A Virtual Machining Model For Sustainability Analysis

Sustainability has become a very significant research topic since it impacts many different manufacturing industries. The adoption of sustainable manufacturing practices and technologies offers industry a cost effective route to improve economic, environmental, and social performance. As a major manufacturing process, the machining system plays an important role for sustainable manufacturing on the factory floor. Therefore, technologies for monitoring, analyzing, evaluating, and optimizing the sustainability impact of machining systems are critical for decision makers. Modeling and Simulation (M&S) can be an effective tool for success of sustainable manufacturing through its ability to predict the effect of implementing a new facility, process without interrupting real production. This paper introduces a methodology that provides a traditional virtual Numerical Control (NC) machining model with a new capability – to quantitatively analyze the environmental impact of machining system based on Life Cycle Assessment (LCA). The objective of the methodology is to analyze the sustainability impacts of machining process and determine a better plan for improving the sustainable performance of machining system in a virtual environment before work orders are released to the shop floor. Testing different scenarios with simulation models ensures the best setting option available can be chosen. The virtual NC model provides the necessary data for this assessment. In this paper, a list of environmental impact indicators and their metrics has been identified, and modeling elements for sustainable machining have been discussed. Inputs and outputs of the virtual machining model for sustainable machining are described. A case study to experiment the proposed methodology is discussed. INTRODUCTION This section describes the concept of sustainable manufacturing, sustainable machining, Modeling and Simulation (M&S), and Life Cycle Assessment (LCA). Current problems in industry are also addressed. Sustainable Manufacturing Sustainable development has become an important part of approaches to integrate economic, environmental, and social aspects. The Department of Commerce recently identified sustainable manufacturing as one of its high-priority performance goals, defining sustainable manufacturing as the “creation of manufactured products that use processes that minimize negative environmental impacts, conserve energy and natural resources, are safe for employees, communities, and consumers and are economically sound [1].” As such, sustainability related issues such as energy consumption, emissions and other environmental impact issues are becoming a more integrated part of operational and long-term planning decisions in manufacturing [2]. Machining is a major manufacturing operation and it involves a number of sustainability factors that have a big potential for environmental impact. These factors include tool life, usage of coolant and lubricant, waste chips and energy consumption. Therefore, the analysis of machining systems and optimization of these input factors (controls and constraints) and outputs (objective functions) has significant implication for sustainable manufacturing. It has been observed that the relationship between machining technologies and environmental impact remains insufficiently discussed and the environmental impact due to this manufacturing activity has been little evaluated [3]. Currently, there is no ideal method that can analyze the environmental impact of machining processes systematically to 1 Copyright © 2010 by ASME predict the outcome of the machining operations and optimize the process. This paper is a step towards to this direction. Modeling and Simulation for Manufacturing M&S has been proven to be an effective tool for reducing costs, improving quality, and shortening the time-to-market for products in the manufacturing industry. The virtual machining models simulate the kinematics, dynamic, mechanical, control, other behavioral characteristics of machining system. A virtual machining model allows visualizing and analyzing the functionality of a machine tool, the machine’s Computer Numerical Controlled (CNC) controller, and the material removal process. It can be used for emulating, validating, and optimizing the NC machine processes. The virtual environment enables the engineers to perform program prove-outs off line, while keeping the actual machine tool in production and avoiding expensive machine down time and possible tool crashes. It can also be used to evaluate machine tool’s capabilities and setups, debug, and test new post-processors, design fixtures, determine the best suited machine tool for a given job, and detect near misses and collisions. It is also useful for process evaluation of the conditions that cause cutter and tool breakage, or gouges and undercuts. The analysis of data collected during the machining cycle is used to optimize the program cycle time, tool life, and surface finish. Using a virtual machining model, all experiments can be performed in a safe environment [4]. Likewise, M&S will be an essential ingredient for sustainable machining through its ability to predict the effect of implementing certain facility, process and product actions and analyzing the environmental impacts. Kibira and McLean have developed a vision for M&S of sustainable manufacturing [5]. However, little work has been done to incorporate sustainability indicators in simulation models. It is especially important if the current existing virtual machining model can be extended to include the new functionality of sustainability analysis. Since there has not much demand for simulation technology to deal with sustainability features, software vendors and analysts have not typically addressed these issues. Therefore, models for sustainable manufacturing have to be built using existing simulation engines. The new aspect greatly complicates the simulation modeling. But with a systematic approach, the sustainability impacts of machining system and production process can be incorporated and evaluated. Life Cycle Assessment Methods Life Cycle Assessment (LCA) is a very useful methodology for analyzing sustainability of manufacturing systems, using multiple commensurable (i.e., measurable in similar units) aspects of quantifiable systems, based on systems thinking. Normally, LCA methods analyze parameters such as energy, resources, impact and outputs, e.g., waste, emissions, electricity, and total energy. The results from this analysis provide guidance on the relative impacts of different types of products, materials, services, or industries with respect to resource use and emissions throughout the supply chain. For example, the effect of producing an automobile would include not only the impacts at the final assembly facility, but also the impact from mining metal ores, making electronic parts, forming windows, etc. that are needed for parts to build the car [6]. The simulation of machining operations in combination with LCA and sustainability related data can be utilized to estimate the environmental performance measures of a machining system. The improvement of such performance measures includes optimizing material use, minimizing the use of cutting fluids, and reducing cutting energy [7]. This paper proposes a methodology that uses a virtual model of a machining system to analyze the environmental impact of the process. The objective of simulation system, scope, model elements, and its input and output requirements are discussed. This approach allows assessing the environmental impact in the virtual environment using real world data, specification data, and simulation data as input and providing