PREDICTION OF TOOL LIFE IN MACHINING WITH RESTRICTED CONTACT TOOLS

A semi-empirical method is described for predicting tool life in orthogonal machining with restricted contact tools. The method uses a well established machining theory to predict cutting forces, tool-chip contact length and cutting temperatures for the corresponding plane face tool i.e. tool having the same cutting edge geometry but no restricted contact. These predicted parameters and a set of empirical relations are then used to calculate the cutting temperatures and tool life for the restricted contact tool. A comparison has been made between predicted and experimental results obtained from the literature and from tests carried out by the authors.     

THE OXLEY MODELING APPROACH,ITS APPLICATIONS AND FUTURE DIRECTIONS

Abstract

The Oxley machining theory which allows for the high strain-rate/high temperature flow stress and thermal properties of the work material is described. It is shown how the theory that was originally developed for the orthogonal process and later extended to oblique machining, can be used to predict cutting forces, temperatures and subsequently built-up edge formation conditions, tool life and cutting edge deformation conditions. It is also shown how the theory can be applied to obtain predictions in machining with restricted contact tools and in intermittent cutting processes, and to obtain work material properties using machining test results. Finally, some consideration is given to the future directions of machining research at UNSW. The Oxley Model can be used for predicting the performance parameters for different machining processes by taking into account the fundamentals of the chip formation process.     

Experimental and numerical investigation of cutting forces in micro-milling of polycarbonate glass

Abstract

Polycarbonates have found important applications in various types of industries including optical, automotive, aerospace, biomedical, and defense manufacturing industries. Conventional mechanical machining has the capability to create complex multi-scale parts and components for various materials including polymeric materials. This study investigates the cutting forces generated during the machining of polycarbonate glass using the micro-milling process. The goal of this research is to machine high quality micro-channels in polycarbonates for microfluidic applications. Both experimental investigation and numerical simulations using the Finite Element Method (FEM) have been carried out to assess the cutting forces generated in three directions during machining of polycarbonate. The effectiveness of tool coating on the reduction of cutting forces has been investigated. It was found that with the careful combination of depth of cut and feed rate, the ductile mode machining of polycarbonate can be achieved, which produces lower cutting forces, that could result in improved surface finish and low tool wear. Both lower and higher of depths of cut were found to generate higher cutting forces due to dragging action and higher tool-workpiece contact area respectively. The Finite Element Method (FEM) was found to be effective in simulating the cutting forces with acceptable range of errors, and thus, could be used to predict cutting forces at the parametric combinations beyond the capacity of the machine or without carrying out further expensive experimentation, for which the chances of tool failure are higher.     

INDUCTION-HEATED TOOL MACHINING OF ELASTOMERS—PART 2: CHIP MORPHOLOGY,CUTTING FORCES,AND MACHINED SURFACES

ABSTRACT

The induction-heated tool and cryogenically cooled workpiece are investigated for end milling of elastomers to generate desirable shape and surface roughness. Elastomer end milling experiments are conducted to study effects of the cutting speed, tool heating, and workpiece cooling on the chip formation, cutting forces, groove width, and surface roughness. At high cutting speed, smoke is generated and becomes an environmental hazard. At low cutting speeds, induction heated tool, if properly utilized, has demonstrated to be beneficial for the precision machining of elastomer with better surface roughness and dimensional control. Frequency analysis of cutting forces shows that the soft elastomer workpiece has low frequency vibration, which can be correlated to the surface machining marks. The width of end-milled grooves is only 68 to 78% of the tool diameter. The correlation between the machined groove width and cutting force reveals the importance of the workpiece compliance to precision machining of elastomer. This study also explores the use of both contact profilometer and non-contact confocal microscope to measure the roughness of machined elastomer surfaces. The comparison of measurement results shows the advantages and limitations of both measurement methods.     

