Sustainable Manufacturing E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 272

Veröffentlichungsjahr: 2013

Beschreibung

According to the NACFAM (National Council for Advanced Manufacturing, USA) Sustainable Manufacturing is defined "as the creation of manufactured products that use processes that are non-polluting, conserve energy and natural resources, and are economically sound and safe for employees, communities, and consumers." The book covers Sustainable Manufacturing techniques such as materials and manufacturing for renewable energies; clean manufacturing technology; ecological manufacturing; energy-efficient manufacturing; remanufacturing; recycling of materials; environmentally conscious design and manufacturing processes; sustainable advanced manufacturing systems; manufacturability in sustainable product design; education and training for sustainable manufacturing.

Leseprobe

Table of Contents

Preface

Chapter 1. Environmental Impact in Micro-device Manufacturing

1.1. Introduction

1.2. Role of LCA

1.3. Energy consideration in micro-manufacturing

1.4. Energy consideration in micro-end-milling manufacturing

1.5. Conclusions

1.6. References

Chapter 2. Cutting Tool Sustainability

2.1. Introduction

2.2. Statistical reliability of cutting tools as quantification of their sustainability

2.3. Construction of the probability density function of the tool flank wear distribution with tool test results

2.4. Tool quality and the variance of tool life

2.5. The Bernstein distribution

2.6. Concept of physical resources of the cutting tool

2.7. References

Chapter 3. Minimum Quantity Lubrication in Machining

3.1. Introduction

3.2. The state-of-the-art research for MQL in machining

3.3. Case studies on MQL in machining

3.4. Summary

3.5. Acknowledgments

3.6. References

Chapter 4. Application of Minimum Quantity Lubrication in Grinding

4.1. Introduction

4.2. Minimum quantity lubrication

4.3. Results

4.4. Conclusions

4.5. Acknowledgments

4.6. References

Chapter 5. Single-Point Incremental Forming

5.1. Introduction

5.2. Incremental sheet forming processes

5.3. Analytical framework

5.4. FE background

5.5. Experimental

5.6. Results and discussion

5.7. Examples of applications

5.8. Conclusions

5.9. References

Chapter 6. Molding of Spent Rubber from Tire Recycling

6.1. Introduction

6.2. State of the art of tire recycling

6.3. Direct molding of rubber particles

6.4. Experimental results

6.5. Concluding remarks

6.6. References

First published 2010 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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Library of Congress Cataloging-in-Publication Data

Sustainable manufacturing / edited by J. Paulo Davim.       p. cm.   Includes bibliographical references and index.   ISBN 978-1-84821-212-1 1. Production management--Environmental aspects. 2. Manufacturing processes--Environmental aspects. I. Davim, J. Paulo.   TS155.7.S856 2010   658.5--dc22

2010003697

British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library ISBN 978-1-84821-212-1

Preface

According to the NACFAM (National Council for Advanced Manufacturing – USA), sustainable manufacturing is defined “as the creation of manufactured products that use processes that are non-polluting, conserve energy and natural resources, and are economically sound and safe for employees, communities, and consumers”. In other words, sustainable manufacturing is developing technologies to transform materials and products with reduced emission of greenhouse gases, reduced use of non-renewable or toxic materials, and reduced generation of waste. Sustainable manufacturing includes the manufacturing of “sustainable” products (e.g. manufacture of renewable energy) and the sustainable manufacturing of all products.

The purpose of this book is to present a collection of examples illustrating the state of the art and research developments in sustainable manufacturing. Chapter 1 of this book provides the environmental impact on micro-device manufacturing. Chapter 2 contains sustainability aspects of cutting tools. Chapter 3 covers minimum quantity lubrication (MQL) in machining. Chapter 4 contains information on the application of MQL in grinding. Then, Chapter 5 focuses on single-point incremental forming. Finally, Chapter 6 is dedicated to molding of spent rubber from tire recycling.

