HELICAL MILLING: AN ENABLING TECHNOLOGY FOR MACHINING HOLES IN FULLY HARDENED AISI D2 TOOL STEEL (2006)

The machining of hardened steel is becoming widespread throughout the manufacturing industry owing to the benefits derived in the form of manufacturing flexibility, better product quality, and the prospect of dry machining. This coupled with developments in super hard cutting tool materials and machine tools, and better understanding of hard machining technology translates into significant economical benefits and faster turnaround times. The majority of applications involving hardened steels comprising AISI D2 tool steel relates to the manufacturing of die and mold wherein the current trend of hard machining is restricted by the limitations imposed by hard drilling. Being the final operation in many manufacturing applications, it is imperative that the process of drilling be robust and reliable to enhance the value already added to the product. Problems associated with inherent deficiencies in the drilling process kinematics, combined with poor machinability of hardened D2 tool steels due to the presence of hard and abrasive carbide particles in its microstructure that lead to catastrophic drill failure, constitute the single major process chain bottleneck in realizing hard part manufacture. The research work presented in this thesis focuses on the application of helical milling as an enabling technology for hole making in hardened AISI D2 tool steel in comparison to conventional drilling, which is not feasible at the current level of developments in drilling technology. Helical milling employs a rotating end mill of a diameter smaller than the hole, and traverses a helical path to generate a hole. The novel process derived by simple modification of tooling and process kinematics offers an excellent avenue to successful machining of precision holes in hardened D2 tool steel. In order to compare the performance of helical milling against drilling, four types of conventional twist drills intended for drilling hardened steels were employed in the machining experiments while, end mills commonly used in the die and mold industry, were chosen for helical milling. The processes were evaluated in terms of tool life, wear progression, wear mode, cutting forces and hole quality. Accelerated wear followed by catastrophic fracture of the cutting edges at the periphery of the drill was observed to be the primary tool failure mode in conventional drilling as opposed to uniform progressive flank wear in helical milling. The innovative helical milling method is found to facilitate1 hole-making in hardened D2 tool steel with an order of magnitude improvement in tool life. The helical trajectory of the tool in helical milling facilitates material removal near and at the center of the hole by cutting rather than extrusion as seen in drilling, thereby reducing the excessive thrust forces that cause work material breakouts at the hole exit in conventional drilling. Furthermore, chip evacuation in not problematic in helical milling considering that chips can be removed across the radial clearance between the tool and the hole as opposed to through the flute space in conventional drilling. This implies that an air blow could be employed to assist chip transport in helical milling facilitating dry machining, considering that in many drilling applications cutting fluid is merely used to flush the chips away from the cutting zone. The intermittent cutting action in helical milling further provides respite to the cutting edge from the imposed mechan ical and thermal loads and offers exceptional chip control. The process represents an enabling technology with additional benefits of superior hole quality thus rendering the elimination of an additional reaming process.