Urban Wemmerlöv

Cellular manufacturing in the U.S. industry: a survey of users

This paper reports the findings of a survey study of 32 U.S. firms involved with cellular manufacturing. Areas covered include the reasons for establishing cells, benefits achieved, types and sizes of cells, extent of cellularization in the plants, methods used to design the cells, changes to planning and control systems, labour-related issues and important experiences gained by the companies. The data presented here were collected as part of a larger study of various group technology (GT) applications in which 53 companies participated. GT applications other than cellular manufacturing are described in Hyer and Wemmerlov, 1989, Group technology in the U.S. manufacturing industry: a survey of current practices, International Journal of Production Research.

Cellular manufacturing at 46 user plants: Implementation experiences and performance improvements

Despite the increasing popularity of cellular manufacturing, by now a firmly entrenched approach to factory work organization, the number of large-scale systematic studies aimed at investigating issues surrounding implementation is still relatively sparse. This paper reports on a survey study of plants involved with cellular manufacturing. A target population of high-probability users received mail questionnaires designed to collect responses related to characteristics of industry cells and the firms that have implemented them (a companion paper, based on the same survey data, describes procedures and issues in cell design). Forty-six plants supplied detailed data on 126 of their cells, including reasons for establishing them, types of operations performed in the cells, problems faced and lessons learned during implementation, and achieved performance improvements. This study provides confirming evidence that manufacturing cells, which may house a large variety and many combinations of processes, can prov...

Procedures for the Part Family/Machine Group Identification Problem in Cellular Manufacturing

Abstract One of the first, and most important, problems faced in the design of a cellular manufacturing system is the identification of part families and machine groups and the simultaneous or subsequent evaluation of the associated cell properties. This cell formation problem is embedded in a larger cell design process. This article first discusses the general cell design process and its objectives and then proposes a framework for structuring the cell formation problem, which encompasses four basic solution approaches. These approaches are based on the fundamental ways part families and machine groups are identified and matched within the cell formation process. The framework is then used to classify descriptive and analytic procedures for the part family/machine group identification problem found in the literature. In all, over 70 such contributions are categorized and briefly reviewed. Promising procedures are identified and areas where additional research is needed are highlighted. It can be concluded that cell formation, despite the reliance on computerized procedures, is a process that is difficult to formalize and that requires a large portion of human insight and decision making.

Research issues in cellular manufacturing

Abstract Both industry and academia are currently devoting considerable interest to cellular manufacturing systems. Despite this growing attention, the existing literature does not adequately address a number of issues related to the successful adoption of these systems. This paper identifies a large number of research topics related to cellular manufacturing, discusses the need for their investigation, and suggests appropriate methodologies for their study. This research agenda is presented in the context of four important decision areas for new technology adoption—applicability, justification, system design, and implementation. The volume and variety of topics identified allow us to conclude that cellular manufacturing is an area with many researchable issues, spanning several academic disciplines, and requiring a multitude of research methodologies.

Designing a Cellular Manufacturing System: A Materials Flow Approach Based on Operation Sequences

Abstract This paper presents a cell formation method which integrates the issues of cell formation and with in-cell material flows. A coefficient which assesses the similarity of two parts in terms of their operation sequences is proposed and used to form part families and cells. Further, the within-cell machine sequence and machine loads are explicitly considered in the design process. The methodology has been operationalized through several computer programs )written in FORTRAN 77) and implemented on an IBM/PC, An application of the design methodology to a hypothetical cell formation problem is provided.

Lot-sizing under uncertainty in a rolling schedule environment

SUMMARY This paper presents results from a simulation experiment evaluating 14 different single stage lot-sizing procedures. Uncertainty in the form of forecast errors was used as one factor in the experiment. In order to prevent different service levels from confounding cost comparisons, enough safety stock to achieve a 100% service level in all situations was introduced. The ranking of the rules under uncertainty turned out to be very different from the ranking when no demand uncertainty was present. Statistical differences were found between the six best lot-sizing procedures when there was no uncertainty. When forecast errors were present, however, no differences existed for the six best rules in this situation. Thus, uncertainty not only changed the relationship between the lot-sizing rules, but also the character of this relationship.

