High-Speed Machining for Aerospace
Successful high-speed machining requires attention to the cutting tool, spindle, and machine dynamics
By Bruce Morey
Enabled by advances in tooling and control capabilities, high-speed machining is finding wider acceptance in aerospace applications. Although especially good for aluminum, the technique is finding its place in composites and hard-metal machining as well.
Competitive pressures are constantly forcing manufacturers to machine parts more efficiently. At the same time, builders of aerospace structures are requiring stronger, lighter structures with closer tolerances. High-speed machining (HSM) can cut cycle time while also allowing manufacturers to cut finer, thinner parts than ever before.
HSM is a term used loosely by many people, according to Wayne Reilly, applications manager, Haas Automation Inc. (Oxnard, CA). While some say it is any spindle speed over 10,000 rpm, others have a little more complex definition, according to Reilly. "It really depends on the context of the term," he says. "A tool manufacturer might define it by rpm, and a machine-tool manufacturer might define it as some large-block look-ahead in the CNC controller. The technique tends to use fast speeds and feeds and lighter cuts, while conventional machining is usually slower with heavier, deeper cuts." For example, VMCs from Haas offer HSM options with spindle ratings up to 30,000 rpm and drive systems rated up to 30 hp (22.4 kW).
"Rather than discuss spindle speed alone, I like the term high-performance machining," says Randy Von Moll, platform manager-aluminum, MAG Cincinnati (Hebron, KY.) His definition includes the dynamic response of the machine as well as spindle speed. He describes five parameters that define high-performance machining: i) spindle rpm, ii) spindle power, iii) high feed and toolpath rates, iv) high acceleration and decelerations and v) high accuracy. The last three terms uniquely define the dynamic response of the machine, rather than spindle properties. "You really need the combination of the high-performance spindle with the high dynamic response of the machine tool to efficiently cut alloys like aluminum," says Von Moll.
Categorizing aircraft parts into two broad categories of 'thin-plate' and 'thick-plate', he believes what defines HSM for parts up to 50-mm thick are spindle rotations of 30,000 rpm and spindle ratings to 80 hp (60 kW). High speed for thick plates—thicknesses greater than 50 mm—is 18,000 rpm and spindle ratings to 135 hp (100 kW).
"The high dynamic response parameters are not really different for machines cutting thin or thick," explains Von Moll. "For either, acceleration/deceleration should be about 0.5 g, and you should offer as rapid a [noncutting] traverse as possible, at least 1500 ipm [38 m/min]."
The acceleration/deceleration (acc/dec) significantly influences the cutting time in complicated pocketed parts, where the tool has to make many direction changes.
The traverse time affects cutting time, especially 'parasitic' time, which can be as high as 20% of total cycle time in cutting aluminum. Parasitic time includes positioning the tool for a new cut or moving to a tool changer. "In lean terms parasitic time is an easy waste to eliminate. Achieving rapid-traverse rates in combination with high acc/dec rates is what led MAG Cincinnati to introduce our HyperMach Vertical series of profilers several years ago. These machines have a rapid-traverse rate of 4000 ipm [101 m/min]. Nobody can machine that fast—that speed is for reducing parasitic loss," explains Von Moll. Overall, the HyperMach Vertical profilers boast X-axis travel up to 33 m, Y-axis up to 3500 mm, and Z axis up to 1250 mm with additional A and B or C axes, and a spindle speed as high as 30,000 rpm. Most HyperMach Vertical Series installations feature twin independent spindles carried on a common X-axis gantry structure. "In response to the market demand for increased efficiency for monolithic parts up to 2000 x 4000 mm, the HyperMach Horizontal Series is being introduced and demonstrated at IMTS 2008," says Von Moll.
"Take a small chip and go as fast as you can," is the way Alan Hollatz, proposal engineer for Makino Corp (Mason, OH) defines HSM. "A smaller depth of cut at high speed leads to less cutting heat transferring either to the part or to the tool," explains Hollatz. "It also transfers less cutting force to the part and the machine." He describes conventional machining methods larger cuts with lower spindle rpm—as prone to distorting parts that are as thin as 0.030" (0.76 mm) in modern designs. Less force transferred also means reduced fixturing requirements.
