Hole in Four… or More

Effectively drilling stacks, which have multiple layers of composites and metals, takes the right combination of tool and machining parameters.

From the F-35 Lightning II to the mammoth Airbus A380 and the Boeing 787 Dreamliner, composite/metal stack materials, most often consisting of multiple layers of carbon fiber-reinforced plastic (CFRP) composites and aluminum or titanium, have made a big a splash in the aerospace industry.

The Dreamliner, for example, is made up of (by weight) 50 percent composite, 20 percent aluminum, 15 percent titanium, 10 percent steel and 5 percent other materials. By volume, the aircraft will be 80 percent composite. Each 787 contains approximately 35 tons of CFRP, a good portion of it stacked in conjunction with aluminum or titanium alloys.

Courtesy of Boeing

Constructed of 80 percent composites by volume, each Boeing 787 Dreamliner contains 35 tons of CFRP, and a good portion of it is stacked with aluminum or titanium alloys.

Courtesy of Novator

An example of a metal/composite stack stack.

Aircraft manufacturers like stack materials because they combine metal’s high strength with composite’s low weight and corrosion resistance. The variety of stack materials is increasing nearly as fast as the applications, according to Glenn Sheffler, project engineer at the National Center for Defense Manufacturing and Machining (NCDMM), Latrobe, Pa.

“Composite stack materials are replacing aluminum honeycomb materials, which consist of honeycomb paper sandwiched between layers of aluminum,” Sheffler said. “Often, stacks have various layers of composite, or a composite/metal stack combination, or foam or other core materials, and are then wrapped with composites.” Some stack materials may also incorporate a thin copper mesh designed to protect against lightning strikes, or other materials, he added. Overall stack thicknesses can vary from less than ¼” to several inches.

“Composite layup techniques also make a difference in machinability, and there could be multiple types of composites as well,” Sheffler continued. “To achieve the required strength in a composite structure, many times carbon fibers will require weaves and different layups in multiple directions, depending on the application.”

NCDMM’s involvement with stack materials began several years ago during development of manufacturing processes for the F-35. Since then, the organization has continued to work with defense manufacturers to develop drilling processes for stack materials.

“The material could be for an aircraft skin, it could be for weapons systems components—wherever they are trying to reduce weight,” said Joe Slusarcyk, project engineer. “As composites evolve, there will be more and more applications.”

Hole Strategy

NCDMM takes sample stacks sent to them by defense manufacturers and develops the best holemaking process for the materials. Engineers rarely see anything but test panels and for proprietary reasons may have only a generic idea of the materials making up the stack—something that makes the task of developing optimal drilling processes all the more challenging. “We drill holes until we find the solution,” Sheffler said. “That could be five holes or 5,000.”

Engineers use dynamometers to track cutting forces during drilling, and the optimal process is usually a tradeoff between low cutting force and tool life. “You want light cutting forces to minimize composite delamination and stress on the material,” Sheffler said. “Once you find a drill that gives you light cutting forces, it may, however, wear out after a minimum amount of holes. So we look for a happy medium between low thrust and tool life when developing a tool.”

Courtesy of Amamco Tool

Potential quality issues in drilling composite/metal stacks include size changes from one material to another as well as fraying and delamination. From top: clean hole, frayed hole, delaminated hole.

The process is complicated by variables other than the workpiece. Quality requirements, the order of materials in the stack and even whether the holes will be drilled using a CNC machine or by hand all make a difference in the needed tools and machining parameters. “Inspection requirements differ depending on the application,” Slusarcyk said. “Sometimes it might be only visual inspection as opposed to ultrasonic or some other technique.”

If a metal skin is on one surface, the drilling direction makes a difference. “Depending which side you enter, it can change the whole process,” Slusarcyk said. “Sometimes, if there’s a backing material, it is pretty stable. But in other cases you might have a thin skin that just blows out completely when you start pushing it. So fixturing is a very important piece of the puzzle.”

Tool Talk

Fortunately, cutting tool suppliers work with NCDMM and other organizations to develop the tools needed for effective drilling of composite/metal stacks. A frequent partner of NCDMM is Amamco Tool, Duncan, S.C.

“In drilling, cutting tool geometries vary based on the composition of the stack, and the geometries are endless,” said Peter Diamantis, Amamco plant manager. But, he added, there are some general guidelines.

According to Diamantis, the widely differing properties of materials in a composite/metal stack are the main obstacle to productive drilling. “Long point angles and long tapers work best when the drill exits the composite material,” he said. “Composites build up a lot of heat, which is not an issue in relatively thin material, but in thicker materials you need narrower flutes, wider gullets and tighter spirals to allow faster penetration.”

Courtesy of Cajero

The need to drill disparate materials such as CFRP and titanium results in unique tool geometries. This drill from Cajero has open, polished flutes to facilitate chip flow and double margins to support alignment and shave hole walls on materials that contract when machined. The combination point geometry is aimed at clean holemaking in fibrous materials.

Long taper angles tend to smear aluminum and cause drill loading, so knowing the placement of the aluminum in the stack is critical. “If the aluminum is on the back side of the stack, a high-shear drill with a sharp point angle will minimize the burr height of the exit hole,” Diamantis said. “If you are exiting on the composite material, you need to adjust drill geometry as well as feed rate. This is where drill/reamer combination tools and multifaceted drill points can work well.”

Diamantis calls composite/titanium stacks “the mother of all headaches” when it comes to drilling. “When drilling titanium, you typically want low reliefs, rake angles, feed rates and spindle speed—everything composites hate,” he said. “So you have to take into account everything, including the thickness of the stack and the number of holes you wish to achieve. The setup, the drill motor, using coolant or dry, hand or machine drilling—the entire process has to be evaluated.”

According to Diamantis, Amamco strives to produce holes in a single operation. “We do everything in one shot—we don’t drill and then come back and ream,” he said. “A ¼” stack is not too hard, but thicker stacks become more difficult. So you have to provide tools that are specifically designed for drilling holes with a depth four and five times the diameter of the drill.”

CNC or Manual?

Alex Harding, operations manager for toolmaker Cajero Ltd., Kent, England, echoes many of Diamantis’ comments. “It’s essential that the end user work closely with the cutting tool supplier in identifying the correct drill for the application,” Harding said. “You have a responsibility to the customer, their machines and the $50,000 part that nobody wants to scrap!”

Harding emphasizes that the type of drilling operation—CNC or manual—can make a big difference in the cuting tool selected. “For CNC drilling of composite/titanium stacks, the best solution will likely be solid-carbide tools with through-coolant, although CVD diamond coating could also be considered,” he said.

Although coolant or mist is not generally used when drilling CFRP, which can swell because of coolant absorption, it can be preferred when drilling titanium, which tends to workharden.