MACHINING STUDIES OF UD-FRP COMPOSITES PART 2: FINITE ELEMENT ANALYSIS

ABSTRACT

A number of parameters and an exhaustive material development and experimental procedure to determine the response variables like cutting forces, surface damage restricts the expensive experimental research. In this context, Finite Element Method (FEM) analysis can be used as a tool for the prediction of the various machining responses. A finite element analysis of the orthogonal machining of Uni-directional Glass Fiber Reinforced Plastic (UD-GFRP) laminates is presented in this study to understand the complex relation between fiber orientation, tool geometry, depth of cut on cutting forces and sub-surface damage.     

An integrated prediction model for the dynamics of machine tool spindles

ABSTRACT

The predictility of dynamics of the machine tool spindles is essential for machining precision. During machining, the machine tool components and the cutting process interact with each other. Accordingly, it is necessary to take the process-machine interaction effects into account in order to predict the spindle's dynamics accurately. This paper presents an integrated model for the prediction of a spindle's dynamics. The model synthesizes the interactive influence between machine dynamics and forces in grinding process. The thermo-mechanical model of the spindle with angular contact ball bearings was built by using the finite-element method. The analytical model was used to calculate the process forces. A coupled simulation was adopted to accomplish the interactive process between the two models. Basing on the integrated model, the bearing stiffness, the natrual frequency, the spindle tip stiffness and deformations of a grinder's spindle were investigated. The prediction of the deformation fluctuations at the spindle tip due to process-machine interaction was also achieved.     

END MILLING SYSTEM COMPLIANCE AND MACHINING ERROR

Abstract

Tool deflection resulting from cutting forces places a constraint on the achievable precision and productivity in machining. This paper presents an analytical model of machining error, in terms of part form deviation in end milling due to the elastic compliance of cutting tool. Based on the relationship of local cutting forces and chip thickness, the shear loading and bending moment on the tool cross section are presented in terms of cutter angular position. The tool deflection resulting from the bending moment is then established from the principle of virtual work. The resulting deflection of workpiece and machine tool structure is also considered through shear loading analysis. The expression for machining error is derived as a closed-form function of the machining parameters, cutting configuration, material characteristics, and machine receptance. End milling experiments were conducted to verify the analytical model under various cutting conditions. Error maps are presented to illustrate the effects of process conditions on the achievable part accuracy.     

MANUFACTURING PROCESSES AND EQUIPMENT

ABSTRACT

For the past fifty years researchers have developed various machining models to improve cutting performance. Several approaches have been taken including analytical techniques, slipline field solutions, empirical approaches and finite element techniques. Of these, the finite element approach provides the most detailed information on chip formation and chip interaction with the cutting tool. Finite element models have been developed for calculating the stress, strain, strain-rate, and temperature distributions in both the chip and the workpiece. In addition, tool temperatures, machining forces and cutting power requirements can be determined. This information is extremely, useful for developing more fundamental understanding of complex machining problems. This paper presents a critique of finite element approaches used for simulating machining processes. Several applications of the finite element technique for simulating various machining problems are also reviewed. A new application for determining diffusion wear rates in cutting tools is described, and future directions for finite element modeling of machining processes are discussed.     

A comparative study of multilayer and functionally graded coated tools in high-speed machining

Multilayer-coated tool systems have been effective in controlling mechanical and thermal loads, especially in high-speed cutting regime. In this study, cutting performance of tungsten carbide tools with restricted contact length and multilayer chemical vapour deposition deposited coatings, TiCN/Al₂O₃/TiN (in series) and TiCN/Al₂O₃–TiN (functionally graded), was investigated in dry turning. Cutting tests were conducted on low carbon alloy steel AISI/SAE 4140 over a wide range of cutting speeds between 200 and 879?m/min. Results including cutting forces, chip compression ratio, shear angle, contact area inclusive of sticking and sliding phenomena and tool flank wear are presented. In particular, prediction of heat partition into the cutting tool inserts was carried out using a combination of experimental tests and the finite element method. The results show that coating layouts and cutting tool edge geometry can significantly affect heat distribution into the cutting tool. The paper clearly shows the role and potential benefits of applying different top coats on the rake and flank faces with regards contact phenomenon, impact on thermal shielding and tool wear. An appropriate coating layout selection is crucial in controlling tool wear, especially in high-speed machining.     

