The Machining of Polymers

Abstract

Machining is a non-linear process and involves the consideration of variables such as inelastic deformation, high temperatures and contact conditions. This thesis focuses on investigating the contact conditions between cutting tools and polymeric materials during orthogonal cutting. A test rig has been developed to allow cuts to be taken from a rectangular workpiece, such that the rake angle (the angle of inclination of the tool surface) can be varied. In this work the rake angle was varied from -20o to 30o, and the cutting was performed at rates of 0.01ms-1 and 0.1ms-1 on PMMA, nylon 4/6 and nylon 6/12. As part of a round robin investigation, tests were also performed on HIPS, ABS and LLDPE. The experimental method developed required the measurement of forces in two directions (the direction of cutting, Fc and transverse to the direction of cutting, Ft). The rig allowed for the careful control of the depth of cut, h. After each cut, the thickness of the off-cut chip hc was also measured. A series of cuts were taken at depths varying between the range of 0.02mm to 0.25mm and the forces were measured. A cutting theory has been applied to the experimental data to determine the fracture toughness Gc, and the yield stress σY of the material. The Coulomb friction μ and an adhesion term, Ga representing sticking at the tool-chip interface, were also deduced. Independent fracture mechanics tests were performed at a range of temperatures and rates on the different polymers. Tensile tests were also performed, to compare standard values to the material parameters determined in cutting. The values of Gc and σY deduced were independent of the rake angle however, μ and Ga were not. The calculated values of Gc were typically within 5% of the standard values however, σY was found to be up to 5 times higher. The existence of work hardening is believed to be the cause of these elevated values. The cutting analysis was also applied to some previously published metal cutting data and produced constant values of Gc.