a platform to evaluate different options for an optimal decision making. The paper is subdivided into five main sections. The next section discusses related work, followed by the introduction of the proposed methodology. Then the procedure of a potential case study is described to test the methodology. Finally, a summary is provided and future work is discussed. RELATED WORK This section discusses the related research work, techniques, and tools for the proposed methodology. Even though sustainable machining is crucial to sustainable manufacturing in shop floor operations, there is still insufficient research performed on this topic [3]. The approaches to solve the problem vary from finding alternative material removal methods to changing of material to optimization of the various inputs to machining process. The main objective of published research listed in the reference is to optimize environmental factors such as the usage of material, cutting fluids, and energy. Alternative material removal technologies include cryogenic machining and high pressure assisted machining, as opposed to conventional machining, that are deemed to be more environmental friendly for machining of materials with unique thermo-mechanical properties such as Nickel and Titanium alloys [8]. Environmental consideration of machining Optimization of process parameters introduced in [9] analyzes the environmental impact of turning operation of American Iron and Steel Institute (AISI) 1040 Steel. This analysis is based on using sensors and instruments on the machine tools and calculation – along four indicators (global warming, acidification, eutrophication, photo-chemical oxidant) for machining operations. It proposes using already existing technology while choosing the best process parameters and practices. A general description of approaches that can reduce environmental burden is presented in [10]; the authors 2 Copyright © 2010 by ASME describesdifferent approaches involving design for sustainability, developing sustainable production management systems, and alternative machining processes. Another research conducted environmental analysis of machining operations from a systems perspective [7]. In this approach, the work focuses on minimizing environmental impact of machining and associated activities such as tool preparation, material production, material removal, and cleaning among others. This analysis aims to determine a distribution of energy usage through various activities. Akbari and others carry out the lifecycle of machine tools from manufacture, use to disposal. During operation it is found that more energy can actually be used for running peripheral devices such as coolant pump, hydraulic pump, and control devices than actual cutting [11]. [12] describes a methodology using an environmental-based process model for computing energy use and wastes in addition to traditional parameters like process time and yield of a product design to facilitate systems planning for machining operations. [13] proposed a generic energy consumption model that can be applied to describe how the energy consumption and the energy efficiency of machines and production systems relates to the way they operated. Simulation methods for sustainability analysis of machining Narita and others developed a machining simulator, which consists not only of the simulation of the production of the geometry of the product but also the physical system to extract information such as energy consumption, coolant, and lubricant used [14]. An extension of previous methods that evaluate the difference between dry, minimal quantity lubricant (MQL), and wet machining approaches and include such factors as depth of cut, feed rate, spindle speed, and tool path pattern to find one with the least environmental burden of a machine tool operation has been developed [15]. The environmental burden is calculated after converting all the measures to CO2 equivalent. Narita and Fujimoto developed algorithms to calculate the “environmental burden” due to machine tool operations. They calculated the environmental impact of dry, MQL, and wet machining on the five impacted categories of global warming, acidification, eutrophication, photochemical oxidants, human toxicity, and ecotoxicity [3]. In another approach a prediction system based on LCA is used where “environmental burden” is evaluated using the inputs: workpiece model, cutting tools model and NC program while inputting information associated with the machine tool and machining process. Then the electric power consumption, cutting tool status, coolant quantity, lubricant oil, metal chips, and other factors can be calculated [16]. In general, the sustainable machining research was carried out in an ad hoc and piecemeal manner without methodology to systematically consider available model and data and guide decision making to optimize machining operations for minimal environmental impact in a virtual environment. This paper proposes such a methodology. PROPOSED METHODOLOGY Based on the needs described in previous sections, an innovative methodology that proposes procedures to allow the modeling, simulation, and analysis of sustainability indicators is introduced. It is described in the following subsections. Procedures The methodology involves several activities as shown in Figure 1. It includes defining a problem, setting up objectives of the study, selecting software, identifying modeling methods, developing model, executing the model, and analyzing the simulation results. The detailed steps are list below. 1. Define a machining problem by setting up a set of objectives and scope 2. Identify the key performance indicators that includes sustainability indicators and their metrics for machining processes 3. Create the virtual machining model, that is, to develop a simulation model of the machine tool under study 4. Define a test part for the study 5. Develop models of workpiece, fixtures, and cutting tools 6. Identify and develop the model for each sustainable indicator such as energy consumption 7. Collect the relevant data from a real machine if necessary 8. Collect data from the machine specification 9. Collect data from the LCA database 10. Develop methods to extract intermediate data from the virtual machining model 11. Process the raw data to extract the useful subset for modeling purpose 12. Format and input appropriate sustainability data according to the input data requirement of the simulation software 13. Create test scenarios 14. Execute the NC program in the virtual environment 15. Generate simulation results that include virtual part and NC validation report 16. Modify NC program if necessary 17. Repeat step 14 until the NC is validated 18. Generate sustainability report by executing the correct NC program 19. Analyze the key sustainability performance indicators such as total emission Some of steps such as step 3 to step 5 are the typical steps in current machining simulation. To simulate and analyze the sustainable machining, extra work is required, e. g., steps 2, 6, 9 are specifically for sustainable machining. Figure 1 shows main modeling procedure of the proposed methodology. In order to successfully perform the sustainability analysis using the methodology, the following questions need to be answered: How can the sustainable indicators be integrated with the virtual machining model? What role does tool wear play for environment impact? What real machine data needs to be collected? What data needs to be collected from the specifications? What data needs to be collected from LCA database? In what format is the collected data presented? How 3 Copyright © 2010 by ASME should the data be processed? How can the data needed from virtual machining model be expressed?