Investigation on tool wear and tool life prediction in micro-milling of Ti-6Al-4V Open Access

The manufacture of 3D geometric features and the ongoing tendency for miniaturization of products with a wide choice of materials, including metallic components, are increasingly in demand by micro-manufacturing industries, such as medical devices and micro-molds. Material removal in micromachining is restricted by the capability of micro-tools used in the material removal process, where the tool cutting-edge radius and material properties of the workpiece directly affect the quality of finished products.¹ The determination of the ratio of the minimum chip thickness to the cutting-edge radius is critical in the micro-cutting process, where the phenomenon of change from the material removal process to the plowing effect once the cutting-edge radius exceeds the minimum chip thickness value changes the dynamic performance of machining.^2–7 The effect of cutting-edge on chip formation illustrated through the comparison of chip formation shows that perfectly sharp tools result in the generation of an intact individual chip.⁸ Observation of workpiece material hardness shows that it has a direct effect on micro-tool life as the cutting-edge radius increases more rapidly on machining of hard materials, such as titanium and steel, in comparison with aluminum, brass, or copper. Titanium alloys are widely used in a variety of industries, such as biomedicine, transportation, power, marine, and aeronautics.⁹ In particular, Ti-6Al-4V is a favorite material because of its low density and high strength-to-weight ratio, corrosion and erosion resistance, and biocompatibility.⁹ Despite the desirable properties of titanium alloys, the poor wear resistance,¹⁰ high friction coefficient, and low thermal diffusivity¹¹ have limited the application of titanium, which in turn results in the short tool life of micro-tools because the stress of the cutting forces is concentrated on the cutting-edge, leading to excessive wear and premature failure.¹² Low thermal diffusivity results in high cutting temperature, which induces the adhesion of the workpiece material to the cutting face of the tool,^13,14 leading to the short tool life, pronounced chipping, and premature failure of the tool cutting-edge.¹⁵ Over the years, researchers have investigated different methods to reduce tool wear by optimizing the selection of machining parameters, reduction of machining temperature, and estimation of the micro-tool life to regulate tool change and improve the surface roughness of finished parts in micromachining. The previous works are summarized as follows.

The effect of feedrate on the wear rate of coated and uncoated tools was experimentally investigated on aluminum 6061,¹⁶ where cutting force and burr formation were used to assess the performance of the tools. Experimental data indicated an insignificant difference in the performance of the uncoated and coated tools. Moreover, coating did not shorten the tool life. However, low burr formation was observed for tools coated with fine-grained diamond during the early stage of machining. A similar method to compare the wear rate of uncoated and coated tools was used in machining of titanium alloy Ti-6Al-4V, and the results showed that the coated carbide tools outperform the uncoated tools in terms of tool wear and cutting temperature.¹⁷ 

Wear on micro-tools using high-speed milling of titanium alloys has been analyzed by several researchers.^{13,14,17–19} Titanium-coated carbide tools were used in the manufacturing of parts for aero engines, where abrasive wear and adhesive wear were reported to be the dominant wear mechanisms.¹⁸ Notch wear, micro-chipping, and fatigue cracks were observed to be the primary failure modes.¹³ Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses confirmed that the primary wear mechanisms of PCBN tools are attrition, adhesion, and diffusion, whereas the tool wear of micro-tool is described as stochastic because of the tool size and insufficient strength of the tool cutting-edge.²⁰ Tool wear rate was described by Wang et al.²¹ and divided into three stages, namely, initial, stable, and sharp wear stages. The initial wear stage is dominated by flank wear as the primary wear type. The stable stage is dominated by chipping and adhesion as the primary wear types, which gradually increase and become severe. The sharp wear stage is described as a combination of flank wear and severe chipping and adhesion, with flank wear identified as the primary wear type in micromachining.²² The effect of cutting speed was assessed by measuring flank wear and using the trend of the plot of flank wear versus cutting speed to estimate the useful tool life of PVD TiN-coated tool. Low thermal conductivity and high temperature strength of the workpiece material are the main reasons for tool wear and machining efficiency. The effect of different cooling types on micromachining was compared,¹⁸ and the pneumatic mist jet impinging cooling (PMJIC) was shown to be more efficient than the commonly used total flood cooling (TFC). PMJIC was identified to be more suitable than TFC because of its capability to direct cooling into the cutting zone. A high-pressure jet is also capable of breaking the thin skin of steam around the cutting zone, enhancing the cooling efficiency and directly improving the tool life.

