April 2016 Reprinted with permission from Shop Metalworking Technology | www.shopmetaltech.com
By Mary Scianna
Selecting a machine to tackle a tough metal alloy
If you want to machine titanium there are three things you need: the right machine, the right cutting tools with the right cutting technique, and the right CAM software. If you remove one from the equation, you may fail.
Titanium poses some significant challenges. It’s a difficult alloy. If you’re focused on removing as much material as possible in the shortest amount of time to achieve high machining efficiencies, the forces generated put a lot of pressure on your machine tool and on your cutting tools. Titanium is also heat resistant, so the heat generated in the forces used to cut the alloy gets redirected into the cutting tool, resulting in faster tool wear and, ultimately, breakage.
The right machine
Walk into Mazak Corp. Canada’s lobby in Cambridge, ON, and you’ll see a titanium guitar on display. The guitar was machined during a titanium workshop the company held in Spring 2015. Mazak partnered with Sandvik Coromant to machine the guitar.
Mazak used its Variaxis i600 five axis CNC VMC equipped with its Smooth control technology and Sandvik Coromant high feed milling cutters.
“Traditional thinking is that you need a slow geared head super high torque machine when it comes to titanium,” explains Tim Scott, application engineer with Mazak Corp. Canada. “We partnered with Sandvik to cut that titanium guitar and we found that using high feed end mills on the Variaxis, we were able to maintain low feed rates and high metal removal rates.”
A key benefit Mazak was able to demonstrate was improvement on non-cutting times, primarily due to the company’s rapid motion function on its Smooth control.
Just as important though, Mazak demonstrated that you don’t necessarily need super high geared spindles. “We found that as long as you select the right tools for your machine, yon can cut titanium with no problems on a 40 taper machine and maintain longer tool life,” says Scott.
High torque machines for titanium is an “old school of thought,” says Vince D’Alessio, vice president of Elliott- Matsuura Canada Oakville, ON, which supplies several machines for titanium machining, such as five axis Matsuura milling machines and Nakamura-Tome multi-tasking turning machines.
“With titanium, the idea was to take a big tool with massive rigidity and lots of torque and plow through the material at slow speeds, taking big volume cuts, but that’s not necessary with today’s machine technology. You still need good torque and rigidity, but people are moving to machines that can handle high feed milling, so tools that can run at high speeds with faster feeds but take less depths of cut so the end result is that you’re putting less stress on the machine and the material isn’t producing as much heat and you’re getting better material removal rates.”
Even if you machine titanium with lower torque machines, the rigidity of that machine is still critical, say suppliers.
“Even in high speed and finishing applications, some machines are weldments. So they’re steel compared to Meehanite granite and they will create vibration. If your machine isn’t rigid during roughing and finishing it will create problems,” says Mark Larson, head of Makino’s Titanium research and development group in Mason, OH. The group focuses on testing and developing machines designed specifically to address titanium machining challenges. The company has developed what it calls its “Advantage Technology” for complex aerospace machining. Available on the company’s T-Series machinng centres, the concept encompasses five elements: a rigid machine construction, a high power, high torque tilting spindle (HSK-125, 4,000 rpm), high pressure, a high flow coolant system, a vibration damping system, and collision safeguard and autonomic spindle technologies.
Five axis machining is another consideration for titanium, adds Larson.
“There’s a big push to five axis machining and it does several things. One, it reduces the number of setups; you mount and clamp the part one time and you get better quality. Five axis allows you to move the part to an orientation not possible on other machines, you can keep the tool length shorter with little overhang to give you more stability and of course it allows you to machine part complex part geometries and features.”
With titanium and other heat-resistant alloys, machines designed with coolant delivery systems are essential. You want a machine that allows the maximum amount of coolant through the spindle.
If Sandvik Coromant’s MacNeil were to select a machine for machining titanium, the former machinist says in addition to selecting a rigid machine, “it would be equipped with a dual contact spindle such as a Capto HSK or BIG Plus and it would have the maximum amount of coolant pressure and volume through the spindle I could afford. It’s important to note that the tool you choose has the ability to deliver the coolant available from the machine. This can include adjustable size coolant nozzles as options.”
Indeed, in Mazak’s workshop in which the company machined a titanium guitar, “through-coolant or high pressure coolant was key in the success of machining titanium,” says Scott.
Makino’s Larson adds that coolant delivery in the machine is an important piece of the titanium machining puzzle.
“You need a machine that can deliver a lot of coolant at high pressures and high volume to the tool for better tool life and for higher metal removal rates.”