EMPIRICAL FINDINGS IN MANUFACTURING CELL DESIGN

Cell design is the most researched topic in the area of cellular manufacturing. The preponderance of studies has focused on the development of models to assist with cell formation, i.e. the initial stage in the cell design process where the parts and equipment are determined. On the other hand, fairly little is known about the broader context in which cell design takes place, and the processes that users follow. The purpose of this study was to learn more about the methods, goals, considerations, and constraints that industrial users apply to cell formation and cell layout, as well as the actual outcomes of those processes (such as cell configurations and staffing patterns). This paper adds to the sparse literature on empirical cell design by reporting on findings derived from a survey study of cell users predominantly drawn from the metalworking industry (related data have previously been published in Wemmerlov and Johnson (1997)). Supporting evidence is primarily taken from survey and field studies by Wemmerlov and Hyer (1989), Harvey (1993), Choi (1996), Suri et al. (1996), Olorunniwo and Udo (1996), Marsh et al. (1998), Johnson (1998), and Hyer and Brown (1999). The findings should be of interest to both students and practitioners of cellular manufacturing, and could serve as guidance for researchers seeking to develop more effective methodologies for solving the cell design problem for industrial users.

Assemble-to-order manufacturing: Implications for materials management

Abstract A companys manufacturing is often characterized as either make-to-stock (MTS), make-to-order (MTO), or assemble-to-order (ATO). This classification relates to the degree of interaction between the technological core and the market, with MTS involving the least amount of interaction and MTO the highest degree of contact. ATO represents a hybrid manufacturing strategy for which parts and subassemblies are made according to forecasts while the final assembly of the products is delayed until customer orders have been received. It is evident that each manufacturing philosophy has strategic as well as operational implications. This paper focuses on ATO manufacturing and, in particular, on the design and operation of the manufacturing planning and control system. The relative differences between MTO, ATO, and MTS strategies, and the reasons why a company may decide to be an ATO manufacturer, are discussed first. Several problem areas, which must be addressed by a company that chooses this form of manufacturing, are then identified. It is found that the ATO philosophy requires special system design considerations, particularly in the areas of master scheduling, bills of material structuring, order entry/order promising, final assembly scheduling, and buffering against demand uncertainty. Examples of important issues discussed are the selection of appropriate master schedule units and the associated consequences for the structuring of the bills of material; the choice of efficient procedures for reliable order booking using the combined information from forecasts, confirmed orders, and the master schedule; the relationship between the final assembly schedule and the master schedule; and various techniques that can be used to counter the effects of forecast errors related to the demand for customer options. Following the discussion of all the issues raised above and the presentation of some common solutions, actual industry applications taken from several companies are used to illustrate various aspects of ATO manufacturing. These case illustrations complement the other material since many of them were chosen to reflect alternative procedural approaches to some ATO-related problems. Finally, areas for future research into the design and operation of ATO manufacturing systems are suggested. It is particularly noted that few normative models for systems design exist in this area.

Job and Family Scheduling of a Flow-Line Manufacturing Cell: A Simulation Study

Abstract This paper compares four part family scheduling procedures, which seek to avoid setups by sequencing similar jobs consecutively, with four job scheduling procedures which are oblivious of part family affiliations. Of these eight scheduling procedures, four are applied intermittently to finite job sets in stochastic and dynamic environments while the other four rules are dispatching heuristics applied dynamically in the same environments. The shop configuration is a 5-stage pure flow-line manufacturing cell. Besides determining the ranking of the procedures in various operating environments, the impact on relative scheduling performance of the mean time between job arrivals, the number of part families processed, and the family setup-to-job processing time ratio is investigated. Significant interaction effects between all these factors were detected. It can be concluded that, for the scheduling procedures and conditions used in this study, family-based scheduling approaches can generate marked imp...

The behavior of lot-sizing procedures in the presence of forecast errors

Executive Summary This paper discusses lot-sizing in time-phased order point systems under three different conditions: with no forecast errors present, with forecast errors present but no safety stocks, and finally, with forecast errors present but with safety stocks introduced to counter the effects of the demand uncertainty. Fourteen different single stage lot-sizing procedures have been observed during simulation experiments where each of these operating environments have been modeled. The existence of forecast errors radically affects the behavior of the lot-sizing procedures compared to situations without forecast errors. For example, forecast errors not only lead to stockouts, they also induce larger inventories. Introduction of safety stocks, in turn, generates even larger inventories and also more orders. These environments, therefore, are so different, as are the associated lot-sizing performances, that the relevance of previous research which has not considered stochastic environments must be questioned. One must conclude that, despite the enormous interest in lot-sizing research over the years, the “scientific” body of knowledge, as it can be applied in practice, remains relatively undeveloped. Few guidelines with respect to lot-sizing can, therefore, be offered to the practitioner by the research community. More relevant, i.e., more realistic, lot-sizing research should alleviate such a situation.