Polycrystalline Cubic Boron Nitride Tool Research Articles

Improving the Performance of the Machining Process by Using Ultra‐Advanced Tools in a Clean Turning of Inconel 686 Using the Minimum Quantity Lubrication Method

ABSTRACTThe high tensile strength and high resistance of nickel‐based superalloy 686 against high temperatures and corrosion rates have made it a widely used in important applications such as the aerospace industry, high pollution‐ and corrosion‐resistance equipment manufacturing and petrochemical industry. Therefore, the machining of this advanced alloy with its unique properties is extremely important and can be challenging. Significant increase in input parameters levels, reduction of machining costs, improvement of surface and subsurface properties and clean production are among the issues that should be considered in dealing with Inconel 686 turning operations. Simultaneous application of advanced tools such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) and optimised minimum quantity lubrication (MQL) method and evaluating the results obtained with a wide range of output parameters related to machining process performance and tribological properties can be proposed as an innovation and a solution to this problem in this article. This study analyses several output parameters with different speeds and feeds to evaluate the effect of cutting insert type on machining process performance and tribological properties. The output parameters include tool wear, residual stress, cutting zone temperature, surface smoothness, machining forces and workpiece surface defects. The results indicated that using the optimised MQL method reduces the size of lubricant droplets and increases the surface covered by cooling. With these changes, the performance of the machining process and the parameters related to the surface integrity increase significantly. Among the parameters associated with the performance of the machining process, the PCD tool reduces the cutting zone temperature by 23%, the tool wear by 19% and the machining forces by 18% compared to the PCBN tool. In the parameters related to surface integrity, this method reduces the residual stress by 19% and the surface roughness by 9% compared to the PCBN tool. From the production index perspective, the PCD tool can significantly increase the cutting speed and feed rate, reducing production time and costs.

Composition, microstructure and mechanical properties of PcBN composites with Al- Zr binder

The cBN, Zr, and Al powders were mixed to synthesize PcBN (polycrystalline cubic boron nitride) tool material under high-temperature, ultra-high-pressure conditions. X-ray diffraction and scanning electron microscopy were used to analyze the phase composition and microstructure of the tool material. The mechanical properties and microstructural trends of the tool material at various temperatures were investigated. Research indicates the formation of new phases in the PcBN material following high-temperature and ultra-high-pressure sintering. However, there is no significant difference in the types of formed new phases, all of which consist of BN, ZrB2, ZrN, and AlN. The sintering density of the tool material was higher at 1500 °C and 1600 °C, and the mechanical properties improved with increasing sintering temperature. The highest number of tool processing parts made of PcBN tool material synthesized at 1600 °C is 232.

An experimental approach to characterize the performance of PCD and PCBN tools in milling nano Al-8081-Zr/Mg/TiO2 metal matrix composites using multi-sensor data fusion

To prevent the quality of the finished product from declining, precision manufacturing procedures need reliable cutting tool wear detection. Cutting tool material, workpiece attributes, cutting conditions and conditions, and excessive cutting forces creating vibrations causing chatter impacting tool wear finally leading to tool failure all have an impact on tool performance. This paper presents a multisensory data fusion approach that assigns sensors at nearfield sites during the machining process to monitor the tool condition. The study investigates the performance of polycrystalline diamond (PCD) and Polycrystalline Cubic Boron Nitride (PCBN) cutting tools during the machining of nano metal matrix composites reinforced with Zr/Mg/TiO2 (15%). The method correlates signal features with experimental results to provide a reliable empirical approach to monitor the cause of tool flank wear and displacement, leading to failure. The experimental investigation it is found the cutting forces showed significant effect on flank wear affecting surface finish and tool life. Tool performance was successful monitoring and predicted instantly based on the signature analysis of vibrations and forces during machining helped accurately analyzed factors affecting tool wear at uncertain cutting conditions using FDA analysis. The study provides insights into the PCD and PCBN tools’ performance characteristics.

