Influence of Water-Miscible Cutting Fluids on Tool Wear Behavior of Different Coated HSS Tools in Hobbing

The present paper describes the influence of water-miscible cutting fluids on tool life (flank wear) and crater wear of various coated cutting tools and finished surface roughness, as compared with the cases of dry cutting and wet cutting using cutting oil in hobbing in an attempt to improve the working environment. Experiments were conducted by simulating hobbing by fly tool cutting on a milling machine. The following results were obtained. (1) In the case of an uncoated tool, cutting oil was more effective than dry cutting in reducing flank wear. Cutting oil and water-miscible cutting fluids were more effective in reducing flank wear than dry cutting using TiNand TiAlN-coated tools. The use of water-miscible cutting fluids in conjunction with TiSiNand AlCrSiN-coated tools prolongs tool life. (2) For all coated tools, the use of cutting oil or water-miscible cutting fluids were effective in reducing crater wear. Especially, water-miscible cutting fluids were effective for TiSiNand AlCrSiN-coated tools. (3) Regarding the finished surface roughness, in the case of dry cutting, the finished surface roughness was similar for various types of coating films. When using cutting oil or a water-miscible cutting fluid, the finished surface roughness improved compared with dry cutting, independent of the type of coating film applied. The finished surface roughness obtained using water-miscible cutting fluid was approximately the same as or smaller than that obtained using cutting oil. (4) With respect to flank wear, crater wear, and finished surface roughness, the water-miscible cutting fluid of emulsion type containing a large amount of synthetic lubricating additives was suitable for the AlCrSiN-coated tool.


