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Milling Titanium Takes Muscle and Smart Planning

Posted by: 7. March 2016

Fives Giddings & Lewis recently demonstrated the capabilities of its high-torque T-Rex spindle on an HMC 1600 by milling out 33.1 cubic inches per minute from a block of 6AL-4V titanium. Driven at 245 rpm at a 3" axial depth of cut and a 1.0" radial depth of cut, the 3"-diameter Harvi indexable-insert end mill produced good surface finish with no sign of chatter and virtually no vibration, says Pete Beyer, director of product strategy and development.

Here’s a video of the demonstration:

The T-Rex Spindle

“The power of this spindle is so great that it easily handled the full Y-axis thrust of the machine, which can be equipped with higher torque axis motors to produce even higher metal removal rates,” Beyer adds. Both machines in the rigid, four-axis HMC series—the HMC 1250 and 1600—are equipped with the T-Rex spindle shown in the video. The 80.5-hp T-Rex delivers as much as 1,918 foot-pounds of torque to achieve its high metal removal rates on large titanium, Inconel and Carpenter 465 workpieces. Its design incorporates 5.1" ID bearings to handle those machines’ 9,500-lb cutting thrust capability. As you can see in the video above, the demonstration includes cutting both horizontally and vertically.

Smart Milling of Titanium in Your Shop

Obviously, big airplane parts require a certain hefty minimum of muscle. But as Mark Albert of Modern Machine Shop has noted, there’s far more to effectively milling titanium than raw power. In an article titled “A Practical Approach to Milling Titanium,” he quotes Mike MacArthur, VP of engineering at cutting tool manufacturer RobbJack, who points out that regardless of the machine, the rules for milling titanium don’t change. “What have changed,” MacArthur says, “are the options available for controlling variables that these principles are based on.”

For example, Albert writes, “toolpath algorithms that ensure constant engagement of the cutter in the workpiece material are especially effective—and necessary—in titanium. Super-accurate toolholders with highly concentric, rigid gripping force address another requirement for titanium machining. A high-pressure coolant delivery system on a machine is advantageous. Of course, the range of end mills designed for various machining operations in titanium are now well developed and available from many cutting tool manufacturers.”

With the help of MacArthur, Albert laid down a few guidelines for managing the cutting process:

  • Tool paths: To protect the cutting edge from heat produced by milling titanium, it can be helpful to formulate tool paths that restrict how much of the tool radius comes in contact with the workpiece at any time.

  • Number of flutes: One rule of thumb is to choose an end mill with fewer flutes if they will likely contact the workpiece surface, and more flutes if not.

  • Toolholder: Highly accurate toolholders are a must, since insecure clamping can lead to chatter and compromise the rigidity of the entire cutting tool assembly. Make sure the toolholder is well-balanced.

  • Shank roundness: Along with making sure the toolholder is accurately clamping, check that the tool shank has a high level of roundness. Shank roundness and toolholder quality go hand-in-hand, MacArthur says, especially with smaller-diameter tools.

  • Coolant: A high-pressure, through-spindle coolant system promotes chip evacuation to further ease titanium cutting. Additional external coolant ports are a plus, too.

The full article goes into much more detail with each of these points.

Learn more about Five Giddings & Lewis’ machining centers and compare models here at Techspex

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