Comparison of the performance of the internally cooled tool in closed circuit against standard PCBN tools in turning AISI D6 hardened

Machining is a widely used manufacturing process that involves removing material from a workpiece. During this process, more than 95 % of the mechanical energy from the machine tool's electrical motor is converted into heat. This can cause the cutting tool's temperature to soar to extremely high levels, reaching up to 1200 °C or even higher, especially when working with hardened steels. To cool down the machining region, cutting fluids are commonly employed in industries. However, these fluids are not considered sustainable because they pose environmental pollution risks, are costly, challenging to dispose of properly, and can also be hazardous to operators' health. Moreover, in some applications, cutting fluids are not recommended. Considering these issues, this research aims to propose and test a more sustainable approach. The method involves using a closed-circuit system where a mixture of water and ethylene glycol circulates internally in polycrystalline cubic boron nitride (PCBN) tools. These tools are employed in the turning operation of AISI D6 hardened steel and are compared with conventional dry machining. The study utilizes a design of experiment (DOE) and analysis of variance (ANOVA) to plan and analyze the results with statistical reliability. Key output variables considered include cutting temperature, tool life, cutting forces, and surface roughness. The implementation of the internally cooled tool (ICT) system has shown significant benefits. It notably reduces the temperature of the tool's rake face, as measured using a thermographic camera. Despite not being the main variable affecting the maximum temperature, the cooling system plays a crucial role in achieving these improvements. The ICT system also substantially increases the tool life in comparison to conventional dry machining across all tested cutting conditions. While the feed rate remains the most influential factor affecting cutting forces due to increased cutting area, the cooling system also plays a significant role. The ICT system, with reduced temperatures, performs better in this regard. As for surface roughness, the condition of the tool edge emerges as the sole significant factor. Surprisingly, the worn tool provides better surface finishing due to the presence of crater wear. With conventional dry machining, higher cutting temperatures lead to increased tool wear, primarily caused by abrasion and attrition.

The influence of microtexture tools on the geometric morphology of serrated chips based on stress triaxiality

PurposeThis study aims to investigate the effect of microtextured tools on the geometric morphology of serrated chips, and further improve the cutting performance of polycrystalline cubic boron nitride (PCBN) tool and extend the tool life and the surface quality of the machined surface.Design/methodology/approachA three-dimensional finite element cutting model of hardened steel AISI D2 with microtextured PCBN tools were established using the finite element software Abaqus, and cutting tests were carried out. Furthermore, the stress distribution in the primary deformation zone was investigated based on the triaxiality of stress, and the influence of microtexture on the geometric morphology of serrated chips and crack development was researched.FindingsThe results show that compared with nontexture tools, elliptical pits and wavy grooves microtexture tools have lower serrated degree Gs, higher serrated frequency f per unit length and more miniature serrated step Pc. The serrated phenomenon is intensified because the tensile stress zone of chips generated by nontextured tools is longer than that of elliptic pits and wavy grooves microtexture tools. Simultaneously, the maximum value of triaxiality in the tensile stress zone achieved by nontexture tools is larger than that of the two microtexture tools, and chips obtained by nontextured tools are more susceptible to propagation fractures.Originality/valueThis paper mainly studies the effect of microtexture on chip microgeometry, which is relatively little studied at present. At the same time, this paper has a certain engineering significance for PCBN tool turning hardening steel.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2023-0149/

Effect of different working conditions on tribological behaviors of PcBN/440c stainless steel in vacuum environment

Polycrystalline cubic boron nitride (PcBN) is a widely used tool material for cutting hard materials under high-speed and heavy-duty cutting conditions. As the need for in-orbit manufacturing and service in vacuum environment of outer space grows, understanding the tribological behavior of PcBN tools during vacuum operation becomes increasingly important. This study investigates the effect of different working conditions on the tribological behavior of PcBN discs and high‑carbon chromium alloy steel (440c) balls in vacuum. The tribological behavior of tribopair is analyzed. The experimental results reveal that the average of coefficient of the friction (CoF) is generally lower at 100 r/min than at 300 r/min and decrease with increasing load at two different speeds. The CoF of the 300 r/min group exhibits greater ups and downs at different load cases. The wear rate increases and then decreases with increasing load at different speeds. These fluctuations in CoF and wear rate are attributed to the transformation of the wear form and the effect of frictional heat build-up. Based on the SEM and EDX results, the wear mechanism is predominated by abrasive wear, gradually transitioning to adhesive wear with increasing loads. These findings provide insights into the tribological behavior of PcBN composite materials in vacuum. The results aim to inform the development of more efficient and dependable PcBN tools for in-orbit manufacturing and service applications.