This book can be used as a textbook for final-year undergraduate engineering students or as a subject on sustainable manufacturing at the postgraduate level. Also, this book can serve as a useful reference for academics, manufacturing and materials researchers, manufacturing, mechanical and environmental engineers, professionals in manufacturing, and related industries. The scientific interest in this book is evident for many important centers of research, laboratories, and universities throughout the world. Therefore, it is hoped that this book will encourage and enthuse others research in this recent field of science and technology.

The Editor would like to acknowledge his gratitude to ISTE-Wiley for this opportunity and for their professional support. Finally, I would like to thank all the authors for their availability for this work.

Chapter 1

Environmental Impact in Micro-device Manufacturing1

This chapter deals with the environmental impact of manufacturing used in the production of micro-devices. It focuses on methods to assess the environmental impact and is largely directed toward the Australian manufacturing industry but can be applied to the micro-device manufacturing industry in general.

The relevance of life cycle analysis (LCA) and thermodynamic considerations are presented which form the current trends for analyzing the sustainability of manufacturing.

This chapter concludes with a simple study of varying the spindle and slide speeds in the micro-end-milling production of T-junctions as an example that offers energy optimization in the micro-device manufacturing industry.

1.1. Introduction

Micro- and nano-technology have been advocated as emerging areas where substantial reduction in energy consumption and greenhouse emissions can be achieved and have been endowed with the prospect of being a candidate for sustainable technological development. In 2002, the U.S. manufacturing industry required a total input of 24,100 PJ [EPA 03] equivalent to 3.94 × 109 barrels of oil, which is more than the total U.S. domestic production of oil (3.11 × 109 barrels) [CIA 08]. Electricity accounts for 40% of the total energy consumed by the U.S. manufacturing industries.

Figure 1.1.Energy usage (PJ) in the last three decades by the manufacturing sector in Australia [ABA 08]

In Australia, the manufacturing sector used 1,300.5 PJ in the year 2007–2008. Over the last two decades, its average annual growth rate of 1.4% [ABA 08] has been steady after a decline in 1982–1983, as shown in Figure 1.1. Although manufacturing in Australia consumes 22.5% of the total energy used, it only contributes 10% of the economic output. The greenhouse equivalent CO2 (e-CO2)1 gas emission, based on the Kyoto accounting method, for the manufacturing and construction industry, is shown in Figure 1.2. The emission represents on average about 9% of the total greenhouse gas emissions for Australia. Although the greenhouse gas emission from the manufacturing and construction industry has increased in the last decade, the overall percentage has been steady due to large increases in emissions from the mining industry. Over the last two decades, the proportion of electrical energy used in the manufacturing industry has increased significantly. Overall, the manufacturing industry is considered an energy-intensive sector with a significant CO2 footprint.

Figure 1.2.Energy usage in the last three decades by the manufacturing sector in Australia [COM 09a, COM 09b]

The Australian manufacturing industry consists of more than 75,000 business organizations employing 1 million workers, and changes in the energy competitiveness of the industry can be quite significant for job stability in the manufacturing industry. Manufacturing Skills Australia has recognized the need for the manufacturing industry to respond to climate change, as pointed out by the chief executive of the Australian Industry Group, Heather Ridout [MSA 08]:

Responding to climate change will require a fundamental shift in Australia’s approach to management and workforce skills. Reducing greenhouse gas emissions means new processes for industrial and agricultural production, new research and investment in low-emission technologies, new patterns of consumption, and innovative thinking in almost every aspect of business life.

We know little about the carbon footprint and the impact of micro- and nano-technologies on the environment. The U.S. Department of Energy recently released the following claim for nano-manufacturing [USD 09], “Advances in cost-effective nano-manufacturing can deliver diverse energy benefits”. A recent analysis of the environmental cost of a single DRAM chip [WIL 02] showed that it had a high-energy intensity and processing material usage, suggesting that the converse is true. This energy-consuming outcome was attributed to the highly organized structure of the DRAM. Similarly, micro-devices manufactured with micro-or nano-technologies have highly organized structure and as one would expect, a high-energy cost per unit. Another important factor is that micro-devices are mostly manufactured with large-scale machines that are inefficient for micro-device manufacturing, a legacy of the techniques brought over from the manufacturing industry. There has been only a few LCAs of micro-and nano-manufacturing processes, and most of the energy efficiency claims have been extrapolated by the belief that anything that is small will use less energy. Thus, micro- and nano-technologies have been advocated as emerging areas where substantial reduction in energy consumption and greenhouse emissions can be achieved and have been endowed with the prospect of being a candidate for sustainable technological development. This premise is currently being challenged by several studies. A more important question is whether it is possible to optimize the manufacture of micro-devices to select energy-efficient routes with minimum environmental impact? This question is in urgent need for an answer with the rise of large-scale micro-device manufacturing.

