International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 Process Planning - new process plans for agile manufacturing Abdelouahhab Jabri, Abdellah El barkany, and Ahmed El Khalfi Department of Mechanical Engineering Sidi Mohammed Ben Abdellah University, Route d’imouzzer, BP. 2202, Fès, Morocco Abstract— The agile manufacturing system should be simple, flexible, reconfigurable, reliable and responsive to market changes. The objective of this paper is to develop a new approach that aims to achieve this goal by adapting manufacturing resources to customer requirements in terms of lead time and reducing manufacturing costs. Based on Design for Production (DFP) and Activity Based Costing (ABC) methods, Process Planning for Agile Manufacturing (PPAM) approach is used by process planners to compare different process plans and select which could respond to the objective of the company. DFP method is employed in this approach to estimate manufacturing cycle time and resource capacity, as for ABC tool, it is used to estimate manufacturing cost based on information generated by the former method. Optimal batch size is firstly determined in this new approach and used as input data to DFP method. A case study and a software application program are presented to illustrate this approach and support process planners successively. Keywords— ABC, DFP, Lead-time, Manufacturing cost, Agility, CAD/CAM system, Batch-sizing. I. INTRODUCTION Manufacturing systems should be able to produce a variety of components at low cost and in a short time period. In another hand, process planning is an activity for designers to evaluate manufacturability and the manufacturing cost in the early design stage for mechanical parts production. Since major manufacturing costs of a product are committed in early product development, it is critical to be able to assess manufacturability and cost as early as possible in the design process [1]. Agility has been defined, in terms of outcomes, as dynamic, context specific, aggressively change embracing and growth oriented succeeding winning profits, market share and customers. In other words, agility is the ability of a business to grow in a competitive market of continuous and unanticipated change, to respond quickly to rapidly changing markets driven by customer-based valuing of products and services. By focusing on the output, Yusuf [2] asserted that an agile organization can quickly satisfy customer orders; can introduce new products frequently in a timely manner; and can even get in and out of its strategic alliances speedily. ISSN: 2231-5381 However, further insights into agility could be gained by looking at the specific and operational issues. In response to the challenge of agile manufacturing, this study focuses on: Lower costs and short manufacturing lead times for productions of a variety of components. Manufacturing lead time is defined as the time interval from the starting time to the completion time of the first product in production. Lead time of manufacturing each product is reduced. To date, many researchers have focused on process planning and its impact on the manufacturing processes. In his work, Lee [3] considered agile manufacturing in the early design of components and manufacturing systems. A design for agility rule is formulated. The design rule reduces manufacturing lead times in consecutive changes of product models. Hundal [4] discussed design rules aiming at the reduction of production costs. Jianxin [5] proposes a pragmatic approach to product costing involving two stages: the preparatory stage and production stage, this approach uses the ABC method to estimate manufacturing cost. Ciurana [6] presented a model integrating process planning and scheduling tool in metal removal processes. Alaa [7] presented an approach of a quality/cost-based conceptual process planning. The manufacturing cost was estimated by taking into account the risk cost associated with the process plan. Quality function deployment (QFD) method was used to select the process alternatives by incorporating a capability function for process elements. Kusiak and Weihua [8] suggested rules that designers can follow to reduce a product's manufacturing cycle time. These rules attempt to simplify the production scheduling problems that plague most production systems. For example, the rules state that one should minimize the number of machines needed to manufacture a product (which yields fewer moves and less queue time) and allow the use of substitute manufacturing processes (which gives the production system the flexibility to route an order to another operation to avoid a long queue at a bottleneck resource or unavailable machine. Jeffrey [9] proposed a systematic approach based on the DFP method to reduce manufacturing cycle time during the product development stage. Feng [1] developed a prototype for the preliminary assessment of manufacturability in the early stages of product design. The literature on agile manufacturing is abundant; however it seems that very few works have considered