Variation of the coefficient of thermal expansion with the volume ratio of TiB2-Sialon and its influence on the overall performance of PcBN

In this study, the primary factors affecting the performance of PcBN (Polycrystalline cubic boron nitride) tool materials were investigated by high temperature/high pressure sintering through the variation of TiB2-Sialon volume ratio, the strength of TiB2-Sialon binder itself, as well as the matching of the average coefficient of thermal expansion (CTE) with cBN (Cubic boron nitride). The effects of TiB2-Sialon on the phase composition, microstructure, and mechanical properties of PcBN composites were studied by X-ray diffraction (XRD), universal testing machine, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the properties of TiB2-Sialon binder are analogous to those of the corresponding composites. The phase composition of PcBN composite is not affected by the change of volume ratio. It was found that prepared composite, have TiB2 rod crystals growing along the (100) plane and Sialon rod crystals growing on the (001) plane. The influence of the volume ratio of TiB2-Sialon on the mechanical properties of the composites is principally concerned with the density, mechanical properties, and thermal expansion coefficient of the used binder. The extent to which the binder matches the coefficient of thermal expansion is favorable to the strength of the composite. When the volume ratio of TiB2 to Sialon was 1:3, the hardness (25.21 GPa, 37.44 GPa) and toughness (7.09 Pa m1/2 and 7.48 Pa m1/2) of TiB2-Sialon binder/TiB2-Sialon-cBN composites reached the maximum value, and the optimal impact resistance was also achieved. The density of the binder and the composite material was maximized when the volume ratio is 1:4. The average thermal expansion coefficient of the binder hoc tempore matches comparatively with cBN, and the interface bonding is better, displaying the best flexural strength (805.30 MPa and 1059.60 MPa) and wear ratio (2005). Therefore, adjusting the volume ratio of TiB2-Sialon within a certain range is beneficial to further enhance the binder's strength and mechanical properties.

Chip formation, cutting temperature and forces measurements in hard turning of Gcr15 under the influence of PcBN chamfering parameters

Hard turning is a key technology for difficult-to-cut materials with high precision and efficiency. However, the turning of GCr15 hardened steel leads to elevated cutting temperature and forces even at very low cutting parameters. Therefore, cutting parameters, tool geometric parameters, and cooling mediums are generally used to improve the machinability of such hard materials. This research aims to investigate the hard turning of GCr15 by varying the chamfering angle and chamfering width of polycrystalline cubic boron nitride (PCBN) at constant cutting conditions. The workpiece material and PCBN tool parameters were designed and modeled using 2D simulations using DEFORM®. Through simulation analysis and experimental research, the cutting force, cutting temperature, and the development of saw-tooth chips were studied and compared. The results achieved from experiments and simulations are very close with a maximum error of less than 5% for cutting forces and 6.3% for cutting temperatures. It is important to mention that 2D Finite Element Modeling (FEM) of hard turning is effective and efficient to forecast and measure the cutting temperature.

Investigation on preparation of composite vitrified bonding ultrafine diamond grinding wheel and machining of PcBN inserts

The grinding of polycrystalline cubic boron nitride (PcBN) is hard owing to its high hardness and superior wear-resistance capacity. Machining of PcBN tools via vitrified diamond grinding wheels with a size above 10 μm may lead to brittle fracture instead of a ductile machining because of the poor toughness of cubic boron nitride. A uniformly dispersed M0.5/1.5 diamond grinding wheel with a composite vitrified bonding was fabricated to improve the surface roughness of PcBN inserts. It is demonstrated that the preparation of composite vitrified bonding with various additions of vitrified bonding produced by the melting-quenching technique (VB-MQ) has little effect on the performance of vitrified bonding, such as bending strength, CTE and phase and achieves the uniform dispersion of M0.5/1.5 diamond as the addition of VB-MQ is no greater than 50%. Both the grinding ratios and the surface roughness of PcBN inserts are enhanced.

Experimental Investigation on the Machinability of PCBN Chamfered Tool in Dry Turning of Gray Cast Iron

Polycrystalline cubic boron nitride (PCBN) tools are widely used for hard machining of various ferrous materials. The edge structure of the PCBN cutting tool greatly affects the machining performance. In this paper, dry turning experiments were conducted on gray cast iron with a PCBN chamfered tool. Both the cutting temperature and the cutting force were measured, and then the surface quality and tool wear mechanisms were analyzed in detail. It was found that the cutting temperature and cutting force increased with the increase in feed rate, depth of cut, and cutting speed. The surface roughness firstly decreased, and then increased with an increase in feed rate. The minimum surface roughness was obtained with a feed rate of 0.15 mm/r which exceeded the tool chamfer width. The PCBN tool wear mode was mainly micro notches on the rake face and micro chipping on the tool chamfer, while the adhesion wear mechanism was the main tool wear mechanism.

