On Some Tribological Effects in Machining of Compacted Graphite Iron (CGI)

This paper addresses the tribological challenges in the machining of compacted graphite iron (CGI) through an investigation of the effects of tool material, local tool-surface topography and Minimal Quantity Lubrication (MQL) on machining performance. CGI is a material that is being considered as an alternative to aluminum and gray cast iron (CI) for diesel engine applications — a market with large potential. Solutions for continuous cutting operations like turning and boring of CGI are constantly being sought. Prior limited work has attributed the tool wear problems encountered during CGI machining to the absence of a lubricating layer of manganese sulphide (MnS) on the cutting tool during continuous cutting at high speeds. With the aim of understanding the tribology of the tool-chip interface, experiments were undertaken using four different tool-inserts (flat-coated carbide, grooved-coated carbide, grooved-coated cermet and chamfered ceramic tool inserts) under dry and minimal quantity lubrication (MQL) conditions. Two cutting speeds (250 m/min. and 400 m/min.) with constant feed and depth of cut were tested. Post-machining analysis was conducted to determine the chip flow angle and the resultant force in each case. Results show that at low speed, the cermet tool produces a significant reduction in cutting forces in comparison to coated carbide. At high speed, cermet shows a minor increase in forces. MQL causes a decrease in forces only at low speed in all tools except the flat-coated carbide, in which case it shows a minor increase. Resultant forces show an increase with MQL usage at the high speed, suggesting a negative influence on the CGI machining performance. The work reveals that there are several complex effects observed due to the interaction between tool coating, tool topography, and lubricant action.