Frictional interactions in machining: comparisons between transparent sapphire and steel cutting tools

Abstract

This paper is concerned with the frictional interactions at the chip/tool interface in machining. Lead, aluminium, copper, iron, and nickel have been machined over a range of cutting speeds and times. Steel, cemented carbide, and transparent sapphire cutting tools have been used; the last allowing the interface to be observed directly. When used in this way in an earlier investigation it was found that relative motion, or sliding, occurred between chip and tool. However, a wider range of experiments has now shown that the conditions can vary between interfacial sliding and full sticking friction or seizure. The physical differences between sliding and seizure are discussed in detail. In the latter case the bond between chip and tool is sufficiently strong for relative motion to occur within the chip, and material is transferred to the tool face. When cutting aluminium with the sapphire tools, comparatively long cutting times and high speeds promote transfer. The key feature of such a process is the absolute removal of organic contaminants. When cutting with metal tools, the transfer will be further enhanced if the chip can also remove the oxide films from the rake face and form metal-metal bonds with the parent tool material. The difference in hardness between tool and work material is the major factor in this process. At the rear of the chip-tool contact length, the interaction between the atmosphere and the chip can cause transfer of a different nature. However, this may be eliminated if the cutting speed is increased.