process plans in http://www.internationaljournalssrg.org Page 92 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 the concept of agile manufacturing. This has motivated the present work aiming to develop a new approach based on DFP and ABC methods that aims to roughly estimate manufacturing costs and help process planner to establish process plans that meets customer requirements in terms of lead-time. As shown in Fig. 1, the role of the Process Planning for Agile Manufacturing (PPAM) approach is to carry out an evaluation of the resources selected during the conceptual process planning. These resources include machines involved in the processing the part, cutting tools, fixtures, etc. Cutting conditions are also among the input data of the PPAM method, and then gives feedback about the performance of the manufacturing system such resource utilization and manufacturing cycle time and cost. These data are useful to validate the manufacturing process and avoid any problem related to the capacity of the workshops. This paper is organized as follows: the following subparagraphs present an overview DFP and ABC methods. The second section presents the PPAM approach. A prototype system is presented in section 3. A case study of the application of the proposed approach is presented in section 4. Finally Section 5 concludes this paper. A. Activity Based Costing (ABC) ABC assumes that cost objects (e.g., products) create the need for activities, and activities create the need for resources. Accordingly, ABC uses a two-stage procedure to assign resource costs to cost objects [10]. In the first stage, costs of resources are allocated to activities to form Activity Cost Pools. Tolerance standards Product Design CAD Engineering requirements techniques Selected resources Preliminary Process planning Raw Material Resource availability Process cost Tools & Fixtures These activities are allocated in the second stage to cost objects based on these object’s use of the different activities. In order to differentiate between the different allocations at the two stages, the first-stage allocation bases are termed ‘‘resource cost drivers’’ and the second-stage bases ‘‘activity cost drivers’’ [11] - [12] - [13]. Fig. 2 illustrates the concept of the ABC method. Consume Consume Resources Activities Resource drivers Activity drivers Fig. 2. The concept of ABC B. Design For Production (DFP) DFP refers to methods that determine if a manufacturing system has sufficient capacity to achieve the desired throughput and methods that estimate the manufacturing cycle time of a new product. DFP can also suggest improvements that decrease capacity requirements (which can increase the maximum possible output), reduce the manufacturing cycle time, or otherwise simplify production. These methods require information about a product’s design, process plans of existing products, and production quantity along with information about the manufacturing system that will manufacture the product [14]-[15]. The manufacturing system is characterized by the machines performance like the mean time to failure (mfj) and mean time to repair a machine (mr j). The products are characterized by the job size (number of parts) and the desired throughput (D i) (number of parts per hour of factory operation). Time methods Standard process alternatives Manufacturing cycle time Resource utilization Process Planning for Agile Manufacturing Manufacturing resources database Product Cost methods Manufacturing cost Batch size Detailed Process Planning Process plans Fig. 1.The role of PPAM ISSN: 2231-5381 http://www.internationaljournalssrg.org Page 93 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 The sequence of machines that each job must visit; the mean setup time (per job) at each machine (sij) and its variance (cs ij); the mean processing time (per part) at each machine (tij) and its variance(ctij); the yield at each machine that a job must visit (yij) (the ratio of good parts produced to parts that undergo processing). The squared coefficient of variation (SCV) of a random variable equals its variance divided by the square of its mean. Other notations used are as follow: I set of all products Ri sequence of machines that product i must visit Rij subsequence that precedes machine j Yij cumulative yield of product i through Rij Yij cumulative yield of product i through Ri xi release rate of product i (jobs per hour) Aj availability of a machine j Vj set of products that visit machine j cri SCV of batch inter-arrival time of part type i t+ij total process time of product i at machine j c+ij SCV of the total process time t+j aggregate process time at machine j c+j SCV of the aggregate process time t* j modified aggregate process time at machine j c*j SCV of the modified aggregate process time The cumulative yield is the product of the yields at each machine that the product visits is calculated using the following equations. (1) (7) 2) Processing time aggregation The mean processing time (per part), the mean setup time (per batch), batch size, desired