Achieving optimum cubes per minute is a function of the total machining system.
It’s not too difficult to understand the importance of machining aluminum for aerospace applications. High volumes of aluminum are used, principally for structural components. Machining systems are purpose-built for high metal removal rate (MRR) production because of the overwhelming amount of material that must be removed quickly.
The 7000 series of aluminum is made with a combination of zinc and magnesium or zinc and copper and pose significant challenges to increasing MRR and throughput while improving quality. Series 7000 aluminums are typically used for aerospace structural components and skin. They require high-capability machines, typically five-axis units with 30,000 rpm spindle capabilities and 80 to 100 hp (60-75 kW).
Sources contributing to this article agreed that the solution involves maximizing the performance of the
total machining system, including the machine tool, cutting tools, and toolholding systems.
Systems Approach to MRR
For machining higher tensile strength aluminum alloys like 7075 and 7050, where zinc is the main alloying element, Walter USA LLC (Waukesha, WI) offers custom tooling. According to Luke Pollock, product manager, “Three-flute solid-carbide tools do the best job of damping and quieting the harmonics of spindle vibration. These tools aren’t your everyday aluminum machining tools. The MB 266 solid-carbide finishing and MB 265 solid-carbide roughing tools are designed to handle high throughput on dedicated aluminum machining centers with 30,000+ rpm spindles,” Pollock said.
They are able to run at the elevated surface footages that Series 7000 aluminum requires for high-productivity deep pocketing of wing structures and other structural components. “These tools are designed to run at elevated surface footages, typically 6,000 to 7,000 sfm and as high as 10,000 sfm,” said Pollock. “Without spindle speeds above 30,000 rpm, it’s difficult to maximize the machine’s MRR with a smaller tool, so they’ll run larger 2″ or 3″ (51 or 76 mm) tools to achieve the higher surface footage.”
While MB 266 and MB 265 are solid-carbide tools, the M2331 is an indexable insert cutter with either 15 mm or 20 mm carbide inserts that feature up sharp edges with high rake and clearance angles and polished face.
“We recommend using a shrink-fit holder. By its very nature, an end mill holder would have a lot of imbalance because of the bolt on one side. Using the shrink-fit holder, the whole tool and toolholder should be dynamically balanced as one unit to prevent pull-out due to high centrifugal forces.”
To avoid built-up edge (BUE), Pollock said inserts should be polished to a very smooth, almost mirror-like finish. Together with a high rake angle and high clearance angle, the up sharp tools keep the aluminum from sticking to the edge of the cutting tool and creating BUE. “Also, in most instances we prefer an uncoated insert because coatings, however thin, can reduce the sharp edge, increasing the coefficient of friction and making the edge rougher.”
The main challenges in machining Series 7000 high tensile strength aluminum are not too different from other aluminums. “Throughput and cycle time that depend on high MRR are the goals of advanced machining processes. Tool life isn’t a big concern as it is with machining steel, for example,” he said.
Walter focuses on providing total solutions for specific customer parts. “We recommend which tools to use as well as the processes for roughing, finishing the bottom of the pocket, blending the corner radius between the sidewall and the bottom of the pocket, and which process can give the best cycle time and part quality at the lowest cost,” said Pollock.
When machine performance improves and actually outperforms the tool, a tool is upgraded. “That was the case with our M2331, which is an upgrade of the M2131, because we want to push the machine to its limits,” he said.
It makes a big difference if you’re working with the latest high-speed, purpose-built machining centers with spindle speeds of 30,000 rpm and up, or with legacy machines with lower horsepower ratings. Just taking a high-speed spindle and installing it on a general-purpose machine for aluminum isn’t likely to produce optimum results.
Demand is high right now for machines with the right design characteristics for machining Series 7000 aerospace aluminums, according to David Ward, product marketing manager, Makino Inc. (Mason, OH). “The vast majority of parts require machining monolithic structural components. If you peel the skin off a jet airplane, you reveal a skeletal structure with ribs linked by stringers and leading edges. All of those components start out as a billet of aluminum, a large aluminum plate that is hollowed out by pocketing to reduce weight.”
