Tool path generation for ultra-precision machining of free-form surfaces

The generation of tool paths for ultra-precision machining is still a limiting factor in the manufacturing of parts with complex optical surfaces. In conventional machining as well as in complex five axes machining the application of CAD- and CAM-software for the generation of tool paths is state of the art. But these software solutions are not able to generate tool paths according to the high requirements of ultra-precision machining. This paper describes possible ways to generate tool paths for ultra-precision machining when the optical surface can be analytically described or when the surface data is derived from optical design software. Ultra-precision milling experiments with different tool paths have been carried out and the quality of the machined geometry has been evaluated concerning the achievable form accuracy.     

New cooling approach and tool life improvement in cryogenic machining of titanium alloy Ti-6Al-4V

Titanium alloy Ti-6Al-4V, a difficult-to-machine material because of its extremely short tool life, has been a major subject for cryogenic machining research. However, the approaches reported in past publications are inherently flawed. This study reviews how the temperature affects Ti-6Al-4V properties, and compares different cryogenic cooling strategies. Based on these findings, a new economical cryogenic cooling approach is proposed. Using a minimum amount of liquid nitrogen (LN2), this innovation features a specially designed micro-nozzle. Formed between the chip breaker and the tool rake face, the nozzle lifts the chip and injects focused LN2 into the chip–tool interface at the point of highest temperature. As the nitrogen evaporates, a nitrogen cushion formed by evaporating nitrogen lowers the coefficient of friction between the chip and the tool. An auxiliary mini-nozzle that sprays LN2 onto the flank at the cutting edge further reduces the cutting temperature. The study finds that the combination of these two micro-nozzles provides the most effective cooling while using the lowest LN2 flow rate. Improving the position of the nozzle/chip breaker further enhances the performance. Our cryogenic machining tests show that tool life increases up to five times the state-of the-art emulsion cooling, outperforming other machining approaches.     

The relation between chip morphology and tool wear in ultra-precision raster milling

In the field of ultra-precision machining, the study of the relation between chip morphology and tool wear is significant, since tool wear characteristics can be reflected by morphologies of cutting chips. In this research, the relation between chip morphology and tool flank wear is first investigated in UPRM. A cutting experiment was performed to explore chip morphologies under different widths of flank wear land. A geometric model was developed to identify the width of flank wear land based on chip morphology. Theoretical and experimental results reveal that the occurrence of tool flank wear can make the cutting chips truncated at both their cut-in and cut-out sides, and reduce the length of cutting chips in the feed direction. The width of truncation positions of the cutting chip can be measured and used to calculate the width of flank wear land with the help of the mathematical model. The present research is potentially used to detect tool wear and evaluate machined surface quality in intermittent cutting process.     

High-speed machining of titanium alloys using the driven rotary tool

Machining of titanium at high cutting speeds such as from 4 m/s to 8 m/s is very challenging. In this paper, a new generation of driven rotary lathe tool was developed for high-speed machining of a titanium alloy, Ti–6Al–4V. The rotary tool was designed and fabricated based on the requirements of compact structure, sufficient stiffness and minimal edge runout. Cylindrical turning experiments were conducted using the driven rotary tool (DRT) and a stationary cutting tool with the same insert, for comparison in the high-speed machining of Ti–6Al–4V. The results showed that the DRT can significantly increase tool life. Increase in tool life of more than 60 times was achieved under certain conditions. The effects of the rotational speed of the insert were also investigated experimentally. Cutting forces were found to decline slightly with increase of the rotational speed. Tool wear appears to increase with the rotational speed in a certain speed range.     

Modelling tool wear in cemented-carbide machining alloy 718

Tool wear is a problem in turning of nickel-based superalloys, and it is thus of great importance to understand and quantitatively predict tool wear and tool life. In this paper, an empirical tool wear model has been implemented in a commercial finite element (FE) code to predict tool wear. The tool geometry is incrementally updated in the FE chip formation simulation in order to capture the continuous evolution of wear profile as pressure, temperature and relative velocities adapt to the change in geometry. Different friction and wear models have been analysed, as well as their impact on the predicted wear profile assessed. Analyses have shown that a more advanced friction model than Coulomb friction is necessary in order to get accurate wear predictions, by drastically improving the accuracy in predicting velocity, thus having a dramatic impact on the simulated wear profile. Excellent experimental agreement was achieved in wear simulation of cemented carbide tool machining alloy 718.     