1.1.1. Sustainability in micro-manufacturing

There are several manufacturing methods that are used in the micro-device manufacturing industry. Apart from the standard techniques used in the semiconductor industry, these also include soft lithography on photoresistant polydimethylsulfoxide copies from master templates, focused ion beam, laser micro-machining, water jet milling, microelectrodischarge machining, LIGA, diamond milling and micro-machining, chemical etching and micro-machining, silicon machining, mechanical micro-machining, and other less common manufacturing methods. An analysis of the energy consumption of a micro-end-milling machine versus a conventional CNC milling machine showed that there are significant energy-reduction advantages with a micromachine designed specifically for micro-device manufacture, as most of the energy is consumed by moving the slides and associated services rather than in the actual milling process itself [LIO 09].

In micro-device manufacture, the development of sustainable manufacturing requires the disengagement of the link between increased micro-device manufacture and the rise in greenhouse gas emissions. It will require the replacement of old technology and manufacturing processes with new technology that is less energy intensive per unit of manufactured product. This, in turn, will require re-skilling of the workforce to adapt to the change in technology as well as a paradigm shift to a “greener” thinking by both the management and workers.

Sustainable manufacturing in the micro-device industry requires more than just a product-centered approach, whereby only the steps in the making of a product are analyzed in detailed. A more comprehensive approach is to include the complete product development involving not only all the processes that contribute to the development of a product but also the environmental and life cycle cost of the manufacturing equipment. Such an approach is useful when comparing the relative environmental footprint of different micro-device manufacturing methods, all of which have been marketed as environmentally friendly by the manufacturers. It is important to distinguish between the current environmental and energy-efficiency claims regarding micro-devices and the actual environmental and energy costs associated with the manufacture, usage and life of a single micro-device. The disconnection between the product use and the manufacturing and disposal cost is highlighted with the personal computer (PC). When the micro-processor revolution began in the 1970s, it was seen as the revolution that would reduce paper usage, increase work efficiency for the energy used, and lead to smaller and less energy-consuming machines. The present status is an explosion in PC usage, a mounting disposal problem, large energy usage by the semiconductor manufacturing industry, and the generation of large quantities of chemical waste. The PC, similar to the motorcar, is now a permanent fixture in modern life and the focus has now shifted onto making the complete life cycle of the PC much more environmentally friendly and sustainable. Similarly, the continuous rise in the use of micro-devices is predicted and the future society will be more dependent on such devices for its well-being. The challenge is to ensure that not only the manufacturing processes used are sustainable with minimum environmental footprint but also the use and disposal of the micro-devices have minimum environmental impact. This challenge is not easy to accomplish, as energy requirements per unit mass of micro-devices is significantly higher than that for a macro-device and micro-devices may yet pose problems that have not arisen or been identified that awaits their more prevalent use.

1.2. Role of LCA

Life cycle analysis is a management tool for quantitatively assessing the impact of a product on the environment through its complete life cycle. It has been used extensively in the chemical industry [BRE 97], where management of the impact of chemicals has been a major environmental issue since Silent Spring was published [CAR 62]. Applications of LCA to micro-device manufacturing need to satisfy the demands of governments, environmental groups, and the public in the areas of waste minimization, recycling, reduced or zero emission, reduced e-CO2 reduction, and utilization of renewable energy and material resources. A holistic approach must be used if a realistic and genuine effort is made to understand whether micro-device manufacturing is a sustainable form of manufacturing. The current boundary for the LCA study is usually limited to the production of the micro-device itself. This is expected, as stretching out the boundary often requires information that is not easily available. Including waste and emissions as well as the contribution from the life cycle of the equipment used in the manufacture is important when comparing different manufacturing methods because of differences in energy and material impacts. Large-scale equipment, used for production runs, is highly inefficient for prototyping small quantities. Moreover, many of the new products from laboratories lack substantiation when claims are made that a new micro-product or manufacturing method will be sustainable environmentally.