Performance and wear mechanisms of different PcBN tools when machining superalloy AD730

Nickel-based superalloys are known to be difficult to machine. These alloys are generally machined with cemented carbide tools under low productivity process conditions. Superhard polycrystalline cubic boron nitride (PcBN) tooling offers the possibility of increased production rates, however, a thorough understanding of the material performance and degradation is required before a transition from carbide tools occurs. This study investigated the performance and wear mechanism of three different PcBN tools with low (50 vol. %), medium (65 vol. %), and high (90 vol. %) cBN content, when turning AD730, a new Ni–Co-based superalloy. Due to the novelty of the material, little research has been done on its machinability with carbide and even less with PcBN tooling. The results showed the high-cBN grade was the best performer in terms of machining time. However, an undesirable wear morphology is generated, leading to excessively high cutting forces. Both the medium-cBN and low-cBN showed promise with stable rake and flank wear, but with notching as a drawback. Degradation analysis of medium-cBN grade revealed diffusional dissolution of cBN and formation of a reaction layer atop the cBN grains. The reaction layer, which acts as a diffusional barrier, consisted of three sub-layers: (1) Al2O3, (2) (Ti, Nb, Zr)N, and (3) (Ti, Cr, Nb, Zr)N. The low-cBN grade also showed a reaction layer; however, the smaller cBN grain size of this grade did not allow the layer to sufficiently stabilize.

Innovative comparison on performance of PCBN tool and uncoated tool in CNC green turning of LDX2101 duplex stainless steel to minimize surface roughness

The goal of this research is to minimize the surface roughness (SR) by comparing Polycrystalline Cubic Boron Nitride (PCBN) tool and uncoated (cemented carbide) tool during CNC turning of LDX 2101 duplex stainless steel. PCBN inserts were used as the experimental group, whereas uncoated inserts were used as the control group in this research. The selected machining factors were speed of cutting, rate of feed and cut depth. The total number of groups were two and the sample size was 27 samples for each group were used for conducting experiments. The obtained mean SR value for PCBN insert is 3.826559 µm and for uncoated insert is 4.066111 µm. The significant difference between the two inserts is 0.01 (p < 0.05). Within the confines of this research, when LDX2101 duplex stainless steel is machined with PCBN insert it gives lower SR and when the same specimen is machined with uncoated insert it gives higher SR.

FEM Modelling of Tool Wear in Hard Turning Operations

Hard turning is a machining operation of finishing performed on material with a hardness higher than 45 HRC and it has been demonstrated to be a profitable alternative to grinding process allowing, respect to this latter, better surface quality, improved process control reliability, and faster production rates. The high pressure and sliding velocity at the tool-chip interface, in addition to the deformation energy generated, lead to the formation of high temperature zones localized in the cutting region. These thermal loads enhance harmful aspects on both cutting tool and machined material. On the tool, the abrasive-diffusive wear mechanism is promoted. On the workpiece, a local surface heat treatment of quenching is observed, upgrading the formation of a thin layer of over-quenched martensite, called white layer, due to its appearance under microscopical observation, that is undesirable due to its brittleness and related cracks generation. In addition to this, hard turning is usually performed in dry condition further augmenting machining temperature and tool wear rate. Because of the extremely high hardness of the machined materials, the employment of expensive ultra-high hardness cutting tools is mandatory. Thus, to make the process valuable, it is essential to optimize the tool usage by avoiding catastrophic failure and being able to forecast the tool wear evolution during cutting time as a function of the applied process parameters. One of the possible approaches for predicting the wear behaviour is finite element (FE) simulations of the turning process. In this work, the wear of polycrystalline cubic boron nitride (PCBN) tools when turning AISI 52100 hardened steel is investigated. An abrasive-diffusive wear model has been developed and implemented in a FE engine, by means of a dedicated subroutine able to update the geometry of the worn tool as a function of the cutting parameters, with the intent of simulating the tool wear behaviour, in terms of crater and flank wear extension. For the calibration of the model, the tool wear measurements achieved from an experimental campaign of orthogonal cutting tests have been used. The influence of process parameters on the wear rate has been examined, underlining that the most affecting parameter on tool wear is the cutting speed: the higher the cutting speed the higher the tool wear. Furthermore, it has been observed that the tool breakage was due to an excessive depth of the crater wear that modified the initial negative rake angle to a positive one. This modification changed the stress state of the tool from compressive to tensile, not well tolerated by PCBN tools, bringing to catastrophic failure. The tool wear simulation results are in good agreement with the experimental ones, validating the usability of the proposed computational methodology for crater and flank wear prediction. Therefore, the application of this technique allows the optimization of cutting parameters from a preventive FE simulation analysis avoiding the need of exploiting new costly and time-consuming experimental tests.