Although mostly coated tools are preferred in micromachining as reported in the literature, experimental data show an insignificant difference in the machining performance and wear rate of uncoated and coated tools; however, low burr formation was observed for tools coated with fine-grained diamond during the early stage of machining.¹⁶ SEM and EDX analyses confirmed that the primary wear mechanisms of PCBN tools are attrition, adhesion, and diffusion.¹⁸ Although the low wear rate of coated tools is reported to result in a low overall cutting temperature favorable in micromachining, the coating layer is considered to increase the cutting-edge radius, which directly promotes the size effect in micromachining.¹⁷ 

Hence, this study focuses on predicting and improving the useful tool life of uncoated carbide tools by experimentally investigating the effect of different cutting speeds and feedrates on tool wear. The standard procedures set by ISO 8688-223 have been used and modified for micromachining to predict the useful tool life of uncoated tungsten carbide end mills during slot milling of titanium alloy Ti-6Al-4V. A novel method is used to quantitatively evaluate the effect of different cutting speeds and feedrates with reference to volumetric loss of the tool and change in micro end mill geometries, wear type, and finished surface roughness to estimate the useful tool life during micromachining. This work contributes to the development of machining knowledge in the process planning stage for the selection of tools and estimation of tool change intervals.

In this study, experimental work was conducted on a standard precision CNC machining center (HURCO-VM10) to ensure industrially feasible results, as shown in Fig. 1. A high-speed spindle (NAKANISHI-HES810) with electric drive and ceramic bearings was retrofitted to the main spindle. The spindle is capable of continuous power output of 350 W, and the output torque of 3 N⋅m over the speed range of 20,000–80,000 RPM(r/min) allows a high cutting velocity for tools with a small diameter. Ultra-precision collets were used to clamp the micro-tool, and spindle runout was controlled at 1 µm. Spindle error is considered to have a significant effect on the surface roughness. In this study, the main spindle was on mechanical lock throughout the experiment so that the spindle error will be limited to the vibration and runout of the high-speed precision spindle. This experimental setup ensures that the primary spindle error, such as vibration and runout, will be minimized. Therefore, the primary spindle error will not be considered in the analysis.

In this experiment, uncoated tungsten carbide micro-square end mills with a nominal diameter of 1 mm were used. A 3 mm ultra-precision spindle collet was used to fit a nominal tool shank diameter of 3 mm. A large tool diameter was selected to prevent premature tool failure due to the harsh machining environment. In each set of experiments, a new tool is used to ensure the endurance of the tool without excessive tool wear and chipping. Tools were selected from the same batch to reduce randomization error due to the dissimilarity of micro end mills caused by manufacturing errors.

Slot milling was conducted at 0.1 mm axial depth of cut (a_p) machined on a block of titanium alloy Ti-6Al-4V (Grade 5). Fig. 1 illustrates the experimental setup and schematic diagram of slot milling.

The top surface of the block was prepared before the experiment to ensure the flatness of the top surface and consistent depth of the cut throughout machining. Each slot has a length of 20 mm, which is 20 times that of the diameter of the cutter, as recommended by ISO 8688-223 for tool life testing. Total flood synthetic fluids were used as lubricant to reduce the heat generated during the machining operation. To determine the tool life of end mills, various cutting speeds and feedrates were used to compare the influence of each parameter on tool life experimentally. The effect of different machining parameters was observed by monitoring the flank wear (VB), and tool geometry every 60 mm interval (i.e., three slots) for each tool. Flank wear is characterized by loss of tool material from the tool flanks during the machining operation and is categorized into uniform wear (VB1) across the active cutting-edge radius, nonuniform wear (VB2) with an irregular width on the active cutting-edge, and localized wear (VB3) developing on a specific part of the tool, as shown in Fig. 2.