Aluminum is a horsepower-constrained material. This means the more power you can apply, the greater the MRR. Makino’s purpose-built MAG series features a 130 kW (174 hp), 33,000 rpm spindle. “But it’s not just horsepower—it’s where the horsepower is deliverable to the cutting tool,” said Ward. “We’ve designed the motors in such a way that they ramp up and reach full horsepower at 28,000 rpm and stay level all the way to 33,000 rpm, maximizing MRR. Reaching full horsepower at 28,000 rpm is beneficial when roughing with inserted milling cutters. These cutters often have rpm limitations.”
Where the geometry of the parts isn’t so open and parts are made up of multiple pockets and thin walls, the tool of choice is the 1″ end mill, according to Ward. “If you have a small pocket, say 1.5-2″ (38-51 mm) on each leg, you will spend a lot of time just spiraling into these pockets and moving around the periphery of these pockets. The ability to feed fast, say at 62 m/min, is important, but so are accel/decel rates of the linear axes.”
In machining a 2″ pocket, “you’re never going to get up to that speed because as soon as you start ramping up to the speed you have to make a right-angle change and go up in Y and over in X because of the part geometry,” Ward noted. “The ability to accelerate at 1 G for linear axes and change the rate of acceleration, called jerk, smooths the lead into and out of the linear move.”
Range of motion of the horizontal spindle in MAG machines that tilt up and down allows chips and coolant to flow away under gravity, according to Ward. The A axis allows the spindle to tilt up and down ±110º. The C axis is behind the spindle and is an infinite controlled rotary axis for positioning around pockets. This range of rotary axis motion allows operators to access five sides of the part in a single clamping.
Shrink-fit toolholding and balancing the tool in the toolholder are critical. “At 33,000 rpm, a small amount of imbalance becomes a massive force to the cutting system,” said Ward. “All tools have to be balanced to 2.5 G. HSK A—which has two drive keys, one positioned lower than the other—should be avoided and HSK F without drive keys should be used,” said Ward.
Reducing HP Per Cube Removed
Joel Radner, sales manager, solid tools for Niagara Cutter LLC (Reynoldsville, PA), and Eric Gardner, senior product specialist, solid tools for Seco Tools LLC (Troy, MI) outline a strategy for maximizing MRR for machining aerospace aluminum as diverse as 7000 series, as well as newer abrasive materials like aluminum lithium. Niagara Cutter’s primary focus is on the broader market where spindle speeds of less than 20,000 rpm are encountered. For high MRR aluminum applications, typical of aerospace structural components, where spindle speeds of more than 20,000 rpm and up to 30,000 rpm are utilized, Seco Tools Jabro solid-carbide cutters are recommended.
Seco’s Jabro solid-carbide end mill product line features two main geometries: a serrated edge rougher and non-serrated edge used for roughing and finishing. “The cutter and toolholding system are selected based on their ability to maximize MRR in application-specific machine tools that have spindle speeds approaching 30,000 rpm, up to 120 kW of power and high accel/decel capabilities,” said Radner.
“Machining Series 7000 aluminum for structural aerospace applications usually begins with roughing larger exterior features with indexable insert milling cutters,” he continued. “Then, remaining interior part features are roughed and finished with solid-carbide end mills. The key to reducing horsepower required per cubic inch of material removed is found in the right combination of machine spindle, up sharp and high rake angle cutting tools, and properly balanced toolholders. Every tool must be balanced in its holder as an assembly. High-quality toolholding systems with excellent concentricity that provide high gripping force are necessary to drive today’s tooling at the recommended parameters.”
When it comes to machining abrasive aluminum lithium, which is significantly more difficult to machine than 7075 or 7050 aluminum, in addition to up sharp cutters, shops need a tool with a wear-resistant coating, according to Gardner. “Many carbide inserts for indexable cutters are up sharp with high rake angles, but without wear-resistant coatings, they tend to wear out rapidly when machining aluminum lithium,” he said.
To ensure wear resistance and long tool life while maintaining free-cutting, sharp edges, Gardner recommends a submicron grade solid carbide be selected. Coatings with high hardness and lubricity ensure that chips flow freely over the cutting surface and do not adhere to the cutting tool, in turn preventing BUE or galling that reduces tool life and negatively affects part quality. High-quality toolholding systems provide concentricity to minimize runout and have a lot of mass and a short gage length to maximize MRR.