When machining aluminum, Hollatz advises going as fast as possible in finish cuts. "If we have a 30,000-pm spindle, we are going to try and run at 30,000-rpm," explains Hollatz. "We also try to limit the diameter of the tools we use. The faster we go, the smaller the tool diameter because of centrifugal force causing imbalances." As an example, one of the larger Makino machines has a top spindle speed of 33,000 rpm with a 107-hp (80-kW) motor where he does not recommend using any tool bigger than a 50-mm-diam cutter. For most cuts, a 25 mm or smaller diam cutter is most efficient.
Like most other machine tool providers, Hollatz recommends hollow shank (HSK) toolholders over CAT type at higher spindle speeds. He notes that at high rpm there can be issues with accuracy in the Z-direction with the CAT type. According to Hollatz, it is not unheard of in extreme cases for CAT-type toolholders to get jammed into the spindle when operated at these high spindle speeds. The HSK design features a dual-contact taper and face, providing a controlled accuracy in the Z-direction. "You could run CAT up to 20,000 rpm but at 30,000 rpm, you really don't have a choice, you need to use HSK," he says.
Another key enabler for HSM is the CNC controller and its ability to move the machine accurately enough at high speed. Controllers with 'look-ahead' features that control the present speed and acceleration or deceleration of the tool based on where the tool is going to go is, by all accounts, just as important as driving the spindle at high rates.
The standard look-ahead on Makino controllers is about 60–80 blocks of G-Code ahead. Their Super GI.4 controller package is designed specifically for HSM and looks 180 to 250 blocks ahead, according to Hollatz. For the same cutter path geometries, Super GI.4 is 15–30% faster than the Makino SGI.3 controller it replaces.
Haas has a high-speed machining control option, according to Reilly, the applications manager for Haas. The Haas HSM option allows faster feed rates and more complex toolpaths without hesitation or starving the machine. Using a motion algorithm called "acceleration before interpolation" combined with full look-ahead of up to 80 blocks, the HSM option provides contouring feeds up to 500 ipm (13 m/min) without risk of distortion to the programmed path. "The biggest thing that this does is look ahead in the program during execution, and keep velocity as fast as possible for any change in direction," explains Reilly, "So, if the change in direction is only slight, then velocity needs to change very little. The change to velocity is proportional to the change in direction."
The standard look-ahead on Makino controllers is about 60–80 blocks of G-Code. Their Super GI.4 controller package is designed specifically for HSM and looks at 180 blocks ahead, according to Hollatz. The Super GI.4 is 15–30% faster than the Makino SGI.3 controller it replaces.
Machining composite materials is becoming a more urgent need in aerospace, as new airplanes use more of it to reduce weight. The Boeing 787, with fuselage and wing structures made of composite, is an extreme example of the trend. While HSM for aluminum may be a standard technique before long, its application for other common aerospace materials also seems to make sense. Certainly for composites. "While composites are made to near-net shape, achieving accuracy of mating parts, joints, and pockets means machining," according to Jeff Crick, platform manager for composites machining, MAG Cincinnati. "For instance, the original layup can create an access port in a wing skin, but only to approximately ±0.5-mm accuracy. The layup produces that accuracy. A secondary process like machining achieves better accuracies where they are needed, exactly as if you were finish-machining aluminum, titanium, or steels."
HSM for composites requires less horsepower and torque than for aluminum, according to Crick. The machine itself does not need to be massive, as it must be to cut titanium, but still needs to be rigid to overcome vibration and harmonics. He reports that most spindle speeds are 10–13,000 rpm, although they can go much higher. He cites one example of composite HSM at a large domestic aerospace OEM where cuts in composite material of 0.012–0.016" (0.3–0.4 mm) are made on a 24,000-rpm machine.
Units originally designed for cutting metals are used to machine most composite materials at present. Crick believes the ultimate goal is to build a composite machine tool that is lighter and purpose-built for composites. One trend such a machine has to address is the growing size of composite parts in aerospace.
