The Experimental Evaluation of Environmentally Friendly Cutting Fluids in Micro-Milling
Date
2013-08-30
Authors
Zhang, Yanqiao
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Abstract
In manufacturing, cutting fluids promote machining performance by removing heat, lubricating the cutting zone, flushing away chips, and preventing in process corrosion. To synthetize conventional metalworking fluids (MWFs), aside from choosing from a selection of base oils, an array of additives are also typically added. In traditional cutting fluid applications, the cost of waste fluid treatment is enormous. Moreover, the treatment is not always effective and disposal may lead to unexpected environmental contamination. The bacteria and chemical elements in the waste liquids may also introduce health and safety concerns. For the milling process at the micro-scale, i.e., micro-milling, traditional flood cooling may not be suitable. Since the cutting zone between the tool flank and workpiece is in the order of micrometers, the liquid surface tension of flood coolant would impede effective cooling and lubrication of the cutting fluid especially at a high spindle speed for tools. So for micro-milling, some researchers have tried to use minimum quantity lubrication method to apply cutting fluids. Other semi-dry methods like atomization method based on an ultrasonic atomizer have also been tested. However, even though these systems are able to decrease the amount of cutting fluids, the atomization of conventional cutting fluids with harmful surfactants (especially water miscible MWFs) and additives inside would still pose problems related to health hazard and contamination. Thus, new systems and/or green cutting fluids that eliminate the use of undesired surfactants or additives need to be developed. In this thesis, efforts to solve these problems for micro-milling operations are presented.
Firstly, canola oil is selected and used to be emulsified in distilled water through ultrasonic atomization without any surfactant. Then, the emulsified water and oil solution is applied as cutting fluid in micro-milling, and the cutting performance results are compared to those with dry machining and traditional cutting fluid – 5% TRIM aqueous solution. The experimental results show that smaller chip thickness, and burr amount are observed with canola oil-in-water emulsion compared to conventional MWF. Reduction of almost 30% in cutting forces has also been achieved.
Secondly, development of a new atomization-based cutting fluid system is introduced. Both cooling and lubricating capabilities of the cutting fluids are achieved using air-mixed water and oil mists, requiring no surfactants. Experiments are then conducted to evaluate the new system and the air-mixed jet of independently atomized water and oil sprays and compared to results with water only, oil only, and conventional cutting fluid (5% TRIM) conditions. The results reveal the mixture of water and oil leads to best performance in cooling and lubrication during micro-milling. The new system is proved to be effective in cooling and lubricating the cutting zone for both Al6061 and steel 1018. This atomization system is considered as a novel application method to apply totally green cutting fluids.
Finally, a novel environmentally friendly additive was added to conventional cutting fluids. In this thesis, lignin powder obtained from wood is considered as one kind of these “green” additives. It is firstly tried to be dissolved in 5% TRIM aqueous solutions in 8 different concentrations through injection and atomization methods. Then, those lignin containing cutting fluids are used to run micro-milling experiments and compared with 5% TRIM. Nine MWFs are all nebulized by a nebulizer to cool and lubricate the workpiece. The results show that the concentration of 0.015% lignin leads to the least cutting forces, tool wear and burrs. The obtained solution (f) with 0.15% lignin inside causes cutting forces that are just 50% in value of those with 5% TRIM. Considering lignin’s anti-oxidative characteristic and its performance in improving machining processes, it is a promising additive in MWFs.
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Keywords
Micro-milling, Environmentally friendly cutting fluids, Surfactants, Green additives, Mixed Jet, Lignin