In this experiment, the average width of wear progression across the active cutting face of the tool was measured. The weight of the tool was also recorded to monitor volumetric loss due to abrasive wear as an effect of machining. Before the weighting process, all of the tools were ultrasonically cleaned for 15 min in isopropene and dried using nitrogen to ensure the removal of lubricant and debris from machining. The average surface roughness and width of the slots every 4 mm were measured on the machined surface using an optical 3D measurement surface profilometer (Alicona InfiniteFocusSL) with vertical resolutions of 50 nm. Average measurable machined surface roughness (R_a) was recorded at the bottom of each slot. The machining parameters used in this experiment are listed in Table 1.

Different wear rates were observed on the tool flank face when using different cutting speeds, where the nonuniform wear land extended parallel to the cutting-edge. Fig. 3 shows the average length of the wear land across each tool flank face when using different cutting speeds during slot milling of 360 mm-long slots. Fig. 4 shows the SEM image of the tool flank face machined at various cutting speeds.

The data plotted in Fig. 3 show a linear increase in average flank wear for all cutting speeds. The wear lands shown in the SEM images in Fig. 4 indicate that the main cause of tool wear is adhesive wear due to adhesion of the workpiece material. As shown in Fig. 4, the average wear progression was measured across the active cutting-edge, where a low flank wear rate was observed at a low cutting speed because a low cutting temperature directly reduces the built up edge. Even though a high cutting speed results in an increase in cutting temperature, a large average flank wear land and a reduced wear rate were observed at a high cutting speed. Fig. 3 shows a low progression trend with an increase in cutting speed. The plot of VB2 for 30,000 RPM shows a significant change of up to 50% throughout the experiment, indicating the suitability of low cutting speed in micromachining. The overall trend for machining with low cutting speed is described as two stages, i.e., a slow steady wear rate and a sudden increase after 0.2 m slot milling followed by a gradual increase. In comparison with flank wear data using the maximum cutting speed of 70,000 RPM, the flank wear was reduced to 0.015 µm by the end of machining on the flank face. Machining using low cutting speeds of 40,000 and 50,000 RPM showed a rapid increase in flank wear land, whereas machining using high cutting speeds of 60,000 and 70,000 RPM exhibited a low wear land expansion rate. The wear land progression indicates that a critical range of cutting speeds results in significant tool wear. Given the fragility of micro-tools, predictable and gradual wear is desirable over a long tool life as a sudden increase in tool wear could result in sudden tool failure. Hence, low cutting speeds in micromachining are unsuitable even though the experimental results showed a slow wear rate as result of using low cutting speeds. The increase in cutting speed resulted in the reduction of flank wear, where a negative trend was observed over the cutting length of 0.2 m. Given the irreversible wear process, the negative trend indicates that abrasive wear replaces the main wear type identified as a critical transition point because the average flank wear of the tool begins to decrease. The effect of feedrate on the rate of flank wear was experimentally investigated, and the measured data are presented in Fig. 5.

