High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.
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4 8 | H I G H - P E R F O R M A N C E C O M P O S I T E S FEATURE/MACHINING UPDATE I n the aircraft industry, where me- chanical fastening of joined com- ponents is a necessity and the drill- ing of many thousands of holes per aircraft is, therefore, unavoidable, stacking the parts in joint position and drilling them in a single operation not only saves time, but also ensures proper hole alignment when fasteners are in- serted. So-called one-shot drilling can produce a high-quality hole if all the joined parts are either all-metal or all- composite. When carbon fber-reinforced polymer (CFRP) and aluminum and/or titanium parts are stacked, however, the story is much different. Drilling composite/metal stacks, therefore, has usually involved multistep operations to permit the use of drill tools optimized for each mate- rial. This requires either tool changes or the use of multiple drill motors. De- spite these precautions, poor-quality holes are still common. And in the best circumstances, all drilling, to date, has required minimum quantity lubricant (MQL) — minute amounts of high-qual- ity lubricant applied, ideally, only at the cutting interface. This requires a method for controlled delivery of the lubricant, typically through channels inside the cut- ting tool. And that, in turn, increases the cost of an already costly drill tool and complicates the drilling process. This was the state of the composite/ metal stack drilling art until cutting tool manufacturer Mapal (Aalen, Germany) developed a promising alternative: A simpler, one-shot, dry (yes, lubricant- free) drilling process for stacked materi- als that reportedly cuts tool cost by up to 50 percent, increases tool life by up to 200 percent and produces high-quality holes (specifcally, H8 with a CPK of 2.4, see "Hole quality defned," p. 51). Why dry drilling? "MQL was developed to lower the buildup of material on the cutting tool and aug- ment chip removal, but these are func- tions a good tool design should achieve," contends Dr. Peter Mueller-Hummel, se- nior manager for Mapal's aerospace and composites business unit, adding that lubricants hide shortfalls in tool capabil- ity. "We are using MQL to span gaps in tool performance." Mueller-Hummel, who has sought a dry drilling option since his career be- gan, says the use of MQL compensates for some problems, but actually causes others. First, the lubricant actually wets carbon dust, keeping it in the hole rather than allowing the tool futes to drive it out. "Often, too much oil is introduced into the tool's airstream," he adds, "caus- ing signifcant overspray from the hole exits, to the point where jigs and even foors are slippery. OEMs then become concerned about contamination in the part, and cleaning is added as a second- ary operation to machining." Dry drilling eliminates all this, and eliminates the need to design lubricant channels into the tool. "If you eliminate MQL," he maintains, "you cut tool cost in half, right off the bat." Further, MQL tools must be fash- ioned from special materials that, he ar- gues, are not compatible with diamond- One-shot dry drilling of stacked materials Tool design innovations tighten tolerances and cut costs for those who drill composite-metal assemblies. BY GINGER GARDINER Hole-ly focused on composite-metal stacks In this example hole drilled in a trailing edge component for the Airbus A350 XWB wing produced by GKN (Filton, U.K.), the different layers in the stack are clearly visible. Source: Mapal 0514HPC_MachiningUpdate-OK.indd 48 4/22/2014 3:19:16 PM