And while suppliers recommend investing in machines designed with coolant delivery systems, chip blaster coolant systems for higher pressures can be retrofitted on older machines, adds Larson.
The right cutting tools
No matter how well your machine tool is designed, you won’t succeed machining titanium unless you use the right cutting strategies.
“A lot of practices you use for high speed machining of aluminum, like soft entries, soft exits and maintaining a constant tool engagement, are also applicable to titanium. If you don’t follow these rules, you’ll break tools because titanium is less forgiving,” says Mazak’s Tim Scott.
Since titanium is resistant to heat and the cutting process generates heat, you need cutting tools that can help to reduce that heat.
“With aerospace applications, you’re looking for high metal removal rates in a very difficult material,” says Brian MacNeil, milling product manager with Sandvik Coromant in Canada. The best tools are high feed or high shear cutters. Indexable tools or button cutters are also suitable.
“You’re able to get a higher metal removal rate if you can use tools like these that thin the chip at the cutting edge and produce less heat. This will allow you to increase cutting speeds while increasing tool life.”
High feed milling poses some challenges, but the benefits outweigh the challenges, says Todd Miller, milling product manager for Seco Tools.
“The process offers amazing productivity, nearly triple the metal removal rate of conventional methods, and it increases tool life.”
When you select your tooling, ensure you use inserts with ground edges and PVD coatings, advises MacNeil. “The rule of thumb for inserts for machining titanium is sharp geometries and thin PVD coatings that help keep those insert edges sharp. Sandvik’s PVD inserts undergo a secondary sintering process and we’re able to produce an insert that has a very hard substrate and a very tough edge line that takes a lot of abuse.” Sandvik’s next generation PVD coating is called Zertivo. MacNeil says the company has seen up to a 40 per cent increase in tool life over older PVD coatings.
Tool engagement is important, adds Miller, which means you need to keep your high feed mills as engaged as possible “across the full diameter or less than half the insert width. One of the reasons these mills work so well is because their cutting forces are directed at the machine spindle in the axial direction to create balance. If you use the same cutter and only engage it 50 or 60 per cent of the diameter, you will experience push and increased vibrations because the cut is unbalanced.”
The right CAM program
A good CAM program, when combined with the right machine, the right cutting tool and cutting techniques, will help you succeed at titanium milling.
“For titanium, there are some key factors to look at beyond just high feeds,” says Daniel Remenak, product manager for GibbsCAM software products from 3D Systems. “If you’re looking at toolpaths, titanium tends to work harden so as you pass the insert through the part in a roughing application, you’re constantly hitting the same edge and this will damage the tool. CAM software suppliers like GibbsCAM offer a notch ramping function for turn roughing. So the program will ramp the tool instead of cutting straight on the path and this allows the contact point to move across the tool, which means the tool won’t wear out in one single location.”
Feeds and speeds are other key factors to consider. “Titanium has a narrow working window and cutting too slow or too fast will cause problems,” explains Remenak. To address this, GibbsCAM software has a function called CutData, essentially a library of speeds, feed and tool material that provides standard speeds and feeds for a given material and tool combination. “The software provides recommendations on depth of cuts that have been programmed and the tools being used.”
Another key factor to look for in your CAM software is to ensure it has an option for thin wall machining strategies. Titanium has the tendency to flex easily because it’s elastic. So when machining thin features, you typically reduce the axial depth of cut so you’re not applying pressure on the workpiece and that can lead to inaccuracies, says Remenak. “To address this we have a function called Advanced 3D Milling.” The software automatically creates fillet surfaces along any concave intersection so that the resulting toolpath has curved sections instead of sharp inside corners.
Increasingly, machine tool builders are developing CNC controls and software that work hand-in-hand with CAM software and, in some instances, perform many of the functions available with CAM platforms. Mazak’s new Smooth CNC software platform is one example, says Scott.
“A lot of CAM companies use dynamic motion type toolpaths versus a conventional zigzag roughing approach, which is archaic and slow. Dynamic motion allows you to round corners smoothly while maintaining speed. Our Smooth control is actually doing a little bit of this as well. It will round corners based on the tolerances you give it and maintain that speed. Together with the CAM system, our control works to make it more efficient for machining titanium and aluminum.”
Seco’s Todd Miller adds that cutter path optimization through proper programming is critical. “A good rule of thumb is to program an arc that is 50 per cent larger than the cutter diameter. If using a 2.0 in. cutter, program a 3 in. diameter arc. Programming an arc in the pocket corners reduces the angle of engagement and avoids overloading the cutter.”