Vimal Dhokia

A review of hybrid manufacturing processes – state of the art and future perspectives

Today, hybrid manufacturing technology has drawn significant interests from both academia and industry due to the capability to make products in a more efficient and productive way. Although there is no specific consensus on the definition of the term ‘hybrid processes’, researchers have explored a number of approaches to combine different manufacturing processes with the similar objectives of improving surface integrity, increasing material removal rate, reducing tool wear, reducing production time and extending application areas. Thus, hybrid processes open up new opportunities and applications for manufacturing various components which are not able to be produced economically by processes on their own. This review paper starts with the classification of current manufacturing processes based on processes being defined as additive, subtractive, transformative, joining and dividing. Definitions of hybrid processes from other researchers in the literature are then introduced. The major part of this paper reviews existing hybrid processes reported over the past two decades. Finally, this paper attempts to propose possible definitions of hybrid processes along with the authors’ classification, followed by discussion of their developments, limitations and future research needs.

State-of-the-art cryogenic machining and processing

This article is a state-of-the-art review of the use of cryogenic cooling using liquefied gases in machining. The review is classified into two major categories, namely cryogenic processing and cryogenic machining. In cryogenic processing also known as cryo-processing, the cutting tool material is subjected to cryogenic temperatures as a part of its heat treatment process. The majority of the reported studies identify that cryo-processing can considerably increase cutting tool life especially for high speed steel tools. It also identified that, in cryogenic machining, a cryogen is used as a cooling substance during cutting operations. The cryogen can be used to freeze the workpiece material and/or cutting tool. This article concludes that cryogenic cooling has demonstrated significant improvements in machinability by changing the material properties of the cutting tool and/or workpiece material at the cutting zone, altering the coefficient of friction and reducing the cutting temperature.

Surface roughness prediction model for CNC machining of polypropylene

Cutting strategy research has traditionally been focused on hard materials that are intrinsically difficult to machine. An increase in the desire for personalized products has led to the requirement of the direct machining of polymers for personalized products. Little research is evident in the literature on the analysis of optimal machining parameters for machining materials such as polypropylene. One of the vital factors that affects the quality of polypropylene products and the respective machining strategy is surface roughness. This research is aimed at extracting information on the machining of polypropylene materials. A surface roughness predictive model based on neural networks has been developed. The design of experiments approach is used to obtain an adequate predictive model for the process planning which is further utilized as an input to the predictive model. The model mainly hinges on three independent variables namely spindle speed, feed rate, and depth of cut. Extensive experimental work on different network topologies and training algorithms has been performed to predict the behaviour of the surface roughness for machined polypropylene products. The results illustrate the benefits of being able to determine surface roughness values. This allows for the determination of optimal cutting strategies and tooling for the required surface roughness. The performance predictive model has been found to be satisfactory over the dataset for polypropylene machining. Hypothesis testing has also been carried out to identify the confidence of the predictive model.

A Methodology for the Estimation of Build Time for Operation Sequencing in Process Planning for a Hybrid Process

The on-going industrial trend toward production of highly complex and accurate part geometries with reduced costs has led to the emergence of hybrid manufacturing processes where varied manufacturing operations are carried out in either parallel or serial manner. One such hybrid process being currently developed is the iAtractive process, which combines additive, subtractive, and inspection processes. The enabler for realizing the hybrid process production is the process planning algorithm. The production time estimation for the additive process, namely build time, is one of the key drivers for the major elements in the algorithm. This paper describes a method for predicting build times for operation sequencing for process planning of the iAtractive process. An analytical model is first proposed, theoretically analyzing the factors that affect build times, which is used to help with the design of four test parts together with 64 sets of variations. The experimental results indicate that part volume, and interactions of volume and porosity, height and intermittent factor have significant effect on build times. Finally, the build time estimation model has been developed, which were subsequently evaluated and validated by applying a wide range of the identified influential factors.

Adiabatic shear band formation as a result of cryogenic CNC machining of elastomers

The traditional method for producing polymer-based products is the use of moulding technologies such as injection moulding. CNC machining methods are predominantly used for metal part manufacture. The use of CNC machining methods for direct machining of polymers has been discussed in previous studies, particularly for hard polymers such as polypropylene. The CNC machining of soft elastomers, such as ethylene vinyl acetate (EVA), even at significantly reduced temperatures presents a number of challenges, with one being the formation of a machining phenomenon termed adiabatic shear bands, which can lead to increased part surface roughness and reduced part quality. The adiabatic shear band is an area on a chip where the ductile properties of the material being machined have been exceeded and the heat generated does not have sufficient time to be removed. This can lead to permanent material damage resulting in reduced fatigue resistance. The adiabatic shear formation has the potential to be even more evident with the machining of elastomers, leading to rapid material degradation, poor surface finish characteristics, and reduced material attenuation. In this paper the state of the art in cryogenic manufacturing is described and the concept of cryogenic CNC machining of elastomers is discussed. The experimental work consists of machining EVA and Neoprene utilizing the described cryogenic CNC machining facility. Sample chips are taken from the experimental testing and analysed using a scanning electron microscope to illustrate the adiabatic shear band formation in some cases. It is observed that in order to reduce this effect, correct machining parameters corresponding to the elasticity values need to be used and the glass transition temperature needs to be maintained.

