Mueller-Hummel, P., Sripathy, P., and Atarsia, A., "Fail Safe Drilling of CFRP/Titanium and /Aluminium Stack with H8 Quality for Aerospace Applications," SAE Technical Paper 2013-01-2223, 2013, doi:10.4271/2013-01-2223.
Ever since the advent of fiber reinforced polymer materials in the field of Aerospace, Metal-FRP stacks started to gain importance due to their superior fatigue performance, phenomenal low weight and good specific strength. However the machining, specially drilling these multi stack materials has always proved to be a challenge for the field of manufacturing and assembly. Drilling holes in only metal with a drill (metal drill), the material removal is through a process of clear shearing since the tool is much harder and sharper than the base material. The tools hence wears at a much slower and gradual rate, also the malleable properties of the machined metal compensate to the reduced cutting capability of the worn out drill. These properties of the machined metal act like a ‘FAIL SAFE’ mechanism during the machining process assuring a trouble free fail safe environment during the drilling process. However, drilling FRP composites is altogether a different story. The very properties that make the FRP composite superior make it a challenge for machining. The mechanism of machining viewed here is more or less fracturing and breakage of the fibers present in the system. This breakage of fiber also induces fractures in the tool resulting in accelerated tool wear. These characteristics render the process unreliable. Also the temperature during the machining of composites is higher than while machining a metal due to the lower thermal conductive properties of the composite. When two such extremely different elements are brought together in the form of a FRP-Metal stack, various challenges are faced to achieve required hole size, quality, tool life and most of all a FAIL SAFE Process.This article elaborates about methods to achieve a FAIL SAFE process during the drilling of CFRP/Titanium and Aluminum stacks just by adding and optimizing certain geometric features to the drill. The article also goes through simplified theoretic models which show how CFRP/Titanium stacks should be machined “Fail Safe”, inside the required Cpk tolerance, without scratches and burn marks even when the drill is during its last stages its life. The concept of Micro Reamer Technology (patented by MAPAL) is discussed and the application of this technology to achieve the required hole quality, burr and other important parameters is elaborated. The only apparent indicator for the end of tool life would be a linear increase in the burr height.