D.A. Axinte

Surface integrity of hot work tool steel after high speed milling-experimental data and empirical models

Abstract High speed milling (HSM) using small diameter solid or indexable insert tungsten carbide end mills, with coatings such as TiAlN is now an established technology for manufacturing moulds and dies in a range of hardened tool steels. The paper presents experimental results and corresponding empirical models for workpiece surface integrity (SI) of hardened AISI H13 hot work tool steel, when HSM using solid carbide ball nose end mills coated with TiAlN. The influence of cutting speed, feed rate and workpiece angle on SI (workpiece surface roughness, microstructure, microhardness and residual stress) was studied using a full factorial experimental design with two levels of each factor. Where appropriate, empirical models were proposed in order to predict SI based on cutting parameter inputs. No significant changes in microstructure and microhardness below the machined surface were found. Within the range of cutting parameters tested, the operating parameters that gave the lowest workpiece surface roughness did not result in the most compressive residual stress distribution below the machined surface. A compromise in parameter selection is therefore necessary to achieve desired roughness and integrity.

Tool condition monitoring in broaching

Abstract The paper reports on research which attempts to correlate the condition of broaching tools to the output signals obtained from multiple sensors, namely, acoustic emission (AE), vibration, cutting forces and hydraulic pressure, connected to a hydraulic broaching machine. Broaching tools in a variety of conditions, namely, freshly ground teeth, one or more chipped teeth, one or more weakened teeth, one or more broken teeth and tools showing uniform wear, were investigated. Single tooth and multiple teeth broaching tools were employed during the broaching trials and the sensory signals were analysed both in the time and frequency domains. The results show that AE, vibration and cutting force signals are all sensitive to tool condition and a correlation can be made between the broaching tool condition and sensory signals using a variety of signal analysis techniques. A brief review of the advantages and the disadvantages of each sensor/signal and its associated analysis technique is presented. It is concluded that the most sensitive sensors to changes in tool condition are not necessarily those that are easy to mount or incorporate in a broaching machine. This can impose restrictions on the types of sensors that can be retrofitted to current machines. The condition monitoring system is currently being extended to incorporate sensor fusion techniques to increase the flexibility of the monitoring system and improve confidence in decision-making.

Reliable tool life measurements in turning : an application to cutting fluid efficiency evaluation

Abstract The paper proposes a method to obtain reliable measurements of tool life in turning, discussing some aspects related to experimental procedure and measurement accuracy. The method (i) allows an experimental determination of the extended Taylors equation, with a limited set of experiments and (ii) provides a basis for the quantification of tool life measurement uncertainty. The procedure was applied to cutting fluid efficiency evaluation. Six cutting oils, five of which formulated from vegetable basestock, were evaluated in turning. Experiments were run in a range of cutting parameters, according to a 23–1 factorial design, machining AISI 316L stainless steel with coated carbide tools. Tool life measurements were associated to an estimation of their uncertainty, and it was found that by taking three repetitions the uncertainty calculated with a coverage factor of two was on average three times bigger than the experimental standard deviation.

Turning of advanced Ni based alloys obtained via powder metallurgy route

Nickel based alloys (RR_X) manufactured via powder route are considered the next generation materials that can offer increased efficiency of gas turbine engines. Their chemistry and mechanical properties indicate even lower machinability than the current disc alloys. The paper reports on specifics when rough and finish turning these new Ni based alloys. Tool life, surface finish, workpiece surface integrity and residual stress distributions have been used as multi-objective quality criteria to assess the capability of shaping RR_X alloys. The optimised cutting parameters/route was then employed to produce surfaces in low cycle fatigue samples to demonstrate machining capability to Airworthiness Authorities.

Assessment of the effectiveness of a spindle power signal for tool condition monitoring in machining processes

Less expensive and ‘readily available’ process monitoring techniques are needed to be effective in industrial machining processes. Spindle motors on modern computer numerical control machine tools allow easy access to the monitoring of spindle power. Whilst a spindle power signal fulfils the requirements for simple process monitoring, such a signal can trigger ‘machine alarms’ when process malfunctions occur. Little analysis has been done to assess the sensitivity of a spindle power signal relative to interrupted/continuous cutting processes. This paper aims to assess the effectiveness of a spindle power signal for tool condition monitoring in three machining processes: milling, drilling and turning. Based on cutting force/torque, the cutting power was calculated and a comparison between the theoretical cutting power and the spindle power signal was performed. Tool condition monitoring using spindle power could be successful in continuous machining processes (turning and drilling), while for discontinuous machining operations (milling), the spindle power signal showed reduced sensitivity to detect small uneven events such as chipping of one tooth. The results were used to define the sensitivity limitations when using a spindle power signal for tool condition monitoring on different computer numerical control machining centres where continuous and discontinuous machining operations are performed.