Tool life determination based on the measurement of wear and tool force ratio variation

Non-linear regression analysis techniques are used to establish models for wear and tool life determination in terms of the variation of a ratio of force components acting at the tool tip. The ratio of the thrust component of force to the power, or vertical, force component has been used to develop models for (i) its initial value as a function of feed, (ii) wear, and (iii) tool lifetimes. Predictions of the latter model have been compared with the results of experiments, and with predictions of an extended Taylor model. In all cases, good predictive capability of the model has been demonstrated. It is argued that the models are suitable for use in adaptive control strategies for centre lathe turning.     

Cutting performance of diamond tools during ultra-precision turning of electroless-nickel plated die materials

This study is an attempt (a) to observe the wear characteristic of diamond tool with 200 km cutting distance and to study the effects of wear on the surface roughness and cutting forces and (b) to optimize various cutting parameters such as depth of cut, feed rate, spindle speed and phosphorus content. The experimental results showed that tool wear was not so significant although some defects on rake face were observed after cutting 15.6 km. Further cutting showed that the surface roughness increases with cutting distance, and that the cutting forces were larger than thrust force at the beginning of cutting, but after cutting 130 km, thrust force became larger and increased rapidly. It was also observed that forces increase with the increase of depth of cut, spindle speed and feed rate, and decrease with the increase of phosphorus content of the plating. Depth of cut has no significant effect on surface roughness, while it increases with increase of feed rate and decreases with the increase of percentage of phosphorus content in the workpieces. In case of spindle speed, surface roughness decreases with the increase of spindle speed up to a certain value and then starts to increase with the increase of spindle speed.     

A mesoplasticitiy analysis of cutting friction in ultra-precision machining

The control and minimization of cutting force variation is of prime importance in obtaining a consistent surface finish and form accuracy of a machined workpiece in ultra-precision machining. However, most continuum theories do not take into account the effect of crystallographic anisotropy that causes variation in the shear plane at the grain level and hence of the cutting force. The periodicity of the fluctuation of cutting forces is found to be dependent on the frictional condition during cutting. However, investigation of the in situ relationships among the cutting friction, the crystallographic orientation of workpiece and the periodic fluctuation of cutting forces has received relatively little attention. In this paper, a mesoplasticity approach is proposed to access the crystallographic and frictional effect on the fluctuation of micro-cutting forces in diamond turning of crystalline materials. The predictions were able to explain the experimental results based on the power spectrum analysis of the cutting force variation. The research findings throw light on the possibility of an indirect in situ assessment of the frictional condition in ultra-precision machining.     

Tool life and tool wear in the semi-finish milling of inclined surfaces

Functional die and mold components have complex geometries and are made of high hardness materials, which make them difficult to machine. This work contributes to a better understanding of this type of process and of the wear mechanisms of tools used in semi-finishing operations of hardened steels for dies and molds. Several milling experiments were carried out to cut AISI H13 steel with 50 HR_C of hardness using the high-speed milling technique. The main goal was to verify the influence of workpiece surface inclination and cutting conditions on tool life and tool wear mechanisms. The main conclusions were the inclination of the machined surface strongly influences tool life and tool wear involves different mechanisms. At the beginning of tool life, the wear was caused mainly by abrasion on the flank face plus diffusion and attrition on the rake face. At the end of tool life, the mechanisms were adhesions and microchipping at the cutting edge.     

Determination of tool friction in presence of flank wear and stress distribution based validation using finite element simulations in machining of titanium and nickel based alloys

Tool friction plays a very important role in machining titanium and nickel-based alloys and is an important parameter in Finite Element based machining simulations. It is the source for the high amount of heat generation, and as a result, the excessive flank wear during machining these materials. The worn tool is known to create poor surface qualities with high tensile surface residual stresses, machining induced surface hardening, and undesirable surface roughness. It is essential to develop a methodology to determine how and to what extent the friction is built up on the tool. This study facilitates a determination methodology to estimate the stress distributions on the rake and flank surfaces of the tool and resultant friction coefficients between the tool and the chip on tool rake face, and the tool and the workpiece on tool flank face. The methodology is applied to various tool edge radii and also utilized in solving stagnation point location on the tool edge. Predicted friction results are further validated with comparison of predicted stress distributions from FE simulations for machining of titanium alloy Ti-6Al-4V and the nickel-based alloy IN-100. It was found that tool stresses and friction are mainly influenced by tool rake angle, edge radius, and tool flank wear and are slightly affected by the cutting conditions in the ranges that were considered in this study.     

