Mueller-Hummel, P., "Process Understanding of Dry Drilling CFRP/Aluminium and AL/AL Stacks in IT8 Quality," SAE Int. J. Aerosp. 9(2):217-222, 2016, doi:10.4271/2016-01-2116.
Drilling holes into metal with MQL (Minimal Quantity Lubrication) is a normal procedure, because the drill is designed for drilling metal and the malleable capability of the metal compensates for the insufficient cutting capability of a worn out drill. Drilling composite materials using the same drill (designed for drilling metal) is a different procedure, because composite fibers are not malleable like metal at all. Due to this fact the tools become very hot trying to forge composite fibers like metal. The elastic behavior of the composite and the delamination inside the hole makes the tool temporary smaller than the diameter of the drill.The hole in the metal part of the stack remains slightly larger due to the heat and the thermal expansion rate. This paper shows how to drill metal and composite with the same diameter, so that achieving H8 quality is no longer a dream. Besides drilling H8 with cpk higher than 1.7, the dry drilling is one other important goal for aerospace assembly ever since. This paper will show how the dry drilling of aluminum/aluminum and composite/aluminum is feasible and well established in the serial production at the Airbus Bremen plant. The nice side effect of this new technology is the stability of the bores quality and diameter. Even when the tool reaches the end of it′s life the cpk and the absolute value remains the same. The slightly linear increase of the burr height acts as an “In Situ Measure Indicator” of the tool life.