Cutting performance of micro-textured PCBN tool

To study the effect of micro-texture on the cutting performance of polycrystalline cubic boron nitride (PCBN) tools, five types of micro-textures (circular pits, elliptical grooves, transverse grooves, composite grooves, and wavy grooves) were applied to the rake surface of PCBN tools by an optical fiber laser marking machine. Through a combination of three-dimensional cutting simulations and experiments, the influences of micro-texture on chip–tool contact area, cutting force, chip morphology, shear angle, and surface roughness during the cutting process were analyzed. The results indicated that the chip–tool contact area and cutting force of both non-textured and micro-textured tools increased with increasing cutting speed, while the shear angle decreased with increasing cutting speed. The chip–tool contact area and cutting force of the five types of micro-textured tools were smaller than those of the non-textured tool. The chip–tool contact area and cutting force obtained by the wavy-groove micro-textured tool were the smallest. The chip radius produced by the five types of micro-textured tools was smaller than that produced by the non-textured tool, and the chip morphology was more stable. The transverse-groove micro-textured tool had a better chip breaking effect. The chip radius generated by the elliptical-groove micro-textured tool was 0.96 cm, while that generated by the wavy-groove tool varied from 0.55 cm to 1.26 cm. The presence of a micro-texture reduced the surface roughness of the workpiece by 11.73%–56.7%. Under the same cutting conditions, the five types of micro-textured tools gave a smaller chip–tool contact area, cutting force, chip radius, and surface roughness and a larger shear angle than the non-textured tool. In addition, the elliptical-groove and wavy-groove micro-textured tools had better cutting performance.

The impact of the retained austenite in the case-hardened steels on the crater wear formation of the PcBN tools

The finishing operation of transmission components in the automotive industry is widely performed by the hard part turning process, using polycrystalline cubic boron nitride (PcBN) as cutting tool material. Carburizing steels are used in shafts and gears, and after the heat treatment processes, the components have typically 10-30 vol% of retained austenite in the carburized layer. The hard-turning is essentially a process involving high temperatures. The temperature at the tool-chip interface typically reaches the austenitization temperature range of the machined steel. In carburizing steels, the consequence is the generation of crater wear in PCBN tools by the chemical wear mechanism. In this context, this research aims to determine the impact of the retained austenite in the carburized steels on the crater wear formation in the PcBN tools. The 18CrNiMo7-6 steel was used to manufacture the workpieces. During the heat treatment, two carbon potentials (0,64% and 0,72%) and two tempering temperatures (160 °C and 180 °C) were applied to reach two levels of hardness (710 HV and 740 HV) and amount of retained austenite (20% and 27%). To study the wear, hard-turning tests were performed, applying industrial cutting parameters. The PCBN inserts were analyzed by a focus variation microscope (FVM) and a scanning electron microscope (SEM). For the same level of hardness, an increased phase fraction of the retained austenite in the steel increases the crater wear of the PcBN tools. Two complementary hypotheses were framed to explain the results: i. the high fraction volume of retained austenite increases the overall oxidation potential of the iron on the steel, accelerating the chemical degradation of the cBN grains; ii. the high amount of retained austenite reduces the formation of the protective layer on the chip tool interface. The in-depth investigation of chemical degradation is a largely unexplored subject on the wear evaluation of the ultrahard cutting tool.

