Machinability study on GCI brake discs - Effects of material variations and process behavior on tool life

Abstract: Grey cast iron is one of the most used materials in the automotive industry. The material has a range of properties that makes it suitable for machining applications, but manufacturers still experience machinability problems. Automotive Components Floby (ACF), who currently experience machinability issues with brake discs, initiated this study to gain further understanding of tool deterioration in machining of grey cast iron. They provided two brake discs with known relative machinability for the experiments. Machining tests were performed where dynamic cutting forces were recorded, and chip segments were collected. Hardness measurements with nano indentation and SEM microscopy for structure analysis were also performed. The cutting forces and hardness values were analyzed statistically to get representative models for the results. The cutting force analyses showed that a high segmentation frequency is desirable for improved machinability. The deformed geometry of the chip segments showed that the material with bad machinability had to travel longer into the workpiece to produce one chip segment. This confirms that a high segmentation frequency is desired. The relation of the feed force and the main cutting force also showed to be of importance. A low feed force compared to the main cutting force reduces the contact surface on the inserts flank face, which improves machinability. The relative hardness between the materials showed that high hardness in general, and high pearlitic hardness in particular are desirable properties. This holds if the cutting tool is harder than the workpiece matrix and its inclusions. The microstructure analysis gave some unexpected results. It showed that the hardest inclusions were found in the material with good machinability. It also showed that presence of steadite could help with chip breaking, improving machinability. This holds as long as the tool material is harder than the hard steadite. More expected results were that machinability was improved by a uniform graphite structure and small grain size. Worn cutting tools were collected from the production line at ACF. These were analyzed under microscope to clarify the wear problems at hand. The results showed abrasive wear, chemical wear and adhesion of workpiece material to the cutting edge. It was assumed that the wear problems were in part cutting data related, so attempts to suggest development paths for ACF were made. The suggestions were based on a literature survey, and although promising examples of cutting data optimization were found, no real confident proposals could be done. Their machinability issues are experienced on a batch-to-batch basis, which indicates that each batch needs a unique set of cutting data. That is hard to accomplish, as long as the company does not have the ability to analyze each batch in house.