Effect of Pre-Cooling Low Carbon Steel in Liquid Nitrogen on Productivity of a Milling Operation

Abstract

In machining operations, coolant is applied to convey away heat to extend tool life and ensure a good surface finish of the workpiece. While conventional cooling methods make use of soluble oils, cryogens have found applications for difficult to machine materials. Selection of an appropriate treatment plan when using a cryogen during machining of steel workpieces has an impact on shopfloor productivity. This research sought to study the impact that pre-cooling of mild steel had on human and machine productivity. Identical mild steel workpieces were milled using a 3-axis computer numerically controlled (CNC) Vertical Milling Centre (VMC). Each sample was subjected to one of four different conditions of cooling; viz., dry, pre-cooling, pre-cooling and raising to ambient temperature and soluble oil. Pre-cooling was achieved by submerging mild steel workpieces in a 10-litre Dewar of liquid nitrogen for five minutes. Results of the pre-cooling treatment plan presented three significant effects on productivity. Firstly, handling time of workpieces at-40°C was higher than when the parts were at ambient temperature. Secondly, the pre-cooled workpiece itself served as the heat sink. Heat from the cutting zone was not adequately dissipated away from the cutting zone. Thirdly, the machine did not have any feature that could compensate for material contraction/expansion due to temperature. The research demonstrated the importance of accounting for the thermal contractions and expansions during the CNC programming. Reduction in human productivity when handling workpieces cooled to sub-zero temperatures was also an important factor