A complete LCA should follow the life cycle of a micro-device, from the extraction of the raw materials used to its final disposal, which includes the material inputs, transportation, energy generation, use, reuse, maintenance or single use, and recycling. The approach to conducting LCA is outlined in the Australian Standards based on the ISO standards. These are as follows:

1. [ISO 98] AS/NZS ISO 14040 “specifies the general framework, principles and requirements for conducting and reporting life cycle assessment studies”.

2. [ISO 99] AS/NZS ISO 14041 “specifies the requirements and the procedures necessary for the compilation and preparation of the definition of goal and scope for a LCA, and for performing, interpreting, and reporting a life cycle inventory analysis (LCIA)”.

3. [ISO 01a] AS/NZS ISO 14042 “describes and gives guidance on a general framework for the LCIA phase of LCA, and the key features and inherent limitations of LCIA”.

4. [ISO 01b] AS/NZS ISO 14043 “provides requirements and recommendations for conducting the life cycle interpretation in LCA or LCI studies”. In particular, it provides guidelines on what should be done if information, as well as sensitivity and consistency checks, are incomplete. Examples of life cycle interpretation are provided in its appendices.

Figure 1.3 shows the inter-relationship between the different stages in carrying out an LCA. The scope of an LCA varies with the changing boundaries and although larger and more inclusive boundaries provide a clearer picture of the energy and material usage, the lack of information or the difficulty in isolating all the contributions and losses may make it difficult to achieve. Most of the LCAs conducted are currently at the factory stage where the machining process can be evaluated under known conditions.

Figure 1.3.The life cycle stages for micro-manufacturing

The LCA involves mass and energy balances. Mass balance is often difficult when chemicals are used and re-used in the manufacturing of multiple micro-devices, as the single use of the chemicals is wasteful. Energy balance is more common, as electricity is often a large component of the energy source and can be quantified easily.

1.2.1. Energy considerations in micro-device manufacturing methods

The process of micro-device manufacturing consists of several different sub-steps, which are different for every manufacturing process used. In a micro-manufacturing factory, several different manufacturing processes are often used. An LCA of the production of an insert for micro-injection molding [DEG 07] required the use of five major processing steps; plasma vapor deposition, electrical discharge machining (EDM), laser cutting and milling, chemical deposition and dissolution, and cleaning and activation.

The most common approach to the evaluation of the environmental cost of a micro-device manufacturing method is to determine the energy cost for the production of a specific micro-device. As the complexity of the micro-machining required differs from micro-device to micro-device, comparing the energy cost of methods in which micro-devices can be manufactured from start to finish provides a baseline for evaluating the environmental footprint of different micro-machining methods. As the manufacturing can be done in a variety of ways, comparing different manufacturing routes can become a major task, particularly when several machining processes are involved that result in a large number of possible variations.

The energy consumption by machining processes at the production level can often be dominated by static requirements; energy requirements that are inherent in the process irrespective of whether any product is produced. For example, Gutowski et al. [GUT 05] showed that the energy use breakdown from a large Toyota machining center identified that a maximum of 14.8% of the energy is used in removing material. This dynamic energy requirement increases linearly from zero to the maximum value as the number of vehicles produced increases. The static energy consumption included cooler, mist collector, etc. (15.2%), oil pressure pump (24.4%), coolant (31.8%), and centrifuge (10.8%). Kordonowy [KOR 01] analyzed several milling machines and found that the energy used in material removal varies from 48% to 69%. The most efficient milling machine was a manual milling machine, as there was less energy requirements from automated auxiliaries (Figure 1.4), whereas the least efficient was an early 1988 Cincinnati Milacron automated milling machine.