Introducation
In recent years, the use of large amounts of water-immiscible cutting oil has been scrutinized because of problems related to the working environment and global environmental pollution, e.g., because of problems such as the deterioration of the work environment due to mist and odor and the risk of ignition, even in hobbing. As an alternative, there is an increasing demand for the use of water-miscible cutting fluids as well as dry cutting and semi-dry processing (Weinert et al., 2004;Winkel 2010). The authors have been working on the development of water-miscible cutting fluids for hobbing, and the following results were obtained. (1) The water-miscible cutting fluid (emulsion type) used in the experiment prolonged the tool life, and the finished surface roughness is equivalent to that obtained by the conventional water-immiscible cutting oil for hobbing, even if the cutting speed is changed (Matsuoka et al., 2002). (2) Water-miscible cutting fluid containing a polyalkylene glycol synthetic lubricating additive was the most effective among the various emulsions (Matsuoka et al., 2006). Moreover, the results for the pH of the water-miscible cutting fluid indicate that the pH should be 8.0 in order to counter tool wear (Matsuoka et al., 2009).
With the development of coating technology, a titanium-based ceramic coating film (Sakurai & Terao, 2007) and an aluminum-based coating film without titanium (Furuno, 2005) with improved heat resistance and wear resistance,  is higher than re, we herein f nd attempted t ormance of wa ness was inve tting.
tions lating hobbing ctual hobbing hips produced b e same shape a y tool is shown nized that the r -immiscible cu ), generally agr est was recogn ng experiment bing on a hobb perimental setu hob cutting e n that in wet c focus on a wa to verify how w ater-miscible c estigated in co with fly tool c is complicate by the top cutt as the single h n in Figure 2. results, e.g., fl utting oil (kat ree with those nized by investi (Rech, 2006) Table 6 shows the Vickers hardness and oxidation temperature of various coating films. In the case of dry cutting, as shown in Figure 5, the tool life tends to increase as the hardness and oxidation temperature of the various coating films increase, and the tool wear appears to be related to the hardness and the oxidation temperature of the coating films. When using cutting oil (Oil A), the tool life increases compared to that for dry cutting for all of the coated tools considered herein, and the tool life tends to increase in coated tools with relatively low hardness. In particular, in the low-hardness TiN-coated tool, the tool life is the longest, which appears to be due to the lubricating effect of Oil A. When using the three water-miscible cutting fluids, the tool life increases compared with dry cutting for all of the coated tools considered herein. In particular, the tool life increases in the TiSiN-and AlCrSiN-coated tools, and it is inferred that in addition to the relatively high oxidation temperature of these coated tools, the cooling effect of the water-miscible cutting fluid is related to the reduction in the cutting temperature. Moreover, when comparing the TiN-and the TiAlN-coated tools, although the TiAlN-coated tool has high hardness and high oxidation temperature, the reason for the short tool life for both cutting oil and water-miscible cutting oil is the occurrence of delamination of the coating film surface layer at the beginning of cutting (see Figure 6).
Among the coated tools used in the experiments, the cutting performance of the AlCrSiN-coated tool is particularly excellent, so we hereinafter consider the flank wear behaviour of the AlCrSiN-coated tool.
Although the cutting temperature was not measured in these experiments, in order to examine the cooling performance, the cutting temperature was estimated based on the color difference of the chip that was formed. Figure 7 shows the color of the chip at the beginning of cutting (cutting groove length: 0.5 m) using the AlCrSiN-coated tool and the three water-miscible cutting fluids. In the case of dry cutting, the color from a portion of the beginning of the cut of a chip to a portion of the end of the cut is dark blue-purple. However, in the case of Oil A, the color from the beginning of the cut to the end of the cut of a chip is dark brown. When water-miscible cutting fluids are used, the chip turns brown to pale brown in the order of Fluid B, Fluid C, and Fluid D. Based on the chip color, the cutting temperature decreases in the order of dry cutting, Oil A, Fluid Fluid C, and Fluid D. Figure 8 shows the color of the chip after cutting groove of 15 m. The order of the chip color is the same as in the case of the beginning of cutting. Fujimura reported that the chip color is the interference color of the oxide film formed on the chip surface in an extremely short time, which is rapidly heated by the cutting heat and is quenched by the atmosphere. These interference colors are indicators of the cutting temperature, under the same cutting atmosphere (Fujimura, 1991). In this experiment, however, although the atmosphere differs in dry cutting and wet cutting, there is a high possibility that the chip color differs as a result of cooling by the cutting fluid. It is thought that the temperature at the tool edge during cutting depending on the presence or absence of cutting fluid does not change so much, but when the fluid exists at the cutting region, the cutting edge temperature is instantaneously lowered and the chip itself is also cooled, so it is presumed that the difference in chip color has appeared because of the difference in temperature.
In the case of the AlCrSiN-coated tool, the longer tool life obtained using water-miscible cutting fluid (as compared with the cutting oil) is considered to be due to both the lubricating and cooling effects of the water-miscible cutting fluid. Moreover, although the cutting temperature when using the emulsion (Fluid B) is higher than when using other water-miscible cutting fluids, the longer tool life obtained using Fluid B is considered to be due to the excellent thermal and oxidative stability of synthetic lubricating additives and because of the high affinity to the metal surface and high lubricity at high temperature. Figure 9 shows the progress curves for the maximum wear width obtained using various cutting fluids when using the AlCrSiN-coated tool. In the case of dry cutting, the wear dramatically increases until reaching a cutting groove length of 2.5 m, and the average wear rate (increment of maximum wearwidth per 1 m of cutting groove length) was 0.127 mm/m. In the case of Oil A, initial wear was not observed, and the wear increased substantially and linearly from the beginning of cutting. The average wear rate was 0.019 mm/m. For three types of water-miscible cutting fluids, initial wear was not observed and the wear increased approximately linearly from the beginning of cutting. The average wear rates were 0.014 mm/m, 0.012 mm/m, and 0.009 mm/m for Fluid D, Fluid C, and Fluid B, respectively.
mer.ccsenet Table 6. C   Therefore, lubricating becomes l that for Fl contact len contain lu friction co of cutting water-misc tool and th added, and however, t using wate (Okabe, Figure 12, the e of the coatin med using too m on the flank ting film havin oating film we flank face, an r words, it is e he coating film haracteristics o y be due to the n the water-mi utting oil, due t ears to have b fluids, the cr ble, and the con ntioned consid ng fluid and th ces and cutting o measure the Mechanical e and crater we and wet cuttin friction condi ricating additiv may be the sh of synthetic lub followed by F ed on this co ake face when excellent lubr c extreme pres s of chips, in w med on the fr ing Oil A wa or this reason, 1985). Corros nomenon. rature is assum fluids, i.e., the e crater wear w ng film applied ols without a co face and the w ng a thickness ars out or beco nd the substrat xpected that th m (see Table 6 of the rake fac e difference in iscible cutting to the cooling been generated ater wear is ntact length is derations, the he occurrence g temperature cutting forces  Figure 18 Fluid B in groove, su can be obs small. A g Figure 19) although c and wet cu side cuttin cutting edg and/or vibr is used. W of dry cutt very simil also very s Figure 1 t.org shows the fin n conjunction w uch as a scratch served at almo glossy smooth ). When using cutter marks ca utting using O ng edge may be ge shape. In th ration of the cu When observed ting and wet cu ar to that at th similar.  Vol. 8, No. 2;2018 This is considered to be due to the effect of the deposited metal on the finished surface roughness at the beginning of cutting, and the flank wear may be affected at the end of cutting.
In this experiment, there are notable differences in the adhesion amount of deposited metal on the side cutting edges and side cutting edge wear due to different cutting fluids and coated tools as factors influencing the finished surface roughness. In addition, the finished surface roughness is thought to be influenced by the fluctuation and vibration of the cutting tool singly or in combination. Regarding the difference in finished surface roughness due to differences in cutting fluids and coating films, the amount of deposited metal on the side cutting edge affects the finished surface roughness by observation of the cutting edge. When the adhesion amount of the deposited metal is large, the finished surface roughness tends to be large. The properties of the deposited metal generated are expected to differ based on the reactivity of the components contained in the cutting fluid, the coating film, and the work material, either singly or in combination. Therefore, more detailed analysis and observations will be necessary in the future.

Conclusions
In fly tool tests that simulated hobbing, the cutting performance of various water-miscible cutting fluids for various coated tools was investigated in comparison with dry cutting and water-immiscible cutting oil in terms of tool life (tool wear) and finished surface roughness. The main conclusions of the present study are summarized as follows: 1) Regarding flank wear, in the case of the uncoated tool, water-miscible cutting oil was more effective than dry cutting. For the TiN-and TiAlN-coated tools, water-immiscible cutting oil was effective. For TiSiNand AlCrSiN-coated tools, using water-miscible cutting fluids prolonged tool life and was effective.
2) Regarding crater wear, in the case of the uncoated tool, water-immiscible cutting oil was effective. In the case of the TiN-and TiAlN-coated tools, water-immiscible and water-miscible cutting fluids were more effective than dry cutting. In the case of the TiSiN-and AlCrSiN-coated tools, water-miscible cutting fluid was effective and generated less crater wear.
3) Regarding finished surface roughness, in the case of dry cutting, the surface roughness was approximately the same for all of the coated tools. When water-immiscible cutting oil and water-miscible cutting fluids were used, the finished surface roughness was improved for all of the coated tools. When the water-miscible cutting fluids were used, the finished surface roughness was equal to or smaller than that obtained using the water-immiscible cutting oil. 4) From the viewpoints of flank wear, crater wear, and finished surface roughness, the emulsion-type water-miscible cutting fluid containing a large amount of synthetic lubricating additives was suitable for the AlCrSiN-coated tool.