"You can have very large composite parts, for instance wing skins that are up to 100' [30-m] long," says Crick, "even whole fuselage sections such as the new 787 with barrel sections that are more than 20' [6-m] in diam and 30' [9-m] long. The interfaces have to be machined to tight tolerances between one fuselage section and the other on these large structures. Other parts can be long and 'stringy,' such as spars, stringers, struts, and floor beams."
To address machining these long, thin parts that have a lot flexion—Crick describes them as acting like wet noodles—MAG Cincinnati has developed a purpose-built extrusion mill. Adaptable to either aluminum or composites, it is a feed-through process with a 13 x 8' (4 x 2.4-m) work zone, a spindle rated at 24,000 rpm that uses up to 12 tools no more than 25-mm in diam. Parts can be up to 40' (12-m) long.
"While nearly all applications and materials can benefit from HSM, free-cutting materials, like aluminum or composites, tend to benefit most," says Reilly, the applications manager for Haas Automation. "But even hard mold steels benefit from HSM because of the trend to hard milling, where high feeds and speeds are used with light cuts." Titanium, a material growing in importance in aerospace applications, is certainly one of those metals.
"Aluminum cutting machines are more like a Formula 1 race car and titanium-cutting machines are more like a bulldozer," says Dan Cooper of MAG Maintenance Technologies' Productivity Solutions (Hebron, KY). "There is a drastic difference in spindle rpm. Although the principles of high-speed machining—a shallow cut at high rpm—is still sometimes relevant for titanium." In particular, thin-walled parts are best machined using HSM principles, says Cooper, describing for example, a customer part with dimensions of 0.030" (0.76-mm) thick and up to 3" (76-mm) high. "Such a thin, high wall cannot be roughed with the old, conventional process. Deep cuts at low rpm and high torque distort the part and deflect the cutting tool. This is especially true for newer components machined from 5553 Titanium alloy," says Cooper.
Titanium's low thermal conductivity and a high modulus of elasticity, combined with its strength, makes it a difficult material to cut, according to Cooper, "While torque and dynamic stiffness may not be as important to composites and aluminum, they are very important for machining titanium," says Cooper, limiting how high a speed one can achieve when compared to aluminum.
Rather than spindle speed, Cooper uses surface speed and feed rate as a measure of HSM. The surface speed is a function of spindle rpm and cutter diam; the feed rate is a function of rpm and flute density. This makes cutting tool design vital, because more flutes at a higher surface feet/minute (SFM) means higher feed rates.
Describing MAG's new carbide tooling, which runs at 390 fpm "With a 1" [25.4-mm] cutter and maximum number of flutes, we may only be running at 1500 rpm and 100 ipm [2.5 m/min], but that is very high-speed machining in titanium," says Cooper.
Others are looking ahead to the newer, harder materials like titanium as well, seeing that the technique has proven itself for aluminum.
"High-speed machining aluminum is becoming almost standard today," says Rudy Canchola, application manager for Mazak Corp.'s Western Regional Headquarters and Aerospace Technology Center located in Gardena, CA. For him, the greater machining challenge today is in the high-temperature alloys, like stainless 15-5 or titanium 5553 or 6Al4V. These are materials finding more and more use in aerospace. His recent projects using a variety of tools on Mazak equipment included titanium tests on the Mazak VCN-510 C VMC. "We have demonstrated machining titanium with solid carbide end mills up to 500–600 fpm. We think that is impressive," says Canchola.
They also tested 15-5 stainless steel using the Mazak Vortex 815-II five-axis machining center. Tools from Seco, Ingersoll, Kennametal, and Sandvik were tested on the stainless material. Using a climb cut method; he reports speeds of 400–600 fpm.
"Most of our machines have the ability to perform at these high surface feed rates. If the customer is going to be cutting this type of material, we provide them with the information we developed in these tests," says Canchola.
For machining higher-temperature alloys, the look-ahead feature of the controller is not as important as in aluminum, since the speeds simply are not as high. A feature of control that is important is the ability to measure the loads on the spindles and axes and adjust accordingly. Mazak equipment senses electrical feedback from the servomotor and adjusts speed to match machining conditions—even stopping the machine and changing the tool if needed.
This article was first published in the March 2008 edition of Manufacturing Engineering magazine.