Tool wear in friction drilling

This study investigates the tool wear in friction drilling, a nontraditional hole-making process. In friction drilling, a rotating conical tool uses the heat generated by friction to soften and penetrate a thin workpiece and create a bushing without generating chips. The wear of a conical tungsten carbide tool used for friction drilling a low carbon steel workpiece is studied. Tool wear characteristics are quantified by measuring its weight change, detecting changes in its shape with a coordinate measuring machine, and making observations of wear damage using scanning electron microscopy. Energy dispersive spectrometry is applied to analyze the change in chemical composition of the tool surface due to drilling. In addition, the thrust force and torque during drilling and the hole size are measured periodically to monitor the effects of tool wear. Results indicate that the carbide tool is durable, showing minimal tool wear after drilling 11,000 holes, but observations also indicate the progressively severe abrasive grooving on the tool tip.     

Experimental observation of tool wear in rotary ultrasonic machining of advanced ceramics

As one of the cost-effective machining methods for advanced ceramics, rotary ultrasonic machining (RUM) has attracted much attention and there exist numerous publications on the process. However, few investigations on tool wear in the RUM process have been reported. This paper, for the first time in literature, presents an experimental observation on tool wear in RUM of silicon carbide (SiC). It first reviews some related wear mechanisms for grinding wheels and some techniques for studying the wheel wear mechanisms. After describing the experimental procedures, it presents and discusses the results on tool wear and cutting forces in RUM of SiC. It also discusses some practical implications of the findings from this study.     

A review of machine-tool vibration and its influence upon surface generation in ultra-precision machining

Vibration in ultra-precision machining (UPM) is an intrinsic physical phenomenon, which is a key factor influencing surface generation. With a focus on passive vibration, this paper reviews the latest research into vibration characteristics and the effect of vibration on surface generation in UPM. The opportunities and challenges facing researchers are also discussed and suggestions are made for future related studies. It is found that active vibration can possibly be employed to improve surface quality influenced by passive vibration in UPM.     

A review of surface roughness generation in ultra-precision machining

Ultra-precision machining (UPM) is capable of manufacturing a high quality surface at a nanometric surface roughness. For such high quality surface in a UPM process, due to the machining complexity any variable would be possible to deteriorate surface quality, consequently receiving much attention and interest. The general factors are summarized as machine tool, cutting conditions, tool geometry, environmental conditions, material property, chip formation, tool wear, vibration etc. This paper aims to review the current state of the art in studying the surface roughness formation and the factors influencing surface roughness in UPM. Firstly, the surface roughness characteristics in UPM is introduced. Then in UPM, a wide variety of factors for surface roughness are then reviewed in detail and the mechanism of surface roughness formation is concluded thoroughly. Finally, the challenges and opportunities faced by industry and academia are discussed and several principle conclusions are drawn.     

Tool-life and wear mechanisms of CBN tools in machining of Inconel 718

The demand for increasing productivity when machining heat resistant alloys has resulted in the use of new tool materials such as cubic boron nitride (CBN) or ceramics. However, CBN tools are mostly used by the automotive industry in hard turning, and the wear of those tools is not sufficiently known in aerospace materials. In addition, the grade of these tools is not optimized for superalloys due to these being a small part of the market, although expanding (at 20% a year). So this investigation has been conducted to show which grade is optimal and what the wear mechanisms are during finishing operations of Inconel 718. It is shown that a low CBN content with a ceramic binder and small grains gives the best results. The wear mechanisms on the rake and flank faces were investigated. Through SEM observations and chemical analysis of the tested inserts, it is shown that the dominant wear mechanisms are adhesion and diffusion due to chemical affinity between elements from workpiece and insert.     

