As composite parts increase in size and complexity, so do requirements for trimming, finishing and drilling holes for fasteners. Abrasive machining methods can be tough on composites; because the high-strength fibers don’t break easily, they tend to be pulled by the cutting tool. This can lead to microcracking and delamination. And overheating at the drilling site can take the resin above its glass transition temperature and damage the laminate.

Added to these risks are those associated with machining composite/metal alloy hybrids or “stack” materials. Each material in a stack exhibits different machining characteristics. The drilling solution, then, must provide a clean hole through each material, without causing damage to the part or leaving splinters or burrs that require additional processing.

Dormer Tools Ltd. (Sheffield, U.K.) has developed cutting tools for drilling applications that involve stacked materials in aircraft wingskin assemblies. In one case — a stack of 10-mm/0.4-inch thick aluminum and 4-mm/0.16-inch thick carbon fiber for a proprietary client — Dormer’s European key account manager for aerospace and composites, Ricky Payling, says the customer ordinarily would have to cut a smaller-diameter hole, first, and then gradually enlarge it to the finished size. This sequence requires considerable time and several different cutting tools.

“However, with Dormer’s new R790 micrograin carbide composite drill,” he says, “we were able to machine through-holes with no delamination or splintering of the carbon fiber and no exit burr on the aluminum.” The R790 is available with optional coatings, including a True Diamond coating, and it has a unique patented point geometry designed to reduce thrust and torque and, as a result, cutting temperatures. It is also available with optional internal cooling for minimum-quantity lubricant (MQL) drilling processes.

Another application involved drilling through a composite stack with a woven fiberglass exit face. “With this type of material,” explains Payling, “it is extremely difficult to produce a hole with a perfectly clean periphery.” For this challenge, Dormer’s patent-pending solution features a special eight-facet geometry and True Diamond coating. This tool is said to consistently bore holes with excellent exit quality, showing no signs of delamination, splintering or fraying of the glass cloth.

The company reports that it offers tool geometries that can be tailored to specific customer applications. Additionally, Dormer Tools offers a range of tools made with veined polycrystalline diamond (PCD) for use in high-rate production environments.