throughput and machine availability are used at this stage to calculate the aggregated processing time. Batch processing time The mean batch process time is the sum of the mean batch setup time and the mean total processing time. The mean total processing time is the mean single-part processing time multiplied by the mean number of parts in the arrived batch, equation (8). (8) Assuming the batch setup time and single-part process time are independent, the variance of batch processing time is the sum of the variance of setup time and total processing time. The variance of total processing time is contributed by the variance in single-part processing time and the variance in the arrived batch-size, equation (9). (9) Aggregation The aggregate process time of jobs at machine j is the weighted average of all the jobs that visit machine j. Each product is weighted by its release rate, equation (10). Equation (11) calculates the mean of the square aggregate process time, which can be used to determine the SCV (c+j). (10) (2) 1) Arrival aggregation The batch arrival rate of a part type is its demand divided by the average batch-size arriving at the first machine, and adjusted by the overall yield rate to fulfil the demand, equation (3). (11) The aggregated batch arrival rate at the machine j is the sum of the batch arrival rates of all part types is calculated with equation (4). Downtime adjustment Equation (10) gives the SCV of aggregated processing time at the machine without considering machine unavailability. However, due to the machine failures or (3) downtime (e.g., scheduled maintenance), the actual processing time will take longer thus needs to be adjusted. The percentage of time that a machine is available is Aj, equation (12). (4) (12) (5) The adjusted mean aggregated time and SCV of aggregated time become: Assuming the mean batch arrival rates for all part types are of the same order of magnitude, the SCV of aggregated interarrival time at the first machine can be approximated by the weighted average of the SCV of batch inter-arrival time of all part types, equation (6) [14]. (6) ISSN: 2231-5381 (13) (14) 3) Flow variability propagation and cycle time calculation http://www.internationaljournalssrg.org Page 94 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 At this step, three factors are determined, which are: Machine utilization, cycle time at station j multiple. It can be seen that the queuing time is composed of three factors: variability, utilization, and processing time. The mean system cycle time is the sum of the machine cycle time: Machine utilization The average utilization rate uj at a machine j is the percentage of time that it is busy. It is calculated by the following equation. (18) (19) (15) The variability of inter-departure time at each machine is propagated from the variability of inter-arrival and processing time. It can be approximated by the following equation. II. PPAM APPROACH The proposed PPAM approach is a decision support tool for the process planner which aims at estimating manufacturing cycle time and cost. It is based on the methods previously presented ABC and DFP. As shown in Fig. 3, the steps for PPAM process are: (16) (17) Generation and simulation of tool paths; Approximation for cycle time calculation With all the information about (caj), (c*j), xi and (t*j) through the manufacturing system, they can be used calculate the cycle time at each machine. The first term on the right hand side of equation (18) is the approximated queuing time. Determination of optimal batch size; Manufacturing cycle time estimating; Manufacturing cost estimating. Process plans of existing products Product design CAM Systems Generation and simulation of tool paths Machining operations times Process Planning for Agile Manufacturing Lead time Consideration Optimal batch size determination Optimal batch size routine Optimal batch size Manufacturing cycle time estimating Manufacturing Cycle-time Manufacturing cost estimating Resource utilization DFP method Cost consideration ABC method Manufacturing process cost Detailed process planning Fig. 3. The process of AMPP ISSN: 2231-5381 http://www.internationaljournalssrg.org Page 95 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 A. Generation and simulation of tool paths The first step of this approach uses the CAM systems such as CATIA V5, SolidWorks, TopSolid or ProEng, to generate and simulate machining tool paths. The process planner have to take into account at this stage the customer requirements like dimensional and geometrical requirements and roughness, creates a sequence operations to carry out in the same machine, selects cutting tools and cutting conditions and establishes cutting speed and feed rate values. At this stage all machines, fixtures and tools involved in the manufacturing process are gathered and each operation is finally provided and serves as input data for the following steps. B. Determination of optimal batch-size The number of units produced per order (the lot size) is an important managerial