As shown in Fig. 5, the flank wear rate measured experimentally for a range of feedrates indicates that a low feedrate leads to a significant reduction of the flank wear rate, whereas a high feedrate leads to a significant change of the flank wear rate followed by rapid growth. Given the significant behavior changes within the range of 50 mm/min to 100 mm/min, the data indicate a critical feedrate range for micromachining where the transition point results in a significant change in wear rate. The wear rate behavior is divided into three stages. In the first stage, tools using a high feedrate exhibit significant wear and wear rate stabilization up to 200 mm of machining. In the second stage, wear starts after 200 mm of machining, where a sudden jump was observed in VB2, indicating a transition point to a high wear rate followed by a stable gradual increase. A sudden jump from the early phase of the first stage to the second stage, which is considered to be the third stage, could result in unstable machining, directly affecting the effective tool life and resulting in sudden tool failure, which are unfavorable in micromachining. The dynamic behavior of machining varies depending on different feedrates. Sudden jumps in wear rate between 50 mm/min and 100 mm/min indicate a transition point, which could be described as the optimal feedrate range for a specific material type and cutting speed to maintain the stable machining operation. The analysis of measured flank wear data indicates that a high cutting speed and a low feedrate are suitable for micromachining as it results in less resultant flank wear and a low and steady trend of wear rate, which all directly improve machining stability.

Cutting-edge radius and tool diameter were also measured during the experiment. Fig. 6 and Fig. 7 show the plots of the measured tool diameter throughout machining for different machining speeds and feedrates.

The measured tool diameter shown in Fig. 6 indicates a substantial reduction of the tool diameter during machining, where the rate of reduction varies for different cutting speeds used during machining. The measured tool diameters for tools using cutting speeds of 60,000 and 70,000 RPM show a similar consistent reduction, indicating a similar abrasive wear rate throughout machining. However, high cutting speeds result in a large tool diameter reduction. In terms of the effect of low cutting speeds, 30,000 RPM shows a lower tool reduction of 974 µm in comparison with 978 µm. However, the trend of reduction significantly changes after 120 mm of machining with significant tool reduction for the remainder of the machining operation. Middle-level cutting speed of 50,000 RPM shows the lowest tool reduction of 982 µm, indicating a range of cutting speeds that could provide the optimum machining conditions that could reduce the tool wear rate. Notably, tool diameter reduction using high-level and low-level cutting speeds is approximately the same. However, the steady trend of wear at high cutting speeds due to the improved stabilization of material removal is preferred in micromachining. Fig. 7 shows the cutting tool diameter reduction of tools using various feedrates. The trend of tool diameter reduction at a low feedrate of 50 mm/min proved to be steeper than that at a high feedrate of 150 mm/min at a gradient of 3.67. The rate of reduction of tool diameter indicated by the gradient of the trend line for the feedrate of 150 m/min was reduced to 2.4, indicating that a high feedrate would result in a low wear rate that directly reduces the tool diameter reduction rate. Even though the trend of tool diameter reduction of a high feedrate shows a significant improvement, premature tool failure experienced after 250 mm of machining indicates that abrasive wear is not an element that causes premature tool failure.

The width of the channels machined using different machining speeds was also measured as shown in Fig. 8, where the cutting speeds of 40,000 and 70,000 RPM resulted in substantial runout; hence, these cutting speeds are unsuitable for micromachining. The low cutting speed of 30,000 RPM leads to a slow tool diameter reduction and an improved geometric accuracy in comparison with other cutting speeds. Tool diameter reduction becomes significant as the cutting speed increases because the channel width range changes from 15 µm to 44 µm by the end of machining. The sudden decrease in channel width indicates rapid tool diameter reduction. However, the change from a negative trend to a positive trend indicates a substantial increase in cutting temperature, resulting in the adhesion of the workpiece material to the cutting face of the tool as the tool diameter increases.

Fig. 9 shows the width of the channels machined using different feedrates. The width of the channels machined using feedrates of 50 and 100 mm/min indicates rapid abrasive tool wear up to 180 mm of machining followed by a significant increase in tool diameter. However, given that tool wear is an irreversible process, the increase in tool diameter explained by the process of adhesive wear, indicating a significant increase in cutting temperature with the high adhesion of the material observed.

Figs. 10 and 11 show the measured tool cutting-edge radius for different cutting speeds and feedrates.

