Andrew Mantle

The paper compares the performance of two bio-ester and two mineral-oil emulsion metalworking fluids (MWFs) in finish turning an Inconel 718 alloy bar with a high hardness (HB 397 – 418). In this study, a coolant with a lean concentrate diluted at 6.5% to create an emulsion with stabilised water hardness was used to prepare each MWF. The finish-turning method used a small tool nose radius (0.4 mm) and small depth of cut (0.25 mm) to turn down 52.2 mm diameter bars in multiple passes to reach a maximum tool flank wear of 200 µm. In each MWF turning test, the tool flank wear, cutting forces, and surface roughness were measured against cut time. Chips from each MWF turning test were also collected at the same cut time instances. The surface and subsurface integrity on a workpiece obtained from each MWF turning test were compared by using a new unworn tool. Overall, for the machining parameters studied, the findings suggest the bio-esters were capable of equivalent machining performance as the mineral-oil emulsions, apart from one bio-ester that displayed improved surface roughness. Common to all MWF turning tests was a change in the chip form at low flank wear, which is discussed. Further findings discussed include the sensitivity of the concentration of the MWF diluted in the emulsion and the effect of the workpiece hardness within the batch used, with useful recommendations to improve the finish-turning method for the assessment of MWFs.

This paper discusses a method of finish turning Inconel 718 alloy to compare machining performance of a naturally aged and used metalworking fluid (MWF), which had been conventionally managed through its life cycle, with the same new unaged product. The MWF concentrate was a new-to-market bio-ester oil, diluted with water to produce an emulsion. In the experiments, 50 mm diameter bars were turned down with multiple passes at a 250 μm depth of cut to reach a tool flank wear of 200 μm. The machining was interrupted at several stages to measure the flank wear and compare the chip forms for the aged and unaged MWF. The method of finish turning used a small tool nose radius and a small depth of cut that was found to be sensitive in detecting a difference in the flank wear and chip forms for the aged and unaged MWF. On the chemistry, the findings suggest that higher total hardness of the aged MWF was the cause of reduced lubricity and accelerated flank wear. This paper discusses the state of the art with the insights that underpin the finish turning method for the machinability assessment of MWFs. The findings point to stabilization of the MWF chemistry to maintain machining process capability over an extended sump life.

The paper discusses an instrumented tapping test method using a CNC machine tool to compare the lubricity of MWFs by cutting threads in a Ti-6Al-4V alloy at low speed. The method uses a spiral flute tap size typical of industrial practice. A soft synchronous tap holder and spindle mounted dynamometer were incorporated on the machine to measure torque and thrust force. The tapping test method was demonstrated on three groups of MWFs that were commercially available and classified by ASTM E2523-13:2018. The method developed stable results free of chip clogging in tool flutes which could otherwise mask their comparative lubricity. The fully synthetic (FS) group displayed the best lubricity and within this group the FS from renewables (FS-bio) was the best overall. The method was shown to be effective in mitigating biasing effects on lubricity performance due to the generous tool chamfer angle tolerance and was practical and economical to implement. The significance of the results is discussed enabling an understanding of friction effects in tapping using a soft synchronous tap holder. A life cycle assessment of each MWF group found total Greenhouse Gas emitted from the FS group was 17% of the hydrocarbon group whilst FS-bio emitted just 7%.