An investigation of machining amorphous brittle materials.

Abstract

Although ductile-regime machining technique provides a higher productivity than conventional polishing and lapping processes without loss of high surface quality, the operation requires a cutting depth of the order of less than one micron, confining the material removal rate in a low range. Aiming at a high productivity, an investigation is conducted in this work to explore the possibility of a <italic>rough</italic>---<italic>semi-finish</italic>---<italic> finish machining</italic> sequence for brittle materials as is general practice for ductile materials. The chip formation and surface damage during rough and semi-finish machining of amorphous brittle materials are studied both experimentally and theoretically. The machining experiments were carried out with an orthogonal-cutting configuration using geometrically-defined cutting tools. Different machining modes are identified along with the increase of cutting depth as: <italic> pure edge-rubbing</italic>, <italic>ductile-mode cutting</italic>, <italic> semi-ductile-mode cutting</italic>, and <italic>brittle-mode chip spalling </italic>. The essential factors for transition between ductile-mode cutting and chip spalling have been identified. A quantitative evaluation of the ranges of cutting depth at which the four machining modes occur under different rake angles has been performed. This evaluation reveals that the transition between modes occur at larger depth of cut when rake angle becomes more negative. The experimental identification of the machining modes and their dependence on the depth of cut and rake angle supports the use of geometrically-defined tools to machine glass using rough, semi-finish and finish machining sequence. Simplified two-dimensional problems are analyzed to understand the formation of surface cracks and chip spalling and to correlate loads with fracture damage. The results of the analyses suggest the possibility that only a minor portion of normal (thrust) load involves in the development of surface cracks while a major portion of the load is supported by the material behind the crack mouth and does not influence the cracking events. The nonlinear geometry effects induced from displacement of the material above the surface crack causes the crack to deviate towards the free surface and form a spall. The correlation between the load and the damage is established. A reasonably good agreement between the numerical prediction and experimental observation indicates that the mechanism described in this work is an appropriate one for describing the formation of spalling chips.