Investigation of Machinability and Dust Emissions in Edge Trimming of Laminated Carbon Fiber Composites

Abstract

The use of carbon fiber reinforced plastic (CFRP) systems for product structures has been steadily increasing because of superior material properties, including high strength, low weight, and corrosion resistance. CFRP parts may be molded to near net-shape geometry; however, machining operations are often necessary to provide final shape. Edge trimming by the milling process is commonly employed to provide required surface finish and dimensional accuracy and can be performed wet or dry. CFRP does not machine similar to ductile metals as the chip formation mechanism consists mostly of a series of brittle factures by shearing of the fibers and matrix where ductile metals produce a uniform chip through plastic deformation. In dry machining, CFRP produces powder like chips or dust which are electrically conductive and currently classified as a nuisance particulate. Further machining challenges of CRFP include delamination, fuzzing, fiber pullout, and short tool life due to the inhomogeneity of the material and abrasive nature of the fibers. In this work, experimental studies were performed to determine methods best suited to analyze dust particles generated from machining of composite materials. Data was initially collected using cascade impactors where mass median aerodynamic diameter (MMAD) was found to be .12 µm - .35 µm when machining at 0o to 135o of fiber direction without using local exhaust ventilation. Real-time particle analysis instruments were introduced into the test protocol which demonstrated the ability to reduce sampling times from greater than 6 minutes to as short as 30 seconds without the need of an additional 48 hours of filter conditioning. Dust profiles where found consistent between six real-time instruments evaluated for this application including four used in a unique isokinetic configuration designed specifically for dust analysis of particle producing manufacturing processes. A major development from this research is a process to safely and completely analyze all dust produced from machining of carbon fiber laminates using isokinetic sampling of particles as they travel from the work piece to a dust collection system with short sample times. This is a significant improvement over existing methods which involve open air sampling or recovering dust from vacuum filters. Dust was analyzed using both cascade impactors and with parallel use of real-time instruments. Dust concentrations from typical edge trimming conditions were found to exceed permissible limits of 5 mg/m3 for nuisance dust at the source, thus meriting the use of fully self-contained systems for collection and analysis. Supporting analysis tools were developed that enable comparison of dust concentrations reported by different instruments and measured gravimetrically. A method to determine cutting force at the cutting edge and cutting coefficients used in a mechanistic model for cutting force prediction of multi-directional composite laminates was also demonstrated. In this method milling force cutting coefficients are empirically determined for all cutting orientations using a series of milled semi-circular slots that incorporate the effects of fiber orientation in unidirectional laminates. In limited testing, linearity was found with a correlation coefficient greater than .99 using tangential forces from a rotating dynamometer for determination of tangential and edge milling coefficients and an average of .77 using radial force for radial and edge coefficients.