The analysis showed that improvements in the energy usage progressed with each newer model of the automated milling facility, resulting in the current suite of newer machines producing as much useful machining work as the older manual milling machines. As seen in Figure 1.4, the work from the specific electrical energy is a linear function of the load; hence, we can write the power (P) per mass rate () as:

[1.1]

where P0 is the idle power and k is a constant.

Similarly, for micro-device manufacturing, the use of conventional CNCs is highly energy inefficient [LIO 09]. The concept of micro-factories whereby micro-components are produced by smaller scale equipment that is designed to handle the small components in possibly clean environments has been studied and implemented in the last two decades [KUS 02, OKA 04]. Micro-factories would occupy less space, provide better environmental control, and consume less energy through a reduction in the energy lost through friction and heat. Thus, the use of micro-milling equipment designed with micro-machining in mind should result in significant energy efficiencies.

Figure 1.4.Machining energy breakdown for the 1998 Bridgeport automated milling machine with 5.8-kW spindle motor (left) and the 1985 Bridgeport manual milling machine with a 2.1-kW spindle motor (right) [DAH 04]

Because of the size of the micro-device, the amount of material removed during the final fabrication of the device is usually small. Most of the materials removed during the process are in the preparatory stages where conventional manufacturing methods are used. In the micro-end-milling of material, the chip sizes that are formed during machining are a few orders of magnitude smaller than those in conventional CNC milling because of the small chip load used, usually of the order of 1 μm. As more exposed surfaces are formed for a given mass of material removed, the energy required to create those surfaces per unit volume of material removed increases. This is similar to the crushing and grinding of ores for which the required energy increases rapidly as the passing screen size decreases. Laser-based techniques are energy intensive, as the material is fully (with CO2 lasers) or partially (excimer lasers) vaporized, which requires substantial amounts of energy for overcoming the latent heat of vaporization [MOR 07].

In material processing, Gutowski et al. (GUT 09) estimated the electricity requirements for 20 different processes as shown in Figure 1.5. As the process rate decreases, the electricity requirement per kilogram of material increases, with the limits within the following range:

[1.2]

Given the large amount of electricity used in the micro-device manufacturing processes, it is imperative that alternative processing paths be evaluated to identify low-energy and material usage routes for large-scale manufacturing of a micro-device.

Figure 1.5.Specific electrical requirement for various manufacturing processes showing the exponentially increasing requirements as process rate decreases (reproduced from [GUT 09])

1.3. Energy consideration in micro-manufacturing

One of the major procedures in LCA is energy balance. Energy is consumed in manufacturing, most of which is through electricity. The generation of electricity in Australia is largely through coal-fired power stations, and in Victoria, through brown coal, which has a lower energy and higher waste content. Indirectly, it contributes to the depletion of non-sustainable energy source as well as a major contributor of greenhouse gas.

The energy used in the micro-manufacturing process consists of the energy required for processing the materials, energy required for the micro-manufacturing process, and the energy losses during the conversion of energy sources into useful work for the processing. Each aspect can be subdivided into smaller sections to identify where energy is lost. The output is the usefulness of the transformed material. Mass and energy balances provide the means to analyze and thermodynamics will allow us to draw useful conclusions from the energy use and losses for these processes.

1.3.1. Mass and energy balance

This analysis closely follows the standard textbook thermodynamics [MOR 08]. The mass balance for a control volume is given as follows:

[1.3]

where m is the mass of the material in a control volume (kg), t is time (s), and is the mass rate in kg/s. The energy balance is:

[1.4]

where E (J) is the energy, (W) is the heat flow rate, (W) is the rate of work done, and h (J/kg) is the enthalpy of the materials in the control volume. The potential and kinetic energies of the control volumes have been ignored, as the manufacturing system is generally stationary and located on the factory floor.

The entropy balance for a control volume is:

[1.5]

where S (J/K) is the entropy change of the control volume, s (J/(kg·K)) is the entropy accompanying the mass flow, T (K) is the temperature, and (W/K) is the rate of entropy production due to irreversible processes within the control volume.

