Lubrication in metal cutting under built-up edge conditions
The effect of a mineral oil lubricant and sulphur (an EP additive) on machining of medium carbon steel has been investigated. The lubricant was applied with several different concentrations an additive containing sulphur. The concentrations were 0%, 1 %, 2%, 3%, 4% and 5% of the additive. The results were compared with dry conditions. The effect of cutting speed and feed rate was also investigated. During the tests cutting force, transient surface temperature and surface finish were measured. Cutting force and surface finish were measured using traditional equipment. measure the temperature of the workpiece a new method was developed. The temperature of the transient surface was measured at three different positions below the cutting edge during the cut using an infrared sensor. The results were then extrapolated to obtain the temperature of the transient surface close to the cutting edge (the maximum temperature of the surface). The temperature results can be used to calculate the cooling rate of the surface. Quick stop tests were also used to obtain chip samples and information about BUB size and shape. The lubricant had no effect on forces, temperature or BUB size, but improved surface finish. This works suggests that the enhanced surface finish obtained when lubricant is applied is not due to lubrication on the rake face (preventing a BUE), but action on the machined surface after it has been formed. The BUE is squeezed parallel to the cutting edge and touches the workpiece altering the surface finish, by a rubbing action and also because periodically part of the BUB bonds to the surface. The lubricant works on this rubbing action and can avoid bonding of the squeezed BUE and also of debris particles that are inevitably formed during the operation. It has been shown that a very low volume of lubricant is enough to improve the surface finish because the area that requires lubrication is small.