Definitions of high-speed machining as well as the benefits to be achieved through use of HSM, have both previously been documented. With regard to water-based metalworking fluids used in HSM operations, while an understanding currently exists of the importance of fluid properties, such as, coolant stability and foam behavior, less is known about the demands on the fluid for lubrication and cooling, and how these demands may differ from a fluid’s use in conventional lower speed machining. With the knowledge that under high-speed conditions lower machining forces and improved machined surface finish can be achieved, do the metalworking fluids used need to be as effective and high-quality as those currently used at lower speeds, specifically with regard to the lubrication and cooling provided?
High versus conventional speed machining
To better understand the influence of metal working fluids in aluminium high-speed machining, machining tests were performed at both, lower conventional speeds and at high-speed conditions. In considering some of the history of the origins of HSM, Dr Carl Salomon, in his original investigations on high-speed machining, determined that the heat generated between the chip and the cutting tool would increase with increasing cutting speed, up to a critical speed, dependent upon the metal being cut. With further increase, a critical speed would be reached, at which point the chip removal temperature would decrease with further increasing speeds. Given this analysis, and the presumption that machining performance (forces, BUE formation, tool wear, etc) are all largely influenced by the heat generated at the tool chip interface, it would be expected that overall machining performance would decrease with increasing cutting speeds prior to the peak cutting speeds, and then begin to improve as speeds exceed the peak value. To investigate this premise, machining tests were performed, using cast 380 aluminium at cutting speed values below, equal to, and above the peak cutting speed value, which Dr Salomon plotted for nonferrous metals. Using a 0.25” diameter carbide step drill, machining of Al 380 was performed, using spindle speeds of 2,900 RPM, 10,000 RPM, and 18,000 RPMs, with these cutting speeds corresponding to one below, one at, and one beyond the critical speeds, as they relate to chip removal temperatures.
To assess the machining performance at these three different cutting speeds, the axial machining forces, tool flank face wear, machined surface finish, and hole dimensions were measured. The axial machining forces, while providing a measure of the energy required for the operation, also provide a useful indirect measure of the mechanical and thermal demands on the tooling and the potential tool life to be expected in a given operation, which shows the mean axial machining forces measured at the three cutting speeds, the machining forces climb considerably when speeds are increased from 2,900 RPM up to 10,000 RPMs. However, as the speeds increase further to the HSM conditions (18,000 RPMs), the cutting forces level off and actually start to decrease. Thus, the mechanical and thermal demands on the tooling are reduced at HSM conditions and improved tool wear will likely be obtained.
While tool wear is an important issue in aluminium machining, the amount or degree of built-up edge formed on the cutting tool can be an equally or often a more critical parameter to be considered. Built-up edge, when formed, often leads to a degradation of the machined surface finish, as well as loss of accuracy of size or dimensions of the holes produced. To assess the impact of HSM conditions on this parameter, the degree of BUE formed on the cutting tools, and subsequently the hole finish and form, were measured for each of the three cutting speeds utilised. While BUE formation is an extremely dynamic process, examination of the tooling following the machining operation still offers a useful assessment of the tendency for this to happen. As the machining operation tends to increase cutting speeds, the overall quality of the tool and the hole produced improves. Thus, high-speed machining offers benefit with regard to the quality of the operation and part produced, as well as the gains in productivity which can be obtained.
Metalworking fluids in high-speed aluminium machining
With an understanding that at high cutting speeds, lower machining forces, reduced tool wear, and improved machined surface finish can be obtained, a question to be asked is: do the metal working fluids used, need to be as effective and high quality as those currently used at lower speeds, specifically with regard to the lubrication and cooling provided?
To address this question, high-speed machining tests were conducted to assess the properties of various water-based aluminium machining fluids and determine if machining performance could be influenced by the quality of the fluid used. For this study, four fluids, currently widely used in the industry and considered to represent the state-of-the-art in fluid technology for aluminum machining operations, were each tested under high-speed machining conditions. While all four of these water-based fluids are considered to be effective, there are observable performance differences between them when utilised at lower, more conventional machining speeds. Such differences may arise from the composition and type of lubricating additives, their emulsion properties, or a combination of such factors. Nevertheless, it was felt that if the fluid used can be a significant factor in the level of machining performance obtained in HSM, then differences in their machining performances should be observed at the high-cutting speeds of 18,000 RPMs.
Also the use and selection of the metalworking fluid can impact the machining performance and potentially yield further improvements in the quality of the part produced as well as the tool life obtained. The fluid performance differences are likely a result of compositional differences between the fluids giving rise to varied levels of the lubrication, cooling, and chip removal capabilities.
The results of machining tests conducted at lower, more conventional cutting speeds, and also at high-speed machining conditions, show that along with gains in productivity under HSM conditions, improvement in the machining operation and quality of the part produced can be obtained. Such improvement is seen in the reduced wear and built-up edge observed on the cutting tool used at the 18,000 RPMs, as well as in the improved machined surface finish obtained at HSM conditions. While improved machining can be obtained at higher speeds, it was also seen in the test results obtained, that the machining fluid used can still have a significant influence on important measured parameters, such as tool wear and part quality. Thus, it is felt that the composition and resultant performance properties of the metalworking fluid will continue to play an important role in the quality of the operation, as the use of high-speed machining continues to grow in industry.