decision, which affects both the lead times and the throughput [16]. In our approach we adopt the model of the queuing behaviour of a multi-item multi-machine job shop developed by Karmarkar [16]. This model is based on the node decomposition technique for approximate analysis of open queuing networks. Each node is described by a simpler queuing model with a single stream of arrivals and the characteristics of the departing stream are deduced from this model. The arrival stream at each node is the superposition of departure streams from the other nodes in the network that feed it. It is assumed (heuristically) that the arrival process of batches at machines is Markovian. Since processing times at machines vary by item, a general service time distribution is assumed and machines (nodes) are modelled as M/G/1 queues with first come first served (FCFS) service discipline, [17][18]. The mean time spent in queue (Wj) at machine j is given in equation (22). (22) Where, E [t2j] is the second moment of processing time at machine j. (23) The mean time spent at machine j by batches of item i (Tij) is the sum of the second moment of processing time and the mean time spent in queue at machine j, equation (24). The objective of this step is to evaluate the capacity of workshops and estimate manufacturing cycle time using DFP method based on the results of previous steps. DFP method takes into account data generated by previous steps, namely: processing times, optimal batch size. Consider a flow line manufacturing system consisting of n machines (1< j < n) which manufacture m types of parts and all these parts go through every machine in the system without skipping. Part type i (1< i < m) has a desired throughput (Di), and arrives in batches randomly with predetermined batch-size (Bi), and a SCV for its batch arrival (caj). Each batch of part i is processed on one machine j with mean setup time, (sij), mean single-part process time, (tij), and mean yield rate yij. The average cycle time for a batch of any part type spending in machine j, equation (18), is TTj. The total manufacturing cycle time is estimated by equation (19), and the resource utilization using equation (15) taking into account the availability and the yield of each machine. D. manufacturing cost estimating The total manufacturing cost is estimated using the ABC method. The manufacturing process is broken down into activities based on a decomposition of Feng and Song [1], [19]. The total manufacturing cost is the sum of activity costs, equation (26). (25) (26) Cma is the cost of manufacturing activities; N is the total number of activities involved in the manufacture of the part. Cimachining is the machining cost of activity i, it is calculated by equation (27). The first term of this equation is the cost related to machines involved in the manufacturing process, as for the second term it is the cost related to the tools performing each operation, the formula is as follows: (27) Where: (28) (24) The lot-sizing problem is formulated as Optimization problem, where the objective is to minimize the inventory costs. Let hii be the holding costs for units of item i waiting or being processed at the machine and let hif, be the holding cost in finished goods. The expected total cost function is formulated as follows. (p) (i) (ii) uj(B) < 1 , j=1, …, n Bi>1 , i=1, …, m. C. Manufacturing cycle time estimating ISSN: 2231-5381 Tk1 and Tk0 are the cutting time and tool life time respectively of the tool k. Ck1 and Ck2 are the cost per hour related to cutting labour and tool labour respectively of the tool k, [6]. Ciload_unload is the load and unload activity. Cisetup is the setup cost of activity i. Cihandling the handling cost of activity i. Handling is a batch-level activity. Ciprogramming_testing the programming and testing cost of activity i. Programming-testing is a product-level activity. Cioverhead the overhead cost of activity i. It is a facility-level activity. http://www.internationaljournalssrg.org Page 96 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 III. A PROTOTYPE SYSTEM Based on the methodology described above, a prototype has been developed to support the process of estimating manufacturing cycle time and costs, resource utilization with optimal batch size. The machining time of each operation, cutting tools involved in this operation are imported from the CAMWorks module of SolidWorks. Indeed, this information which is stored in XML files is automatically extracted and displayed on the corresponding tables. The user introduces the different information about the manufacturing system like the desired throughput (units/day) of item i to be manufactured in the workshop, holding costs, etc. The Solver Module of Microsoft Excel is then called to determine the optimal lot-size of items, and then resource utilization manufacturing cycle time and costs are estimated. Fig. 4 presents the structure of this prototype. IV. CASE STUDY To illustrate the PPAM approach we present in this section an example of a machined part to be manufactured on a