Shrink-fit toolholding is recommended and a dynamic harmonic signature should be established with TAP testing (an evaluation tool) for vibration analysis, he said.
Radner also recommends vibration analysis to establish the stability lobe zone, or area with chatter-free machine harmonics, which is absolutely essential for maximum MRR. Gardner noted that the higher the rpm, the narrower the stable spindle speed zone. “In fact, changing the spindle speed by 500 or 1,000 rpm can take the machine from a completely stable cut to a high-chatter condition. If you have two different tools with a 1″ [25.4 mm] difference in stick out of the tool, there will be an entirely different stability chart,” Gardner noted.
High-Productivity Al Machining
Typically, almost any machine can be used for cutting aluminum; however, machines with a high-speed spindle are ideal, especially when high productivity is desired, according to Tyler Hashizume, engineering production manager, OSG USA Inc. (Glendale Heights, IL). “The size of the machine will depend on the size of the parts,” he said. “For example, large aerospace parts like ribs are most efficiently machined on powerful, high-speed machines like Makino’s MAG machines.”
Cutting tools must be designed specifically for these machines to maximize their capability, he said. “Uncoated tools are commonly used to machine aluminum because they can be very sharp. However, care should be taken to select proper cutting speed and to use a good coolant to control heat. Regardless of whether the tool is coated or uncoated, water-soluble oil is best because it provides a balance of lubrication and cooling.”
Hashizume explained that the two most important things to prevent welding are tool coatings and coolants. “For aluminum alloys with lower silicon content, DLC [diamond-like carbon] coating is suitable for machining due to its high lubricity and low friction coefficient. For cast aluminum with a higher silicon content, DLC wears easily because of the material’s abrasiveness. Diamond coating is more suitable than DLC coating because it is much harder and it prevents cutting edge wear.”
To efficiently evacuate chips, large flute pockets are required. “Without good chip evacuation, chips may become packed in the flutes and then soften due to the cutting heat and weld to the tool. Another way to improve chip evacuation is to polish the flute, which provides a smooth surface. To minimize cutting heat, cutting geometry should be as sharp as possible.”
An Aerospace Material Outlier
Ask anyone in the aerospace machining supply chain about titanium aluminide (TiAl) and you’ll get everything from a knowledgeable discussion to a gasp of recognition about the difficulties of machining the intermetallic material. TiAl offers great promise for jet engine air foils because it’s lighter and can take a lot of heat. Jet engines, like the LEAP and Gear Fan can burn fuel more cleanly and more efficiently. The material, however, poses significant challenges to machine tools, including
turn mills and five-axis vertical machining centers that must be stiffer and control heat at the cutting edge more consistently.
Carbide cutting tools with special edge prep are having the most success in machining TiAl, which is highly abrasive, brittle and subject to being distorted or twisted under machining pressure.
According to Edwin Tonne, technical trainer, Horn USA Inc. (Franklin, TN), the biggest challenge in machining titanium aluminides is to find the appropriate edge geometry to avoid BUE. “For more widely applied materials such as Ti6Al-4V (grade 5), normal sharp geometries with small edge prep are appropriate and will work on alpha/beta and beta phase titanium alloys economically. From my past experience, titanium aluminides reject normal conventions and are readily machined with sharp geometries and careful selection of grades.”
Tonne said that his past testing revealed that ground geometries with sharp cutting edges are the most economical. He advises avoiding fully sintered inserts or cutting tools, specifically because the edge prep is normally too large, and looking instead at options with less than 0.025 mm radial hone.
“Micrograin carbides with hardness in the ISO S10-S30 range are suitable and will retain edge sharpness at lower cutting speeds,” he said. “Coatings with very low friction coefficients, smooth surface and good adhesion at the edge are the most appropriate. Titanium diboride (TiB2) coatings fit these requirements well. They can be applied to very sharp cutting edges without risk of spalling or delamination. In general, when using cutting tools with sharp edges, parameters similar for beta titanium like Ti1023 can be used to machine titanium aluminide.”
Editor’s Note: A thorough discussion of the challenges of machining TiAl, “Cutting Tools for Use with New Aluminum Compound Materials,” can be found in the January 2017 issue of Manufacturing Engineering.