MODELING THE PHYSICS OF METAL CUTTING IN HIGH-SPEED MACHINING

ABSTRACT

Physical modeling of metal cutting was carried out to provide an understanding and prediction of machining process details. The models are based on finite element analysis (FEA), using a Lagrangian formulation with explicit dynamics. Requirements for material constitutive models are discussed in the context of high-speed machining. Model results address metal cutting characteristics such as segmented chip formation, dynamic cutting forces, unconstrained plastic flow of material during chip formation, and thermomechanical environments of the work-piece and the cutting tool. Examples are presented for aerospace aluminum and titanium alloys. The results are suited for analysis of key process issues of cutting tool performance, including tool geometry, tool sharpness, workpiece material buildup, and tool wear.     

Finish Machining of Nickel-Base Inconel 718 Alloy with Coated Carbide Tool under Conventional and High-Pressure Coolant Supplies

Single-point turning of Inconel 718 alloy with commercially available Physical Vapour Deposition (PVD)-coated carbide tools under conventional and high-pressure coolant supplies up to 20.3 MPa was carried out. Tool life, surface roughness (Ra), tool wear, and component forces were recorded and analyzed. The test results show that acceptable surface finish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures. The highest improvement in tool life (349%) was achieved when machining with 11 MPa coolant supply pressure at higher speed conditions of 60 m · min^?1. Machining with coolant pressures in excess of 11 MPa at cutting speeds up to 40 m · min^?1 lowered tool life more than when machining under conventional coolant flow at a feed rate of 0.1 mm · rev^?1. This suggests that there is a critical coolant pressure under which the cutting tools performed better under high-pressure coolant supplies.

Cutting forces increased with increasing cutting speed due probably to reactive forces introduced by the high-pressure coolant jet. Tool wear/wear rate increased gradually with prolonged machining with high coolant pressures due to improved coolant access to the cutting interface, hence lowering cutting temperature. Nose wear was the dominant tool failure mode when machining with coated carbide tools due probably to a reduction in the chip-tool and tool-workpiece contact length/area.     

LUBRICATION MECHANISMS OF LN2 IN ECOLOGICAL CRYOGENIC MACHINING

ABSTRACT

Cryogenic machining is considered an environmentally safe alternative to conventional machining where cutting fluid is used. In cryogenic machining, liquid nitrogen (LN2) is well recognized as an effective coolant due to its low temperature, however, its lubrication effect is less well known. Our previous studies of the change in cutting forces, tool wear, chip microstructure, and friction coefficient indicate a possible lubrication effect of LN2. This paper proposes two mechanisms on how LN2 can provide lubrication in the cutting process. To verify these proposed LN2 mechanisms and distinguish them, idealized disk-flat contact tests were performed. A low temperature can alter the material properties and change the friction coefficient between the specimens. However, from the test results, this lubrication mechanism was dependent on the material pairs. An uncoated carbide insert with a low carbon steel or titanium alloy disk test showed reduction of friction under LN2 cooling, but a coated insert increased the friction force. LN2 injection to form a physical barrier or hydrodynamic effect between two bodies is always effective in reducing the friction force.     

Cutting force prediction for ball nose milling of inclined surface

Ball nose milling of complex surfaces is common in the die/mould and aerospace industries. A significant influential factor in complex surface machining by ball nose milling for part accuracy and tool life is the cutting force. There has been little research on cutting force model for ball nose milling on inclined planes. Using such a model ,and by considering the inclination of the tangential plane at the point of contact of the ball nose model, it is possible to predict the cutting force at the particular cutting contact point of the ball nose cutter on a sculptured surface. Hence, this paper presents a cutting force model for ball nose milling on inclined planes for given cutting conditions assuming a fresh or sharp cutter. The development of the cutting force model involves the determination of two associated coefficients: cutting and edge coefficients for a given tool and workpiece combination. A method is proposed for the determination of the coefficients using the inclined plane milling data. The geometry for chip thickness is considered based on inclined surface machining with overlapping of previous pass. The average and maximum cutting forces are considered. These two forces have been observed to be more dominating force-based parameters or features with high correlation with tool wear. The developed cutting force model is verified for various cutting conditions.     

