Drilling composite/metal stacks has usually involved multistep operations to permit the use of drill tools optimized for each material. This requires either tool changes or the use of multiple drill motors. Despite these precautions, poor-quality holes are still common.
In the best circumstances, all drilling, to date, has required minimum quantity lubricant (MQL) — minute amounts of high-quality lubricant applied, ideally, only at the cutting interface. This requires a method for controlled delivery of the lubricant, typically through channels inside the cutting 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 developed a promising alternative: A simpler, one-shot, dry (yes, lubricant-free) drilling process for stacked materials that reportedly cuts tool cost by up to 50 percent, increases tool life by up to 200 percent and produces high-quality holes.
This hole drilled in a trailing edge component for the Airbus A350 XWB wing is produced by GKN. The different layers in the stack are clearly visible. (Source: Mapal)
Mueller-Hummel, who has sought a dry drilling option since his career began, 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 flutes to drive it out. Too much oil introduced into the tool's airstream causes significant overspray from the hole exits, to the point where jigs and even floors are slippery. OEMs then become concerned about contamination in the part, and cleaning is added as a secondary operation to machining. Dry drilling eliminates all this, and eliminates the need to design lubricant channels into the tool.
MQL tools must be fashioned from special materials that are not compatible with diamond-coating technologies. MQL system must be calibrated to ensure proper air and oil flow. The calibration process requires special training and must be completed before machining can begin. If the test fails, the whole system must be recalibrated. According to Mueller-Hummel, all of this time and expense can be eliminated by a shift to dry drilling. With no necessity to deliver lubricant, drilling equipment could be simpler and less costly. By eliminating MQL they could also use smaller, lighter drilling equipment, which is more reliable and requires less maintenance. 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.
In order to dry-drill H8 holes in composite-metal stacks in a single operation, the problems traditionally solved by using different tools, multiple drilling steps and lubricant had to be addressed in tool design and in control of drilling process parameters, specifically, rotational speed and feed rate. This required a fundamental understanding of the drilling processes for metals and composites.
The drill tool action to cut through composites vs. metals is very different. Developed before the advent of widespread use of composites, metal-drilling tools evolved from standard metals-cutting technology, where heat at the cutting interface actually 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. 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.
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). 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.