Fig. 10 shows the plot of the measured cutting-edge of the tools throughout machining. Notably, a sharp increase in tool cutting-edge radius was observed in the initial stage of machining. This initial increase in cutting-edge radius is determined to be proportional to the cutting speed as a high cutting speed results in a significant wear rate on the cutting-edge radius. The wear trend line of different cutting speeds shows that 60,000 and 70,000 RPM exhibit a steady gradient at 1.42 and 1.44, respectively, indicating that high cutting speeds result in low edge wear. However, the initial wear measured in the first stage exhibited a sudden jump because of a significant increase in cutting-edge radius from 3 µm to 23 µm. All of these findings indicate that the initial substantial cutting-edge deterioration leads to an unstable machining environment and tool failure. After 350 mm of machining, the tool using a cutting speed of 40,000 RPM has the least cutting-edge wear where the cutting-edge radius was measured to be 16 µm, indicating the inverse relationship between cutting-edge radius wear rate and cutting speed. Fig. 11 shows the measured cutting-edge radius using various feedrates, which resulted in a rapid increase in tool cutting-edge radius. The trend of wear is determined to be proportional to the feedrate as the increase in feedrate resulted in the rapid increase in wear rate. However, the wear trend at feedrates of 50 and 100 mm/min initially increases and subsequently levels off with the steady progress of the machining operation. By contrast, the wear trend at the feedrate of 150 mm/min showed a different behavior in comparison with the low feedrates as rapid wear was observed in the initial stage, which led to tool failure in the third stage. The experimental data show that the tool cutting-edge radius has a significant effect on tool life, where a low feedrate would increase the useful tool life.

The machining parameters are considered to have a substantial effect on surface roughness, which is used as a critical reference to compare and assess the suitability of different machining parameters. Tool wear and cutting radius are considered to have a direct effect on the surface finish. Hence, in this experiment, surface roughness was measured every 60 mm for various cutting speeds and feedrates. The measured values are presented in Fig. 12 and Fig. 13.

Fig. 12 illustrates a rapid increase in surface roughness after 240 mm of machining for all cutting speeds <70,000 RPM, which exhibited a leveling off trend. By contrast, the highest cutting speed of 70,000 RPM resulted in a linear increase in surface roughness throughout machining. As the cutting speeds increases, the measured surface roughness gradually decreases to a critical length of 240 mm. The trend of the measured surface roughness indicates that a high cutting speed improves the surface roughness. However, apart from the critical duration of machining, the data also indicate a critical range of machining speeds that can be used in micromachining, with 60,000 RPM resulting in high surface roughness. Fig. 13 shows the surface roughness measured during machining using different feedrates. The trend indicates that a high feedrate improves the surface roughness significantly. However, the trend cannot be described as linear as a high feedrate of 150 mm/min increases the surface roughness by approximately 100 nm. Moreover, the tendency of the plot indicates a critical feedrate range for individual cutting speeds, resulting in an improved machining surface roughness and a long tool life.

Fig. 14 shows the plot of volumetric change measured for each tool at an interval of 60 mm during machining. Tools using the cutting speeds of 60,000 and 70,000 RPM have the lowest volumetric losses of 2.287 and 2.043 µg, respectively. All tools exhibit an initial sudden volumetric loss followed by a stable and gradual weight loss throughout the experiment. However, significant weight gain was observed for tools using the cutting speeds of 50,000–70,000 RPM, indicating substantial adhesion of the workpiece material to the cutting face of the tool. The data of the subsequent stage follows the same trend as the first two stages. A tool using the cutting speed of 50,000 RPM exhibits a faster rate of volumetric loss than other tools. Once the material is lost through abrasion, it cannot be gained back. Meanwhile, the weight increase of the tools indicates the adhesion of the workpiece material, where cutting speeds higher than 50,000 RPM clearly lead to weight increase. The weight gain rate has been proven to be proportional to the cutting speed as material adhesion increases by 0.064, 0.074, and 0.180 µg for 50,000, 60,000, and 70,000 RPM, respectively. The large material adhesion indicates the high machining temperature that can be attributed to the well-known size effect, where the material removal process is replaced by the plowing effect. Hence, the rubbing effect increases the machining temperature. Fig. 15 shows the volumetric data measured for tools using feedrates of 50, 100, and 150 mm/min. The trend of volumetric loss indicates the gradual wear of all tools using a range of feedrates, with similar behavior across all tools indicating that the feedrate has nearly no effect on abrasive tool wear.

