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Date of Award

8-2002

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Professor M. A. Elbestawi

Abstract

The maching of hardened steels is becoming a viable technology. At the inception of the present research, this technology for milling processes was in its infancy. Advancements in cutting tool materials such as poly-crystalline cubic boron nitride (PCBN) have enhanced the ability to machine these difficult to cut alloys. The machining of hardened tool steels have been explored in the recent literature to a great extent because of the possible gains and benefits this technology has to offer. Near net shape manufacturing can become a viable technology and save the die and mold industry considerable investment by reducing the lead time for production of dies and molds.

In this thesis, an experimental investigation is presented regarding the optimal process parameters that make high speed hard machining a viable technology. The experimental investigation shows that hard machining of AISI H13 (55 HRc) is possible, and is an extremely effective technology if the proper conditions are used. Because of the nature of hard machining, the fundamental aspects of chip formation, tool wear and life is explored through a detailed investigation. It was confirmed that the ball milling of hard materials produce segmented saw toothed chips even at low chip loads for specific cutting conditions. It was noticed that the chip morphology was very repeatable and consistent and therefore formed a preliminary basis for the modelling strategy. Tests were also performed on hardened AISI D2 tool steel (62 HRc), which showed that this material in its hardened state challenges the ability to machine this material in the fully hardened state. The primary tool failure modes are outlined and a detailed analysis of the chip formation mechanisms is reviewed.

Owing to the difficulty associated when machining hardened AISI D2 tool steel, the development of an analytic force model was attempted. The modelling methodology required a correlation of the flow stress (the mechanical response of the material) with the cutting conditions in the form of kinematic parameters derived from chip morphology. The hardened material was characterised using high strain rate ballistic impact tests (using the compressive split Hopkinson pressure bar) in a punching shear configuration. This configuration was chosen as it represented the shearing process in metal cutting more accurately in terms of strain and strain rate. The tests were modified and performed on the fully hardened tool steel. The ultimate result was a precise representation of the flow stress of the hardened material in shear as a function of the strain, strain rate and temperature where the fitted correlations represented the experimental data with an accuracy of approximately 10%. The temperatures in this part of the investigation exceeded 600 °C with strains in excess of unity and strain rates approaching 50000 s^-1.

Having a correlation of the flow stress of the fully hardened material, the force model was derived using the chip morphology and chip formation kinematics to represent the governing strain and strain rate conditions during machining. The resulting formulations allowed for a time domain orthogonal machining simulation represented by specific inputs such as cutting speed and feed rate. The orthogonal formulation was verified against experimental data and showed good correlation with observation.

The orthogonal formulation was extended to the ball milling process (an oblique cutting configuration) to test the validity of the force model. Good correlation was realized between the experimental and predicted results. The ball milling process challenged the validity of the force model by applying the modelling strategy to small chip loads and low cutting speeds. The predicted results also rationalized the tool failure mode that was observed in the ball milling investigation.

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