The output is based on the factory conditions, which has a reference temperature of T0. Rearranging and eliminating the term gives:

[1.6]

where (J) is the exergy of the process and (W) is the rate of exergy generated. The exergy provides a maximum theoretical work obtainable from the overall process as it comes into equilibrium with the environment. In calculating the exergy of a manufacturing process in which the material is removed, a large proportion of the exergy flowing into the control volume exits with the product (output). When manufacturing removes a small amount of the material but at a high energy cost per unit volume, the efficiency of the process is not optimized when the amount of starting material is varied, since for the same amount of energy expended, the exergy through the control volume is different. There exists a stream of exergy that flows through the process unaltered and a stream of exergy that is acted upon and active in the control volume considered. The exergy that is acted upon is called the “utilizable exergy” by Sorin et al. [SOR 98], and it provides a comparable basis for differentiating the efficiency of different micro-device manufacturing processes.

The efficiency of a process is often defined as:

[1.7]

where the factory environment with temperature T0 is taken as the minimum temperature at which heat is exhausted. However, for manufacturing processes in which the processing often occurs at the ambient temperature and the products are also at the ambient temperature, there is no work output. Instead, the use of the minimum amount of work required to perform the machining operation to the exergy destroyed provides an efficiency that is much more useful for comparison. Hence, an efficiency of removal as defined by Gutowski [GUT 08] is:

[1.8]

According to Branham et al. [BRA 08], the minimum work input required to effect the transformation of material to product is just the difference between the exergy outputs minus the exergy inputs. The efficiency of the machining process can then be estimated by taking the ratio of the exergy of the useful output and dividing it by the exergy destroyed. Szargut et al. [SZA 88] used a slightly different ratio, ηP, calling it the “degree of perfection” defined as:

[1.9]

The degree of perfection has been calculated for several different manufacturing processes by Gutowski et al. [GUT 09]. Values obtained include 0.7 to 0.9 for injection molding and induction melting of iron, 10–4 to 10–7 for semiconductor processes, and 10–3 to 10–7 for processing of nano-materials. The amount of useful work relative to the work input is very small for micro-device manufacture, which does not support the fact that micro-device manufacture is as sustainable as claimed.

1.3.2. Minimum work

The calculation of the minimum net work input required to effect the transformation should be independent of the transformation itself. However, every machining process results in a different transformation of the material even if the product is similar, or has similar use and application. Even when only material is removed, the removal process results in different forms of the waste being produced. The least amount of work occurs if the material can be removed unchanged with only cleavage at the planes where the waste separates from the workpiece. This is related to the generation of the minimum amount of new surface area during the process of producing a micro-device. In laser micro-machining, the material is vaporized or ablated from the surface and the extra energy required for melting and vaporizing the material is wasted because the removed material loses all of the energy on refreezing. In electrodischarge machining, energy is also lost through the vaporization of material. In micro-end-mill machining of the material, the removed material is chipped off, resulting in small chips and a large increase in the energy required for creating the extra surface area. Similarly, with water jet machining, the energy abrades the material, similar to grinding, and this incurs a high-energy cost – the efficiency of grinding is at least one or two magnitudes less than CNC machining.

Estimation of the minimum work required for the removal of material in CNC-type machining depends on the models used, and theoretical estimates can be substantially less than the measured values.

1.3.2.1. Fracture of material

[1.10]

where a is the separation between two atom planes. The values of the true surface energy are in the range of 1 to 5 J/m2 for brittle material and less than 1 J/m2 for plastics and salt. A list of true surface energies for several materials used in micro-machining is listed in Table 1.1. For elastic material, it is necessary to include the effects of plastic deformation and the surface energy is a sum of the true surface energy and the work of plastic deformation (γp). The value of γp can be increased by suitable changes in the microstructure of the material. In particular, plastics show very large discrepancies between γs and γp. Guy [GUY 72] provided an example for poly(methyl methacrylate), where γp/γs ≈ 1,000, which means that the minimum work required is increased by three orders of magnitude.

Table 1.1.True surface energy for materials used in micro-machining manufacture of micro-devices [GUY 72, COT 75]

High-strength materials, such as alloys of iron, titanium, and aluminum, are often characterized by low toughness, and the fracture toughness (Kc) is used to associate the material with the fracture stress. The fracture toughness is related to the work of plastic deformation as follows:

[1.11]

The work for plastic deformation is likely to yield a more realistic value for the minimum work for the machining of metals. The minimum work obtained is based on the area of new surfaces formed.