Influence of cutting tool material on machinability of Inconel 718 superalloy

Appropriate selection of tool material is very important for effective machining of ‘difficult-to-cut’ aerospace superalloy Inconel 718, especially, under dry condition. Present work aims at investigating performance of physical vapor deposition (PVD) multi-layer (TiN/TiCN/TiN) coated cermet, and PVD TiAlN coated PCBN (Polycrystalline Cubic Boron Nitride) brazed tipped carbide inserts on machinability of Inconel 718. Machinability is assessed based on cutting force magnitude, maximum tool-tip temperature attained, tool wear morphology, chip macro/micro morphology, and end part surface integrity. Later includes detailed analysis on morphology of the machined surface, roughness, and depth of hardened layer. Results obtained thereof, are compared to that of conventional uncoated WC-Co tool. It is evidenced that coated cermet outperformed other two inserts in purview of lower cutting force, better surface finish, and tiny white layer. WC-Co causes extreme cutting temperature than other counterparts. Up to 86 m/min cutting velocity, PCBN tool exhibits minimal flank wear; afterwards, tool is severely affected by huge notch wear. On the contrary, cermet tool suffers from lower flank wear than WC-Co for all cutting speeds tested. Both cermet and PCBN inserts are found remarkably affected by notching, and coating dissipation. Attrition wear is evidenced only in case of WC-Co insert. Coated cermet, thus, appears as a better choice for dry turning of Inconel 718.

Experimental investigations on machining force in turning of AZ91D magnesium alloy using PCBN tools

The present work aims to study the behavior of AZ91D Magnesium alloy during turning with Polycrystalline cubic boron nitride (PCBN) tools via investigation on machining force with variable speed, feed, depth of cut, and nose radius. Experimentation is planned and executed as per L16 orthogonal array. The parametric effects and their interactions were discussed using main effects plot and interaction plot. Results concluded that Machining force is significantly influenced by depth of cut followed by feed, speed and nose radius.

Pulsed laser micro ablation of polycrystalline cubic boron nitride

Polycrystalline cubic boron nitride (PcBN) is the second hardest material after diamond. Because of its unique properties, PcBN is extensively used as a cutting tool material for the machining of hardened steels. Pulsed laser ablation (PLA) offers the possibility to generate tailored microgeometries on PcBN tools and can thereby increase tool performance. However, the thermal response of PcBN materials to the laser ablation and the laser effeciency in laser machining is not known in dependency of certain PcBN grade characteristics. In this paper the influence of different laser sources in the short and ultrashort pulse duration regime is investigated for a wide number of PcBN grades. Thermally induced surface alterations are examined in detail and connected with the functional performance of the material. It could be shown that a careful parameter definition is crucial for enhancing the performance of laser prepared PcBN cutting tools.

Investigation on surface morphology and crystalline phase deformation of Al80Li5Mg5Zn5Cu5 high-entropy alloy by ultra-precision cutting

The materials with high precision，easy machining and excellent micro-machinability are considered significant on the fabrication of precision components, the outstanding comprehensive properties of Al80Li5Mg5Zn5Cu5 high-entropy alloy just meet these requirements. Firstly, our study found that the feed rate parameters have the greatest influence on the surface quality of material, and the surface roughness can reach 6 nm at the lowest level. After ultra-precision cutting the material, we found that the cutting process of high-entropy alloy has no obvious damage to the cutting tools and two distinct phases (FCC α-Al phase and Al/Al2Cu/MgZn2 quaternary eutectics phase) were formed on the finished surface under the observation of scanning electron microscopy. By comparing the size of two phases and the morphology of undeformed chips cut by diamond tool and polycrystalline cubic boron nitride (PCBN) tool, the material surface cut by PCBN tool was subjected to greater extrusion force and friction during machining, leading to more remarkable increase in surface microhardness.

Failure and Control of PCBN Tools in the Process of Milling Hardened Steel

The polycrystalline cubic boron nitride (PCBN) milling tool can be used in the mold industry to replace cemented carbide tools to improve machining efficiency and quality. It is necessary to study the tool wear and failure mechanism to increase machining efficiency and extend tool life. Cr12MoV is used to analyze the failure form of PCBN tools in the interrupted cutting of hardened steels at low and high speed conditions in milling experiments. Experimental results show that the failure forms of PCBN tools include chipping and flank wear at low speed, and the failure modes at high speed are flank wear, the surface spalling of the rake face, and the fatigue failure on the flank face. The failure mechanism of different failure forms is analyzed by observing the surface morphology of the tool and using the theory of fracture mechanics. The results show that a high cutting speed should be selected to avoid the early damage of low speed and achieve better application of PCBN tools. At high cutting speed, tool failure is mainly caused by mechanical wear, diffusion wear, and oxidation wear. Moreover, a fatigue crack will occur at the cutting edge on the chamfered tool under thermal–mechanical coupling because of the intergranular fracture of the CBN grain and binder. A large area of accumulated fatigue damage may appear due to the influence of alternating mechanical stress and thermal stress. Finally, the control method to avoid tool failure is presented.