Using formal methods to model hybrid manufacturing processes

The on-going industrial trend towards high value sustainable manufacturing has led to the emergence of hybrid manufacturing processes and resources. This new generation of processes and resources combine the capabilities of a number of older technologies on a single platform. This increase in capability, however, comes at the cost of increasing complexity. Manufacturing processes, in general, can be categorised into subtractive processes, additive processes and transformative processes. While traditional machines supported a single type of these processes (e.g. a turn-mill machine that supports multiple metal removal - subtractive - processes), the new hybrid machines combine multiple types in one device. As a result, the current process and resource models have many shortcomings in representation of hybrid processes and hybrid machines. In this research formal methods are used to construct a new type of model for hybrid manufacturing processes. Formal methods are mathematically based techniques that allow clear and nonambiguous specification, development and, most importantly, verification of software and hardware systems. This paper utilises the ISO-standardised Z notation (named after Zermelo-Fraenkel set theory) to construct a formal model for hybrid manufacturing processes. The capabilities of the model in specification and verification of a hybrid device is then discussed through the use of a case study based on a prototype parallel kinematics hybrid manufacturing platform.

A novel decision-making logic for hybrid manufacture of prismatic components based on existing parts

The on-going industrial trend towards high value sustainable manufacturing has led to the emergence of hybrid manufacturing processes. This new generation of processes combines the capabilities of a number of individual manufacturing processes on a single platform. Despite the fact that the drawbacks of individual processes have been significantly reduced, the application of hybrid technology has always been constrained by the capabilities of their constituent processes, in particular the capability to utilise various raw materials in terms of shape and size. This paper introduces a novel concept of hybrid process, which consists of combining additive, subtractive and inspection processes. A feature-based decision-making logic is developed, enabling the hybrid process to reuse existing parts/legacy products. An existing part is first measured for obtaining its geometrical information. Feasible manufacturing strategies are provided and then additive, subtractive and inspection processes are utilised interchangeably to add and/or remove material, transforming the existing part into the final part. This indicates that the iAtractive process is not restricted by raw material geometries. Three identical test parts were manufactured from three existing different shaped parts, demonstrating the efficacy of the proposed hybrid process and the decision-making logic in material reuse. New features can be added onto the existing parts and existing features can be removed or further manufactured, giving these parts additional lives, new uses and increased functionality.

A novel process planning approach for hybrid manufacturing consisting of additive, subtractive and inspection processes

In recent years, hybrid manufacturing technologies that combine different processes (e.g. additive and subtractive processes) together have gained significant attention. This is due to their ability to capitalise on the advantages of each individual process, whilst minimising their disadvantages. However, there are limited process planning methods that are able to effectively utilise manufacturing resources for hybrid processes. In this paper, a hybrid process entitled iAtractive, combining additive, subtractive and inspection processes, along with part specific process planning is proposed, aiming to provide the designer with greater manufacturing capability and flexibility. This novel process planning approach enables a plastic part to be manufactured in a proper way in terms of process capabilities, production time and material consumption. This approach can be also adopted to remanufacture and reincarnate exiting parts or legacy products into other products. In this paper, the principle and framework of this process planning approach is presented and a test part was manufactured to validate this innovative approach.

A new algorithm for build time estimation for fused filament fabrication technologies

The manufacture of highly complex and accurate part geometries with reduced costs has led to the emergence of hybrid manufacturing technologies where varied manufacturing operations are carried out in either parallel or serial manner. One such hybrid process being currently developed is the iAtractive process, which combines additive (i.e. fused filament fabrication, which is sometimes called fused deposition modelling. However, the latter term is trademarked by Stratasys Inc. and cannot be used publicly without authorisation from Stratasys) and subtractive (i.e. computer numerical control machining) processes. In the iAtractive process production, operation sequencing of additive and subtractive operations is essential. This requires accurate estimation of production time, in which the fused filament fabrication build time is the determining factor. There have been some estimators developed for fused deposition modelling. However, these estimators are not applicable to hybrid manufacturing, particularly in process planning, which is a vital stage. This article addresses the characteristics of fused filament fabrication technologies and develops a novel and rigorous method for predicting build times. An analytical model was first created to theoretically analyse the factors that affect the part build time and was subsequently used to facilitate the design of test parts and experiments. The experimental results indicate that part volume, interaction of volume and porosity and interaction of height and intermittent factor have significant effects on build times. Finally, the estimation algorithm has been developed, which was subsequently evaluated and validated by applying a wide range of identified influential factors. The major advantage of the new proposed algorithm is its ability to estimate the build time based on simple geometrical parameters of a given part. The key factors that drive the algorithm can be directly obtained from part dimensions/drawings, providing an efficient and accurate way for fused filament fabrication time estimation. Test part evaluations and analysis have clearly demonstrated that estimation errors range from 0.1% to 13.5%, showing the validity, capability and significance of the developed algorithm and its applications to hybrid manufacture.

Comparative investigation on using cryogenic machining in CNC milling of Ti-6Al-4V titanium alloy

ABSTRACT Ti-6Al-4V titanium alloy is one of the most important materials in industry, 80% of which is used in aerospace industry. Titanium alloys are also notoriously difficult-to-machine materials owing to their unique material properties imposing a major bottleneck in manufacturing systems. Cryogenic cooling has been acknowledged as an alternative technique in machining to improve the machinability of different materials. Although milling is considered to be the major machining operation for the manufacture of titanium components in aerospace industries, studies in cryogenic machining of titanium alloys are predominantly concentrated on turning operations. To address this gap, this article provides an investigation on the viability of cryogenic cooling in CNC end-milling of aerospace-grade Ti-6Al-4V alloy using liquid nitrogen in comparison with traditional machining environments. A series of machining experiments were conducted and surface roughness, tool life, power consumption, and specific machining energy were investigated for cryogenic milling as opposed to conventional dry and flood cooling. Analysis revealed that cryogenic machining using liquid nitrogen has the potential to significantly improve the machinability of Ti-6Al-4V alloy in CNC end-milling using solid carbide cutting tools and result in a paradigm shift in machining of titanium products. The analysis demonstrated that cryogenic cooling has resulted in almost three times increased tool life and the surface roughness was reduced by 40% in comparison with flood cooling.