An optimization method of the machining parameters in high-speed machining of stainless steel using coated carbide tool for best surface finish

High-speed machining (HSM) has emerged as a key technology in rapid tooling and manufacturing applications. Compared with traditional machining, the cutting speed, feed rate has been great progress, and the cutting mechanism is not the same. HSM with coated carbide cutting tools used in high-speed, high temperature situations and cutting more efficient and provided a lower surface roughness. However, the demand for high quality focuses extensive attention to the analysis and prediction of surface roughness and cutting force as the level of surface roughness and the cutting force partially determine the quality of the cutting process. This paper presents an optimization method of the machining parameters in high-speed machining of stainless steel using coated carbide tool to achieve minimum cutting forces and better surface roughness. Taguchi optimization method is the most effective method to optimize the machining parameters, in which a response variable can be identified. The standard orthogonal array of L₉ (3⁴) was employed in this research work and the results were analyzed for the optimization process using signal to noise (S/N) ratio response analysis and Pareto analysis of variance (ANOVA) to identify the most significant parameters affecting the cutting forces and surface roughness. For such application, several machining parameters are considered to be significantly affecting cutting forces and surface roughness. These parameters include the lubrication modes, feed rate, cutting speed, and depth of cut. Finally, conformation tests were carried out to investigate the improvement of the optimization. The result showed a reduction of 25.5% in the cutting forces and 41.3% improvement on the surface roughness performance.     

Experimental investigation of machining characteristics and chatter stability for Hastelloy-X with ultrasonic and hot turning

Ultrasonic-assisted machining is a machining operation based on the intermittent cutting of material which is obtained through vibrations generated by an ultrasonic system. This method utilizes low-amplitude vibrations with high frequency to prevent continuous contact between a cutting tool and a workpiece. Hot machining is another method for machining materials which are difficult to cut. The basic principle of this method is that the surface of the workpiece is heated to a specific temperature below the recrystallization temperature of the material. This heating operation can be applied before or during the machining process. Both of these operations improve machining operations in terms of workpiece-cutting tool characteristics. In this study, a novel hybrid machining method called hot ultrasonic-assisted turning (HUAT) is proposed for the machinability of Hastelloy-X material. This new technique combines ultrasonic-assisted turning (UAT) and hot turning methods to take advantage of both machining methods in terms of machining characteristics, such as surface roughness, stable cutting depths, and cutting tool temperature. In order to observe the effect of the HUAT method, Hastelloy-X alloy was selected as the workpiece. Experiments on conventional turning (CT), UAT, and HUAT operations were carried out for Hastelloy-X alloy, changing the cutting speed and cutting tool overhang lengths. Chip morphology was also observed. In addition, modal and sound tests were performed to investigate the modal and stability characteristics of the machining. The analysis of variance (ANOVA) method was performed to find the effect of the cutting speed, tool overhang length, and machining techniques (CT, UAT, HUAT) on surface roughness, stable cutting depths, and cutting tool temperature. The results show both ultrasonic vibration and heat improve the machining of Hastelloy-X. A decrease in surface roughness and an increase in stable cutting depths were observed, and higher cutting tool temperatures were obtained in UAT and HUAT compared to CT. According to the ANOVA results, tool overhang length, cutting speed, and machining techniques were effective parameters for surface roughness and stable cutting depths at a 1% significance level (p ≤ 0.01). In addition, cutting speed and machining techniques have an influence on cutting tool temperature at a 1% significance level (p ≤ 0.01). During chip analysis, serrated chips were observed in UAT and HUAT.     