CNC machines, Fig. 5. The raw material of this part is low alloy steel preformed bar 110x110mm² and cut into 35mm. This part will be manufactured in a work shop which manufactures another product. The objective of this section is to illustrate with this example the methodology followed to perform an analysis of the conceptual process planning. Two processes are evaluated using the PPAM approach. Fig. 5. Drawing part A. Generation and simulation of tool paths The machining of this part has been simulated with the CamWorks module of SolidWorks. For the two Alternative Groups AG1 and AG2, two machine centres are selected to perform all operations of a part machining: NC milling machine and NC lathe centre. Operations to be performed and fixtures are the same for these two Alternative Groups. However, Cutting Conditions, and cutting tools are not the same, therefore processing times are not the same. Table 1 summarizes the different machining operations on these two machines selected to manufacture the part. Generate XML files Product design CAD/CAM Operations & Machining-time (XML files) Generate and simulate tool paths SolidWorks CamWorks Extract data MS Excel Solver Newton method Determinate optimal batch-size Extracted data Manufacturing cycle time estimating Manufacturing cost Estimating Fig. 4. A structure of the ABC-DFP prototype TABLE 1 Conceptual process planning Alternative Group 1 Alternative Group 2 ISSN: 2231-5381 CNC Machines Processing time (min) Setup time (min/lot) Lathe centre Milling machine centre Lathe centre Milling machine centre 7.50 6 8.57 6.66 30 20 30 20 http://www.internationaljournalssrg.org Page 97 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 We suppose that the setup time for this part is 30 minutes on the lathe centre and 20 minutes on the milling machine centre. Fig. 6 is the “products” screen sheet, which contains: Processing time table, Setup-processing time table, products table and process plans table. The processing time are extracted from XML files and displayed in the processing time table. The setup-processing time table contains the processing rate (parts / day), the setup time and the SCV of setup time and processing time which are equal to 1.The desired throughput for the product to be manufactured is 24parts/day. Information related to the products to be manufactured is gathered in the same layer. B. Optimal batch size The second step of this method is to determine the optimal batch size that minimizes the holding costs of batches to be manufactured. Processing times determined in the previous step are used at this stage. Assuming that the holding cost for units of items i waiting or being processed at machine j is 0.2 € and the holding cost in finished goods is 0.1 €. In order to find an optimal solution, the optimization problem was solved using Solver, a built-in optimization routine of Microsoft Excel, it is called from the application layer. Optimal batch size of the two items found is: B1 =13 and B2 =21 for AG1 and B1 =15 and B2 =25 for AG2. C. Manufacturing cycle time estimating Manufacturing cycle time of each product is estimated with DFP method. Optimal batch size of the two products and processing times determined in the previous step are used to estimate manufacturing cycle time at this stage of PPAM approach. In the next, we present an example of calculation for the first alternative group AG1. Cycle time estimating of AG2 is similar to AG1. D. Manufacturing cycle time estimating Manufacturing cycle time of each product is estimated with DFP method. Optimal batch size of the two products and processing times determined in the previous step are used to estimate manufacturing cycle time at this stage of PPAM approach. In the next, we present an example of calculation for the first alternative group AG1. Cycle time estimating of AG2 is similar to AG1. Arrival Aggregation Release rate is firstly calculated. Desired throughput of the items is 24 (unit/day) for the first item and 40 (unit/day) for the second one. Using equation (3), for AG1, the release rate is 1.85 (batch/day) and 1.9 (batch/day) of the first and the second product, respectively. The SCV of aggregated inter-arrival time at the first machine is determined using equation (6). We need to determine the SCV of inter-arrival times for each product (cr i) using equation (7). Fig. 6. Processing time and aggregate process time. ISSN: 2231-5381 http://www.internationaljournalssrg.org Page 98 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 For the AG1: Processing time aggregation The batch processing time is the sum of the mean batch setup time and the mean total processing time (t+ij). For the AG1. the mean batch processing time of the first product (i=1) at the lathe centre (j=1) and the milling machine centre (j=2) are: Hours For the AG2: Resource utilization table in the manufacturing system sheet, fig. 7, contains the values u1 and u2 of the two alternative groups, and the chart is automatically updated. The SCV of inter-arrival times at the second machine is: Hours The mean