Laser assisted micro milling with minimum quantity lubrication and vortex cooling

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Kuila, Pushparghya Deb
Melkote, Shreyes N.
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As applications in medical, electronics, and aerospace industries continue to demand miniaturization of parts, manufacturing processes capable of creating microscale features are necessary. For part features of moderate complexity and low aspect ratios, micro milling is usually the most practical option because of its high material removal rates and relatively low capital cost. Micro milling refers to milling with end milling tools less than 1 mm in diameter. Micro end mills are prone to breaking easily due to their small size. Additional limitations in micro milling arise from the well-known size effect, which leads to increased ploughing and reduced tool life. Workpiece and tool life limitations also arise when micromachining difficult-to-cut metals such as a high nickel content alloyed steel A-286 (~ 42 HRc), which is the work material of interest in this thesis. Considerable research has been conducted to overcome these limitations and to generally enhance micromachinability. Prior investigations consider the use of wet assist, minimum quantity lubrication (MQL), vortex cooling, and laser assisted micro milling (LAMM) to overcome some of the limitations. In particular, prior investigations of LAMM have shown that by using a laser in situ to thermally soften the workpiece in front of the cutting tool, stresses in the tool are greatly reduced thereby reducing tool wear and improving the dimensional accuracy. However, a systematic comparison of the micromachinability of wet assist and LAMM is lacking. In addition, there is no prior work comparing the performance of MQL and vortex cooling on the tool condition and micromachinability in LAMM. Therefore, the first objective of this thesis is to analyze and compare micro milling under dry cutting, wet cutting, and LAMM. The second objective is to evaluate the performance of LAMM in the presence of MQL and vortex cooling. A limited duration full factorial micro milling experiment was carried out using a range of cutting parameters to understand the micromachinability of a difficult-to-cut high nickel content alloyed steel A-286 (~ 42 HRc) in dry cutting, wet cutting, and LAMM. Results showed that LAMM maintained a more precise depth of cut, yielded lower resultant cutting forces than dry cutting, and reduced the tool diameter wear by 29% compared to dry cutting and by 22% compared to wet cutting. However, LAMM resulted in built-up edge on the tool faces. An extended duration LAMM tool life test was conducted using the best cutting conditions identified in the factorial experiment. As the tool wore, the grooves developed a tapered and rounded cross section, and non-uniform burrs formed at the groove edges. The tool wore rapidly in the first 50 mm3 of material removal but reached a steady state thereafter. Built-up edge formation was prevalent and tended to increase over the test duration. Finally, the use of MQL and vortex cooling methods were experimentally investigated to ameliorate the built-up edge caused by LAMM. The addition of MQL further enhanced the groove dimensional accuracy of LAMM, lowering the change in groove width by 41%. The addition of MQL to LAMM prevented built-up edge formation. Vortex cooling allowed for better chip removal and cooling of the tool and decreased the average resultant cutting force by 15%. Overall, the LAMM+MQL II case (corresponding to an air flow rate of 50 l/min) was found to be the best test condition with the least tool wear, least built-up edge, and the least change in groove dimensions. Physically-based explanations for these results are given. The thesis concludes by describing potential topics for future research.
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