Analysis of tool-chip interface characteristics of self-lubricating tools with nanotextures and WS<Subscript>2</Subscript>/Zr coatings in dry cutting

The environmental obligations of manufacturing industries have resulted in the development of new cutting tools during metal machining without cutting fluids. According to the green manufacturing principles and to further improve the cutting performance of tools in dry cutting, novel cutting tools combined with nanotextures and WS₂/Zr coatings (AN-AW) are developed, and cutting tests without cutting fluids on hardened steel exhibit that the AN-AW tool is the most effective in reducing the cutting forces compared with the WS₂/Zr-coated tool (AS-W) and conventional tool (AS). Based on the experiments and theoretical models, the tool-chip interface characteristics are further investigated quantitatively to analyze the mechanism of the AN-AW tool. Results show that the AN-AW tool has a significant effect on the tool-chip interface characteristics. The AN-AW tool is the most effective in reducing the friction coefficient and tool-chip contact length; meanwhile, it changes the stress distribution at the tool-chip interface. The reduced tool-chip contact length and sticking-total contact length ratio as well as the lubricant film formed by the WS₂/Zr coatings at the tool-chip interface may be responsible for the changes of friction and stress distribution for the AN-AW tool.     

THE OXLEY MODELING APPROACH, ITS APPLICATIONS AND FUTURE DIRECTIONS

The Oxley machining theory which allows for the high strain-rate/high temperature flow stress and thermal properties of the work material is described. It is shown how the theory that was originally developed for the orthogonal process and later extended to oblique machining, can be used to predict cutting forces, temperatures and subsequently built-up edge formation conditions, tool life and cutting edge deformation conditions. It is also shown how the theory can be applied to obtain predictions in machining with restricted contact tools and in intermittent cutting processes, and to obtain work material properties using machining test results. Finally, some consideration is given to the future directions of machining research at UNSW. The Oxley Model can be used for predicting the performance parameters for different machining processes by taking into account the fundamentals of the chip formation process.     

The Effect of Coolant Concentration on the Machinability of Nickel-Base, Nimonic C-263, Alloy

This paper investigates the effect of coolant concentration on tool performance when machining nickel-base, C-263, alloy with triple coated (TiN/TiCN/TiN) carbide insert at various (3–9%) coolant concentrations and under different cutting speed conditions. Tool life, tool-failure modes, wear rates, component forces and surface finish generated during machining were recorded, analyzed and used to formulate mechanisms responsible for tool wear at the cutting conditions investigated. Analysis of the recorded data shows that tool performance during machining is dependent on coolant concentration. 6% coolant concentration gave the best overall performance as effective combination of cooling and lubrication functions were achieved during machining. Increasing coolant concentration to 9% reduced tool performance due to a reduction of the tool-chip contact length area and the consequent increase in compressive stresses at the tool-chip and tool-workpiece interfaces. This action often leads to pronounced chipping of the tool cutting edge during machining. Friction coefficient between the workpiece material and substrate increases once the coating layer(s) is broken as a result of the direct contact between the tool substrate and the work material. This action increases mechanical wear of the tool, which in turn leads to a significant increase in the cutting force with negligible effect on the feed forces during machining.     

The tribology of orthogonal finish machining — A review

A review of orthogonal finish machining is presented. The relations be- tween material flow conditions in the three distinct flow regions in metal cutting are examined: the deformation zone governs chip flow, the tool-chip contact zone is responsible for tool wear and the tool base rubbing zone controls workpiece integrity. In orthogonal machining the initial sharp tool cutting edge is of importance regarding the integrity of surface finish although tool edge forces have been the subject of more investigations. Material flow near the tool edge is considered with respect to the author's own model.     

An Investigation of the High Speed Machining Process Using a Variable Flow Stress Machining Theory

Abstract

This article investigates the application of the Oxley modeling approach to high speed machining (HSM) process for gaining a fundamental understanding and performance prediction of this process which is gaining increased popularity due to its many economic and technological advantages such as faster metal removal rates, efficient use of machine tools and, improved surface finish and lower cutting forces. Oxley's theory has so far mainly been applied for making machining predictions for plain carbon steels in the conventional speed range. In the present work, this theory has been applied for two plain carbon steels and a low alloy steel under HSM conditions. The predicted cutting forces, chip thicknesses, and secondary deformation zone thicknesses are then compared with the experimental results obtained under identical conditions. Good agreement has been shown between measured and predicted results. In addition, the possibility of applying the theory to predict the tool life and tool deformation conditions is also explored. An ability to predict these process parameters is of paramount importance since catastrophic tool failure under HSM conditions can be extremely costly and dangerous.