of the batch processing time for each item on each machine are then obtained. Since the part processing times on each machine are exponentially distributed. SCV (ctij) and (csij) are equal to 1. The SCV of total processing time on each machine is calculated using equation (8). Manufacturing cycle time is approximated by equation (18). For the lathe centre: Hours - And manufacturing cycle time at the milling machine is: Hours - Since all machines are perfectly reliable (Aj = 1), c*j = c+j and t*j = t+j. The adjusted aggregate process times of jobs on the lathe centre and the milling machine are calculated as follows: Finally the multiple for the two machines and the time spent in the queue are calculated using equation (20) and (22). - - Hours Hours And the SCV of the adjusted aggregate process times are calculated using equation (14). The adjusted aggregate process times are gathered in the process plans table in fig. 6. Flow variability propagation and cycle time Machine utilization is calculated using equation (15). The aggregate process time calculated previously is key information to estimate machine utilization, for the two machines: Hours Hours E. Manufacturing cost estimating In the final step of this approach, manufacturing cost is estimated using the ABC method, the activities involved in the manufacturing process are firstly identified. These activities are: programming and testing, machining, load/unload, setup, handling, inspection and Material. The machining cost is calculated using equation (27). The previously estimated processing time by DFP method is incorporated in equation (27) used to estimate manufacturing cost on each machine. Fig. 8 is cost estimating screen sheet which contains tooling cost and activity tables. The first table summarizes costs related to the cutting tools involved in the manufacturing process; it provides data needed to estimate tooling cost using equation (28). We assume that the hourly cost of cutting tool is 1.3 € / h and the tool life time is 3 hours and the tool cost is equal to 1.5€. Machining cost is estimated with equation (27) and it equals to 9.49€. Finally, manufacturing cost is the sum of activities costs Cma=15.16€. For the AG2 manufacturing cost is 16.03€. Fig. 7. Performance measure. ISSN: 2231-5381 http://www.internationaljournalssrg.org Page 99 International Journal of Engineering Trends and Technology- Volume4Issue2- 2013 Called Process Planning for Agile Manufacturing (PPAM), This approach presents efficiently reduced machining and setup costs early in the product development stage. Fig. 8. Tool cost and total manufacturing cost. F. Process alternative selection Finally, results of the previous steps are gathered in a selection table, manufacturing cycle time, resource utilization, time spent in queue, tool cost and manufacturing cost are the evaluation criteria to be assessed at this stage. The purpose of this stage is to select the best alternative which compromises multiple evaluation criteria is carried out. In the literature, there are a lot of methods of multiple evaluation criteria Saaty [20], Yannou [21], Blanc [22]. In our approach we use the method used bay Almannai [23] and Allaa [7]. The output data of the precedent steps are gathered in the decision table shown in Fig. 9. The process planner is now able to, to prioritize the evaluation criteria, and to score the process alternatives against them. He can evaluate the alternative options in 1–10 range and then calculate the final score in order to identify the most suitable alternative which replies to multiple objectives. This score is not a subjective quantification but it depends on the numerical data results from the PPAM approach. As shown in Fig. 7, the final score of process alternative groups give the advantage to AG1 (52.9%), and rank AG2 in the second place (47.1%). It includes an optimization routine which is responsible of searching the optimal batch-size in order to minimize the number of setups and therefore reduce manufacturing cost. DFP method is used to estimate manufacturing cycle time and helps the process planner to determine how manufacturing a new product affects the performance of the manufacturing system. Information generated by this method is then used to estimate manufacturing cost with the ABC method. A computer programme is developed which serves as an effective tool to estimate manufacturing cycle-time and cost. To illustrate this approach an example of a manufacturing cell with tow machines (a Lathe Centre and a Milling Machine Centre) and tow items is presented. Information generated by ABC and DFP methods are gathered in a selection table and helps the process planner to select the most suitable combined alternatives towards cycle time reducing and cost minimizing. Further works are required to develop a new tool including quality method in order to evaluate the process quality and give useful information about the manufacturing system. REFERENCES Fig. 9. Process alternatives selection V. 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