1.3.2.2. Work under the stress-strain curve

Fracture of ductile material that has undergone appreciable plastic deformation occurs in the necked region. Work is required for the formation of the new surface and the plastic deformation accompanying the removal process. This involves plastic deformation of the area, which may include not only the material that is removed but also the surface that is in contact with the tool. The minimum amount of work can be estimated as the energy required as a result of the plastic deformation. The energy per unit volume is given as follows [GUY 72]:

[1.12]

The integral in the rightmost term is the area under the true stress-strain curve and is much larger for ductile materials than for brittle materials. For a linearly increasing plastic region, Branham et al. [BRA 08] provided the energy per unit volume as follows:

[1.13]

The work obtained from the area under the curve provides a larger value of work than the previous estimate from the true surface energy. This approach provides the minimum work per unit volume based on the work required for plastic deformation and hence fracture the material to be removed. Branham et al. [BRA 08] used this method to obtain a value of 4.7 × 107 J/m3 for the CNC machining of AISI 1212 steel.

1.3.2.3. Shear energy for material removal

Metal cutting involves concentrated shear along a rather distinct shear plane. Mechanistic models for steady-state cutting have been based on the processes occurring at the shear plane, starting with the early models of Merchant [ERN 41, MER 44]. In orthogonal cutting [SHA 05], the total energy consumed per unit time is as follows:

[1.14]

where FP is the force in the horizontal direction and V is the velocity of the tool. The specific energy (total energy per unit volume) of material removed is as follows:

[1.15]

where b and t are the width and depth of cut, respectively. This specific energy is a sum of the shear energy on the shear plane, the friction energy on the tool face, the surface energy due to the formation of new surface area, and momentum energy due to the momentum change associated with the metal as it crosses the shear plane. The energy balance is dominated by the shear and friction energies, whereas the surface and momentum energies are small and often neglected. Hence, this estimate of the minimum energy required will be substantially larger than the earlier estimates. The shear (uS) energy and the friction (uF) energy per unit volume are given as follows:

[1.16]

where FS is the shear force along the shear plane, FC is the force along the tool face, VS is the shear velocity (velocity of the chip relative to the workpiece and directed along the shear plane), and VC is the velocity along the tool face. The specific energy of cutting can be estimated from published values for a specific material. Shaw [SHA 05] provided a value of 4.914 × 109 J/m3 for the milling of stainless steel with a continuous chip of thickness of 0.25 mm, with no built-up edge formation and an effective rake angle of 0°. This is an order of magnitude higher than the minimum work per unit volume obtained from the stress-strain curve earlier. Methods for further estimating the shear and friction forces are given by Shaw [SHA 05].

1.4. Energy consideration in micro-end-milling manufacturing

In a study of energy requirements for a micro-end-milling setup, Liow [LIO 09] showed that the bulk of the energy is consumed by the computer as shown in Table 1.2. The results presented are for a particular setup of the micro-machining process – a series of four T-junctions as shown in Figure 1.6. The study used a particular spindle speed (40 krpm), chip load (1 μm per tooth), and depth of cut (5 μm) to machine the T-junctions. During the micro-machining process, the spindle, slides, and the computer required energy whether or not milling was taking place. The static and dynamic parts of the energy requirements were not isolated. Here, a more detailed study of the T-junction milling is presented with an optimization of the energy requirements as a function of the spindle and slide speeds and a comparison with the minimum energy requirement.

Table 1.2.Energy consumption for a particular case of micro-end-milling T-junctions with a micro-machining setup [LIO 09]

Part

Energy consumed (MJ)

% of energy use

Spindle

0.101

19.7

Slides

0.069

13.5

Lubricant flow

0.077

15.0

Airflow

0.015

2.9

Computer

0.251

48.9

The T-junction channels are 100 μm wide and 50 μm deep cut into stainless steel. The energy requirements of the spindle and slides are shown in