A novel approach to quantifying tool wear and tool life measurements for optimal tool management

It is a common practice in batch production to continually use the same tool to machine different parts, using disparate machining parameters. In such an environment, the optimal points at which tools have to be changed, while achieving minimum production cost and maximum production rate within the surface roughness specifications, have not been adequately studied. The tool wear index (TWI) and the tool life model developed in this study use a novel approach, analyzing wear surface areas and material loss from the tool using micro-optics and image processing/analysis algorithms. With relation to surface roughness, the TWI measures the wear conditions more accurately and comprehensively, and the tool life model enables maximum use of a worn tool and minimum risk for in-process tool failure. The TWI and a surface roughness control model are integrated into an optimal control strategy that shows potential for productivity improvement and reduction of manufacturing cost.     

Performance evaluation of pure CBN tools for machining of steel

Ultra-precision machining is one of the most important machining technologies for the manufacture of precision dies and molds. Typically, single point diamond cutting tools are used to machine molds which are coated with electroless nickel (NiP) for such applications. The high cost of diamond cutters and electroless nickel plating, coupled with problems of pre-mature failure of the coating in service and long lead time are negative factors in this approach. Hence, there is a strong need for the direct ultra-precision machining of mold steel and to develop relevant technologies to address the problem of tool wear. In the machining of alloy steel, cubic boron nitride (CBN) has long been used as an ideal cutting tool material but recently binderless CBN or pure CBN (PCBN) with superior mechanical properties has been developed by Sumitomo Electric Industries in Japan. The objective of this paper is to explore the feasibility of using PCBN tools for direct ultra-precision machining of Stavax, a type of alloy steel from ASSAB. The performance characteristics in terms of surface roughness and tool wear of PCBN (Sumitomo IZ900) and conventional CBN (Sumitomo BN600) under different machining conditions were studied and their results were compared. Based on the experimental results, PCBN has been found to perform better in terms of wear resistance compared to conventional CBN tool. It is also able to achieve near mirror finish of less than 30 nm R_a, and hence it appears to be a promising tool for direct cutting of die and mold materials.     

Theoretical and experimental determination of tool life in hot machining of austenitic manganese steel

In this study, an expression for the tool life of a sintered carbide tool machining heated austenitic manganese steel was developed. The influence of surface temperature, cutting speed, feed rate, and depth of cut on the tool life were investigated. Afterwards, an expression for the effects of cutting conditions on tool life were determined using a mathematical model developed by a factorial regression method.     

New observations on tool wear mechanism in dry machining Inconel718

Tool wear is a problem in machining nickel-based alloy Inconel718, and it is thus of great importance to understand tool wear. Tool wear mechanism in dry machining Inconel718 with coated cemented carbide tools was analyzed in this paper. CCD and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS) were used to study tool wear mechanism. The results show that the main reason which causes cutting tool wear was that the tool materials fall off from the tool substrate in the form of wear debris. In addition,, element diffusion between tool and workpiece and oxidation reaction all accelerate the formation and the peeling of the wear debris. According to analysis of tool wear mechanism, tool flank wear model was established. The optimal temperature in machining Inconel718 with PVD-coated (TiAlN) tool was obtained through the established model. Excellent experimental agreement was achieved in optimal temperature calculated by the established model.     

On temperatures and tool wear in machining hypereutectic Al–Si alloys with vortex-tube cooling

This study investigates dry machining of hypereutectic silicon–aluminum alloys assisted with vortex-tube (VT) cooling. The objective is to reduce cutting temperatures and tool wear by enhanced heat dissipation through the chilled air generated by a VT. A machining experiment, cutting mechanics analysis, and temperature simulations are employed to (1) model the heat transfer of a cutting tool system with VT cooling applied, (2) explore effects of cooling setting and machining parameters on the cooling efficiency, and (3) evaluate VT cooling effects on tool wear. A390 alloy is machined by tungsten carbides with cutting forces and geometry measured for heat source characterizations as the input of temperature modeling and simulations. VT cooling is approximated as an impinging air jet to estimate the heat convection coefficient that is incorporated into the heat transfer models. The major findings include: (1) VT cooling may reduce tool wear in A390 machining depending upon machining conditions, and the outlet temperature is more critical than the flow rate, (2) cooling effects on temperature reductions, up to 20 °C, decrease with the increase of the cutting speed and feed, and (3) tool temperature decreasing by VT cooling shows no direct correlations with tool wear reductions.