High-Performance Composites

MAY 2014

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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M A Y 2 0 1 4 | 4 9 coating technologies. "Without MQL, we can use the best raw material, which enables complete chemical vapor depo- sition (CVD) coating of the tool with dia- mond, which extends tool life by a factor of two." Mueller-Hummel also points out that the MQL system must be calibrated to ensure proper air and oil fow. The calibration process, he explains, "requires special training and must be completed before machining can begin. If the test fails, the whole system must be recali- brated." According to Mueller-Hummel, all of this time and expense can be eliminated by a shift to dry drilling. "One year ago, we started dry drilling with Bombardier on a wing application in Belfast [Northern Ire- land]," he relates. With no necessity to de- liver lubricant, drilling equipment could be simpler and less costly. "By eliminating MQL they could also use smaller, lighter drilling equipment, which is more reli- able and requires less maintenance." The result? He claims, "The cost per hole has been reduced by at least a factor of four, and the process has a higher CPK, with no jamming and sticking of wet carbon dust." Eliminating heat In order to dry-drill H8 holes in compos- ite-metal stacks in a single operation, the problems traditionally solved by us- ing different tools, multiple drilling steps and lubricant had to be addressed in tool design and in control of drilling process parameters, specifcally, rotational speed and feed rate. This required a fundamen- tal understanding of the drilling process- es for metals and composites. "The drill tool action to cut through composites vs. metals is very different," says Mueller-Hummel. Developed before the advent of widespread use of compos- ites, metal-drilling tools evolved from Heat generation zones By analyzing and mitigating heat generation zones, Mapal was able to develop a drill tool design that enables precision holes without MQL. standard metals-cutting technology, where heat at the cutting interface ac- tually melts the metal so that the tool can push through, similar to a hot knife through butter. "But that type of heat can damage composites," he notes. And in stacked drilling the heat "causes the metal to expand." The result is a larger hole diameter in the metal layer than in the composite layer. Thus, Mapal knew it must develop a tool geometry that works with metals at very low cutting temperatures. A fundamental study of drilling metals revealed three main locations where a drill tool generates heat: the tip (nose), the grip (edges/corners) and the chips (material removed from the hole). That stimulated the following develop- mental goals: • The tip. Design a tip that could cut effectively at lower rotational speeds (100 m/min for CFRP, 120 m/min for aluminum) without melting the metal. "It was not easy to develop a tip which heats up only the relevant area and with the minimum heat required without transferring heat to the remaining tool body," attests Mueller-Hummel. (See chart above.) • The grip. Because the tool body reaches its highest speeds at its Ideal situation with same diameters Actual situation with different diameters Dealing with diameters One issue when drilling composite- metal stacks is the larger hole diameter (typically 0.001 inch greater) in the metal layer, due to the fact that it experiences greater thermal expansion than the CFRP layer. Analytical Study on Heat Generation Zones Zones of Heat Generation 1. Tip 2. Grip 3. Chips Thicker chips An oversized hole Addition of micropecking Source: Mapal Margin with Clearance Guiding 2 Guiding 1 Feed 0.1 0.5 vibration 0,1 mm/rev 1 rev ✓ ✓ ✓ Source: Mapal 0514HPC_MachiningUpdate-OK.indd 49 4/22/2014 3:19:43 PM

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