Effect of pre-cooling low carbon steel in liquid nitrogen on productivity

Abstract

In machining operations, the effects of high temperatures are i) reduction in tool life, ii) production of hot chips and iii) inaccuracies in workpiece dimensions. 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 shop floor productivity. This research employed time-study techniques to assess the impact that pre-cooling of mild steel had on human and machine productivity on a shop floor. 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. This brought the temperature of the workpieces down to -40°C. Results of the pre-cooling treatment plan were compared to the other treatment plans. Through the time studies, it was found that handling times of pre-cooled workpieces was comparatively higher than other treatment plans, especially when it came to mounting them in the vice. Unlike the other treatment plans, where the workpieces were handled at temperatures of 24ºC (ambient temperature), the pre-cooled parts were handled at -40°C. This near cryogenic temperature necessitated the use of leather gloves and tongs which slowed down the machine operator, thus in some respect reducing their productivity. The CNC machine does not have an in-built temperature error compensation feature, so it was unable to automatically adjust its tool position. The result of which was that the rough cut of pre-cooled workpiece was observed to be more of a finish cut, with the cutting tool barely making any contact with the workpiece surface and thus very little material being removed. This low material removal rate is considered to be a reduction in productivity from the machining stand-point. From a tribology perspective, the surface texture measurements revealed that pre-cooled parts did not fare well compared to other treatment plans as with each workpiece being cut in sequence, the surface roughness got progressively worse. However, from the analysis of process flow charts, pre-cooled parts had more value addition processes (or operations), compared to other treatment plans, in its processing cycle. Finally from the environmental point of view, the use of liquid nitrogen proved to be safe and low cost for achieving pre-cooling function for small workpieces, via adapting a dewar commonly used for artificial insemination for this purpose. Keywords—Cryogenic, productivity, machining, computer numeric controlled (CNC), surface roughness