titanium components, among them the Boeing 777X, 787 and 737 MAX, the Airbus A380 & A350, Pratt & Whitney's new Pure Power GTF engine, and CFM International's LEAP engine (a joint venture between General Electric Aviation and French manufacturer Snecma). "CFM has almost 12,000 engines on back order right now. Pratt & Whitney also has about 5500 new GTF orders on backlog," he says. "These are all extremely important programs, promising to significantly increase consumption of hard metals."
With all this activity, it might be time to take a hard look at titanium milling. But first you'll need the right toolkit—a supply of sharp but tough cutting tools, zero-slip toolholders, a firm grip on the workpiece and equipment that might be twice the price of that commodity milling machine you were eyeing at the last tool show. Think the process through, program it right and you'll soon be on the road to titanium mastery. SMT