Manufacturability analysis system : issues and future trends

Traditionally, design and manufacturing activities have taken place sequentially rather than simultaneously leading to inefficient and time consuming iterations between design and manufacturing stages. In order to expedite these iterations, manufacturability analysis systems (MASs) have been developed to allow the evaluation of various manufacturability aspects during the design stage and consequently to reduce the costs and time to market of the designed products. The definition, concepts, advantages of the MAS and its current state of development and applicability in the industrial environment are discussed in the introduction of the paper. Then, key issues and technologies related to the construction of a MAS such as methodologies applied in data input mechanisms, techniques for manufacturability reasoning and formats of the outputs generated by MASs are analysed. Furthermore, applicability of a MAS to various manufacturing processes and aspects such as manufacturability and fixturability indices that can be analysed and produced through a MAS are also commented. Disadvantages and limitations emerged from utilisation of MASs are critically analysed and presented. Finally, the paper concludes with discussion and suggestions for some prospective research trends and challenges in building and exploiting MASs. Particular attention is paid to the application of a MAS to micro-manufacturing processes on which, nowadays, both academics and industry are focusing their attentions for identifying future research and technological challenges and opportunities.

Some considerations on tool wear and workpiece surface quality of holes finished by reaming or milling in a nickel base superalloy

Abstract Higher-strength heat-resistant superalloys are continually being developed to meet the engineering requirements for increased efficiency of gas turbine engines. In addition, work on machinability assessment is needed to fill the gap between the material development and manufacturing capabilities before considering the implementation of such materials in production lines. The present paper presents selected results on optimization of cutting conditions for hole-making processes of a new aerospace nickel (RR1000)-based superalloy manufactured via a powder route. The mechanical properties and the chemistry of RR1000 alloy indicate lower machinability when compared with the current disc alloys (e.g. Inconel, Waspaloy, Udimet). The hole-making processes, involving a succession of roughing (drilling) and finishing (normal/special reaming or plunge milling) operations, were evaluated through a multicriteria procedure consisting of the following output measures: tool life, hole accuracy, surface roughness, workpiece surface integrity, and level of cutting torque. It was found that simultaneous fulfilment of multicriteria quality measures is more dependent on the selection of tooling/cutting edge preparation involved in the finishing step rather than the relatively narrow intervals of operating parameters. While drilling and normal reaming lead to surface overheating (white layers) and material dragging, the change of edge preparation (e.g. double-relief angle) on the special reamers or the use of alternative cutting strategies (e.g. plunge milling) can generate finished holes within the required surface integrity standards. An original relationship between a derived measure from output sensory signals (i.e. specific cutting energy per tooth per revolution) and workpiece surface integrity (i.e. depth of strained material) has been investigated to assist further the selection of the ‘best’ finish machining operation. Additionally, this approach opens the avenue towards online surface integrity inspection via ‘key sensory measures’.

Evaluation of cutting force uncertainty components in turning

A procedure is proposed for the evaluation of those uncertainty components of a single cutting force measurement in turning that are related to the contributions of the dynamometer calibration and the cutting process itself. Based on an empirical model including errors from both sources, the uncertainty for a single measurement of cutting force is presented, and expressions for the expected uncertainty vs. cutting parameters are proposed. This approach gives the possibility of evaluating cutting force uncertainty components in turning, for a defined range of cutting parameters, based on few experiments.

A multicriteria model for cutting fluid evaluation

Abstract This paper describes the results of a comprehensive evaluation programme for cutting fluid efficiency, when machining the aerospace ‘superalloy’, Inconel 718. The machining methods used were milling, drilling, tapping and VIPER grinding. Results from three cutting fluids were evaluated: a water-based semi-synthetic, a water-based synthetic and a high-oil emulsion fluid. Cutting forces, torque and spindle power were acquired during machining, while geometrical accuracy, surface texture and surface integrity of the workpiece were analysed afterwards. The experimental results demonstrate the difficulty of identifying the ‘best’ cutting fluid, especially when several different machining methods are to be employed on the same machine tool. It is unlikely that a single fluid will show the best performance on all machining trials and output measures. Therefore, prioritization of the output measures and specification of the relative importance of each machining operation becomes essential, in order to evaluate and rank the cutting fluid efficiencies. A flexible multicriteria model for the evaluation of cutting fluid efficiency, relative to the performance of a benchmark fluid, is proposed and described. The advantage of the model consists in its flexibility and capability to compare the efficiency of cutting fluids across different machining methods and output criteria. An application of the evaluation model is provided, using results from the tested cutting fluids.

Broaching of Ti-6-4 – Detection of Workpiece Surface Anomalies on Dovetail Slots through Process Monitoring

Surface anomalies induced by abusive broaching represent a concern for the safety of critical components of aero-engines. Using acoustic emission and force signals, the paper reports on process monitoring techniques to detect surface anomalies when abusively broaching dovetail slots in Ti-6-4. Parallel surface inspection helped in defining the correlation between the occurrence of uneven events in the output signals and the appearance/location of anomalies on the broached surfaces. The findings support the definition of process monitoring strategies to detect workpiece surface anomalies including surface deformation/ overheating, directed scoring and smearing of parent material.