Tool wear is an irreversible process that significantly affects the quality of the finished product and the reliability of the machining operation. The tool wear rate is observed to be directly dependent on the workpiece material hardness. In this experiment, titanium, which is known to be a hard-to-cut material, is selected and machined using uncoated tungsten carbide end mills. The effect of different machining parameters is analyzed using the method proposed by Wang et al.,¹² which compares wear progression measured throughout the experimental machining operation with the main wear mode. The experimental data indicated the rapid wear of the micro end mills, where the main wear mode has three different stages, namely, primary, secondary, and tertiary. The plots of the measured cutting-edge radius (Fig. 10 and Fig. 11) illustrated an initial significant abrasive wear, which is improved by high cutting speeds and low feedrates. However, in terms of the measured volumetric change (Figs. 14 and 15) at high cutting speeds, the results show a volumetric loss of 0.002 µg at 70,000 RPM, which is lower than the volumetric loss of 0.009 µg at 30,000 RPM. Moreover, low volumetric changes occur at high feedrates, with 0.002 µg measured at 150 mm/min compared with 0.007 µg measured at 50 mm/min. This finding indicates the unsuitability of a low feedrate as it promotes high friction, resulting in high machining temperature at the cutting zone. The volumetric change data indicated an initial volumetric loss and subsequent volumetric gain. Due to the irreversible tool wear, volumetric gain could be the result of high temperature in the cutting zone, leading to the adhesion of the workpiece material to the cutting face of the tool, which effectively reduces the hardness of the tool and increases the wear rate. Flank wear is the dominant wear type in the secondary and tertiary stages of machining, where VB2 is measured at different machining speeds and feedrates. Fig. 3 summarizes the measured average values of flank wear land on the cutting face of tools. High cutting speeds have been confirmed to aggrandize the wear rate, as the plot peaks to 60 µm using 70,000 RPM. As shown in Fig. 5, low feedrates appeared to improve the wear rate significantly as VB2 measured after 400 mm of machining using 50 mm/min was 15 µm, whereas VB2 measured after 400 mm of machining using 100 and 150 mm/min was approximately 40 µm. Machining stability is assessed by measuring the surface roughness (Fig. 12 and Fig. 13) for different cutting speeds and feedrates. The measured surface roughness across the range of cutting speeds up to 60,000 RPM shows a gradual degradation, whereas a rapid change was observed at 70,000 RPM, indicating a transition in machining performance. Surface roughness gradually increased with machining length up to 300 mm, where it started to level off. However, a rapid increase in surface roughness up to 1,000 nm is measured for slots machined using 70,000 RPM. Low feedrates resulted in the gradual degradation of surface roughness throughout machining. Meanwhile, the plot of surface roughness against various feedrates shown in Fig. 13 illustrates a nearly identical machining performance, indicating that feedrate has a negligible effect on surface roughness.

Tool life is the duration of effective cutting time after which the tool could not perform machining that fulfills the required quality of the finished part. Tool wear has a significant effect on effective tool life subject to the workpiece material hardness and machining parameters selected during machining. Data from the experimental examination of tools presented in Section 3 identified that machining parameters, such as feedrate and spindle speed, directly influence the wear rate of cutting tools. The tool wear rate is attributed to nonuniform abrasion along the active cutting-edge, tool cutting face, and tool flank. The plots of the data presented in Section 3 indicate a critical point in the trend of the wear progression rate, where the wear rate starts to increase significantly once the critical point is exceeded. Given that sudden changes in the material removal process during micromachining results in machining instability and premature tool failure, the trend of wear rate is used to identify the stable period of machining with reference to all attributes influencing the wear rate. Tool rejection criteria for tool life testing proposed by ISO 8688-223 for conventional milling were adopted and modified to suit the micromachining requirements to estimate the effective tool life of cutting tools. The selection of rejection criteria was based on experimental observation to identify the key factors that significantly affect the material removal process during machining of titanium. Three factors are proposed as tool rejection criteria, as follows:

1. Flank wear <37.5 µm;

2. Cutting-edge radius < 33 µm;

3. Surface roughness, R_a < 1 µm.

Flank wear was identified as one of the critical factors because of the limited flank face width for the selected tools. The limit to progression of the wear rate was set to 25% across the flank face width because the average wear progression land plotted in Fig. 3 is shown to exhibit a steady trend up to 37.5 µm. Once the average wear land width exceeded this limit, a rapid increase in wear land width was observed. Surface roughness, R_a, was limited to 1 µm, which is the commonly acceptable limit for micro-products. The maximum cutting-edge radius was restricted to the minimum chip thickness required for machining parameters selected to avoid the well-known size effect in micromachining. In this experiment, the criteria set for cutting-edge radius was the first factor that reached the maximum limit across the range of cutting speeds and feedrates tested. Hence, the cutting-edge radius measured throughout the experiment was used to estimate the effective tool life of micro end mills to determine the maximum tool life during micromachining of titanium. Figs. 16 and 17 show the plots of useful tool life for different cutting speeds and feedrates.

The plot of useful tool life shown in Fig. 16 indicates that a high cutting speed in micromachining improves the useful tool life, whereas a low cutting speed in conventional milling is traditionally favorable to improve the tool life. However, the trend of the plot shown in Fig. 12 indicates that a low cutting speed could improve the tool life. The SEM image shown in Fig. 4 illustrates the adhesion of the workpiece material to the tool cutting-edge as a result of the well-known size effect during micromachining, where the material removal process is replaced by the plowing effect. Fig. 17 shows that a low feedrate significantly increases the useful tool life as the experimental data indicated that, by doubling the feedrate, the tool life is reduced by more than half the machining time.

This study investigated the effect of machining parameters on the wear rate of uncoated tungsten carbide tool and concluded with the proposal of tool rejection criteria for micro-milling of hard-to-cut material, such as titanium alloy. The useful tool life of micro end mills was predicted with reference to flank wear rate, finished surface roughness, volumetric tool loss, and cutting-edge radius as guidelines to measure the effect of machining parameters on the tool wear rate. The following conclusions can be made:

1. A consequence of the low axial depth of cut used in micromachining is the concentrated cutting forces on the tip of micro-tools, which result in the rapid increase in cutting-edge radius and surface roughness.

2. Nonuniform flank wear was observed to be the dominant wear mode. The tool wear pattern in micromachining of titanium can be categorized into three stages, i.e., the initial rapid wear at the tooltip, the nonuniform expansion of wear land on the cutting face, and the final rapid abrasive wear causing tool diameter reduction.

3. High cutting speeds and low feedrates have been shown to improve the useful tool life of micro end mills. However, low feedrates have been shown to enhance the adhesion of the workpiece material to the cutting face of the tool, indicating an increase in the machining temperature.

4. A high cutting speed in micromachining results in the gradual rate of flank wear, which is favorable in micromachining to maintain a stable machining environment.

5. Adhesion of the workpiece material to the cutting face of the tool reduces the hardness of the cutting tool, which decreases the effective tool life and leads to premature tool failure.

6. High feedrates result in less friction and rubbing effect, which is favorable in micromachining to ensure a low machining temperature and enhance adhesion, which all contribute to directly improve the machining stability.

The authors wish to thank the Engineering and Physical Sciences Research Council (EP/M020657/1) for the support for this work.